Not only will the discovery of gravitational waves confirm an important prediction of general relativity, but it will be the first test of GR in the strong gravity regime. So far, all the tests of GR have been in the weak gravity regime, where Newton's law of gravity is approximately correct. The precession of Mercury's perihelion, gravitational redshift, the gravitational deflection of light, and the measurement of frame dragging by Gravity Probe B have all been probing similar regions of parameter space --- slow velocities (compared to the speed of light) and weak gravity (i.e. far from the event horizon of a black hole).
If the rumors are true that LIGO has detected the merger of two black holes, the observed waveform will be the first test of the predictions of general relativity in the strong gravity limit. This is the most interesting limit because this is where physics diverges the most from Newtonian gravity. Moreover, it's much easier to detect deviations from general relativity in this limit.
LIGO's detection won't be the only test of GR in the strong gravity limit for too long, however. The Event Horizon Telescope has been trying to directly observe the event horizon of the supermassive black hole at the center of the Milky Way (or its shadow, anyway) and should have its first images within a few years.
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.
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.
If you look carefully at the press release, the group that's presenting on Thursday is the LSC, the LIGO Scientific Collaboration, which includes VIRGO and GEO. It's one big group, as everyone in the field shares hardware techniques and analysis techniques.
An announcement of an announcement is always off-topic here, so we changed the URL from http://www.ligo.org/news/media-advisory.php to that article, which is much more substantive. Thanks!
The announcement itself, of course, will be on topic when it happens, but we can be patient.
> 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.
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 naively.
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.
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:
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.
In the original comment I replied to, the point was "black holes cannot form because we cannot observe it, so black holes don't exist". This is a misunderstanding, and is what I was pointing out.
Saying that something objectively "doesn't happen" because an observer infinitely far away can't see it is ridiculous.
> This is a misunderstanding, and is what I was pointing out.
No. You are misunderstanding things.
> because an observer infinitely far away can't see it
No, it's not that the observer can't see it (too hard to see or something like that). It never occurs in the timeline of the observer.
From the point of view of the observer it never actually exists. That it exists from the point of view of someone else is irrelevant. It's not a semantic game about I can see it you can't.
It literally and actually simply does not exist.
(Not to mention the context is gravity waves, so if it never exists from the point of view of the observer it also can't make gravity waves.)
Pardon me if I am being obtuse, but the point of the original comment was "black holes do not exist in our universe, since time dilation at the event horizon is infinite", yes? This is what I am arguing against.
The event horizon is within the future null infinity of your observer; i.e. the observer may choose to travel towards to black hole, and will asymptotically fall into it and thus undeniably observe it. Hence it does actually exist even for this observer. Moreover, the observer can observe the shadow, effect on nearby masses and the gravitational lensing produced by the black hole, thus indirectly observing the black hole, even though he cannot see the actual event horizon.
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.
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.
Time differences: Exactly that. Which is why having 3 or 4 synchronized detectors is necessary for narrowing down the direction of a source. (3 minimum, 4 to reduce noise.)
It means that at this point, ground based detectors probably won't work due to the massive amount of background noise (activities from people, geological activity, etc). There are plans for detectors in space that would not suffer the same problems.
There's little reason, almost none at all, to think they don't exist. The standard model clearly predicts they should exist, and non-direct evidence has been seen already.
It would be a spectacular conflict with the Hulse-Taylor binary pulsar system, whose orbital decay is exactly in line with the prediction for energy loss through gravitational waves.
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.
I hear what you're saying, but I'm not sure I agree with the argument that this approach is not in their interest (I also use an ad blocker by the way, not preaching just being practical). I would bet that ~50%+ of people are choosing to disable their ad blocker to view Forbes content once they got stopped (I did), and that that the value driven by that increase in ads served outweighs the cost of not having the small % of users who refuse to turn off ad blockers not share their links.
I can also 100% guarantee they split tested this, before rolling it out, to make sure it was the right cost/benefit choice.
> a way to ensure those people avoid Forbes altogether in the future
I have an ad blocker and never ended up doing this. I either forget which websites are "blocking ad blockers", or I just click without checking the source, especially if the link is in the home of a site I trust like HN.
If the rumors are true that LIGO has detected the merger of two black holes, the observed waveform will be the first test of the predictions of general relativity in the strong gravity limit. This is the most interesting limit because this is where physics diverges the most from Newtonian gravity. Moreover, it's much easier to detect deviations from general relativity in this limit.
LIGO's detection won't be the only test of GR in the strong gravity limit for too long, however. The Event Horizon Telescope has been trying to directly observe the event horizon of the supermassive black hole at the center of the Milky Way (or its shadow, anyway) and should have its first images within a few years.