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What Will It Mean If LIGO Detects Gravitational Waves?

theguardian.com

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Re: What Will It Mean If LIGO Detects Gravitational Waves?

#11

Post by ars »

madaxe_again wrote: post #10
mrfusion wrote: post #5

And conversely what would it mean if we never detect gravity waves? Are there any reasonable theories that posit that gravity waves don't exist?

That general relativity is either incomplete or wrong. It would mean that gravity either propagates far more rapidly or far more slowly than we think. Given however that c is considered and so far demonstrably the maximum velocity of information propagation in this spacetime, instantaneous propagation seems unlikely, particularly given the Mercury orbital confirmation. That all said, we may be looking at things naive…

It could also mean that there are no objects large enough to produce gravity waves we can detect.

Black holes for example dilate time, and may either not exist (since they take infinite time to form), or may produce gravity waves that are too time dilated (i.e. slow) to detect.

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#12

Post by Florin_Andrei »

jack9 wrote: post #8
oneworld wrote: post #2

Great explanation of what it might mean: http://www.forbes.com/sites/startswithabang/2016/02/09/what-... Also, how effing cool is it that we are able to detect changes in spacetime itself!

> Sure, it travels through space at the speed of light, but it itself is a ripple in the fabric of space I don't believe that's been proven yet. I was under the impression that this was an experiment to provide evidence of the speed of gravity propagation.

Sure, it's not been actually measured yet, but if it does anything different from that, we would have to do a pretty massive rewrite of several chapters of physics. It's a little unlikely.

That being said, of course, anything could happen. Nature is not bound to follow our assumptions.

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#13

Post by madaxe_again »

ars wrote: post #11
madaxe_again wrote: post #10

Earlier quoted context omitted.

That general relativity is either incomplete or wrong. It would mean that gravity either propagates far more rapidly or far more slowly than we think. Given however that c is considered and so far demonstrably the maximum velocity of information propagation in this spacetime, instantaneous propagation seems unlikely, particularly given the Mercury orbital confirmation. That all said, we may be looking at things naive…

It could also mean that there are no objects large enough to produce gravity waves we can detect. Black holes for example dilate time, and may either not exist (since they take infinite time to form), or may produce gravity waves that are too time dilated (i.e. slow) to detect.

That too. It could also mean that something unknown to us (dark matter halo?) dampens gravitational waves on earth - which is what ESA's eLISA would get around, if it is the case.

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#14

Post by madaxe_again »

camillomiller wrote: post #9
madaxe_again wrote: post #7

I'm curious as to why ligo is the only project ever mentioned, particularly given that they have to work with virgo and geo600 to be able to triangulate their observations - a two axis interferometer only gives you partial information. Better PR team?

In Italy there's a lot of buzz around Virgo (located near Pisa), but I had the impression that it's mostly national talk, too.

Yeah - I visited virgo years ago when they were about to start calibration - it's super cool, but then again, I think michelson-morely interferometers are cool as a category. They were invented for a not dissimilar purpose - testing the properties of the luminiferous aether seeing if light and matter moved at different rates depending on the earth's motion through the aether. The result was negative, and a patent clerk got thinking.

Now his ideas are being put to the test with the same apparatus.

So cool.

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#15

Post by repsilat »

mrfusion wrote: post #5

And conversely what would it mean if we never detect gravity waves? Are there any reasonable theories that posit that gravity waves don't exist?

This experiment not detecting gravity waves might not be too big a deal -- "Oh, there just aren't as many sources of gravity waves floating around as we thought."

Eventually, though, a conclusion that gravity waves don't exist would be enormous. It would completely upend mainstream theories of gravity, which all posit gravity waves (of some sort.)

In fact, I'd say the only reason we're looking for gravity waves is to tick a box -- we don't even consider the possibility that they don't occur, we just think it would be cool to have seen one.

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#16

Post by TTPrograms »

repsilat wrote: post #15
mrfusion wrote: post #5

And conversely what would it mean if we never detect gravity waves? Are there any reasonable theories that posit that gravity waves don't exist?

This experiment not detecting gravity waves might not be too big a deal -- "Oh, there just aren't as many sources of gravity waves floating around as we thought." Eventually, though, a conclusion that gravity waves don't exist would be enormous. It would completely upend mainstream theories of gravity, which all posit gravity waves (of some sort.) In fact, I'd say the only reason we're looking for gravity waves is to…

Regarding reasons for doing this, there's the hope that more sensitive systems could function as "gravity telescopes", enabling charting the locations of massive bodies that would emit gravity waves. This could potentially allow for confirmed observations of physical regimes not accessible by experiment on Earth, much like other astronomical observatories.

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#17

Post by semi-extrinsic »

ars wrote: post #11
madaxe_again wrote: post #10

Earlier quoted context omitted.

That general relativity is either incomplete or wrong. It would mean that gravity either propagates far more rapidly or far more slowly than we think. Given however that c is considered and so far demonstrably the maximum velocity of information propagation in this spacetime, instantaneous propagation seems unlikely, particularly given the Mercury orbital confirmation. That all said, we may be looking at things naive…

It could also mean that there are no objects large enough to produce gravity waves we can detect. Black holes for example dilate time, and may either not exist (since they take infinite time to form), or may produce gravity waves that are too time dilated (i.e. slow) to detect.

> Black holes (...) take infinite time to form

This is wrong, and based on not taking into account that in GR, simultaneity is not only a relative thing, but also a local thing. There is much more detail in several great answers to this Stack Exchange question:

http://physics.stackexchange.com/questions/5031/can-black-ho...

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#18

Post by cornholio »

I hope Forbes realizes that blocking access to users with add blockers is not only a way to get rid of freeloaders, but also a way to ensure those people avoid Forbes altogether in the future and also avoid passing around Forbes links, since not everybody can see them. Alienating your audience is a very expensive mistake and the cost savings related to page serving are negligible.

While I can turn off my add blocker, I won't, my mother can't because she's not aware I have installed one for her and has no idea what it is and what it does.

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#19

Post by ars »

semi-extrinsic wrote: post #17
ars wrote: post #11

Earlier quoted context omitted.

It could also mean that there are no objects large enough to produce gravity waves we can detect. Black holes for example dilate time, and may either not exist (since they take infinite time to form), or may produce gravity waves that are too time dilated (i.e. slow) to detect.

> Black holes (...) take infinite time to form This is wrong, and based on not taking into account that in GR, simultaneity is not only a relative thing, but also a local thing. There is much more detail in several great answers to this Stack Exchange question: http://physics.stackexchange.com/questions/5031/can-black-ho...

Did you actually read the answers you linked to? They say exactly the same thing: A distant observer never sees a blackhole form, but infalling matter does. Because GR has no such concept of simultaneity the two observers can never agree on when the black hole formed.

But since we are here on Earth we care about how it looks to a distant observer, and from our point of view black holes never form. And what the infalling matter sees is irrelevant to us.

NOTE: You do not need black holes to make gravity waves, neutron stars will also make them. But time is still enormously dilated by neutron stars, and I'm wondering how that affects the gravity waves they make.

Re: What Will It Mean If LIGO Detects Gravitational Waves?

#20

Post by ars »

TTPrograms wrote: post #16
repsilat wrote: post #15

Earlier quoted context omitted.

This experiment not detecting gravity waves might not be too big a deal -- "Oh, there just aren't as many sources of gravity waves floating around as we thought." Eventually, though, a conclusion that gravity waves don't exist would be enormous. It would completely upend mainstream theories of gravity, which all posit gravity waves (of some sort.) In fact, I'd say the only reason we're looking for gravity waves is to…

Regarding reasons for doing this, there's the hope that more sensitive systems could function as "gravity telescopes", enabling charting the locations of massive bodies that would emit gravity waves. This could potentially allow for confirmed observations of physical regimes not accessible by experiment on Earth, much like other astronomical observatories.

How can you do that? There is no way to block gravity, so you can not make directional observations, the waves you see are the linear sum of all the waves in the universe.

Although I do wonder if an array of detectors might be able to use time differences in the arrival of the waves to differently located detectors, and some math to get some directional hints.

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