# What can a Gravity Wave Detector do that a . . . . can't

Stacey Baird
Joined: 14 May 06
Posts: 19
Credit: 453,050
RAC: 0
Topic 194680

What can a gravity wave detector tell us that the ocean tides cannot?

Bikeman (Heinz-...
Moderator
Joined: 28 Aug 06
Posts: 3,522
Credit: 688,484,514
RAC: 234,359

### What can a Gravity Wave Detector do that a . . . . can't

Quote:
What can a gravity wave detector tell us that the ocean tides cannot?

The hope is that measuring gravitational waves will open a whole new branch of astronomy, allowing us to get insights into some extreme events like the merger of black holes, that are hard to observe with conventional means of astronomy.

See http://einsteinathome.org/gwaves/motivations/index.html for a very short summary of the motivations for Gravitational Wave Research.

Actually your question might be caused by a misunderstanding: While the ocean tides are caused by changes in the gravitational field because of the movement of the moon, sun and earth, this has absolutely nothing to do with Gravitational Waves. The mechanism that drives the ocean tides can be explained to great precision just with Newtonian physics (the laws of physics as explained by Sir Isaac Newton). Gravitational Waves are a different kind of phenomenon, not explainable just with Newtonian physics. It's a prediction of General Relativity as invented by Einstein in the early 20th century.

Mike Hewson
Moderator
Joined: 1 Dec 05
Posts: 6,537
Credit: 286,211,703
RAC: 108,113

### RE: What can a gravity wave

Quote:
What can a gravity wave detector tell us that the ocean tides cannot?

You can roughly approximate the thinking as follows :

General Relativity = Newton + Special Relativity + Other Stuff

where :

Newton = static/instantaneous

Special Relativity = account for finite delay of effects ( light speed )

Other Stuff = non-linear/curvature of spacetime

Each of these parts is proportional to various powers of the ratio v/c ( v is the velocity of some typical part of the system under study and c is the speed of light ). As v/c < 1 then higher powers are smaller again, like (v/c)^5 <<< 1 [ compare v/c = 0.1 to (v/c)^5 = 0.00001 ]. Other Stuff is in the fifth power or higher.

Mind you if you take v/c = 0.90 then (v/c)^5 ~ 0.59, hence if you are nearby to some, say, close binary neutron star system where such speeds are achievable [ possibly not for very long though! ] then the Other Stuff corrections are about the magnitude of the Newton and Special Relativity terms. An alternative phrase for this is that 'gravitational self energy' is significant, or that 'gravity attracts gravity'. Weird city .... :-)

The LIGO effort is in the Other Stuff category. In a sense we are testing for the existence of the Other Stuff by trying to detect gravity waves.

There is another pre-existing use of the phrase 'gravity waves' which does refer to ocean movements - a vertical oscillation of the water level - but this is not related to general relativity at all.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Stacey Baird
Joined: 14 May 06
Posts: 19
Credit: 453,050
RAC: 0

### Thank you to Bike Man and

Thank you to Bike Man and Mike Hewson for their explanations. I appreciate it. Sorry to take so long to say thank you. My question and their answers were posted to a different forum and I missed it for quite a while.

Mike Hewson
Moderator
Joined: 1 Dec 05
Posts: 6,537
Credit: 286,211,703
RAC: 108,113

### RE: Thank you to Bike Man

Message 96054 in response to message 96053

Quote:
Thank you to Bike Man and Mike Hewson for their explanations. I appreciate it. Sorry to take so long to say thank you. My question and their answers were posted to a different forum and I missed it for quite a while.

Yeah, we moved it. :-)

You ought have got an email - to the address that E@H knows of as per your account details - with any moderation action performed. If you didn't, get back to us in the case there is a problem our end.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal