# The whole Universe jiggles, maybe.

Discussion in 'Personal' started by racroesus, Feb 10, 2016.

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1. ### TCSC47Star commenter

Dark Star! Yes, brilliant! Cheers, I'm going to look it up and find a DVD. I recommend it to everybody as a commentary on the 70's America.

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2. ### MangleworzleStar commenter

Express it terms of shoes, come on man, you're a teacher, get through to her.

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3. ### Didactylos4Star commenter

I was going to make a comic reply but I'd only get Rosie calling me a prevert

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I have rather a lot of questions for horses' mouths (not a test, I simply don't know)

1. What is the wavelength of a gravity wave, and what does it tell us about the originating event.

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5. ### MangleworzleStar commenter

Brian Cox on the radio at the moment.

What was seen was caused by 2 black holes, one of them the size of 30 suns (=about 30,000,000 earths) the other the size of 36 suns (=about 36,000,000 earths) colliding a billion years ago.
They were moving at 1/3rd the speed of light, then accelerated to 2/rds the speed of light in 0.1s.

Blimey.

This has a good video that explain things a bit.

http://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html?_r=0

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Lambda looks like ~ 3 million metres in the example of the results I've seen - but it is not a constant.
It depends on the motion of the source.

It tells you that very massive things had accelerations usually associated with very much smaller things!

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7. ### ScintillantStar commenter

The wavelength is not a fixed quantity

The whole thing is difficult to get your head around. Spacetime itself is hard enough to comprehend

8. ### lanokiaStar commenter

OK so... and please forgive the amateurishness of this post...

But... am I right in thinking that gravity waves are sort of the consistent background of the universe, that they are super stable and that is why we haven't been able to detect them even though they are all around us? Because very little causes permutations?

So it took two black holes colliding 1 billion years ago to create a force great enough to wibble the waves?

9. ### Didactylos4Star commenter

Having listened to quite a few scientist commenting on this that is my understanding Lanokia

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10. ### Didactylos4Star commenter

I could be wrong though

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Conveniently continental sized then. Handy.

12. ### MangleworzleStar commenter

I'm not sure they are super stable, long lasting ripples, though I suppose a billion years is quite a long time to be around, so super stable is not incorrect! We haven't been able to detect them as we never had instruments sensitive enough, I think I read that the LIGO instruments that found them had recently been upgraded to a much higher resolution.

It seems that it takes massive events like black holes colliding to make enough ripples to be detectable.

"LIGO researchers sensed a wave that stretched space by one part in 1021, making the entire Earth expand and contract by 1/100,000 of a nanometer, about the width of an atomic nucleus."

http://www.sciencemag.org/news/2016...nstein-s-ripples-spacetime-spotted-first-time

Not that I know much about it other than having read around a bit today.

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I don't know what that means, so I would suggest the best answer is "No".

The question is, does gravity propagate (as does e.m. radiation), or is it instaneous. In other words, if an object moves, does its gravitational field strength at an exterior point stay as it was for a short time (depending on how far away it is from the object), or does it change instantly.

Einstein hypthesised that it propagated at a finite speed, in other words, there would be a time lag. That's what all the fuss is about.

Are the tides examples of an effect of gravity waves?

14. ### lanokiaStar commenter

What I mean, the impression I've got, is that gravity waves are emanating all around us and interacting with us like a tapestry which we don't notice but which interacts with us.

Which reading the rest of your post does seem to accord with what you say, in that an object has an impact that radiates out. If by impossible chance a super-massive object emerged in close orbit to the Sun, say a neutron star, that'd have an impact on the Earth, maybe 8 minutes later?

Is there a speed of gravity just like light?

15. ### irs1054Star commenter

The difficulty of detecting gravity waves is related to the relative effect that gravity has compared with other forces. The strong force is the strong nuclear force which holds nuclei together. If it has a strength of 1 then the electromagnetic force which holds molecules together has a strength of 1/137 and gravity has a strength of 10-39.

It therefore requires a very large perturbation in gravity to produce a wave that can be detected.
The idea of General Relativity is that this would be the case (though not necessarily the time). Now that we have actually begun to detect gravity waves we might, at some point, find a definitive answer to this question.

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Oh my...

17. ### irs1054Star commenter

Yup, you need to hold onto that 10-39 very carefully in case you fall off the Earth.
Whatever you do, don't lose it!

Actually, the gravitational strength is only 6x10−39 of the strength of the strongest nuclear forces, provided you are using protons as your unit of measurement.

For two protons, each of which has a mass of about 1 GeV/c2 the gravitational force between them has a relative strength of the square of (1/10 million million million), or (10-19)

• αgravity = (10-19)2 = 10-38
Meanwhile for two electrons

• αgravity = (10-19)2 = 3 × 10-46
which, since an electron has a mass almost 2000 times smaller than a proton, corresponds to a force four million times weaker. Even for a pair of top quarks, nearly 200 times heavier than a proton and with the largest masses of any known particles, the gravitational force has a strength of only

• αgravity = 10-34
That’s about 100,000,000,000,000,000,000,000,000,000,000 times smaller than the electric force between two top quarks.

Last edited: Feb 12, 2016