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The Absurdity of Detecting Gravitational Waves (2017) [video] (youtube.com)
108 points by basicplus2 8 months ago | hide | past | web | favorite | 61 comments

Happy to see Veritasium getting some attention on HN. You may have seen his previous video from 2014, Facebook Fraud: https://www.youtube.com/watch?v=oVfHeWTKjag

One of my favorite videos is on his alternate channel, 2veritasum. Survivor Bias: https://www.youtube.com/watch?v=_Qd3erAPI9w

He's a great presenter, not the normal dumbed down documentaries that are fashionable these days. He actually tries to explain things, not just waffle and make the watcher feel good.

He recently made a full length movie:


I got to visit LIGO in Louisiana a few years back. I remember they lamented that it was not as remote as they really wanted. The detector picked up nearby hunting activity, and logging activity in the woods. And they had to shut down measurements for a couple hours every week when the liquid nitrogen truck drove in due to the road vibrations it created.

It’s an awesome experiment. And it requires huge computational resources to process all the data they take in.

The seismic isolation is much better since the Advanced LIGO upgrade completed in 2014 -- when were you there? (I'm also happy to answer any computing questions; that's my area.)

I wonder if gravity wave detection could be ever be precise enough for communication.

I wonder this because it occurs to me that one possible answer to Fermi's paradox is that aliens don't use Radio or other EM to chat.

And maybe they don't leak it because they move energy and information around more efficiently than we do.

Communication is a red herring; the actual Fermi paradox is why huge sections of the night sky aren't black[0] due to all the stars being enclosed by Dyson spheres. Or, for that matter, why the planet hasn't been eaten by von Neumann swarms yet.

0: technically a uniform dim infrared of waste heat

I thought that huge sections of the night sky are empty - we call them Cosmic Voids.

See: https://en.wikipedia.org/wiki/Void_(astronomy)

Empty isn't the same as black. Dyson spheres would be emitting lots of infrared radiation.

Which would be hard to detect, if they had temperature close to that of CMB.

This is called Olbers paradox, and is much older: https://en.wikipedia.org/wiki/Olbers%27_paradox

That's "Why are there any parts of the sky that aren't stars?". The Fermi paradox is "Why are there any parts of the night sky that are (unobstructed) stars?".

To me Dyson spheres seem decidedly low tech. Why do we think a very advanced alien species would a) even need such an amount of power b) use such a primitve method as wrapping a star in solar panels?

The only stuff I can imagine needing such energy levels would be wormhole gates or Alcubiere drives. Powering the former with a Dyson sphere might make sense but the latter would have to fit on a ship.

The interstellar aliens I find more realistic would be those that just mastered cryo sleep or general AI and built seed ships. We don't really have any way of detecting those, do we?

Another answer to the Fermi paradox is that a Kardashev Type 4 civilization is able to harness the power of an entire universe. We wouldn't be able to detect this by definition, since they exist outside of our perceived reality.

At first glance, this seems to be a useless theory, since it's not refutable. But it lends itself to a belief system: by studying the universe, we gain an understanding of whatever created it. This is helpful as a motive: a reason for studying any of this at all, in absence of economic or social incentives.

This seems important. As the centuries tick by, and as we confirm and re-confirm that we are indeed alone and that we do indeed have a mostly-complete model of physics, there will become less and less incentive to analyze the corner cases. It's costly, and takes decades. But at one time, it was costly and took decades to build a cathedral. Yet we accomplished these impressive feats due to a shared belief system.

The reason I brought this up is that we often like to believe there is an advanced alien civilization tucked away in some corner of some galaxy, sending out messages via gravitational waves or neutrinos. But why do humans find this idea so seductive? It's because of an underlying loneliness: we want to believe that we are connected with the universe in some fashion, that our existence has a point, and that there is reason to do anything at all in a universe that will exist long after we've gone, long after our solar system and sun has gone. Because if there were an alien civilization, at least we would not be so alone.

In that context, a solution to Fermi's paradox is simply to believe that our very universe exists due to some higher-order phenomena not knowable within our reality. And by studying the laws of physics, we gain a glimpse into the boundary between our universe and its hypervisor.

> But why do humans find this idea so seductive? It's because of an underlying loneliness: we want to believe that we are connected with the universe in some fashion, that our existence has a point [...]

No. It's simply that statistically, we can't believe we are the first, nor that we are unique. It would be (statistically) extremely odd if we were, and there is a bias against anthropo-centric theories.

Heck, I'd be happier if we are alone (less risk and more free land), but if we appear to be, that seems odd, and worthy of investigation. No?

EDIT: Why do you claim to speak for all our hopes/dreams/desires? "We need this" or "We want that". Frankly, that's a little collectivist and creepy.

> No. It's simply that statistically, we can't believe we are the first, nor that we are unique. It would be (statistically) extremely odd if we were, and there is a bias against anthropo-centric theories.

Fermi's paradox does not ask about civilisations like ours. It asks about interstellar travelling or at least interstellar communicating species. We have not achieved this level yet so we are not the 'first' as you say. You might claim that our radio signals should be able to be detected, but they are so weak and have been travelling for such a short period of time that they may as well not exist to an outside observer. There may be millions of our type of civilisation out there presently and throughout history but they would all be undetectable to us and hence our civilisation doesn't play into the paradox.

Well one answer to the Fermi paradox is we get wiped out but barring that it seems unlikely we won't reach the stars in millions of years.

And you seem to ignore it, but the other part of the paradox is if we can achieve interstellar travel in millions of years, that's like no time at all on a galactic scale. So it would be a major coincidence if we are within 1 million years of the first.

As far as the Fermi paradox is concerned it's a much safer assumption to make that advanced civilisations will achieve interstellar communication before they achieve interstellar travel.

Even so, my point is that human technology tells us nothing about the probility of a civilisation ever developing interstellar communication because we haven't seen it happen yet. We are still at the 0 stage when going from 0 to 1. It doesn't make sense to extrapolate from our position on the technological timeline. It makes more sense to assume tabula rasa that aliens have developed this technology and then explore the implications of that.

Ok. Fair enough. My point was simply that: we might be curious, but most of us aren't lonely or looking for any higher purpose or meaning (well, many may be, but if we were - we wouldn't look to aliens to satifice them)

"we can't believe we are the first, nor that we are unique. It would be (statistically) extremely odd if we were"

Someone did a Bayesian analysis using probability distributions and found that, given what we observe (no aliens), there is a substantial probability we are the first civilization in the universe, or else the only one within galactic distances. They also inferred that whatever makes spacefaring life unlikely is probably in the past, not the future, which seems optimistic if you're worried about AGW or nuclear war.

"The Fermi question is not a paradox: it just looks like one if one is overconfident in how well we know the Drake equation parameters."

"The Fermi observation makes the most uncertain priors move strongly, reinforcing the rare life guess and an early great filter."

They end up with a 40% chance we are alone in the universe and about a 55% chance we are alone in our galaxy.


I’ve heard mathematicians express similar reasons for studying math. It can be something of a universal language.

Personally, I think no matter how often matter-energy arrange themselves into life in the universe life remains vanishingly rare. For that reason, life should be as harmonious with itself as possible - diverse and varied but self supporting. The rest of the universe is rarely calm and stable enough for life to exist.

As for why we haven’t found life yet: we are very new to this game. We just in this last decade started being able to resolve properties of exoplanets. I don’t think we have the information to even posit a question like the Fermi paradox with any level of confidence.

I don't understand how this is any different from believing in God.

No doubt that was your parent poster's intention. There have been many ontological arguments throughout history. This was one such.

If it was, it was mostly accidental. The argument was derived from logic and reasoning, rather than e.g. family beliefs.

This wouldn't really solve the paradox as it implies there are zero type 0, 1, 2 and 3 civilizations that we can detect in our Universe, which is still statistically odd.

There was a HN thread (probably several) not too long ago about someone who did a better analysis of the Drake equation with probability distributions rather than point estimates and found that given that improved methodology, it isn't odd that we see nobody - as it is quite likely we are either the first civilization in the universe or the only one within a galaxy or more. No new facts were used, only a sensible synthesis of the known information.

If you want to communicate with an alien civilization you'd have to emit some kind of signal for a long period of time in order to ensure that they will receive it. Gravitational waves last for an instant and then you've missed them.

Even if you've established communication with another civilization, using gravitational waves to carry on chatting seems like an awful waste of resources. The only scenario that seems plausible for using them is some kind of an SOS signal in which case you ensure that it will reach the vastness of the universe and hope that one of the receiving folks would be able to decode it. But then again that would take billion of years.

> Gravitational waves last for an instant and then you've missed them.

Could you go into more detail? How do they differ from EM waves?

I don't think the OP is right. The waves LIGO detected lasted for an instant because they were from a fleeting cosmic event - two black holes spiraling into each other. But there's no reason as far as I know that a much more sensitive detector couldn't see waves from more mundane, longer-lasting events.

Though - at that point, why not just send EM waves? They travel the same speed. Maybe gravitational waves could penetrate anything, if you're willing to spend planetary amounts of energy to do it?

I'm not a physicist so I can't elaborate much on the subject. From a few articles I've read at the time of the LIGO discovery they mentioned that the signals lasted for a very short period. Other than that I might be totally wrong so don't take for granted my assumption.

Much of the data from LIGO is publicly available. Most scientists have been looking for chirp-like signals indicative of events like colliding black holes. Maybe you can find some other interesting signals?


Note that there's quite a bit of interference from the Earth. (trucks driving about, etc)

What would be the advantage of using gravity waves?

I was about to comment to state that there is no advantage because both gravitational waves and electromagnetic radiation travel at the same speed: "c".

But then it struck me, maybe there will be one advantage, EM can be absorbed, blocked and warped while gravitational waves are not affected by anything, thus guaranteed to deliver the message(?)

You could also use neutrinos for the same reasons and they have the advantage that it's a lot easier to emit them as a beam instead of omnidirectional waves.

Maybe the complexity required in detecting them acts as a form of security by obscurity. It's far more likely that radio waves would be intercepted.

Radio waves were first predicted almost exactly 150 years before the first confirmed detection of gravity waves. Either signal takes millions to billions of years to travel across the universe.

One reason would be that gravitational wave amplitude decays only as 1/distance instead of electromagnetic radiation which decays as 1/(distance squared).

...But man-made gravitational waves are, without some scientific advance eclipsing the discovery of electricity, completely beyond any human capability of being produced at the levels required for extra terrestrial communication.

Gravity follows the inverse-square law. Every conservative vector field does, actually, and gravity behaves like one.


This is incorrect. Amplitude decays as 1/distance, but energy (and power) decays as 1/distance^2. This is the same as electromagnetic radiation.

> gravitational wave amplitude decays only as 1/distance

Wait, what? How? The wavefront still spreads out over the quadratically increasing surface of a sphere, right, just like EM waves?

I thought about objecting to that also. But the case of spiraling black holes is possibly dispersing energy only on the plane of rotation, so it might actually be 1/r?

The amplitude decay certainly would be a very attractive feature for long distance applications. Maybe a gravity wave radar? Do they get reflected? Now we just need to mass produce black holes with the mass of the sun and bump them into each other :-)

Even if tons of intelligent civilizations happened to be radiating gravitational waves (or some other form of radiation which travels at or very close to the speed of light) encoded with information directly towards Earth right now, it's very likely we'd never receive them in the next million years due to the vastness of the universe.

What about all of the intelligent civilizations who were doing it a million years ago? With a million years for waves to fill space, you'd think we'd be able to hear something.

Maybe. Or maybe the time range of broadcasts that reached our solar system was several millions or even billions years ago. Or maybe it was as recent as 10,000 years ago, but we had no way of detecting or decoding them. Or maybe nearly all of the intelligent life existent in the universe doesn't yet have the capability to transmit information via gravitational waves or other resilient media. Or maybe humans really are one of the first instances of intelligent life in the universe, and the broadcasts won't start coming until millions of years from now... or never.

And the most disappointing/intriguing part is there's a good chance humanity will never, ever know the answers and will never have any confirmation or denial of any of those possibilities. Or if they do, it might be so far into the future that humanity as a concept will cease to exist. The universe is just too big.

Not necessarily, immagine scanning all radio frequencies except you don’t know neither modulation nor the symbols transmitted.

Separating something from noise is a challenge on itself especially if signals are compressed or encrypted

Your idea is actually used in "the three body problem" trilogy.

If you like that sort of sci-fi, read the book; it's worth it !

I’ve had similar thoughts and assume aliens would use something like lasers to have point to point communication rather than broadcast RF.

Of course for us we’d never detect those point to point transmissions unless we were incredibly lucky to be along the vector between two aliens communicating.

Even if you assume a "manycast" or "broadcast" type of communication rather than point-to-point, e.g. some kind of technology which could radiate information-packed gravitational waves traveling at light speed in all directions of 3D space simultaneously, and even if you assume hundreds of thousands of independent intelligent civilizations throughout the universe are doing this right now, it is likely humans will never, ever detect any of them, due to the sheer size of the universe.

I figured that one tube is stretched by the gravitational waves and the perpendicular tube is unaffected, so you're measuring the difference between the two tubes. Simple and easy to understand. However, the video mentioned that both light and the detection apparatus are stretched by the same degree and the physicist says "the light does get stretched and that part doesn't do the measurement for us"[1] -- which makes me think that the simple and easy explanation is not quite right. There's some subtlety that I think I'm missing.

[1] https://m.youtube.com/watch?v=iphcyNWFD10&t=376s (6min 16sec into the video)

The subtlety is this:

Imagine you are in the weightlessness of space, and you measure distances by taking a ruler and placing unconnected beads 10 cms apart.

Now a gravitational wave passes through, what do you see? Nothing. The beads don't move. The intervals you use to measure the distance are stretched in the same way as the space you want to measure in the first place.

So what you can do is, every milisecond or so you lay a new string of beads into empty space. And now when a gravitational wave passes through you'll see that when you lay the string of beads at the moment the gravitational wave passes it doesn't line up with the previous ones.

That you can measure. And by observing the miniscule misalignment of the beads you can see stretching that is much smaller than the 10cm intervals.

So the strings of beads are light rays, and their intervals are the wavelength. The difference is that they don't last, but if there is a light ray in there right as the tube is stretched, it get's stretched along with the tube and you would not see a difference with that, whereas the light that's coming right after it will place new beads/amplitude peaks and actually see the stretched distance.

Light takes different amounts of time to travel down the stretched vs squeezed tube.

Yes, the speed of light cannot exceed its limitation, and as one channel stretches in length, it simply requires more time to travel that length at the constant maximum speed.

The fixed channel remains the same, and covers the unstretched distance, so the light, traveling at the same constant speed reaches the end of the unchanged channel within less time than the other channel.

Or invert the same effect for cases where one channel is compressed to a shorter length, since waves oscillate. So both circumstances may be encountered. Some photons will traverse a compressed distance at a fixed speed, other photons will traverse a stretched distance. Meanwhile, the other perpedicular path is neither stretched nor squashed, and the photons in that channel demostrate the difference when compared to the effected channel.

So no matter what happens, the rate of travel is locked at the upper limit of 186,000 miles per second, whether all of space, energy and matter is squashed in a slightly smaller volume, or expanded to fill a slightly larger volume, the relativistic distances do not change, and the rate of passage of time does not change.

No concrete data on actual funding increases, but it appears the success of LIGO and gravity wave detection has kicked off a renaissance in basic research into fundamental theories of gravitation itself. With a space-based gravity wave detector in various stages of development. And the possibility that gravitons have been indirectly observed already.

Is Gravity Quantum?


LIGO is scaled up version of Michelson-Morley experiment, so we should ask very different kind of question.

One thing the book Black Hole Blues (Janna Levin) brought home to me was the absurdity of actually getting this project done. Now that it's complete, it's easy to say that it was obviously a good idea. It's another thing entirely to bet huge budgets on something that doesn't seem very likely to succeed.

This seems like a thing to build on the moon.

Unfortunate to see NASA was forced to withdrawal (for budgetary concerns).

LISA has now been funded by ESA with a small contribution from NASA. It is slated for launch in the late 2020s / early 2030s.

Funnily enough, the moon has recently been used to constrain the gravitational wave stochastic background [1], i.e. the space-time strain created by the stochastic gravitational wave emission of all massive bodies in the universe.

[1] https://journals.aps.org/prd/abstract/10.1103/PhysRevD.90.10...

This is actually a plot point in Greg Egan's Diaspora. A moon-based laser interferometer is used to detect an impending neutron star merger that will have catastrophic results for biological life on Earth.

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