Gravitational waves

X-43D

If the advanced LIGO will not detect gravitational waves then is the theory dead?

ohwilleke

Theories don't die. They just get remodeled.

X-43D

Do you have any idea what could be remodeled?

SpaceTiger

We haven't confidently reached the threshold where we can be sure we should have detected gravitational waves. One of several things could happen:

1) LIGO detects something and everybody believes it. This is looking less and less likely with time.
2) LIGO is a bust, but LISA (a planned space interferometry mission) detects gravitational waves. This is where my money is at the moment.
3) LISA is a bust because of some failed instrumentation and we have to wait for the next generation.
4) LISA registers a non-detection of gravitational waves down to the threshold where theory definitely predicts them.

Rather than "killing" anything, that last option would likely excite the community. The least interesting result would be a complete verification of current theory.

X-43D

In a chance when both LIGO and LISA will fail, we would have to wait for Constellation-X (after 2016) i guess. If they don't exist this would be a big lesson.

Of course there are some theories which postulate that we'll never detect gravitational waves, such as MLET.

SpaceTiger

Constellation-X is an X-ray mission....

X-43D

But isn't it part of the 'Beyond Einstein project'? The project has nothing to do with gravitational waves though...

ohwilleke

Well, and to be clear, I'm talking about a situation where the experiment disproves with undisputed accuracy the presence of gravity waves which theory predicts, you have a number of options:

(1) The wave length of gravity waves is different, probably longer, than expected.
(2) Gravity waves are slower than expected.
(3) Contrary to theory, gravity is particle-like and not wave-like.
(4) Contrary to theory, gravity is instananeous.
(5) The instrument is part of the system in an unexpected way which makes it incapable of detecting gravity ways.
(6) It's all the Dolphins' fault.

X-43D

I think that if all matter has a wave structre than GR is probably correct about gravitational waves.

Phobos

As ohwilleke notes, it may be that the system is not sensitive enough to detect gravitational waves (i.e., the experiment produces a "false negative" result). I don't know the details of the experiment, but presumably it was designed to detect gravity waves as they are best understood via the current theory of general relativity. If gravity waves are weaker than we expected, then we may need to tweak the theory accordingly and a more sensitive detector would need to be built. Any detector has a "detection limit" below which it cannot see, so it's hard/impossible to prove that there are zero gravity waves (an experiment may say there are no gravity waves detectable at X meters...and we can keep lowering X with better detectors).

In order to totally scrap general relativity, we would first need to develop a new theory that explains everything GR can and more (GR has already been shown to be very accurate on a broad range of tests). A scientific "theory" has already passed experimental tests. It's a hypothesis that you can keep/drop based on (essentially) one experimental result.

ohwilleke

Of course, I think that either an inconclusive result or a LIGO confirmation of gravity waves is much more likely, than a definitive contraindication of gravity waves from LIGO.