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Butch

Gravitational waves?

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If a region of space had an oscillating gravitational field, how would this affect a massive body in it's proximity?

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Okay, I know... No such thing, but just for the sake of discussion, let's entertain the idea that there was such an entity and it was oscillating in the range of say 50Ghz and that the affected entity had a mass of say 10 million times that of the average apparent mass of the affecting entity.

What would the affectation be?

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Not sure why you say “no such thing”

Gravity would oscillate in a region of space near any pair of bodies that are orbiting each other. 50 GHz would be unreasonable, however.

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32 minutes ago, swansont said:

Not sure why you say “no such thing”

Gravity would oscillate in a region of space near any pair of bodies that are orbiting each other. 50 GHz would be unreasonable, however.

Ahh, I agree! What if the orbiting system was smaller than a light wave?

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6 hours ago, Butch said:

Ahh, I agree! What if the orbiting system was smaller than a light wave?

What wavelength of light are you talking about?

The wavelength of the gravitational waves produced by two slowly orbiting black holes could be longer than the size of the system, perhaps (would have to check that) if that is what you mean. 

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9 hours ago, Butch said:

Ahh, I agree! What if the orbiting system was smaller than a light wave?

A light wave can be any size, so this doesn't really mean anything.

But if you're going to tackle physics problems, you should be able to answer what the orbital period/frequency is for a system of a given size

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5 hours ago, swansont said:

A light wave can be any size, so this doesn't really mean anything.

But if you're going to tackle physics problems, you should be able to answer what the orbital period/frequency is for a system of a given size

Yes, a very simple calculation? No... at this quantum level time is subjective. It can be said that t =1/g. But before we get into that discussion...

Let me simplify the question... What effect would a gravitational wave with a frequency in the Ghz range (unreasonable as it may be) have on matter. 

 

Edited by Butch

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8 minutes ago, Butch said:

Yes, a very simple calculation? No... at this quantum level time is subjective. It can be said that t =1/g. But before we get into that discussion...

You are not dealing with the quantum level; you are talking about gravitational waves. A classical (non-quantum) phenomenon. And time is not subjective in quantum theory.

8 minutes ago, Butch said:

Let me simplify the question... What effect would a gravitational wave with a frequency in the Ghz range (unreasonable as it may be) have on matter. 

Gravitational waves interact very weakly with matter, so they would pass through without being absorbed. But they would cause the space the matter is in (and hence the matter, I assume) to be stretched and compressed in directions orthogonal to the direction of travel of the gravitational waves. Exactly how depends on the polarization which, in turn, depneds on the nature of the source; some nice images here: http://www.johnstonsarchive.net/relativity/pictures.html

rings.gif

 

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7 minutes ago, Butch said:

Yes, a very simple calculation? No... at this quantum level time is subjective. It can be said that t =1/g. But before we get into that discussion...

At the quantum level you don't have trajectories, and I don't think you can say that t = 1/g

7 minutes ago, Butch said:

Let me simplify the question... What effect would a gravitational wave with a frequency in the Ghz range (unreasonable as it may be) have on matter. 

No idea, though I strongly suspect it also depends on the amplitude.

 

I will say that a 30 GHz wave will have a wavelength of 1 cm, so this would be a more localized effect than what we've been observing.

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13 minutes ago, Butch said:

Yes, a very simple calculation?

You can do a simple calculation based on the orbital speed (and hence time for one revolution) for a system of two masses, just using Newton's equations. This will not be completely accurate for two very large masses in close proximity (like two black holes about to merge, where you would need to use GR) but should be good enough to calculate the frequency and hence wavelength of the generated gravitational waves.

(Note that the accurate result using GR cannot be calculated; you would need to simulate it.)

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23 minutes ago, Strange said:

Gravitational waves interact very weakly with matter

"weakly" is subjective, ask some one who falls off a ladder how weakly they hit the floor...

The question is not how much matter is affected, but rather ion what way?

The Ghz range wave would have a much different effect than earth's gravitational field did on the man falling off the ladder.

18 minutes ago, Strange said:

You can do a simple calculation based on the orbital speed (and hence time for one revolution) for a system of two masses, just using Newton's equations. This will not be completely accurate for two very large masses in close proximity (like two black holes about to merge, where you would need to use GR) but should be good enough to calculate the frequency and hence wavelength of the generated gravitational waves.

(Note that the accurate result using GR cannot be calculated; you would need to simulate it.)

It is good enough because of the size of the system, a system generating gravitational waves in the Ghz range would be incredibly small and these calculations are no longer good enough... But like I said, let us leave this discussion for later.

21 minutes ago, swansont said:

At the quantum level you don't have trajectories, and I don't think you can say that t = 1/g

True and we can discuss this later.

21 minutes ago, swansont said:

No idea, though I strongly suspect it also depends on the amplitude.

The magnitude of the effect would depend on amplitude, not the nature of the effect.

Come on Swan, your mind can do better than "No idea".

34 minutes ago, Strange said:

You are not dealing with the quantum level; you are talking about gravitational waves. A classical (non-quantum) phenomenon. And time is not subjective in quantum theory.

Gravitational waves interact very weakly with matter, so they would pass through without being absorbed. But they would cause the space the matter is in (and hence the matter, I assume) to be stretched and compressed in directions orthogonal to the direction of travel of the gravitational waves. Exactly how depends on the polarization which, in turn, depneds on the nature of the source; some nice images here: http://www.johnstonsarchive.net/relativity/pictures.html

rings.gif

 

Indeed! Do we know of any other forces that have such an effect on matter?

Edited by Butch

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48 minutes ago, Butch said:

 Come on Swan, your mind can do better than "No idea".

You did not provide enough information for an answer, and this is not my area of physics.

If you had asked about the effect of 3 GHz EM radiation on food I would be able to do a lot better, but it's still going to require that you tell me the duration and power of your microwave oven (i.e. amplitude), because running it at 1 W for 1 sec is not going to noticeably heat up your pizza

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29 minutes ago, Butch said:

"weakly" is subjective, ask some one who falls off a ladder how weakly they hit the floor...

That doesn't have anything to do with gravitational waves. Although gravitation generally interacts weakly with matter. Look at the enormous mass that needs to be under the ladder for them to be hurt by the fall.

But if you want to quantify the interaction, you could look at this, for example: https://arxiv.org/abs/1110.0408 or https://link.springer.com/article/10.1007/BF02748651

35 minutes ago, Butch said:

The Ghz range wave would have a much different effect than earth's gravitational field did on the man falling off the ladder.

The effect of gravitational waves is different from gravity. I'm not sure the frequency makes much difference. I'm guessing it would interact less because the matter is not likely able to move (compress and expand elastically) at those speeds.

37 minutes ago, Butch said:

Do we know of any other forces that have such an effect on matter?

I think it is to do with the stress-energy tensor being second order (but at that point I no longer know what the words mean!)

19 hours ago, Butch said:

Okay, I know... No such thing, but just for the sake of discussion, let's entertain the idea that there was such an entity and it was oscillating in the range of say 50Ghz

There has been some theoretical work on very high frequency gravitational waves, for example: https://arxiv.org/abs/1608.03186

Quote

and that the affected entity had a mass of say 10 million times that of the average apparent mass of the affecting entity.

I would assume that if the source of the waves have a very much smaller mass than the object you are considering, then the gravitational waves will also be relatively minute or even insignificant. Therefore you can approximate the effect as ... zero.

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15 minutes ago, Butch said:

"weakly" is subjective, ask some one who falls off a ladder how weakly they hit the floor...

 

Someone falling is responding to the  gravitational field, not gravitational waves.

 

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29 minutes ago, Janus said:

Someone falling is responding to the  gravitational field, not gravitational waves. 

 

Just a matter of amplitude and peridocity, the earth's gravitational field is not static. Gravitational waves are a phenom in the gravitational field... You get the point. What is the nature of the effect, not the amplitude of the effect.

38 minutes ago, Strange said:

That doesn't have anything to do with gravitational waves. Although gravitation generally interacts weakly with matter. Look at the enormous mass that needs to be under the ladder for them to be hurt by the fall.

But if you want to quantify the interaction, you could look at this, for example: https://arxiv.org/abs/1110.0408 or https://link.springer.com/article/10.1007/BF02748651

The effect of gravitational waves is different from gravity. I'm not sure the frequency makes much difference. I'm guessing it would interact less because the matter is not likely able to move (compress and expand elastically) at those speeds.

I think it is to do with the stress-energy tensor being second order (but at that point I no longer know what the words mean!)

There has been some theoretical work on very high frequency gravitational waves, for example: https://arxiv.org/abs/1608.03186

I would assume that if the source of the waves have a very much smaller mass than the object you are considering, then the gravitational waves will also be relatively minute or even insignificant. Therefore you can approximate the effect as ... zero.

Good link! Comes very close to unifying am and g does it not? 

Again what is the nature of the effect, not the amplitude... Perhaps I should not have stated such a relationship.

Edited by Butch

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9 minutes ago, Butch said:

Just a matter of amplitude and peridocity, the earth's gravitational field is not static. Gravitational waves are a phenom in the gravitational field... You get the point. What is the nature of the effect, not the amplitude of the effect.

It's rather more than that. Gravitational waves are not emitted under all circumstances where you have a gravitational source.

 

9 minutes ago, Butch said:

 Again what is the nature of the effect, not the amplitude... Perhaps I should not have stated such a relationship.

It's a stretching and contraction of spacetime, as Strange has already noted.  

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21 minutes ago, Butch said:

Just a matter of amplitude and peridocity, the earth's gravitational field is not static.

It is pretty much static. But the Earth doesn't generate gravitational waves, as far as I know. (Although the Earth-Moon system will.)

22 minutes ago, Butch said:

Comes very close to unifying am and g does it not? 

What is "am"?

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1 hour ago, Strange said:

It is pretty much static. But the Earth doesn't generate gravitational waves, as far as I know. (Although the Earth-Moon system will.)

What is "am"?

Sorry, EM.

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11 minutes ago, Butch said:

Sorry, EM.

It doesn't unify them. But it is another example of the well-known analogies between them.

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1 hour ago, Strange said:

It doesn't unify them. But it is another example of the well-known analogies between them.

You are a thinker Strange, even more so than a learner... a question for you: quantum physics I have seen described as seeking the smallest of the small, the quanta... isn't it more correct today to describe it as seeking the threshold between existence and nonexistence?

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10 minutes ago, Butch said:

You are a thinker Strange, even more so than a learner...

I guess I have to take that as a compliment.

Quote

a question for you: quantum physics I have seen described as seeking the smallest of the small, the quanta... isn't it more correct today to describe it as seeking the threshold between existence and nonexistence?

No. (I guess that would be metaphysics, not quantum physics.)

I think the first definition is rubbish as well. :) 

Quantum physics is really just the study of systems that are described by wave equations with quantised values. It only studies things that exist. And things at a quite a wide range of scales are quantised so it is not just about the "smallest of the small". 

But quantum theory does not seem to be relevant to anything you have discussed; we don't have a quantum theory of gravity. And so gravitational waves are not quantised.

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9 minutes ago, Strange said:

I guess I have to take that as a compliment.

No. (I guess that would be metaphysics, not quantum physics.)

I think the first definition is rubbish as well. :) 

Quantum physics is really just the study of systems that are described by wave equations with quantised values. It only studies things that exist. And things at a quite a wide range of scales are quantised so it is not just about the "smallest of the small". 

But quantum theory does not seem to be relevant to anything you have discussed; we don't have a quantum theory of gravity. And so gravitational waves are not quantised.

Good information as usual... From all of you.

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