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#53875 04/11/15 07:27 AM
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You missed the more interesting story of last week Bill smile

http://phys.org/news/2015-04-absence-gravitational-wave-limit-knowable-universe.html

It will be really interesting if LIGO comes back with a null result as well as many have speculated as cosmology comes increasingly under the spotlight.

Last edited by Orac; 04/11/15 07:56 AM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
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Orac #53876 04/11/15 01:24 PM
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I saw that and thought about passing it on. I decided not to because it wasn't really showing anything definite. So far they didn't find any thing generated by very small black holes. That may just mean that there aren't a lot of very small black holes out there. I did think that it was good that they could make a gravity wave detector that small, even though it is limited to very high frequency gravity waves.

As far as gravity waves are concerned I can see them being very real, even if they are hard to detect. I can even see gravity waves from Newtonian physics. Let's do a though experiment.

Assume you are in a space ship say 1 light year from a star with 1 planet in orbit around it. You just happen to have an extremely sensitive gravity field detector. You point the detector at the star and measure the gravitational field strength. Now when the planet is on your side of the star its gravitational field strength will be larger than when it is on the far side of the star. So the combined gravitational field strength of the star and the planet will vary with time. That sounds like a gravitational wave. I realize that it will probably be more complicated under GR, but it should follow the same general pattern. A planet orbiting a star will produce gravitational waves.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
Bill #53877 04/11/15 02:31 PM
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Indirectly science has already observed gravity waves a nobel prize was awarded for it and I totally accept that smile

I often think you jump to conclusion about what I think. You never actually read what I write and make up your own version which is actually so wrong you give me great amusement smile

You won't bother reading it but for the record my issue has never been GR it is cosmology and specifically Lambda-CDM model as it currently stands.

You probably won't understand or read this but it is Cosmic inflation in (Lambda-CDM) that predicts tensor fluctuations which you would call gravitational waves. Their amplitude is determined by the energy scale of inflation and it that which LIGO is looking for.

So when I back against gravity waves being detected I am backing against Lambda-CDM model not GR. That gravity waves exist I completely accept something you completely fail to grasp smile

All I can suggest is some reading on issues with the Lambda-CDM model. Hey if LIGO does see a gravity wave then Lambda-CDM is right and I completely accept it, I once backed against the Higgs as well smile

Last edited by Orac; 04/11/15 02:43 PM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
Orac #53878 04/11/15 04:54 PM
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You may know what you are talking about. But what I read doesn't tell me that. You need to be a lot clearer on what you mean. All you do for me is to generate a lot of confusion.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
Bill #53879 04/11/15 06:09 PM
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I mean exactly what I said, I can not be any clearer ... the fact you are confused does not surprise me ... it doesn't say what you want it to say smile

Last edited by Orac; 04/11/15 06:12 PM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
Orac #53880 04/11/15 06:45 PM
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Lets take a leap of faith and assume you actually did want to learn something and actually took the time to read something you could do worse than start with this.

http://www.aei.mpg.de/~schutz/download/lectures/AzoresCosmology/Schutz.AzoresLecture1.pdf

You may want to really carefully read the section "Energy and Gravitational Radiation" and that section is not technically hard to understand and explains the problem with energy and gravity waves and directly the problem with your example above and why its all about the tensor metric.

Last edited by Orac; 04/11/15 07:01 PM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
Orac #53881 04/12/15 02:46 AM
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First off there seems to be a problem with your link. It doesn't work. I copied and pasted it got to the lecture notes ok.

The first thing I noticed was
Originally Posted By: B F Schutz
Lectures assume familiarity with relativistic electromagnetism and
with Minkowski geometry.

Well, I am not familiar with them. But I went on down to the section you mentioned.

Well, I can't say that I got much out of it. It did include a formula that was apparently tensor calculus. I had calculus and differential equations in college, but not tensor calculus. But I didn't see anything that jumped out at me and said that gravitational waves are something other than what I think they are. In fact I didn't see anything new. Well, something about energy not being conserved in GR, I think. Anyway there wasn't an explanation of it, just the statement. Anything that was new to me was apparently hidden in the tensors. So you are still not hitting anyplace close to my level of understanding. If you want me to accept that you know so much more than I do you will have to show me something that I can understand, instead of just showboating with a lot of advanced stuff that I don't know if it applies or not.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
Bill #53883 04/12/15 04:05 PM
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Haha sorry Bill I really was having a joke with you smile

You aren't getting it and so lets see if we can explain the joke.

Remember your claim as per your example above that you can practically show me a gravity wave smile

The problem is the stress-energy tensor for a gravitational field has to be set to zero ... you can't calculate it hence why there is no calculation for it

Hopefully you get my little jibe at you now, I thought you would/should know that fact smile

Here is how uni students usually run across the problem and read Peter Donis the staff moderators very careful answer

https://www.physicsforums.com/threads/stress-energy-tensor-for-gravitational-field.619006/

The 2 points you need to take home


1. The stress-energy tensor on the RHS of this equation has *no* contribution from the gravitational field. The reason this is important is that the Einstein tensor on the LHS of this equation obeys the Bianchi identities, i.e., its covariant divergence is identically zero; therefore, the covariant divergence of the RHS is also identically zero. This is how local conservation of energy is expressed in GR, and it *only* works with the equation written the way I wrote it, with no contribution to the stress-energy tensor from gravity.

2.The various pseudo-tensors describing "energy in the gravitational field" are derived by extracting some piece of the LHS of the above equation, moving it to the RHS, and calling it tμν. But there is no one unique way to do that, because there is no one unique way to split up the true Einstein tensor into a "background" part that stays on the LHS, and an "energy in the field" part that goes to the RHS. You describe two possible ways of doing it, taking either the Minkowski metric as the "background" or some more general curved metric (such as the Schwarzschild metric) as the "background" on which small-scale perturbations (such as gravitational waves) are superimposed. There are others. Which method you use depends on what you are trying to do with it; there is no one "right" answer.

In the original link I gave you they distilled this down to a much simpler semi-layman terminology.

Quote:
Energy is only well-defied in certain regimes, which coincide with those for which waves can be cleanly separated from "background" metrics.

So all you need to do now is convince me you can seperate your wave from the background metric in your example .... hopefully you get the joke laugh

I understood what you were sort of trying to do with your layman example but at a technical level it goes absolutely nowhere.

Last edited by Orac; 04/12/15 04:30 PM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
Orac #53884 04/12/15 05:06 PM
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No I don't get the joke. It appears that you are still trying to snow very body on SAGG with gobbledegook answers that require a huge lot of education to be able to interpret. The things you appear to be talking about are unintelligible to me and I suspect to a great many other people who come to SAGG. If you want me to take you seriously you need to make your answers real answers, not just snow jobs. What you have stated is possibly true, but until I get an explanation at the level I can understand you will just have to accept that I will continue to consider you an irritating anomaly who doesn't necessarily know what he is talking about.

And if a change in gravitational field intensity which varies in time and propagates at the speed of light isn't a gravity wave I would like to know what it is.

So give me an intelligible answer or I will just forget about this conversation.

Bill Gill

Last edited by Bill; 04/12/15 05:07 PM.

C is not the speed of light in a vacuum.
C is the universal speed limit.
Bill #53885 04/13/15 12:09 AM
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ROFL you have been told the answer and most intelligent people get it because they try and understand but you make it my fault.

So lets make this real layman level shall we and use brutal simplifications. There is no such thing as a global reference frame in GR I think I have even seen you write that. So there is no way to globally define or write a global stress-energy tensor.

Read: http://en.wikipedia.org/wiki/Stress-energy_tensor => " There is in fact no way to define a global energy–momentum vector in a general curved spacetime."

So to conserve energy in our GR calculation, it can *only* work with no contribution to the stress-energy tensor from gravity it is a requirement to make the theory and calculation to work .... so it is set to zero.

You were insisting I show you that calculation ... so you want me to show you how to calculate zero ... that is by definition a mathematical/physics joke.

So are we clear and do you understand that you can't calculate the stress-energy tensor ... no one is going to be able to show you that calculation under GR. What everyone will do is make there "own" frame of reference and deal with pseudo-tensors. You are allowed to do that because all frames of reference are equal under GR.

Now lets see if you can do the next bit yourself and see if you are getting it.

So you have no global frame of reference in space-time, can you can guarantee that you actually can see every wave in it?

Last edited by Orac; 04/13/15 12:33 AM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
Orac #53887 04/13/15 01:38 AM
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So you still just keep talking gobbledegook to snow us. Bye Bye

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
Bill #53890 04/13/15 01:50 AM
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LOL sure Bill it is really really basic most 12 year old children can get the issue when discussing boats at sea using your layman world but yeah probably beyond you.

No skin off my nose what you understand ... so anyhow bye bye smile

Last edited by Orac; 04/13/15 03:06 AM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
Orac #53891 04/13/15 03:57 AM
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For anyone stupid or brave enough to go on (maybe Bill S who does seem to have enough grasp) you may ask is there a hypothesis that the torsion tensor is other zero as in GR. The answer is yes and they are called Torsion theories there are a number of them probably the most famous/stable (in my opinion) is Einstein-Cartan theory.

Typically the torsion is viewed as caused by the quantum mechanical spin of particles and as such the mathematics is far more challenging. Current measurements and capability Einstein-Cartan is not experimentally distinguishable from general relativity when I last did any reading in the area (like all things please double check current status). You may need to hit a real physics site and ask around.

Where you would get real differences between these two ideas is in extremely high gravity areas like black holes and our lack of data stops us from excluding it.

You can probably guess a wave in a spacetime under torsion is going to look every different to one with a zero torsion.

Thus one of the cool things about being able to directly measure a gravity wave and finding it where we expect it to be ... we know we have the right theory.

If you really want to be adventurous you can play with the time component of spacetime the most popular of which was probably Brans–Dicke theory. In 2003 the Cassini–Huygens experiment basically ruled the theory out but I guess there are scientists around who still play with the idea.

Last edited by Orac; 04/13/15 03:59 AM.

I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.

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