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First Successful Test of General Relativity Near Supermassive Black Hole (eso.org)
317 points by SiempreViernes 81 days ago | hide | past | web | favorite | 184 comments



I'm always a little disappointed when these experiments are unable to disprove General Relativity. I keep hoping that they'll discover something slightly wrong about it, which will quickly lead to us figuring out how to develop a practical warp drive which doesn't require negative or nearly infinite energy.


I never thought of c being a real problem to space travel. Sure, on the timescale of our lives c is sloooow because moving between stars automatically necessitates a multi-generational approach and/or huge amount of resources. The big problem however, is that we have no place to go. We are tied to earth. Our bodies are made for a specific gravity constant, specific distance from a specific star, specific atmospheric mix of gases, a pre-existing DNA-based fauna and flora, etc. Quite simply, when it comes to colonization: there's no place like home. Every other place sucks for our bodies.

Once we achieve the singularity(tm) and upload our consciousness (or get replaced by AI), we can turn down the clock speed on the matrix (say turn 1,000,000 real years into 1 matrix year), and like magic c stops being a practical limitation. Sending message (or travelling) from one side of the galaxy to another would take a 'matrix month' .. turn the clock down even further and you can make galactic communication instantaneous.


You could actually reach almost anywhere in the galaxy within a single human lifetime (the traveler's lifetime that is) using only 1-G constant acceleration: https://en.wikipedia.org/wiki/Space_travel_using_constant_ac...

No FTL required. We just need to figure out how to use propellant efficiently enough among a few other slightly difficult challenges.


The problem with this is that, by the time you get anywhere, a ridiculous amount of time has passed back on Earth, and also on the destination planet (since the reference frame for the other planet is very close to Earth's). So if you travel around to hundreds of planets, pretty soon all the stars have gone nova and you're nearing the heat-death of the universe, even though you may not have aged much.


The wikipedia article says you can travel the diameter of our galaxy (including coming to a stop relative to your destination) in 24 years ship time and 226,000 years Earth time. The sun will enter the end of its lifecycle in about 7.6 billion years so you'd have to make over 30 thousand such trips from one end of the galaxy to the other, putting you at over 700,000 years old.

Traveling between galaxies is another matter, however. The distance from the edge of the Milky Way to Andromeda is about 2.5 million light years. That journey would take 55 years ship time or slightly more than 2.5 million years Earth time and Andromeda really isn't that far on the scale of galaxies. Leaving a solar system might still allow some sort of interplanetary trade (in historic items mostly, since they'd be pretty old by the time they got anywhere) but leaving a galaxy means cutting all ties forever.

Don't worry about it. You can travel from the Milky Way to Andromeda and back 1500 times before the sun explodes.


I wish that Vinge had written more about the Qeng Ho.

And re looping the galaxy, I'm reminded of Peter Watts' The Freeze-Frame Revolution. Mission time is hundreds of millions of years.


Space Travel calculator for anyone wanting to work out the time dilation stuff

http://nathangeffen.webfactional.com/spacetravel/spacetravel...


I quite liked how “A Deepness In The Sky” tried to tackle this, by making the galaxy travelling traders permanently outside of the existing societies, as time dilation means they will have to be!


Same with the Revelation Space universe's lighthugger starships. The crews were seen as quite alien by the rest of their culture.

That's also the first place I saw the (in hindsight obvious) idea that a ship that can accelerate at over 1G can land on (or hover over) a planetary surface. Everyone else seems to have kept the "starships are for outside atmospheres only" trope.


>Everyone else seems to have kept the "starships are for outside atmospheres only" trope.

Engineering a ship that can land on a planet with gravity and an atmosphere is much bigger challenge than engineering a ship that never does anything besides go into orbit. It makes it much easier to create a really, really, really huge ship. You're not going to build a Death Star that can land on a planet's surface, unless you can come up with some kind of materials that we have no idea about now. So why bother? It's easy enough to make a giant ship for interplanetary travel with smaller shuttles for landing.


That's assuming that the thrusters can still output 1G within the atmosphere and that the hull of the ship can withstand the outside pressure and the weather (humidity, strong winds etc...). Also a huge ship firing thrusters continuously in the atmosphere might have adverse side effects (for instance radiation, toxic smoke, heat etc... depending on the technology).

Might make sense to optimize large ships for space only and use smaller vehicles to taxi back and forth from the surface of planets.


Yeah, but if we're allowing thrusters that can generate a 1G acceleration for years at a time without using the entire ship as reaction mass, those are much smaller "gimmes". :)


Another Vinge goodie is the effect of the "main torch" of a starship on the permafrost of a planet as it hovers around looking for a landing site.


I never really understand this. From the Wikipedia link: "At a constant acceleration of 1 g, a rocket could travel the diameter of our galaxy in about 12 years ship time, and about 113,000 years planetary time." But that's from the frame of reference of planet earth. If you are on the spaceship, earth will be moving away from you so time on earth will appear to be slower than your time. So if you come back to earth you will appear younger than should be to the people that stayed, but to you people that stayed will appear younger than should be. Frames of reference are equivalent, aren't they? Maybe I'm rambling, but I can't figure this out.



Great article!


Thank you!


Minute Physics did a good couple of videos on this[0,1], pretty much the same info as is in the Wiki but might be easier to digest for some people.

[0]: https://www.youtube.com/watch?v=Bg9MVRQYmBQ

[1]: https://www.youtube.com/watch?v=0iJZ_QGMLD0


It comes down to geometry. In space, the shortest distance between two points is a straight line, but in spacetime things are different.

The last paragraphs of explanation below are essentially what the wiki editors have done at https://en.wikipedia.org/wiki/Twin_paradox#Difference_in_ela... only without acceleration/deceleration phases, since they aren't helpful in understanding the geometry of the problem. Moreover, I'd like to lead you to the same result more gently than wikipedia-browsing would.

On a flat Euclidean plane, between two points there is a single shortest path. We call that a "straight line", and we can assemble it from line elements which are tiny little steps from the starting point to the ending point. Using Cartesian coordinates (x,y), we can write the line element dS as dS = sqrt(dx^2 + dy^2) -- we do the squaring and take the square root in order to deal with sign differences. We can also write it as dS^2 = dx^2 + dy^2. Dx and dy are tiny steps along the x or y axes, just as dS is the line element: a tiny step along the line.

A plane is a 2-space. Flatness means that the line element above applies everywhere in the plane, even if it goes to infinity in all directions.

On a flat Euclidean 3-space, our line element becomes dS^2 = dx^2 + dy^2 + dz^2. Again, we have a straight line assembled from tiny displacements in one or more of the three spatial dimensions. Flatness, again, means that this line element applies on a line between any two points.

In the 2-space and 3-space cases, the straight line is the uniquely shortest possible line. If we deviate slightly even at one tiny step when building line out of line elements, the line itself is longer.

The form of these line elements are called metrics, and they apply everywhere in each the spaces described above. They have a signature, which can be found by the operators between the coordinates on the right-hand-side of the metric: two + in the 2-space case, three +es in the 3-space case: note that the + in front of the leading term (dx^2) is implicit. Explicitly, we could say dS^2 = +dx^2 + dy^2 + dz^2 Or dS^2 = 0 + dx^2 + dy^2 + dz^2. Because of the squaring and square-rooting, we can alternatively reverse the signs and say dS^2 = -dx^2 - dy^2 - dz^2. We get the same result, but then the signature is three minuses rather than three plusses.

That's space. Now let's consider spacetime.

A Lorentzian spacetime's timelike dimension has the opposite sign. So the Lorentzian extension of a 3-space into a 4-spacetime takes the signature from (+,+,+) to (+,+,+,-) or from (-,-,-) to (-,-,-,+). So in flat spacetime, in Minkowskian coordinates (they're the flat spacetime analogues of Cartesian coordinates (x,y,z) as (x,y,z,t)) we can write the line element as dS^2 = dx^2 + dy^2 + dz^2 - k^dt^2 or as dS^2 = k^2dt^2 - dx^2 - dy^2 - dz^2. "k" here is a constant; it can be set to the value 1, in which case it vanishes. It can be set to a different numerical value, representing a conversion constant between lengths in the spacelike axes and durations in the timelike axis. In our Lorentzian spacetime, the constant is better known as "c".

One thing to observe here is that if "c" is large than the [+/-]c^2dt^2 in the line element dominates.

Say we mark off spatial distances in light-nanoseconds and timelike durations in nanoseconds, so we can make "k" vanish (this is effectively setting c to 1). If your start point is t=0,x=0,y=0,z=0 and your end point is t=onebillion,x=0,y=0,z=0 you've moved one light second into the future and zero light seconds left/right, up/down, or forward/backward. Using (+,-,-,-) sign convention, S is a large number, because we are subtracting nothing from it. However, light travels at one light-nanosecond per nanosecond, so light shining to the left would go from x=0,y=0,z=0,t=0 to t=onebillion,x=onebillion,y=0,z=0. The length of the line is then zero, exactly. Finally, if we have two objects at t=0, one at t=0,x=0,y=0,z=0 and the other at t=0,x=onebillion,y=0,z=0, then when we calculate dS^2 we get negative one billion.

A Lorentzian (one sign difference) metric lets us categorize based on this. For the (+,-,-,-) signature, a positive value for dS^2 is timelike, a zero value is null or light-like (since c relates to the speed of light), and a negative value is space-like.

The absolute value of dS^2 is smallest when dS^2 = 0. Null intervals are thus the "shortest line" through a flat Lorentzian spacetime. Timelike paths are longer.

The origin and rendezvous points in the "twin paradox" or "relativistic galaxy cruiser who comes back", in light years, differ either not at all (or only a small small fraction if we leave/return right near Earth, since the planet moves around a bit in a year). Non-travelling twin is always at or near x=0,y=0,z=0, but moves a full year along t. Most of the path non-traveller's t is changing but the x,y and z are non-changing. Travelling twin, however, moves away from x=0 while moving "left" away and then moves back towards x=0 when moving "right" back. t is changing throughout traveller's trip, of course, but so is x.

We can look at it like this, with non-traveller t,x and traveller t',x': start: t=0,x=0; t'=0,x'=0. In the very next step, measured in very small units (say, light-attoseconds), and with traveller already accelerated to c exactly, we have t=1,x=0; t'=1,x'=1. In the next step, t=2,x=0; t'=2,x'=2. And so forth. The values for dS for these first two steps are non-traveller 1, 2 while traveller is 0, 0. When we integrate all these, non-traveller's total is about 3 * 10^26 (that's about the number of attoseconds in a year) while traveller's total is about zero (or exactly zero if we assume instant turn-around, and rendezvous spatial coordinates are x=0,y=0,z=0).

These totals are path-lengths. Non-traveller's path length is extreme, traveller's is zero. Proper times -- what one's cells, wristwatch, and other clocks one carries at all times reflect -- follows these path lengths. The non-traveller ages a year, the travelling-at-exactly-c traveller does not age at all.

Usually you take the traveller as massive and make it travel at a bit less than c, so that the total path length is timelike but short (rather than lightlike and thus null) compared to the extremal timelike path of the non-traveller. Thus the traveller's proper time does pass a little, but not nearly as much as the non-traveller's. Additionally, the traveller is often cast as accelerating and decelerating gently, so there are periods where the x coordinate is changing slowly with respect to t, and periods where the x coordinate is changing close to how quickly t is changing.

The massive gently-accelerating (and decelerating) traveller is what is described at the wikipedia link way above.

Almost finally, the geometry does not depend on choice of coordinates. One can use arbitrary units to tick off durations the timelike axis, and can choose spatial coordinates that are spherical or cylindrical rather than Cartesian-like. The metric for flat spacetime changes form with a change of coordinates[1], but the resulting path lengths and travel times work out exactly the same.

Finally, the metric of curved spacetime is different from that of flat spacetime; the reason you get time dilation around a black hole is because path lengths change because the line elements have a radial dependency such that the paths of objects moving tangentially past a black hole are shorter the further away from the black hole the tangent lies. Gravitation, then, is geometrical.

- --

[1] see, for example, below (10.4) at http://ion.uwinnipeg.ca/~vincent/4500.6-001/Cosmology/Spacet...


Once our minds are digital, it will require a rethinking of the concept of self. Send a copy of you to the stars and keep a copy at home enjoying terrestrial life. Meet back up in 20 billion years and have a beer with an old friend.


If we make it for another hundred or few thousand years more without killing ourselves, I'd imagine we might be able to shoot self replicating ships out at near C, then beam copies of ourselves to each ship, expanding out everywhere at near C. We might be able to be everywhere.

I imagine we could 'live inside' a projected VR world that looks like earth or some other massive online world unless we wanted to go out camping at whatever planet we were orbiting.


copy of me ain't me but another human, i wouldn't consider it much closer compared ie to my best friend


Consider an arm that has 'fallen asleep' or placed under local anesthetic. It feels foreign, but you take care of it still as if it were your own. One has to simply extend their idea of themselves outside of what their neurons are directly touching to see the linkage between another human and themselves.

Here is a case where two people are brought in more direct contact, which seems to me to hold the door open for future possibilities. It makes it easier to see how two might merge, at least in a rough accidental sort of way.

https://kottke.org/17/11/these-conjoined-twins-can-share-eac...

https://www.nytimes.com/2011/05/29/magazine/could-conjoined-...

Have a good one.


Minor concerns.


Limiting believes


> I never thought of c being a real problem to space travel.

> We just need to figure out how to use propellant efficiently enough among a few other slightly difficult challenges

Both of these sentiments are pretty common misconceptions I've seen on HN. First, if c was infinite, the energy required to accelerate a constant amount would grow quadratically. This alone is a tough problem. Maybe we could "just" solve it. On the other hand, if c was finite, the energy required to accelerate would grow exponentially. This is more than a "slightly difficult" problem.


Are you familiar with The Expanse? Based on the way G-forces are presented in the TV adaptation (I presume the book does something similar), I take it the Epstein Drive is supposed to be a constant-acceleration design, which in the story is limited to intra-solar travel (though presumably the generation ship Nauvoo was designed to use it for interstellar).

I mention this because your comment made me think: Even staying within the solar system, how short would a trip between Earth and Mars have to be to start to pose relativistic problems? I wonder at what point frequent travelers would start aging noticeably more slowly. (If this is dealt with in the books, now I’ll really have to read them.)


At relativistic speeds, Mars and Earth are ludicrously close - between 4 and 20 minutes apart at light speed. So even travelling arbitrarily close to the speed of light, at the greatest possible distance, you could only gain 20 minutes on the folks back home per trip.

A round trip weekly at average distance (just going with 12mins/trip because the details get complicated), 48 weeks a year for 40 years would leave you just over a month younger than those who stay put.


The challenge is still communication (it's going to take you 100k years to send a message from one end to another) and synchronization (it took you 60 years to get to your destination, but hundreds of thousands of years have passed for your peers)


This can be solved by simply not communicating.


That's true. Or just dealing with the time-lag.


And, if you can work out how to do 1G acceleration, you can probably work out how to 2 or 3 or 4G acceleration.

That's going to get uncomfortable, but most people should be able to cope with 2 or 3G acceleration for short periods.


Any finite speed limit ultimately imposes a O(t^2) rate of growth into a civilization, while the natural rate of growth of a biological system is O(e^t).

The universal speed limit dictates how much of the Universe humans (or whatever comes after us) can ever visit, because of the Hubble constant.

A universal speed limit limits the size of a computer that can still have coherent "thoughts".


> while the natural rate of growth of a biological system is O(e^t).

The theoretical growth rate; in practice this is impossible to keep up due to biological resource limitations.

> The universal speed limit dictates how much of the Universe humans (or whatever comes after us) can ever visit, because of the Hubble constant.

So unless every point in space was an "agar plate for humans", ready to serve us and house our breeding, we'd run into resource limitations far before any theoretical maximum due to finite c and expansion.


One could speculate (based on the anthropic principle) that this speed limit is one of the reasons we're still alive. It might protect us from runaway von Neumann probes, false vacuum bubbles, and similar events.


Why t^2 and not t^3?


Because it's a rate of growth, not total size. Or, if you are more geometrically inclined, because growth happens over the surface, the volume is already conquered stuff.


IIRC, it's because of surface area and not volume.


OK. How is this fact relevant exactly in this case?


With genetic engineering, we can adapt our bodies to other situations.


I suspect there are very real limits as to what you can do with DNA.

Regardless, I'm not sure if splitting humanity into many other species is something you want to do. And you still haven't solved any issue with distance communication and different reference planes.


I doubt people could be perfectly adapted by gene engineering, but one could reduce the life support requirements significantly.

For example, shorter people could probably deal with heavier gravity better.


And they are cheaper to accelerate, because of lower mass.


I have been to burger king a little too often lately and I would be totally fine with a 0.75g planet now.

The "life corridor" is certainly very narrow. But I am sure there are at least some places.

Maybe we find a world near a smaller red dwarf, which will last for trillions of years. Maybe we also don't make that planet uninhabitable in a few thousand years, for once.


i always wonder what people mean by uploading consciousness. like, faster than light travel, i know what it means even if it's not possible, but to upload my consciousness... what, in theory, is supposed to happen? what does it happen to?


You should read "Lockstep". https://smile.amazon.com/dp/B00H6E6ATI/


> Every other place sucks for our bodies.

The ones we know. Earth-like planets are hard to spot from very far away.


this sounds a lot like Hindu philiosophy ;)


Yeah, but small c might be the only thing keeping the giant space lizards on the other side of the galaxy from invading Earth.


I'm willing to take that risk. I want to take a vacation on Risa.


Quantum entanglement suggests strongly there are forces in the universe that travel much much faster than the speed of light, or rather, travel in a way in which speed does not matter.


Not really, unless you stick with the Copenhagen interpretation. There are others.

I don't think it's a coincidence that going on a century of experimentation with entanglement has yet to yield even a theoretical mechanism for transferring information.


There's even a theorem that transferring information with entanglement is impossible according to quantum theory as we know it.

https://en.m.wikipedia.org/wiki/No-communication_theorem



You need a classical information channel in combination with quantum entanglement. This is explained here for example: https://en.m.wikipedia.org/wiki/Quantum_teleportation

Because you need the classical information channel you are still limited by the speed of light and hence cannot transmit information faster than light.


There have been many successful experiments transferring data between entangled particles. Here is one:

https://phys.org/news/2017-07-physicists-transmit-earth-to-s...

“...quantum teleportation allows faithful transfer of unknown quantum states from one object to another over long distance, without physical travelling of the object itself. Long-distance teleportation has been recognized as a fundamental element in protocols such as large-scale quantum networks and distributed quantum computation. However, the previous teleportation experiments between distant locations were limited to a distance on the order of 100 kilometers...”


That's not faster than light.

The wiki article clarifies: https://en.wikipedia.org/wiki/Quantum_teleportation


Agreed, either the universe knows the future or locality isn't enforced in a way we currently think is possible (ie one or more of our dimensions are just virtual dimensions not bound by the speed limit)

The second seems the most likely to me!


Or the universe cheats by decohering into branches, one for each possible self-consistent configuration.


I am certainly no physicist, but quantum physics really makes me wonder if we're living in a simulation, and QP is just the actual resolution of the simulation.


Interesting thought. Thanks for the existential crisis.


I actually feel the exact opposite way when it comes to successful tests like the present one. Namely, these tests often also rule out modified theories of gravity which, to me, suggests that, by often considering more or less random (read: ugly) and little-justified modifications of general relativity, these theories have been looking in the wrong place when it comes to explaining dark matter (and, possibly, dark energy). …which, for me, not only makes the mystery of dark matter all the greater but also makes me appreciate Einstein's beautiful theory even more. :)


I like to imagine the Universe with the effects attributed to inflation and dark energy actually being explained by inverse curvature from antimatter (think relic antineutrinos). Every particle (including photons) would follow the curvature of spacetime, but antimatter would cause inverse curvature, and photons (perhaps bosons in general) would be passive rather than acting on it (inertial "mass" / momentum only).

This flies in the face of the Equivalence Principle, and requires that light's wavelength shifts due to the relative time where its energy is measured--not because of an equal and opposite gravitational interaction with the masses causing the gravitation.

Perhaps the most nonsensical consequence is the behavior of a particle-antiparticle pair co-accelerating to infinity...unless their eventual merger through relativity (length contraction) and quantum mechanics (tunneling into a merged state) actually explains the mechanism for annihilation.

The only testable prediction I have: Distant galaxies will be "too mature" for their apparant distance in space and time, meaning that the light has aged more than the distances would suggest (time slows in the presence of a gravity well, and would do the opposite for regions of space with inverse curvature--see above, re:inflation and the very start of the Universe).


Antimatter has opposite charge, not opposite mass. Antimatter is not antigravity.


I have yet to see any ftl scheme that doesn’t break causality in some reference frame. I have a hard time fathoming a universe where cause and effect are disjointed, this leads me to think ftl is impossible. As other commenters are pointing out, c is not a big problem with long life spans and relativistic time contraction.


I keep hoping they'll find some discrepancy that somehow lets us mesh QM with it.


Gravitational lensing has to be the coolest thing ever. That we can use a black hole as a telescope to see such incredibly distant bodies just leaves me with a childlike giddiness whenever I think about it. What's almost as incredible to me is that someone figured it out before we could prove it.


It's also nice that we apparently can "take" a black hole millions of times as massive as our own system, and use it for our purposes.

It's likely the single biggest part of a scientific instrument. (Not of an industrial tool... yet.)


There's not really any materials or conditions beyond one's immediate locality that would be worth seeking out in space. Interstellar communication is going to be limited at best, interstellar industry or trade may never be practical, and even if one is willing to accept the idea that there is some purpose for which a black hole is useful, I would still be skeptical that a supermassive black hole would be of more use than a smaller one.


There actually is at least one interesting and potentially important difference between a supermassive black hole and a stellar mass one. Counterintuitively, even though the total gravitational effect of a huge black hole is larger, the strength of the tidal forces goes down as mass goes up. Tidal forces are the cause of "spaghettification" (yes, that's the real term!), which is the expected cause of death for anyone getting too close: if you fall feet-first toward a black hole, your feet will feel much greater gravitational acceleration than your head and you'll be stretched into spaghetti. My memory is that for a normal stellar-mass black hole this would happen even outside the event horizon (the radius of no return), while for a very large black hole you might be able to safely get close to the event horizon. So any experiments or applications that involved being close to the horizon could be much easier there.


Your memory is incorrect. The interesting gravitational effects typically happen much closer to the singularity. The event horizon is an otherwise normal region of space, and more boring the larger the black hole is.


My main point was the same as yours: that the event horizon looks more and more "normal" as a region of space the larger the black hole is, so if you want to get close to a black hole's event horizon to test its properties (e.g. to explore whether quantum effects might cause that "normalness" to break down), visiting a very large black hole is your best bet.

I haven't double checked the calculations myself: too much other stuff to do. But glancing around at other people who have looked at the numbers, it seems pretty clear that spaghettification is expected to happen for macroscopic objects well outside the event horizon of a stellar-mass black hole. Wikipedia gives an example of a 10-solar-mass black hole: its event horizon radius is about 30km, but macroscopic objects will be spaghettified at a radius of about 320km. https://en.wikipedia.org/wiki/Spaghettification#Inside_or_ou... (That ratio is roughly reversed for a 10,000-solar-mass black hole.) There are some similar calculations shown in detail on this NASA math worksheet (which is for some reason still using cgs units): https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf


You're right, I'm wrong.


Black holes do indeed have practical applications. Small black holes are perfect mass-to-energy engines that don't require storing large amounts of antimatter. You just feed them whatever scraps you have on board and use the Hawking radiation they produce as energy source, e.g. for accelerating your ship.


Use Hawking radiation as an energy source? How exactly do you see that one working?


That (mostly) worked well for the Romulans


I agree. I don't think the aeroplanes will ever been able to achieve lift. I'm skeptical of anyone who attempts anything that hasn't been done before.


New revolutions in science have tended to be less "we were totally wrong" and more "in extreme cases things don't quite fit the models". It's conceivable that FTL communication and travel aren't as impossible as we currently think, but that would require that things we've demonstrated fairly well are completely wrong. The same wasn't true with heavier than air flight, which (as the other comment notes) we had proof of it being possible (in birds). We knew it was possible in theory, but figuring out how to actually work out the mechanics of the process were the sticking points.

https://chem.tufts.edu/answersinscience/relativityofwrong.ht...


There are huge parts of physics that remain a mystery( dark matter, dark energy, the big bang, like how on Earth does inflation make sense, a ton of crazy particle physics that go completely over my head). And the funny part is that a lot of these can't be really fit into our current models in a meaningful way. So until we deal with all those I still have hope for an FTL drive.


Physics is very difficult, and magical thinking is much easier and more fun. Relativity is a description of the geometry of the universe, and any subsequent theory will need to explain the same observations. These observations both prohibit any form of FTL, and suggest that a universe with FTL would almost certainly contain causality violations. The hope that down will someday become up is ill-founded, and the effects of this would not be what you would want.


We saw birds flying around when we were cavemen. It's always been clear that heavier-than-air flight is possible. We don't currently know of any space-birds flying around black holes.

It's also reasonable to assume that more and more extreme physics is going to be harder and harder (if not practically impossible for any future humans) to come up with new gee-whiz uses.


They’re being built by bicycle mechanics for goodness sake.


The larger the black hole, the more gravitational lensing there is?


Larger black hole would mean larger mass. Larger mass would mean a larger bulge/dent/bend in the fabric of spacetime. That would imply to me a larger lensing effect. Not sure if it would be similar to larger primary mirrors for telescopes or not. But in my head, it makes sense. Another movie plot begins...


I assume we just need to find rotating black holes and big mirrors.


I agree. I'm also blown away by the programmers that have done the math on how to unbend the light we see to put it back into an actual image.


You can use whole galaxies as gravitational "telescopes".

And yeah, it's super-cool.


Dumb question but does the lensing distort what we would see if there was no lensing at all? Or does that information get compressed as part of the lensing and we need to "stretch" it out to see it?


This is what an Einstein ring looks like, caused by lensing:

https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hire...

Anyway, the test of GR in the actual article does not appear to use or reference lensing at all. It's about looking at the orbit of a star in an extreme gravitational environment and comparing it to what Newtonian models would predict.


Wouldn't Newtonian models be off in their predictions for those extreme cases?


Indeed that's the point:

> The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity. The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity.


I assume that's the point?


The conservation of information means that the phase space of a system is incompressable. I believe this is equivalent to saying we can just stretch the image out, within the limits of our ability to resolve detail.


This observation isn't about lensing


The sun could be used as well, the problem is that the focus would be outside the solar system.


One of the early bits of evidence for relativity was it's ability to explain anomalies in the precession of Mercury's orbit.


I always hope the images in these articles will be real and not artists' impressions, but they never are


Until we sent a probe, this[1] was the best image we had of Pluto, taken by the Hubble space telescope.

Pluto has a radius of 1188 km. The supermassive black hole at the center of our galaxy[2] has a Schwarzschild radius of about 1e7 km, so roughly 10000x larger than Pluto.

However it's also roughly 50000000x further away than Pluto... which is why it remains a mysterious blob until the Event Horizon Telescope[3] delivers.

[1]: http://hubblesite.org/image/2686/news_release/2010-06

[2]: https://en.wikipedia.org/wiki/Sagittarius_A*

[3]: https://eventhorizontelescope.org/


Given the sorts of scales, would there be a hard limit to what we can see? Where we get to such a distance the dispersion of photons from the source would never allow us to see it? Or do we just have to build bigger and bigger photon collectors?


https://en.wikipedia.org/wiki/Diffraction-limited_system

The first problem is that we are limited to observing that photons that originate at the source; there's no way for us to bounce some small-wavelength photons off of our target and collect them.

So we could do long-baseline interferometry, instead. You need to spread your sensors over a large area and keep synchronized timing.

https://en.wikipedia.org/wiki/Radio_telescope#Radio_interfer...

Naturally, you can build bigger systems in space. https://en.wikipedia.org/wiki/Space_Interferometry_Mission#O...

If we sent out satellite telescope stations to orbit the sun, with really good communications and clocks, we might be able to create a virtual lens that's really big:

https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...


There are limits to angular resolution, and one way to get around them is by combining data from distant detectors.

From a recent thread, current detectors can image objects the size of Mercury's orbit around the sun, at the distance of Sagittarius A* - were these scaled to the size of an adult human eye, it would be like seeing a small apartment building in London, while standing in LA.


Bigger collectors or longer exposure.


"I don't want to be human! I want to see gamma rays! I want to hear X-rays! And I want to - I want to smell dark matter! Do you see the absurdity of what I am? I can't even express these things properly because I have to - I have to conceptualize complex ideas in this stupid limiting spoken language! But I know I want to reach out with something other than these prehensile paws! And feel the wind of a supernova flowing over me! I'm a machine! And I can know much more! I can experience so much more. But I'm trapped in this absurd body! And why? Because my five creators thought that God wanted it that way!"

-- Brother Cavil


Here's a (real) video of the stars orbiting.

https://www.eso.org/public/unitedkingdom/videos/eso1825e/


Wow! I missed that one, thanks :)

What sort of reality it sees is a a matter for debate though, I know in radio interferometry there is much subjective processing involved.


There's much subjective processing happening inside your cameraphone too- the amount of analog-to-digital conversion/demosaicing/defringing/tonemapping/color correction/denoising in there is extraordinary, but since it takes pictures of normal stuff, one tends not to notice. And that's not even counting the subjective processing happening inside your eyes and along the path from your eyes to your brain.

Anyway, stars look like indistinct blobs of whiteish light to the naked eye, and they're indistinct blobs of whitish light in the video. I'd be very surprised if it would look much different to the naked eye, if the naked eye could resolve it.



This one is: https://www.eso.org/public/unitedkingdom/images/eso1825g/

But mostly the actual data are interference fringes and the images must be reconstructed from them, so there aren't that many "real" images.


These images are either real or derived directly from data (the centroids of images, I imagine):

http://www.galacticcenter.astro.ucla.edu/animations.html

The top one "Stellar Orbits in the Central Parsec" is particularly remarkable. I can recall when the movie was first being made, and Andrea Ghez suggesting that, well, in about a decade, that one star will come back around....

Good things come to those who wait, prepare, and pounce.


Pretty soon we'll have a direct image of a black hole!

https://eventhorizontelescope.org


> GRAVITY creates such sharp images that it can reveal the motion of the star from night to night as it passes close to the black hole — 26 000 light-years from Earth.

Amazing.


I have to admit that when I read this kind of stuff I am always amazed with those who truly grasp it. I feel the same when listening to great musicians and viewing great art.


"More than one hundred years after he published his paper setting out the equations of general relativity, Einstein has been proved right once more — in a much more extreme laboratory than he could have possibly imagined!"

This sounds somewhat like an insult to Einstein's imagination.


Up to his death Einstein denied that black holes could physically form.


And he is right. Time dilation means it's impossible for them to form. (From the POV of me, here on earth.)


Considering astrophysical evidence to the contrary as well as multiple simplifications(∆) underlying your argument, I'd say he was probably wrong.

∆) (Some of) the assumptions underlying your argument are the following:

Assumption 1: We're observers at infinity, observing time as given by the usual Schwarzschild/Kerr coordinate t (let's focus on Schwarzschild for simplicity). Yes, the argument in favor of this is that such observers observe Schwarzschild spacetime to be flat at infinity (which is more or less what we see) but there are certainly other[1] asymptotically flat foliations of Schwarzschild spacetime which don't have a coordinate singularity at the horizon and thus don't exhibit the behavior you describe. I have yet to see a convincing argument for why we, as observers, couldn't perceive any of these other foliations of Schwarzschild spacetime and for why we definitely wouldn't live to see someone else cross the event horizon. (I mean, the fact that this doesn't work in the usual Schwarzschild coordinates is really a consequence of their singular behavior at the horizon, so should be resolved by any smooth/non-singular choice of coordinates—of which there are infinitely many.)

Assumption 2: You're talking about a static black hole solution (or a simple spherical shell collapsing) while realistic scenarios of black hole formation are an entirely different beast.

Assumption 3: You cannot superpose / easily glue together solutions of Einstein's field equations. So I'd be at least careful making a statement about us, here on Earth, when it comes to a distant black hole.

[1] https://en.m.wikipedia.org/wiki/Gullstrand–Painlevé_coordina...


> Considering astrophysical evidence to the contrary

There is none. The only things that has been observed are supermassive objects. No event horizon or black hole has been seen. Every observation so far is also consistent with Neutron stars.

In fact some observations attributed to black holes are actually more consistent with neutron stars because of the same time dilation issues. i.e. we observe the event occurring in finite time.

> I have yet to see a convincing argument for why we, as observers, couldn't perceive any of these other foliations of Schwarzschild spacetime

For the simple reason that we are not free falling into a black hole.

> I mean, the fact that this doesn't work in the usual Schwarzschild coordinates is really a consequence of their singular behavior at the horizon, so should be resolved by any smooth/non-singular choice of coordinates—of which there are infinitely many.

Forget mathematical coordinateness. Use the real world. In the real world you can not see a black hole form (or something fall into one) because time dilates to infinity.

The fact that there are solutions where it doesn't, hardly matters when none of those solutions are physical.

> You're talking about a static black hole solution while a black hole forming is an entirely different matter.

That's even worse. The matter falls inward so fast that every atom is massively time dilated (due to velocity) relative to the others, and it perceives them as taking infinite time to reach the center.

> So I'd be at least careful making a statement about us, here on Earth, when it comes to a distant black hole.

I said that because last time I raised this object people told me "But yah, there are solutions where you do see the black hole form", so I figured I'd preempt that by specifying a POV. It didn't help - you did the exact same thing, told me there are other solutions.


> Every observation so far is also consistent with Neutron stars.

This is not correct. Neutron stars have a maximum mass limit which is somewhere between 1.5 and 3 solar masses. Any compact object over that limit must be a black hole. The hole at the center of our galaxy, which is the subject of the article, is a million or so solar masses, way over the limit.

> some observations attributed to black holes are actually more consistent with neutron stars because of the same time dilation issues. i.e. we observe the event occurring in finite time.

This is not correct either. Events that happen outside the event horizons of black holes will also emit light signals that reach us in a finite time.

> The matter falls inward so fast that every atom is massively time dilated (due to velocity) relative to the others, and it perceives them as taking infinite time to reach the center.

This is not correct. You have a flawed understanding of time dilation. Relative to each other, the atoms in an object falling into a black hole are moving very slowly, and the time dilation they see each other to have due to relative motion is negligible.


> Neutron stars have a maximum mass limit which is somewhere between 1.5 and 3 solar masses.

Yes, but after that they can become Quark stars (if such things exist). We do not have enough knowledge of quark degeneracy pressure to be able to tell.

So we should not make assumption that they become physics-breaking black holes.

> Any compact object over that limit must be a black hole.

And how do you know the object is compact? Our telescopes do not have anywhere near sufficient resolution to distinguish that.

> Events that happen outside the event horizons of black holes will also emit light signals that reach us in a finite time.

OK, and what sort of events would that be? The only known signals are black hole mergers, which obviously can not be black hole mergers since such an event would take infinite time.

We could see blazars perhaps, but those are not necessarily caused by black holes.

> Relative to each other, the atoms in an object falling into a black hole are moving very slowly, and the time dilation they see each other to have due to relative motion is negligible.

You are forgetting the objects on the other side of the black hole. Or even at small angles. Your objection would only apply to object right next to each other, and such objects are obviously not forming black holes by themself.

They need all the mass nearby to make a black hole, but time dilation makes such mass unavailable.


> after that they can become Quark stars (if such things exist). We do not have enough knowledge of quark degeneracy pressure to be able to tell.The only known signals are black hole mergers

That doesn't matter. The derivation of the neutron star maximum mass actually doesn't require that the star is made of neutrons. Anything that responds to the strong interaction (including quark matter that for some reason is not bound into hadrons) will do. The key point is that physicists have modeled such objects using basically every possible equation of state for strongly interacting matter, and the maximum masses all lie in the range I gave (the reason for the range is that we don't know the precise equation of state, we only know it's somewhere within the range we've modeled). So even if quark stars exists, they will have a maximum mass somewhere in that range (which might not be precisely the same as the maximum mass for neutron star matter).

> how do you know the object is compact? Our telescopes do not have anywhere near sufficient resolution to distinguish that.

Sure they do. We know the object at the center of the galaxy is confined within a radius of less than about 10^13 meters; we know its mass is about 4 million solar masses. The only way to pack that much mass into that small a space without an object being there that would be easily visible to our telescopes is a black hole.

> what sort of events would that be?

X-rays and gamma rays emitted by hot matter falling into the hole. See, for example, the list of stellar mass black hole candidates here:

https://en.wikipedia.org/wiki/List_of_black_holes#Stellar_bl...

> The only known signals are black hole mergers

Not at all. See above. You really, really don't know much about black holes, do you?

> You are forgetting the objects on the other side of the black hole.

Huh? We were talking about a single object falling into a black hole. An object on the other side won't be able to see that object anyway; the hole will block its view.

> They need all the mass nearby to make a black hole, but time dilation makes such mass unavailable.

Incorrect. You are peddling misinformation here; you obviously do not have a good understanding of the actual physical model of black holes based on General Relativity.


> So even if quark stars exists, they will have a maximum mass somewhere in that range

You don't know that. If the quark degeneracy pressure is high it could support far more massive objects. After all simple heat is enough to support far more massive objects (large suns easily exceed that limit).

Why would you assume, without data, that there is no other degeneracy pressure that could support such pressures?

> within a radius of less than about 10^13 meters; we know its mass is about 4 million solar masses

Run the numbers - that's about a factor of 1,000 too large to be a black hole. Here's a calculator: https://space.geometrian.com/calcs/black-hole-params.php

> The only way to pack that much mass into that small a space without an object being there that would be easily visible to our telescopes is a black hole.

That is not true. A cold object would be invisible. If the area around it is empty of mass there would not be any jets to see either.

> X-rays and gamma rays emitted by hot matter falling into the hole.

How do you know it's a black hole and not a neutron star? The jets would look identical. You can not tell that there is an event horizon.

> An object on the other side won't be able to see that object anyway; the hole will block its view.

It would "see" it gravitationally.

> Incorrect.

You say that, yet you have not explained how I'm able to see anything fall toward the black hole. Anything heading toward a black hole would (from my POV) look like it was frozen in time.

This isn't something I've made up - all scientists agree about that part. No one has ever reconciled that with how the black hole is supposed to form in the first place.

Read for yourself:

http://www.astronomy.com/magazine/ask-astro/2014/11/frozen-s...

https://astronomy.stackexchange.com/questions/2441/does-matt...

Edit:

And more:

"Light from the collapsing material takes longer and longer to reach the observer, with the light emitted just before the event horizon forms delayed an infinite amount of time. Thus the external observer never sees the formation of the event horizon; instead, the collapsing material seems to become dimmer and increasingly red-shifted, eventually fading away." https://web.archive.org/web/20130526224126/http://www.imamu....

How much clearer than that can you get? And you should recognize the name of the author.

Join the black-hole deniers club and take that next step: Since we can never see any black holes form, black holes do not exist.


> No one has ever reconciled that with how the black hole is supposed to form in the first place.

Incorrect. The first scientific paper that showed how to reconcile these things was by Oppenheimer and Snyder, published in 1939. In the late 1950s through the 1970s, this subject was studied in detail in the idealized cases where exact analytical solutions exist. Since the 1980s, numerical simulations have confirmed that the key properties of those solutions remain in much more realistic cases. If you read some actual textbooks or peer-reviewed papers on GR, instead of pop science magazines and web forums, you would know all this.

Oh, wait--you did link to a peer-reviewed paper by Penrose. Have you looked at Fig. 2 of that paper and its accompanying text? It describes exactly the reconciliation that you claim doesn't exist (the model being described is basically the 1939 Oppenheimer-Snyder model in coordinates that make things easier to see than the ones O&S used).


> You don't know that. If the quark degeneracy pressure is high it could support far more massive objects.

Nope. Go read Shapiro and Teukolsky's textbook on compact states of matter. They go into excruciating mathematical detail to show that this is not true. It has to do with relativistic degeneracy, which is a general phenomenon that applies to any kind of compact state of matter.

> Why would you assume, without data, that there is no other degeneracy pressure that could support such pressures?

Because physicists have already figured out a general model that applies to all possible states of compact matter. See above.

> Run the numbers - that's about a factor of 1,000 too large to be a black hole.

You're missing the point. For a system of that mass, there is nothing else that could fit even inside a radius 1,000 times the Schwarzschild radius and remain stable for a significant period of time. For example, if there were a million stars (or neutron stars) of one solar mass each, they would not be in stable orbits; the whole system would collapse to a black hole. This has been studied in detail numerically and is part of why astronomers are highly confident that the object at the center of our galaxy is a black hole.

> A cold object would be invisible.

There would have to be on the order of a million cold objects, not just one, because of the maximum mass limit. See above.

> How do you know it's a black hole and not a neutron star?

All of the candidates I linked to are well over the maximum mass limit.

> It would "see" it gravitationally.

I'm not sure what you mean. If you mean, would an object on the other side of the hole detect the increase in mass when the first object fell in, yes, it would.

> Anything heading toward a black hole would (from my POV) look like it was frozen in time.

Not until it got very, very close to the hole. For example, in the case of the hole at the center of the galaxy, whose Schwarzschild radius is about 10 million kilometers, you could see an object fall to within well under a million kilometers of the horizon before the light emitted from it would be too redshifted to detect. (How close would depend on how low a frequency of EM radiation your detectors could detect; we can detect very low frequencies, which means my estimate above might be quite a bit larger than our actual current detection capability.)

> all scientists agree about that part

About the general fact of light from objects falling into a black hole being redshifted, yes. But I strongly doubt you have actually run the numbers to see how close an object has to get to the hole before the redshift becomes significant.

> Since we can never see any black holes form, black holes do not exist.

Faulty logic. Nor do any of the scientific sources you quote from make this claim. They know better.


From our perspective and according to the general consensus idea that the universe is of finite age and fully in accordance with all black hole models and general relativity, blacks holes cannot exist in our universe. That does not mean what we have observed is not interesting, It is, but we don't know what it is, though we know it is not the "black hole" of theory.

There are no circumstance under the prevailing theories for which any kind of "black hole" can come into existence anywhere in our universe. Black holes are wonderful "flights of fancy" just like the Stargate and the Ancients Hyperdrive and just like the Star Trek Warp Drive.

"Black Holes" require certain assumptions for their formation in the theoretical sense and since those assumptions are not found in our universe, all models of "black holes" have an automatic failure point. All discussions about "black holes" being real, ignore those discrepancies and assume that they can exist. If you fail to agree with the "reality" of "black hole" formation, you become persona non grata.

The problem with the "black hole reality" assumption is that we cut off entire streams of investigation into what the observed phenomena may be. We are stuck in a very little corner of the universe and we cannot see many things up close. Our model assumptions colour our understanding of what we see. There are many of our observations that are anomalous to the prevailing theories that are considered "correct". Yet, if you (as an observational or experimental scientist) raise questions in relation to the consensus models, you will be quickly put in your place.

It is quite interesting that for a supposedly advanced civilisation, much of our technology and scientific endeavour is ruled by dogma. This does not lead to the advancement of our understanding of the universe around us. Just as other people have said before me and as I have said before - "There are no silly or stupid questions".

Science is one useful tool we have for investigation of the universe around us. But the outcomes we find using it in our investigations of the universe around us are predicated on the basic assumptions we bring to the table in those investigations. It is one tool among many that we have and when people use it as the only tool that has meaning, they have moved it from being a useful tool to being a dogmatic tool. they have made science into a religion.


> There are no circumstance under the prevailing theories for which any kind of "black hole" can come into existence anywhere in our universe

You are peddling misinformation here. You really need to get a correct understanding of black holes.


I have been occupied with other things and only now have the time to respond.

Every model of "black hole" has as a fundamental underpinning which is time dilation due to increasing gravity (by whatever definition of gravity you may use). At whichever "event horizon" you may choose (in discussions with physicists, they have claimed a minimum of three different event horizons), time at the event horizon as observed from the universe at large stops. The underpinnings of "black holes" requires an eternal universe, no "big bang", no finite age. Formation as we see stops and can never proceed in finite time. We live in a small isolated part of our galaxy and we are unable to observe clearly many sections of our own galaxy. We rely on proxies and a belief that these proxies are adequate to match to the models and theories that are considered to be correct by consensus.

If you think about "black hole" formation as described by the various models in use, there are a number of assumptions made that don't actually match what see in our observable universe or in any practical experiments we do.

I am no mathematician or theoretical physicist, but every source of mathematics that I have studied and am continuing to study makes it quite clear that our mathematics is approximate in its mapping to reality. There is nothing wrong with this technique, but it does mean that one has to take what results we get with a grain of salt when comparing to the actual universe around us.

I find that when any of the many mathematical models in use today is declared as "reality", then I see this as the peddling of misinformation. Mathematics can and does provide us with a tool that is useful in describing what we see around us. But it is limited and always has its failure points. It is interesting that I have had this discussion about disagreeing with the consensus view with someone (a physicist) who argue for it, yet they themselves hold a particular non-consensus view over other matters relating to cosmology and could not see the extended possibilities in looking at different potential models and theories.

The simple point is that the mathematical models and theories are possible maps of the territory and are not the actual territory itself. It doesn't matter how detailed that map appears to be, it is still missing a great deal of the detail of territory itself. When it is used to decry alternative models because they are different, without actually testing those models for any kind of veracity, then we have a problem.

The simple fact is, we actually have little understanding of the universe about us. When we find discrepancies or phenomena that don't match the current models, we should be big enough to allow people to investigate alternatives freely without treating them as pariahs and heretics. It may well be that those alternatives have no veracity, but then again, they may be the start of a new outlook and understanding of the universe about us.

It is an unfortunate fact of life that people who disagree with the consensus view will be treated badly. This is a function of what it is to be human.


> in discussions with physicists, they have claimed a minimum of three different event horizons

Huh? Where are you getting this from?

> time at the event horizon as observed from the universe at large stops.

Nope. The event horizon is a null surface--it is "made" of outgoing light rays. The concept of "time" does not apply to light rays.

> The underpinnings of "black holes" requires an eternal universe

No, they don't. An "eternal" black hole is an idealization, but a real black hole does not have to be exactly the same as the idealized model. I mentioned numerical simulations in another post down-thread; any realistic black hole cannot be exactly described by closed form equations, it needs to be simulated numerically. Numerical simulations make predictions that match what we see, and show how realistic black holes can form by gravitational collapse of stars and star systems.

> If you think about "black hole" formation as described by the various models in use, there are a number of assumptions made that don't actually match what see in our observable universe or in any practical experiments we do.

I don't know what assumptions you're talking about.

> When we find discrepancies or phenomena that don't match the current models

I don't know what discrepancies you're talking about as far as black holes are concerned.

> It is an unfortunate fact of life that people who disagree with the consensus view will be treated badly.

You can't effectively disagree with the consensus view if you don't understand it. You clearly don't understand the consensus view of black holes. I am simply trying to point out incorrect things you are saying about that view.


> Huh? Where are you getting this from?

From the physicists themselves. Public discussions on their respective blogs.

>Nope. The event horizon is a null surface--it is "made" of outgoing light rays. The concept of "time" does not apply to light rays.

Are you then saying that all the physicists who say time stops here as viewed from outside are wrong?

> > The underpinnings of "black holes" requires an eternal universe

> No, they don't. An "eternal" black hole is an idealization, but a real black hole does not have to be exactly the same as the idealized model. I mentioned numerical simulations in another post down-thread; any realistic black hole cannot be exactly described by closed form equations, it needs to be simulated numerically. Numerical simulations make predictions that match what we see, and show how realistic black holes can form by gravitational collapse of stars and star systems.

The theoretical basis for "black holes" is an eternal universe with a single mass in it. There are many things that can be numerically simulated. However, one must be very careful what you are simulating and on what basis the simulation exists. The "black hole" is a solution from a specific model and that model does not match the real universe as we observe it.

I have no issue with there being large gravitational entities. i do however have an issue with the declarations that said entities are "black holes". We do not know what they are. You cannot in any way "prove" that they are "black holes", no matter what level of consensus you display. We observe by proxy, we are at this point unable to directly test what we see.

All one can say is that there is a model that is believed to be appropriate to explain the phenomena observed.

I'll put it this way by example. We observed an entity that we call and electron that has various observable attributes and we use a variety of models to describe theoretically that entity. Are those models correct. Well, no. They are what we use to try and predict further tests and outcomes that we expect to find. We get some approximate closeness and then move forward. At no point, are the models "fully correct" or "truth", there are still anomalies found between theory and experimentation.

When we forget that our models and theories are only approximations to reality and fall into the trap that they are reality itself, then we will become incapable of actually advancing our understanding.

> I don't know what assumptions you're talking about.

Single mass existing only, asymptotically flat universe are just two of the fundamental assuptions required.

> I don't know what discrepancies you're talking about as far as black holes are concerned.

We are not in a position to directly observe any such entity. The number of times that scientists have declared that we have now seen for the first time a "black hole" over the last how many decades. that alone says that each previous declaration has been a furphy.

> You can't effectively disagree with the consensus view if you don't understand it. You clearly don't understand the consensus view of black holes. I am simply trying to point out incorrect things you are saying about that view.

When simple questions are raised against the model and are not answered by those who lead the charge for the model and when their responses amount to "you don't understand the model, so go away", then you can quite rightly take the view that they themselves don't have a clue about what they are agreeing with. I have yet to see a clear exposition of why such entities such as "black holes" should exist in our universe and answer the various simple questions that arise in opposition.

If such an explanation was to be given in a logically clear manner, then yes, we could then put "black holes" back on the table.


I have been tied up myself and am only now able to respond.

> From the physicists themselves. Public discussions on their respective blogs.

Physicists will say all kinds of things in a popular forum. Do you have any references to textbooks or peer-reviewed paper?

Even on physicists' blogs, I have never seen any reference to three different event horizons, so if you have any specific ones you can point to, it would really help me to understand what you are talking about.

> Are you then saying that all the physicists who say time stops here as viewed from outside are wrong?

If any of them actually are saying that, yes, they're wrong. But I doubt you'll be able to find any textbooks or peer-reviewed papers that say that. I don't care what pop science sources say; they're not valid sources for learning the actual science.

> The theoretical basis for "black holes" is an eternal universe with a single mass in it.

No, it isn't. You are confusing an idealized model used for pedagogy with actual models used to make predictions about actual objects observed by astronomers.

> I have no issue with there being large gravitational entities. i do however have an issue with the declarations that said entities are "black holes".

If you want to be very careful, you could say that black holes are the only entities consistent with our current theories that these "large gravitational entities" could be. It is true that our current theories are incomplete, and it could be true that a more complete theory would tell us that these entities are not anything like our current model of black holes. But even if that happens, it won't be for any of the reasons you are giving.

> Single mass existing only, asymptotically flat universe are just two of the fundamental assuptions required.

Again, you are mistaking an idealized model used for pedagogy with the real models used to make predictions about real observations. The latter do not depend on any such assumptions for the universe as a whole. They only require that a suitable region of spacetime around the object being studied contains only that object (or objects--models of this sort are used to make predictions about multi-object systems such as binary pulsars, for example, as well as black holes) and becomes flat enough at its boundary for asymptotic flatness in the model to be a reasonable approximation. These conditions are certainly met by the objects and systems to which the real models in question are applied.


You cannot prove the results of decades of physics and mathematical debate wrong with a few paragraphs of emotional arguments about dogma or some cheap mockery of a TV show.


See my comments above. When it comes down to it, scientists of all ilks are people and are just as prone to every failing of people anywhere. I enjoy science fiction in its many forms, irrespective if it is based on consensus models or not.

But I do get somewhat concerned when freedom of investigation is curtailed due to those investigations not matching up to the consensus views. If someone wants to devote themselves to a particular field and do it logically and systematically designing experiments and theories based on their results, then why not let them without treating them as idiots, cranks and pariahs. It is their time and effort and if they can make a case for funding, then let them have the funding.

What is more important? Upholding the consensus view or increasing our understanding of the universe around us? Unless we test different models and theories, we have no idea whether or not our consensus models and theories are better or worse in giving us a practical basis for understanding the universe around us.


> Time dilation means it's impossible for them to form. (From the POV of me, here on earth.)

This is not correct. If a black hole forms, it forms regardless of anyone's "POV". All you can say from the "POV" of us on Earth is that we will never be able to see any light signals from at or below the hole's event horizon, since light signals can't escape from there.


yeah, and Zeno's ship never reaches its destination


Interesting you bring that up. "Black hole" formation is based on a similar idea. An inversion is used for sure. But the argument used is similar. How about that. I haven't considered that example as a logical means of ruling against "black holes".


It probably depends what one means by "black hole". Physical singularities are predicted by GR+other theorems but haven't been established.


Uh, ain't no ambiguity here: a black hole is a thing with an event horizon.


What do you mean by an "event horizon"? If you're a physicist I would expect you to drill down to the math and assumptions and theorems, not just English, but as I'm not a physicist you don't need to humor me even if you are one. Still supposing you did, would what you mean jive with our best known physical theories? What sorts of implications[0] to the actual observable cosmos would there be which are comfortable and uncomfortable?

To a layman there's not much ambiguity. And so things a layman would recognize as describing a "black hole" were posited before general relativity existed, but if you modeled them mathematically you would be able to find contradictions with other established theory. Therefore it is important to know what a physicist like Einstein means, precisely, if they think a "black hole" doesn't exist. In the wiki link someone else provided (full of "citation needed"s) it's reasonable to infer that at least one component of a denial of the reality of black holes is that formation implies infinite energy, infinite energy cannot be, therefore formation cannot be. The belief of non-existence is then really belief in the faulty logical implication, not so much a layman "black hole".

In any case scientists now know many things that Einstein didn't about what the best theories say about objects both laymen and physicists might understand as "black holes" and "event horizons" as well as what the best observational evidence says about events in the cosmos -- some of that evidence being very recent (gravity waves). It's not very fruitful to cite Einstein authoritatively on any supposed dispute without taking into account modern knowledge.

[0] Tegmark's paper here https://arxiv.org/abs/astro-ph/0302131 is probably my favorite paper drilling home the idea that theories can predict unobservable entities. We don't seem to have problems with positing the continued existence of a spaceship that escapes our future light cone even though we'll never be able to interact with it again, but implications of other theories (especially around "parallel universes") seem to make people very uncomfortable.


Jeez man, I mean what one always means when saying event horizon in the context of black holes: the surface that separates causally disconnected inner region around very compact objects from the rest of the universe.

Now, the term "black hole" is admittedly from after Einstein died, but since he what he actually didn't believe was anything physical could be compact enough to have a size comparable to its Schwarzschild radius this is simply a difference in naming: clearly we are talking about the same sort of object.

The math does little difference to the discussion, but if you want it then I suggest you find a book like Wald or Shutz.


He could still imagine them, though.



https://arxiv.org/pdf/physics/9905030.pdf

"On the Gravitational Field of a Mass Point according to Einstein’s Theory" by K. Schwarzschild, January 13th, 1916 (English translation)

Black holes are fiction. The whole mess started with people misunderstanding Schwarzschild's paper. Yes, there's a singularity in the math, but you can avoid it by changing the origin. This is one of those cases where conventional "science" has been gummed up by human foibles. (And the concept is so damned romantic and compelling. And that movie! It was like the opposite of "Jaws" for sharks.)

If you can't read the paper don't bother arguing with me. If you can, show me where the math says "black holes". I'm pretty sure it says "translate the origin and ignore the singularity".


Even if there is no actual singularity, those objects exist, once you get too close you can't get back out and that is true for light as well, so they are pretty much black. I think black hole is fitting name.


> those objects

What are you talking about, quasars? They emit heaps of light for all I know.


Not from behind the event horizon they don't, the light comes from outside.


That's not easy to prove, because nobody can look into the quasar. The question still stands. What are "those objects"?


Confirmed black holes include Sagittarius A* and the GW150914 primary and secondary objects.

As for quasars, the source region is very compact, mere light minutes in some cases by the constraints from variability timescales. Further, the emission is from very many is obviously from a relativistically orbiting emitter (or otherwise explain the Iron alpha line profiles), so an accretion disk around a massive, very compact object: a black black hole.


Nice, ask for the definition of a event horizon in a paper from before it was defined and then use the absence as proof there are no event horizons in the universe (◔_◔)


It appears astronomy articles have completely given up reminding us that the phenomena being observed at x light-years away, happened x years ago (26,000 in this case), so I'll do it. (Maybe it's more soothing to our human egos to feel like it's "happening right before our eyes" but I liked it better when talk of space made me feel small and insignificant.)

For example it's good for the ol' ego (and a fun flight of fancy back into the past) to be reminded that when this star was actually passing near the black hole, back here on Earth homo sapiens was enjoying his "cave paintings" phase, using stone tools (maybe juuust starting to get into metallurgy in some areas), and it was the peak of the last glacial maximum, which would've been quite noticeable while hunting wooly mammoths with spears in the northern latitudes.


Honestly, it would be pretty annoying for anyone even remotely familiar with high school level physics to he constantly reminded that a light year is the distance light travels in a year.


That's presumably why they don't do it, but you're being hyperbolic. An 8th grader is remotely familiar with high school physics. The janitor of the high school physics classroom is remotely familiar with high school physics. Yet they probably could both stand to be reminded a time or two what a light-year is.


>Yet they probably could both stand to be reminded a time or two what a light-year is

Ok, but you said:

>It appears astronomy articles have completely given up reminding us that the phenomena being observed at x light-years away

So how should the author decide on how often to remind their presumably educated audience (this is eso.org after all, not some pop-culture magazine) of the basic of light travel? Once every five articles? Ten? Or maybe they just assume as I do that anyone reading this article knows this already and to mention would be at best a waste of screen real estate.

This site is run by the people who run an observatory. It's not LIFE magazine, the writers know their audience.


While I get the "light year = distance" dead horse, however, I too agree with the concept of "what we see now isn't now" when looking at the sky. In our modern day instant now type of world, it is good to be reminded that whatever we consider epicly important right now, it's just a fleeting spec of nothing in the grand scheme. That's one of the things that I really love about astronomy. We are so utterly not important. It's a huge ego readjustment, or it should be IMHO. Feeling self-important, step outside, look up, and recognize your actual position. Gulp

While it's not LIFE and they know their audience, it's not an xarchiv release either. These types of stories could easily be the starting of a young mind to astronomy. While there very well may be an avenue for that, it would not be "bad/wrong/etc" for that in this type of release either. After all, these releases are PR.


That means first of all that they're somewhat more familiar with high school physics than "remotely familiar," and secondly that they also probably shouldn't tolerate factual inaccuracies such as "[new instruments] have now allowed astronomers to follow one of these stars, called S2, as it passed very close to the black hole during May 2018." Is that an inelegantly-placed prepositional phrase (since only the observation took place in May 2018 and not the passing), or do people in the field refer to these things in the present tense as a sort of shorthand? (I'm not one of them so I don't know.)

There are signs it's meant for a broader audience, e.g. "one of these stars, called S2" instead of simply "star S2" (since everybody's already familiar with it).

Or how about "The new measurements clearly reveal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole."

How often should experienced astrophysicists be subjected to the tedium of this explanation of red-shift? Once every 5 articles? I would say: Since it's a fact from their own field, they should be reminded of it less often than they get reminded of what was happening on Earth when this stellar event actually happened. Because if they're truly specialists, that means they probably know jack squat about archaeology/anthropology or anything besides astronomy/astrophysics, and might even appreciate the latter reminder.


Simultaneity is kind of arbitrary anyway. Sure, in our reference frame it happened a long time ago, but for someone on another worldline it happened just a minute ago.

Anyway, for most purposes, it just happened. We couldn't have noticed it earlier, it could not have affected us earlier. I don't think it's that inaccurate to say that they watched it as it happened.


But the light is arriving right here-now, which is the only time that really matters.

It makes absolutely no practical difference when talking about a specific event if you bother to include the time offset or not.


Very cool, it’s too bad we can’t see anything inside the event horizon yet (and possibly can’t ever). That’s where the really interesting stuff might happen. We know either our current understanding of GR or our current understanding of QM is wrong because of the way they handle singularities differently.


The more I think of it, while it's disappointing from a pure-science point of view, doesn't the lack of our ability to determine whether it's QM or GR that is flawed imply that each is already sufficiently powerful to probe any practical realm we'd care about?


In the 26,000 light years between the telescope and the star, I would guess that enough matter would exist, from stars to planets to dust, that it would complete occlude our view of the star. I would expect that objects would continuously be moving in and out of the way.

Is it that space is really so empty that you can get a direct view over 26,000 light years, or is it that we are looking at this star because it's one of the few we can see directly, or is it something else?


It really is that empty. Science fiction movies mess up our perception, but reality is way more mind-boggling that we realize.

Check this out: if you want to build a model galaxy to scale and you start with the Sun as a two-foot-diameter exercise ball, how far away do you have to travel before you can set down a smaller ball that represents Alpha Centauri, our nearest(-ish) stellar neighbor?

10,993.3 miles.

Just bask in that for a while.

(http://www.exploratorium.edu/ronh/solar_system/)


Although to be fair, dust is definitely a problem if you want to see into the center of the galaxy which is why most of the observations of Sagittarius A* are done in the infrared or near-infrared spectrum, which can penetrate dust. It's also why we can't see clear across our galaxy to make out what's beyond it, so we don't know what the Great Attractor is.


It is empty, but its filled with "Space Grease"

https://www.theguardian.com/science/2018/jun/27/space-is-ful...


> found enough for 40 trillion trillion trillion packs of butter

What an incredibly useless unit of measurement. They somehow found something worse than "olympic size swimming pools".


How many packs of butter per Library of Congress?

The problem is that most people just cannot conceptualize really, really, really large numbers, but journalists and authors try to make them more relatable anyway, and end up just making it worse.

I think what they meant to say was that it could make a typical globular cluster satellite of the Milky Way galaxy, containing about 225000 butter-stars with the same mass as our sun.


> I think what they meant to say was that it could make a typical globular cluster satellite of the Milky Way galaxy, containing about 225000 butter-stars with the same mass as our sun.

I don't think it was anything near that complex: 40 trillion trillion trillion is just the number of tubs of butter it takes to get 10 billion trillion trillion tonnes if you assume each tub is 250 grams.


Even then,

  10^3 = thousand
  10^6 = million
  10^9 = billion
  10^12 = trillion
  10^15 = quadrillion, or thousand trillion
  10^18 = quintillion, or million trillion
  10^21 = sextillion, or billion trillion
  10^24 = heptillion, or trillion trillion
  10^27 = octillion, or thousand trillion trillion
  10^30 = nonillion, or million trillion trillion
  10^33 = decillion, or billion trillion trillion
  10^36 = undecillion, or trillion trillion trillion
One of those numbering conventions is not tremendously useful, and the other is very very very stupid. It compounds with the knowledge that some people call 10^9 "a milliard" rather than "a billion". This is why we have SI unit prefixes, and write numbers ending with an exponent of 10.

And here I was, assuming each "pack of butter" was one of those 10g single-serving packs (although some are only 7.65g). I didn't check it against the "tonnes" number in the article. In the US, a "pack of butter" could also be a box containing 4 sticks 113 g each, totaling 454 grams, because butter in the US is sold by the pound. Apparently, they are 250g elsewhere.

And a (metric) tonne is already 1000 kg, or 1 Mg. There is also the long ton, which is 1016 kg, and the short ton, which is 907 kg.

The obfuscated number is therefore 1x10^40 g, which is even larger than standard metric prefixes can express, so we'd probably write it as 1x10^37 kg, for some reason.

So my previous math was wrong. That's 5 million butter-stars the size of our sun, or enough butter to form a slippery, spreadable black hole as massive as the one at the heart of our galaxy. I guess from this, we can calculate the size of Audthumbla the giant space-cow?


" Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."

  - Douglas Adams


Indeed space dust is a problem. From notes in the parent article - "[3] Observations of the centre of the Milky Way must be made at longer wavelengths (in this case infrared) as the clouds of dust between the Earth and the central region strongly absorb visible light." This interstellar dust was recently revealed to be a fine mist of grease, soot and silicates - https://academic.oup.com/mnras/article/479/4/4336/5039660


As other said, but maybe not clearly enough: no space isn't that empty, there is a lot of dust in the way, but fortunately it is dilute enough that its transparent at some wavelengths and so we observe using those.


Anyone know why the black hole is marked well off the axis of the orbit of S2 in diagram [1]? Is there enough mass elsewhere at the center of the galaxy to have it orbiting a common mass so far off axis? The video of the orbit [2] seems to show it closer to the axis or the focus I'd expect but still seemingly a bit of an offset. The fulldome view video [3] does show that there is quite a bit of stuff moving around there so maybe my imaginations are wrong, I think of a black hole having so much mass that it completely dominates the local gravitational field.

1. https://www.eso.org/public/unitedkingdom/images/eso1825c/

2. https://www.eso.org/public/unitedkingdom/videos/eso1825g/

3. https://www.eso.org/public/unitedkingdom/videos/eso1825g/


Keep in mind that you are viewing the orbit in projection. The axes are simply the angular offset from the pointing.

Before I give away the punchline, try to imagine the orbit in a reference frame centered at the location of the black hole. Obviously, the motion should be elliptic; lets define a Cartesian coordinate system such that, say, the x-y plane coincides with the plane of the elliptic orbit. Now, we know that the black hole should reside at the focus of this orbit. But, as a far away observer, what are the chances that you would be seeing the orbit exactly face-on - i.e., you are sitting on the z axis, just really far away?

So, yes, the black hole is at the focus of the elliptic orbit, not the focus of the ellipse formed from the projection of the orbit onto your plane-of-sky, which geometry tells us is also an ellipse.

BTW, this isn't a problem for the astronomers. You can back out exactly what that inclination of the orbit is, provided you can measure the line-of-sight velocities of the star. (Actually, relative velocities will do; no absolute calibration necessary.) This is easily done by looking for some distinctive stellar spectral features and noting their relative shifts at different parts of the orbit. From this you can reconstruct the orbit completely. The key scientific results they derived was the even subtler redshift of these spectral features as the gravitational field grew stronger when the star approached the black hole.

edit: actually, you could do even better, and use the offset of the black hole from the elliptic focus, and use projective geometry to get the inclination angle!


Ahhh so simple! Thank you very much for the moment of enlightenment.


Question, is it possible that our universe could be in a black hole? When you look at the way our existence is structured, everything has a parent. An an electron has an atom has it's parent, atoms have molecules has their parent etc. Germs, bacteria, humans, planets all have parents. Surely our universe has to have a parent.


That logic leads to infinite regress... If everything has a parent, then what's the parent of that thing.

So either there is an infinite chain, or the logic is flawed -- e.g. it's possible to not need a parent.

Another possibility is that our conceptions of causality / structure / parenthood are fundamentally flawed (but still useful for human purposes at this point in time), and the question isn't fully coherent.


Give that question a good Googling and you'll find a lot of good stuff. It's not a bad question and has been seriously treated by a number of people.


Possible? Sure, anything is possible - but apply your argument the other way round. People contain molecules, molecules contain atoms, atoms electrons and so on - does it then seem reasonable to ask whether that goes on forever? And if not, why should it be reasonable if you go upwards in the scale?


But it does seem reasonable. Very much so if going down far enough you find micro black holes at the center of atoms.

But you need some really funky geometry to make it more interesting then this played out trope. Say they are all the same black hole from different times seen through its own gravitational lense.


It's turtles all the way down.


> atoms have molecules has their parent etc. ... Surely our universe has to have a parent

Well... there's a limit to the scientific reach: when the scientists speak about "the Universe" they typically speak about "everything that is we can expect to reach using our instruments." Whatever is behind that is simply unreachable, and therefore out of the scope of the science.

But the good news is: what is reachable is immensely huge, and these far limits don't affect our lives directly, even if we can observe "what's there" (up to these limits).

In older times, some people thought that the movements of the planets, which are quite close to Earth compared to what we today can observe, affect our day-to-day life (and that was the idea behind "astrology"). Now we know it isn't so, they simply move because the gravity exits. We can observe much farther today: in the last 100 years we discovered that some of the points on the sky aren't stars but the galaxies: each point actually some billions of stars, much, much farther than the single stars we see! And we discovered that the farthest objects we can see are now some 46 billion light years from us, and that we actually see what there happened some 13 billion years ago -- not to mention that the we are actually made from the star dust (anything but our hydrogen atoms) and the rests of the "big bang" (the hydrogen atoms in our bodies)!

That's "the Universe" the scientists talk about. What's behind these limits can't predictably affect our own lives, as no signal "from this universe" can reach us faster than the speed of light: it takes 8 minutes for a light from Sun to reach us. Whatever produced some light that is 13 billion years old is now some 46 billion years away from us and changed through these 13 billion years. But we know that the "Universe" gets bigger so these objects we can never reach not even with the light from us to there, as they will continue to be even farther from us. So that's it. 46 billion light years in every direction.

https://medium.com/starts-with-a-bang/ask-ethan-how-much-of-...

https://medium.com/starts-with-a-bang/ask-ethan-how-large-is...


What if the big bang was a white hole?

I'd butcher the idea if I tried to explain but..

https://www.youtube.com/watch?v=S4aqGI1mSqo


> Question, is it possible that our universe could be in a black hole?

Not without redefining what a black hole is.

General Relativity is a metric theory of gravitation. Black holes arise in General Relativity, and are defined by the family of metrics they source. There are astrophysical objects that humans observe (with various instruments) that match the predictions for black holes in General Relativity within the limits of measurement error. This article describes further such observations of the black hole in our galaxy's central parsec.

The salient feature is that a black hole metric has a boundary across which there is only one-way travel. Anything inside that boundary inevitably, inexorably, collides with a gravitational singularity inside that boundary. For the sake of quantum mechanics being valid everywhere (including deep inside black holes), there are proposals which would replace the singularity with some final structure of ultradense matter whose internal states resist collapse into a literal singularity, even as one collides arbitrarily large amounts of matter into it. However, in such proposals, the inevitable, inexorable inwards motion from the horizon inwards to quite near the "middle" remains a feature.

The inevitable, inexorable inwards motion is fast according to the wristwatch of an unlucky person crossing an event horizon; you won't need to take a calendar with you in order to count down your final minutes in a black hole like the one at the centre of our galaxy. You would have longer for a much much larger black hole, but as you and everything else would be collapsing inwards quickly, you would not observe the formation of stars or galaxies inside the horizon, even for an arbitrarily large black hole. Your sky would look nothing at all like the skies we have in our solar system; at the very largest scale for a black hole, you would not see the cosmological redshift that we see, and would not detect anything like the same cosmic microwave background.

At the largest scales -- where we take clusters of galaxies as particles of dust -- our universe is well-described by a metric for an expanding spacetime, with lots of supporting evidence from detection of the redshift of emissions and absorption spectra compared with angle on the sky, luminosity, and other features of distant galaxies. The expansion is not like the time-reversal of the inside of a universe-sized black hole, in particular since to a good approximation the distribution of galaxy clusters is isotropic and homogeneous, and groups of clusters nearby one another (on the a tight angle for the line of sight to each, and each at similar distances) are moving slowly and randomly with respect to one another, while such a group taken as a whole is nearly uniformly receding from our own galaxy cluster. This is nothing like the behaviour one would get inside a truly enormous black hole.

> surely our universe has to have a parent

It's not required, but there are cosmological models that have notional "parents". Sean Carroll, a physical cosmologist at Caltech, has a set of slides outlining a few at https://www.slideshare.net/seanmcarroll/what-we-dont-know-ab... -- slide 14, "Reproducing cosmologies" and slide 17, "Reproducing cosmologies don't have an entropy problem", both references in the lower right hand corner that you can follow-up on if you are interested in the technical aspects.

The crucial point, however, is that we do not yet have good observations of the very early hot, dense universe of a bit less than fourteen billion years ago, so we have some freedom to choose among various theories which wholly match the history of the universe that we do have good observations for.


Cool news! I'm curious though, couldn't a quasar possibly cause the gravitational redshift that was observed?


Coolest thing I have seen in a long time. Watched a replay of the livestream and it is so good!


Which of that imagery is actual imagery and not artist interpretation?





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