People may be interested in experiments to detect these waves. From the Wikipedia page on LIGO:
"LIGO, which stands for the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment aiming to directly detect gravitational waves. [...] At the cost of $365 million (in 2002 USD), it is the largest and most ambitious project ever funded by the NSF.
Observations at LIGO began in 2002 and ended in 2010; no unambiguous detections of gravitational waves have been reported. The original detectors were disassembled and are currently being replaced by improved versions known as "advanced LIGO", scheduled to be operational by 2014.
[...]
Measurable emissions of gravitational waves are expected from binary systems (collisions and coalescences of neutron stars or black holes), supernova of massive stars (which form neutron stars and black holes), accreting neutron stars, rotations of neutron stars with deformed crusts, and the remnants of gravitational radiation created by the birth of the universe."
I was fascinated with LIGO when I learned about it. It seems the best plans for such devices exist in the planned space-bound installations, such as LISA (http://lisa.nasa.gov/)
NASA participation in LISA was removed a couple years ago due to budget issues. The ESA portion went on (it's called NGO now), but I think NGO is pretty much an acronym without funds.
In general, in fact, space interferometers have been held up and moved to ground-based systems. (See also SIM and TPF.)
Waves (such as EM waves) imply the change in some field, which is what they are trying to detect. After all, we already detect gravitational fields - after all, we are standing on the earth - what they're trying to detect are very specific changes to gravitational fields, thus waves.
Weisberg and Taylor showed, with 30 years of data, that the orbital decay of the binary pulsar PSR 1913+16 agrees with the gravitational radiation prediction of general relativity to within 0.2%:
Something very interesting whenever this question is asked - what is the speed of light? It is "the speed at which data propagates through space time". I think that is a fascinating truth to our universe.
The speed of light can be thought of as infinite speed in that context. Since time slows as you get closer and closer to the speed of light, it stops completely at 299,792,458 m/s (hypothetically, if you gave photons a reference frame).
Trying to go faster than light could be compared to a car slowing down for a red light. Once at the red light, the car stops. Going faster than light would be like asking the car to go slower than completely stopped. It just doesn't make sense in a philosophical or mathematical sense.
It makes me think of hanging sand in Minecraft -- you disrupt 1 column and cause it to be recalculated and fall, and this causes the columns next to it to be recalculated and fall, dominoing on until all of the hanging sand has fallen. This is the propagation in action.
The propagation time is a combination of the 'tick rate' and the 'size of the granules'. In Minecraft, I'm pretty sure the tick rate is around 1/10 of a second, and the granule size is defined as 1 cubic meter.
In our universe, the tick rate probably is 1/c, or .00000000333 seconds (please correct my math), and the granule size is similarly infinitesimal.
Since c is a velocity, 1/c doesn't have units of time; it has units of time/distance.
Special and general relativity (at least, as commonly formulated) also assume that space and time are isotropic and continuous, which means there is no "granule", no "smallest increment" in the field equations. That may or may not be true; we're still trying to figure out if spacetime is quantized or not.
Does anyone know what Wolfram would say about the tickrate? I bought the huge book and know that he believes space is quantized, at least, and I assume he believes time is as well, as the rate the universal state machine changes states. But I never read enough to learn what he thinks that rate might be.
Cellular automata violate continuity and isotropy, which means they're inconsistent with the continuous spatial formalism used in special relativity, general relativity, classical electrodynamics, and as far as I know, quantum electrodynamics. We have no experimental evidence inconsistent with continuous models of space. We also have evidence that Lorentz symmetry holds over scales smaller than the Planck length, which suggests spacetime is not quantized.
One of the problems with quantized spacetime is that the cells would have a size--and sizes change depending on your velocity. Were spacetime to be comprised of cells, you'd be able to establish a single, most privileged reference frame--and that doesn't sit well with the principles of relativity.
That said, we really don't have good models for what happens at small spacetime scales yet. Gravity is poorly understood. Theories change as new evidence and mathematics develop. I just wouldn't trust Wolfram's opinion without either a.) experimental evidence or b.) a model which is consistent with (or radically reformulates) significant parts of relativity and quantum electrodynamics.
Ah, here we go. Aaronson 2002 shows that Wolfram's model cannot be consistent with experimental evidence:
In physics, we examine Wolfram's proposal for a deterministic model underlying quantum mechanics, with 'long-range threads' to connect entangled particles. We show that this proposal cannot be made compatible with both special relativity and Bell inequality violation.
I want to point out that Wolfram doesn't think the universe is a giant cellular automaton. He gives CA as examples of how easy it is to produce impermeable complexity. His more serious proposal is a graph-based system, though I haven't perused enough to know if that's also for the sake of example or if it's intended to be taken as a serious physical theory. Most likely it's an earnest attempt at a heuristic towards the goal of, in his terminology, "finding the program of the universe."
My impression is that Wolfram came to such conclusions because he is obsessed with cellular automata and with their mathematical universality in discrete domains. Of course, they are useless in continuous domains, so it would be ruinous to his world-view if the universe were continuous. So it's a bit like ancient metaphysics a la Plato and Co., where the properties of the universe are inferred from first principles (which come from... where, exactly?) rather than from any empirical observation.
If reality propogates into infinite possibility subuniverses, would it not be accurate to assume that each of those subuniverses must come from some "tick" of state (or delta of state, rather)?
I wouldn't say so. Many-worlds is an interpretation of QM, and to my knowledge, all verified QM formalisms treat time as a continuous parameter; not a discrete one.
Which is where quantum mechanics comes into the picture. If we could measure something in Planck units, we might discover that there is a smallest increment. Of course Nyquist implies that we may only be able to measure something at most, twice that unit size... but that is still on a scale far beyond our reach.
The difficultly: Despite the propagation delay, gravity always points to where the object should be, instead of where it was when the gravity was "emitted".
The explanation: Just like the object itself is moving, the gravitational field (or electric/magnetic field) is ALSO moving! It moves at exactly the same speed, and in the same direction as the object which created it.
It's "disconnected" from the source - so if the source suddenly changes direction the field it emitted doesn't know about it, and continues to point to where it expected the object to be.
More technical:
Why wormholes are impossible, and why you can not create a single magnetic monopoles:
What about conservation of momentum? If an object is pulled to where the other object is expected to be, not where it is, then momentum exchange between the two of them would seem to not add up!
But it works because you can never create a gravitational or electric field from nothing. You can only move them around, so the momentum always catches up.
What about a magnetic field? You can create those from nothing - ah, but you can only create magnetic dipoles, with opposing fields, so again it works.
But, you can never create a magnetic monopole because you would suddenly have a magnetic "charge" where none existed before and the momentum would not add up. What you can do is create two monopoles, of opposite poles. (So this implies a conservation of magnetic pole, just like conservation of electric charge - assuming monopoles exist.)
What about wormholes? They have the same momentum problem - an object suddenly appears where none was before and it pulls on other objects. You could then move it away before it gets pulled in turn by those objects, and that would violate conservation of momentum. So you can't do that.
And, faster than light objects would have the exact same problem - they could move out of the area before they properly shared the momentum. So I suspect they can't exist either. (Unless there is some complicated math I didn't think of which "fixes" it.)
Say some advanced civilization can move around some large mass - an asteroid, a planet, a black hole, whatever, we'll just call it "the big mass". They can, at will, fly it back and forth between two distinct positions we'll call 0 and 1.
And let's say some distance away - a light-year, say - they have an facility where they can measure with excruciating precision the force of gravity on a test mass. After isolating out all other known gravity sources, they can use the remaining vector of gravitation force to compute the current position of the original big mass - and whether it's at position 0 or position 1.
Assuming they can drag around the big mass from one position to the other in a short amount of time, shouldn't the people at the remote facility be able to detect where the big mass is long before light could reveal its position? Couldn't they use those observations to receive a low-bandwidth, but faster-than-light message?
The gravitational field points at where the object would be, given its current velocity.
This is not to say that the object will actually end up there. If the velocity changes, the gravitational field changes direction according to the new velocity. The change propagates at the speed of light.
(I made this from the "Moving Charge" java physics applet
at http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/Mo... .
It's a demo of the physics of electric charges, not general relativity, but the essence
of the idea is the same even if the field equations are different. This follows directly from the central idea of relativity: the physics of uniform constant motion is the same as no motion, if you're moving along at the same speed.)
The point charge (mass for gravity) in the center is first moving slowly to the right, then suddenly changes direction to move slowly to the left. (The small red vector indicates velocity.)
The white lines represent the direction of the field, that is, the direction an object would be pulled.
Near the point source, the field lines point to where it is. Far away, the news hasn't arrived yet that it's motion has changed, and so they point to where it would have been. During the brief acceleration when the source changed direction, the field lines connecting these two regions are "kinked" strongly - that's the radiation, which propagates outward at the speed of light.
A second object would feel a sideways pulse as the wave (kink) passed by.
> Assuming they can drag around the big mass from one position to the other in a short amount of time, shouldn't the people at the remote facility be able to detect where the big mass is long before light could reveal its position?
No, the remote facility won't notice the thing moved until the gravity wave hits and that only moves at the speed of light. If it's a light year away, it'll take a year before the remote facility notices the shift.
I think this is actually a brilliant exercise in though. I have no idea what the answer is, and it's likely impossible to ever cancel out the effects of every other source of gravity within a 1 light-year radius, but it seems like in theory, if the force due to gravity propagates faster than the speed of light, your suggestion may hint at a method of communication that exceeds the speed of light.
No. The change in gravitational fields propagates at the speed of light. The gravitational force vector for a star in the sky N light years away points to where the star was N years ago.
> The gravitational force vector for a star in the sky N light years away points to where the star would be if kept moving at constant speed for N years.
Interesting. I don't know enough about physics to have a clue one way or the other, but even if it was the prior case (we base our model on where the star is for gravitational computations based on the light we are receiving), that's just what it is -- a model. So if there is a lapse in our understanding that doesn't take into account that the star's gravitational effect on us actually depends on its current position then that may need to be corrected.
to say it's "pulled in a direction where the big mass no longer is" seems incorrect, because above we have "whenever a gravitating object moves inertially, the gravitational acceleration vector at a point removed actually points at where the object actually is at a given instant", due to the cancellaction and abberation effects.
"Indeed, the vector (2.5) does not point toward the “instantaneous”
position of the source, but only toward its position extrapolated from this retarded data" (which is pretty much the same location)
Typical Sci-Fi wormholes have that problem, but there are nevertheless valid GR solutions that look a heckuvalot like wormholes. And they do obey all those conservation laws. One can think of it as the wormhole mouths being objects (made out of space rather than matter) that accumulate and lose conserved quantities like mass and momentum and charge as objects enter or leave them, or you can think of it as lines of force getting stretched out to pass through the wormhole mouths whenever an object moves through them, since the fields can't just shear off.
Incidentally, this generally means that you have to be careful about balancing the mass flow in each direction through your wormhole, lest one mouth develop negative mass (and presumably antigravity) from too much stuff leaving it and the other mouth end up shrouded in a black hole.
> The difficultly: Despite the propagation delay, gravity always points to where the object should be, instead of where it was when the gravity was "emitted".
[citation needed]
My understanding is that it points to where the object was, and that the change propagates at the speed of light as gravitational waves.
That's what you would expect, but it results in unstable orbits.
Instead as the object moves, the gravitational field it emits also moves, so you end up feeling the gravity where you would expect it to be if the field transmitted instantly.
It's when the object emitting the field changes direction that things become interesting.
It is a prediction of some physical theories that magnetic monopoles could exist from the origin of the universe (and it kinda seems like they should, 'cause other than the existence of one kind of charge and the other, there's no good reason to see either half of electromagnetism as more fundamental than the other- both kinds of field can generate the other, and how much of each one you think you have depends on your inertial frame since they're relativistic transformations of each other), but no one has shown that you can make them, pulling the poles of a magnet apart like you can pull electrons away from protons.
This is the winning comment from the reddit discussion from /u/RobotRollCall
*
This is a far more interesting question than it might seem at first glance, and it deserves some attention because it tells us something fundamental and wonderful and just bloody awesome about the universe.
But I don't know how to tell the story succinctly. So I'm going to do that thing I do. I am very, very sorry. Please feel free to move on if this strikes you as tiresome.
Consider the Earth, and you on it. You're not floating freely, so clearly something's going on. We call that "gravity." We can call it, in the most generic sense, an interaction: you and the Earth are interacting somehow, and that's what's keeping you from floating freely.
We can then ask what the speed of that interaction is by putting it in these specific terms: How much time will elapse between your changing your position relative to the ground and your beginning to fall?
Yes, it's the Wile E. Coyote problem. Wile E. Coyote runs off a cliff, floats in mid-air long enough to hold up a sign that says "Help," then begins to fall.
Clearly that's an exaggeration. But just how much time does elapse, in real life, between stepping off a cliff and beginning to fall?
We can approach the problem naively by remembering that all propagating phenomena in the universe are limited by the speed of light. Given that fact, it makes sense to hypothesize that the time between the moment when Wile E. steps off the cliff and when he begins to fall will be equal to or more than the distance between him and the ground divided by the speed of light. It certainly can't be less, right?
We can then construct a set of very, very precise experiments with very fine tolerances — probably involving electromagnets and lasers or something — to test this hypothesis.
And then we can find that we're totally goddamn wrong.
To the absolute limit of our ability to measure it — and our ability to measure it is really good, since we used electromagnets and lasers and other expensive science things — when an object is dropped, it begins falling instantaneously. Not after a very small interval of time, but absolutely instantaneously. As in zero time elapses between dropping and falling.
This is fairly earthshaking, really. Because it implies that somehow a "signal" of some kind is getting from the ground to Wile E. faster than the speed of light. Which is supposed to be impossible.
I'm going to skip ahead a bit here, because I don't feel like explaining the entire theory of general relativity, and it won't be that useful in answering the question anyway. Suffice to say that no, no time elapses between dropping and falling, but at the same time no, no signal or interaction has to propagate upward from the ground to Wile E. in order to make him start falling. In fact, what's going on is that Wile E. is always falling, due to the curvature of spacetime created by the Earth. Whenever he's standing at the edge of the cliff, on the ground, the ground beneath his feet — paws? — is arresting his fall by, effectively, pushing up against him. The very instant that's removed, he starts falling.
So in that sense, gravity has no speed. Because it doesn't actually propagate through space. One way to look at it is to say the gravitational field fills space, so wherever you are, you're already being affected by it all the time. Another way is to say that gravitational essentially is space, so it affects you simply by virtue of existing. The two are essentially equivalent English translations of the equations that actually describe the phenomenon.
But okay, that's half the problem. The gravity of a static body fills space, or is space, and as such can't be meaningfully said to have a speed. But what about the gravity of a changing body? Like you said, what if "suddenly a black hole appeared?"
Well, the answer of course is that that never happens, ever. Gravitation doesn't suddenly anything; macroscopic things don't just appear out of nowhere, and teleportation is impossible. So we don't have to think about that … and in fact we couldn't get meaningful answers if we tried.
But things do move. The moon's moving relative to the surface of the Earth; we can tell, even apart from the fact that we can see it up there, because the moon is the major contributor to the tides, and the tides rise and fall. But what's the relationship between the moon's position in space and the tidal acceleration on the Earth? Are the two somehow always in perfect sync, or is there some lag? If so, how much, and in what direction?
That's actually a much harder question to answer than you might think. There was a now-infamous paper some years ago by a fellow named Tom Van Flandern (recently passed, God rest his soul) that asserted that the change in gravitational acceleration in a dynamical system actually propagates many times faster than the speed of light — at least twenty billion times faster than the speed of light — but not instantaneously. This got a lot of attention at the time. If the propagation speed of changes in spacetime geometry were equal to the speed of light, that'd be fine. If it were literally instantaneous, that'd also be fine, more or less, though our theory would need some tweaking. But faster than c but still finite? That was really hard to explain.
It turned out not to be a problem though. Because Van Flandern just made a mistake in his paper. See, the relationship between motion and gravitation is not as straightforward as it might seem. In fact — and I'm glossing over this now, because the maths are damn complicated — whenever a gravitating object moves inertially, the gravitational acceleration vector at a point removed actually points at where the object actually is at a given instant, as opposed to where the object's light is seen to be coming from at that instant. So in that sense, we're back to gravitation being instantaneous again!
But is it really? No. Because you see, if the inertially moving object were to come to a stop instantaneously, the acceleration vector would continue to point toward its future position for a time, as if it were still moving inertially, even though the object is actually somewhere else. The sum of effects that serve to cancel out aberration when everything moves inertially would break down, and the acceleration field would point toward empty space for however long it takes for the change in geometry to propagate through space at the speed of light from the gravitating object to the point in question.
Except things don't stop moving instantaneously. Things accelerate, and acceleration requires energy, and when you factor that in, the equations balance out again.
(If you feel up to the challenging of following a lot of advanced mathematics, here's the best paper I know on the subject.) - http://arxiv.org/pdf/gr-qc/9909087v2.pdf
So what does that mean? It means that the "speed of gravity" is the speed of light … technically. Changes in the geometry of spacetime actually propagate at the speed of light, but the apparent effects of gravitation end up being instantaneous in all real-world dynamical systems, because things don't start or stop moving or gain or lose mass instantaneously for no reason. Once you factor in everything you need to in order to model a real system behaving in a realistic manner, you find that all the aberrations you might expect because of a finite speed of light end up canceling out, so gravity acts like it's instantaneous, even though the underlying phenomenon is most definitely not.
> To the absolute limit of our ability to measure it — and our ability to measure it is really good, since we used electromagnets and lasers and other expensive science things — when an object is dropped, it begins falling instantaneously. Not after a very small interval of time, but absolutely instantaneously. As in zero time elapses between dropping and falling.
> This is fairly earthshaking, really. Because it implies that somehow a "signal" of some kind is getting from the ground to Wile E. faster than the speed of light. Which is supposed to be impossible.
I don't know a lot about physics and relativity but this actually seems to me how I'd expect it to be. I don't understand why people expect that there has to be an interaction before gravity starts, or that gravity is directed linearly between two masses (you and the earth). It's already being emitted, and you're stepping out off the cliff into a gravity wave that is already there. You're always in that wave. Now the thing beneath you preventing you from being pulled in is gone. No initial setup was required.
Imagine the same example with a light beam instead of gravity. At the foot of the cliff is a ball as bright as the surface of the sun, radiating light in every which way. If you step off the cliff, it doesn't take a few nanoseconds for the light to hit you, because you're stepping into a lightbeam that is already there. No interaction is necessary to set it up.
I dunno. This seems like the intuitive way to think about it to me. Anything else would seem strange. Maybe I'm misunderstanding?
No, I think you're right. RobotRollCall sets up a straw man with the Wile E Coyote example. Electric attraction travels at the speed of light, yet you would feel it right away after walking off a cliff.
don't forget the 'sixth sense' - there are three little accelerometers in each of your ears, and cool little feedback control system that keeps your eyes constantly cancelling out the tiny variations in position that your head constantly undergoes.
Why would the fact that dropped objects start falling instantaneously show that signals travel instantaneously, either pre- or post-GR? It just shows that the message "fall if you are dropped" is just being continuously emitted by the Earth, including before the object was dropped.
it is called gravitational wave. The GGP strangely didn't mention it. The gravitational wave is a strange creature as it is a disturbance of space-time, thus question of its speed, distance by time, becomes a bit murky - like what distance? what time? - depending on how you answer the questions (and don't forget, talking about astronomical scales we start to face astronomical factors like noticeable spacetime expansion) - you get corresponding result.
>>"fall if you are dropped" is just being continuously emitted by the Earth, including before the object was dropped.
it is gravitational field of Earth. If Earth were to move with acceleration (as any movement without acceleration is as good as not moving at all by suitable choice of coordinates) the object would start to fall toward previous "retarded" location of the Earth, until the gravitational wave caused by the accelerated move of Earth "updates" the gravitational field at the position of the object.
>The question becomes what if the object moves - how long does it take to notice the new location of the gravity.
a moving object gets instantaneous value of gravitational field at its new position (while this value may be already "obsolete" like in the above mentioned accelerated movement of Earth)
Yes, I know, but that's not what the quoted reddit comment claims:
> To the absolute limit of our ability to measure it ... when an object is dropped, it begins falling instantaneously. ...This is fairly earthshaking, really. Because it implies that somehow a "signal" of some kind is getting from the ground to Wile E. faster than the speed of light. Which is supposed to be impossible.
There is no contradiction. The thing you quoted is what happens in normal circumstances (i.e. linear motion) - you instantly start falling with no delay. Which is correct. It's the change in the linear motion that gets propagated at speed of light. But in any case you still react properly because things cancel. Woo!
>Like you said, what if "suddenly a black hole appeared?"
>
>Well, the answer of course is that that never happens, ever.
Can't it, though? It would be extremely unlikely, but pairs of particles and antiparticles pop into existence constantly. Again, it would be so unlikely as to be of only academic concern, but as I understand, it is technically possible that ~10^57 of antiparticles could pop up all over the universe, while their corresponding trillions of particles all popped into existence right inside the sun.
Presumably, in that so-rare-that-it-definitely-won't-happened case, it would take 8 minutes for us to be able to detect that the sun had doubled in mass.
The particles do annihilate, but they don't have to annihilate with each other. If an electron/positron pair appear, the positron can annihilate a different electron, in which case the electron appears to jump.
So if the pairs appeared light years apart, the antiparticles would annihilate something nearby, leaving their partners in the sun, as if particles were spontaneously jumping from all over the universe into the sun.
Preposterous, yes, but as far as I understand, not actually impossible.
i feel any fundamental theory must violate the conservation of mass as we perceive it in order to explain existance (not to say that it isn't a very good approximation with a true underlying conservation law)
The general idea is that negative energy due to gravity (this is hard to wrap one's head around) perfectly cancels out all of the energy in the universe (including energy in the form of mass).
I am not a physicist, but couldn't this be used to transmit information faster than light? Let's say you have object A attracting object B through gravitation, if you could somehow control the inertial movement of object A to encode some information, couldn't this information be retrieved instantaneously at point B by measuring the direction of the gravitational vector pointing towards A ?
I expect the answer to be something along the line of "you can't change the inertial movement of A faster than light would take to go from A to B", but I have no idea why this would be the case. If there's a physicist nearby I'd love an explanation :) .
In order to preserve information causality, spacetime cannot deform instantaneously.
i.e., an object X in a frame with relativistic speed near c compared to an observer O. O will be unable to detect X by any means until X is very close.
Also, my money is on frame-dragging and Einstein's figures being within half a standard deviation. http://arxiv.org/abs/0911.4718
> Yes, it's the Wile E. Coyote problem. Wile E. Coyote runs off a cliff, floats in mid-air long enough to hold up a sign that says "Help," then begins to fall.
Clearly that's an exaggeration. But just how much time does elapse, in real life, between stepping off a cliff and beginning to fall?
Meh, not so much. One is already in a gravitational field before one falls off a cliff. The time it takes you to start falling has more to do with propagation of forces inside your body, which would happen at around the speed of sound.
The speed of propagation of gravitational waves would be more applicable to something like blowing up a planet. What if someone could blow up the earth in such a way that all the debris flew away from Wile E. Coyote at the speed of light? (And also that he has the powers of Superman and would be unaffected by radiation and vacuum.) How long would it take for him to notice that his weight has started to change?
So if we ever invent artificial gravity that we can turn on and off with a switch all that stuff he said breaks down? And we get some really strange behavior?
The Alcubierre drive has other problems (Krasnikov tubes look more likely, for some values of 'likely'), but not this one. There's no "instantly" in Alcubierre drives.
The linked article directly disagrees with this post (except the last paragraph, which itself disagrees with the rest of the post), however:
> The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%. (Measurements of at least one other binary pulsar system, PSR B1534+12, confirm this result, although so far with less precision.)
This was always my problem with RobotRollCall on Reddit - no one could challenge anything she wrote lest they be downvoted into oblivion. Furthermore, she never took criticism very well, and would assert authoritative dominance over the topic of knowledge (how dare you disagree with me, I read research papers!).
Her explanations were good, but they weren't ever very complete and while they were well written, they weren't always accurate.
The linked article directly disagrees with this post (except the last paragraph, which itself disagrees with the rest of the post)
Actually, Baez's article disagrees with the last paragraph of RRC's post; in so far as it talks about what's in the rest of RRC's post, it basically agrees with it (the part at the beginning about the Wile E. Coyote stuff isn't really talked about in Baez' article, except in so far as it's supposed to illustrate that a static gravity field doesn't have to "propagate" at all).
Baez' article, and the paper by Carlip that is referenced in it (and by RRC) basically say that measurements like the binary pulsar show that this statement from RRC's post is, strictly speaking, false:
all the aberrations you might expect because of a finite speed of light end up canceling out, so gravity acts like it's instantaneous
The cancellation is not exact; that's why binary pulsar systems can lose energy by emitting gravitational waves. But it's very close to being exact, closer than the analogous cancellations for electromagnetism; that's why we don't observe "aberration" in the direction of the gravitational force to the same extent that we do for the electromagnetic force.
I think she pulled up stumps and left at some point over issues with how some of the community was responding to her? Something like that. Real shame because she inspired strong interest in so many who read her explanations.
She probably just got tired of being treated the way highly visible women often are on the internet. This is not always visible to everyone else since there are private messages on reddit. It's possible things are better now, it's hard to say. I tried to perform an experiment on reddit a while ago by posting under a handle that was obviously feminine but I had to cancel the experiment because it didn't really work due to my posting style being too abnormal.
I never noticed gender come into it on the public side, at least. A good number of people wouldn't have even been aware she was female. (Think of the Groklaw thread the other day and many assuming PJ is male.)
There was an abrasive way RRC responded to some that fired up discussions and I got the impression that this ultimately frustrated her into ending her participation. I've seen that abrasive style/response feedback loop develop with males just the same.
> To the absolute limit of our ability to measure it — and our ability to measure it is really good, since we used electromagnets and lasers and other expensive science things — when an object is dropped, it begins falling instantaneously.
That's not true. Our ability to measure the "speed of gravity" gives us measurements that say "the speed of light":
> the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%.
I eliminated the cruft so it was more obvious. RobotRollCall's information is older than the linked article (that's from 1998).
>when an object is dropped, it begins falling instantaneously.
Is answering a different question than this:
>the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%.
It is true that things drop immediately when you let go of them. Because they're already in the gravitational field. It is not true that when I drop something, a gravity particle from the surface of the earth (say 1km away) comes up and gets my apple, and until that gravity particle reaches it, the apple magically is suspended in the air. RRC covered this in the post.
It is the changes in the gravitational field that propagate at the speed of light. So if the sun vanished, which it can't, we wouldn't be flung away for 8 minutes.
That's all true, but that's not what I quoted. RobotRollCall's comment said we can't measure the "speed" of gravity, but in the article linked, we do precisely that.
What? No - what you quoted is about measuring the delay between when I let go of something and when it starts to fall. That is what we've used lazers and rockets and microscopes to measure.
I get what is actually going on, what I'm saying is that RRC's comment does not go from A to B. It goes from A to A', then jumps ahead to B, without any reasoning why.
Or, as she puts it, "If you feel up to the challenging of following a lot of advanced mathematics, here's the best paper I know on the subject.".
What she writes is basically a long explanation of why the speed of light is not correct, right up until the end, where she hastily covers it up with what the actual science says.
What would be useful would be a dissection of the "best paper I know on the subject". The rest of it just says, "It's not the speed of light... actually yes it is. Don't look further unless you're smarter than I think you are."
You said the article disagrees with RRC's post, which it does not. It seems now you're just taking issue with the progression RRC made to get to the point? Or that you think she should have gone into the paper she cited? OK. But it doesn't make claims contrary to the OP. And the thing you quoted does not disagree with the other thing you quoted.
The part you quoted does disagree with the article, but not with the part you quoted: RRC said we can measure the delay, and it is 0. The article said we can derive the delay, and it is almost 0.
That aside, yes, RRC's comments are often lacking in one way or another. However, they're also solid attempts at explaining something ad hoc, a very valuable thing on Reddit.
"The net result is that the effect of propagation delay is almost exactly cancelled, and general relativity very nearly reproduces the newtonian result."
It's starting from basic ideas. This was posted in ELI5 or askscience. Talking about what "everyone thinks" about gravity, a complicated science thing, is silly.
Your post boils down to, you don't like how cute it is. OK.
It's a fairly short and clear article, so no need for a Tl;DR. Note that this is NOT yet another "revolutionary" announcement, but just contemplative reasoning from a respected expert in gravitational physics.
Gravity travels at the speed of light, but it pulls you towards where the object is going to be when it left the object, not where it was when it left the object.
> Gravity travels at the speed of light, but it pulls you towards where the object would be if the object kept moving at constant speed, not where it was when it left the object
That's mind bending, never thought of gravity that way (being a force like EM, and subject to a propagation speed).
In my mind, gravity was just some sort of "vacuum" the mass leaves as it moves, and other bodies get "pulled in" because that's the space where the "energy" (mass) levels are smaller, and the system tends to a stable state. But not something like a wave that propagates, just a side effect of the global system, not tied to speed in any way.
Actually, the "gravity" that propagates at the speed of light isn't really a "force"; it's more like changes in a force. A completely static gravity field doesn't have to "propagate" at all, because it never changes; but it can still produce a "force" (i.e., it can still cause objects to freely fall towards the source).
Steve Carlip used to be my advisor in grad school and his style of writing conveys completely his beautiful way of explaining (and thinking about) deep questions in physics.
One think that I believe is important to point out is that this short essay is focusing on the experimental aspect of the measurement of the speed of gravity. It gives an explanation on why this is even an intelligent question to answer and why the answer we commonly accept is that gravity moves at the speed of light. I will never have better words than him so I won't add anything to what he said.
What I would like to point out, based on some comments I read here, is that this essay is not talking about whether or not theoretically would be possible to have a speed of gravity faster than the speed of light.
General relativity and special relativity have been tested on several aspects, and they pretty much are in agreement with all experimental constraint. There is no other theory that can explain everything we see (some theories tend to explain a few things, but not all of them, or they are too vague).
A theory in which the speed of gravity is faster than the speed of light isn't unconceivable per se, but no one has been able to write a completely consistent one yet, mostly because it would have large consequences on what special and general relativity imply for cosmology and particle physics (and causality). If gravity moved faster than the speed of light then we'd have to 'fix' a lot of problems coming down to paradoxes and stuff like that.
So take this essay as a way to appreciate how elegant and fine some experimental questions on gravity (and physics) are.
In general relativity gravity propagates at c. The rate of orbital decay of binary pulsars is, among other factors, dependent on the speed of gravity. The in-spiral rate of one binary pulsar system has been measured and found to agree with the rate predicted by general relativity to within a 0.2% margin of error.
Gravitational waves haven't been directly measured yet though, so there's no direct confirmation. With multiple detectors currently in operation if a signal is detected and able to be tied to a specific location in space the timing delays between when its received between the two primary LIGO detectors (Livingston, LA, USA and Hanford, WA, USA) and the VIRGO detector (PISA Italy) should allow for estimating a propagation speed.
Advanced LIGO, expected to begin collecting data in 2014 is expected to be able to detect a number of signals so hopefully the question will be settled in a few years. However, there might not be any published results for a while after it goes active. The search for pulsar spindown signals with LIGO data is done via the Einstien@Home distributed computing project; and in prior runs several years passed between when the first part of the data set was collected and when papers on it were finally published.
http://physics.stackexchange.com/a/26743/9521
I wonder if the recent nova event visible with the naked eye can be used to do an experimental measurement... or perhaps it has to be a supernova as the article suggests.
Yes, any gravitational radiation from a nova would be much weaker than from a supernova. As the article notes, unless the supernova is very close to us, our current detectors can't detect the gravitational waves from a supernova, so they certainly wouldn't be able to pick them up from a nova.
I remember perplexing a high school physics teacher with this one, many years ago. ;) His answer was that he thought it would propagate at the speed of light, but he wasn't sure. I was just being a smart-ass, trying to figure out if something could go faster than the speed of light.
Yeah that's what it's saying. But it's not saying we get updates as to where the sun is instantly due to FTL gravity. It's saying objects rotate around a projection. If the linear path of the sun were to change, those changes would take a while to get to us to update our path, but also due to how that can't "just happen", things don't end up getting messed up.
But isn't sun rotating around galaxy center? So the velocity of sun isn't line, but an orbit, so it isn't constant (it's always changing direction), and we (Earth) do orbit the place that sun would be if not for the acceleration of sun caused by combined gravity of galaxy?
c is a conversion factor between space and time coordinates when we project them into a space that we can intuitively grasp.
the obsession with the 'speed of light' always seems a bit senseless in this context - its taking a concept which is known to break down in some limit and then applying it right at that limit. this is bound to be confusing.
really the speed of light is like infinity in the geometry of our universe... it just happens unfortunately that the geometry is not as simple as an infinite euclidean space with an independent time coordinate so we end up attributing a bad interpretation to the 'speed' of light.
I always wondered about this. If changes in gravity propagated faster than light, could you build a transmitter by modulating a gravity field by colliding matter with anti-matter?
If you're thinking that the matter and antimatter have gravity and that gravity disappears when they annihilate because their mass is gone, that's not the case. All energy and momentum produces gravity, not just mass.
You could only modulate gravity as fast as you could move energy close to you or far away from you. An implausibly powerful gamma ray flashlight flicking on and off would let you do this. Get the beam powerful enough, and it will have a gravitational field.
Get the beam powerful enough and it will have meaurable gravitational field. All beams of any intensity technically do contribute some, albeit infinitesimal, component to the gravitational field.
It's a fun exercise to calculate just how strong a laser beam you'd need to have to make a black hole entirely out of photons.
A black hole can actually cause an additional singularity due to the blueshifting of infalling light. IIRC, one researcher studied this phenomenon and called it a "blue sheet."
Just to make it clear. If gravity was propagating faster than light, then general relativity for how we conceive it would have to be reformulated.
It would be similar to the problem of if neutrinos were actually going faster than the speed of light (they're not).
...so then we still get to achieve an effective result of reaching a destination faster than light.
...and since the gravity warps space, and the warp drive exploits warped space, does that mean that the quantum mechanism behind the warp drive is somehow directly related to gravity?
the ring would contain exotic matter called
negative vacuum energy, a consequence of quantum
mechanics. The presence of this toroidal ring of
negative vacuum energy is what's required from
the math and physics to be able to use the warp trick.
It doesn't explicitly mention "gravity", but wouldn't this "negative vacuum energy" have some kind of important relationship with the behavior of gravity?
"LIGO, which stands for the Laser Interferometer Gravitational-Wave Observatory, is a large-scale physics experiment aiming to directly detect gravitational waves. [...] At the cost of $365 million (in 2002 USD), it is the largest and most ambitious project ever funded by the NSF.
Observations at LIGO began in 2002 and ended in 2010; no unambiguous detections of gravitational waves have been reported. The original detectors were disassembled and are currently being replaced by improved versions known as "advanced LIGO", scheduled to be operational by 2014.
[...]
Measurable emissions of gravitational waves are expected from binary systems (collisions and coalescences of neutron stars or black holes), supernova of massive stars (which form neutron stars and black holes), accreting neutron stars, rotations of neutron stars with deformed crusts, and the remnants of gravitational radiation created by the birth of the universe."
For more:
http://en.wikipedia.org/wiki/LIGO
http://en.wikipedia.org/wiki/Gravitational-wave_detector
There is a fantastic lecture series by a Yale U. astronomer that touches on this and related topics from a calculus-free perspective:
http://oyc.yale.edu/astronomy/astr-160
Also on youtube and iTunes U.