Gravitational Waves: A Question on Earth's Magnitude & Frequency

[roineust]

Here is a video i have watched:

It made me think of the following questions:

A. Gravitational waves at the scale that emanate from earth:

1. It is doubtful if they exist at all.
2. They are predicted to exist, but at an extremely small magnitude.

B. In case the answer is A2:

1. These gravitational waves are a result of only Earth orbiting the sun.
2. These gravitational waves are a result of only Earth turning around itself.
3. These gravitational waves are a result of only something else.
4. Not only but both B1 and B2 (and perhaps B3).

In case the answer is B4:

What are the predicted differences of magnitude and frequency between B1 and B2 (and perhaps B3) ?

 

[Nugatory]

If I'm understanding you properly, your question is gravitational waves produced by th eearth's orbital motion, and the video is irrelevant to the question?

 

[roineust]

If I'm understanding you properly, your question is gravitational waves produced by th eearth's orbital motion, and the video is irrelevant to the question?

The video talks a lot about gravity and scale of magnitude, this is the reason that it made me think of these questions, which are about gravitational waves and scale of magnitude.

I can delete the video link if it interrupts in reading the thread rather than contributes.

 

[Nugatory]

I can delete the video link if it interrupts in reading the thread rather than contributes.

It doesn't seem to be doing any harm... Leave it up unless someone else objects (and that happens, we'll take care of it )

 

[PAllen]

Here is a video i have watched:

It made me think of the following questions:

A. Gravitational waves at the scale that emanate from earth:

1. It is doubtful if they exist at all.
2. They are predicted to exist, but at an extremely small magnitude.

B. In case the answer is A2:

1. These gravitational waves are a result of only Earth orbiting the sun.
2. These gravitational waves are a result of only Earth turning around itself.
3. These gravitational waves are a result of only something else.
4. Not only but both B1 and B2 (and perhaps B3).

In case the answer is B4:

What are the predicted differences of magnitude and frequency between B1 and B2 (and perhaps B3) ?

A2, yes

B1, yes
B2, to the extent the Earth could be approximated by a spinning black hole of the same mass, no. To the extent the actual Earth deviates from this, the yes GW would be generated but extremely small even compared to the extremely small production from B1. One could say, that on the scale of the B1 waves, the B2 waves are insignificant.

I don't know the quantitative ratio between B1 an B2 waves, but my guess would be many orders of magnitude smaller for B2, noting that B1 is likely too small to ever be detected, in practice.

 

[roineust]

A2, yes

B1, yes
B2, to the extent the Earth could be approximated by a spinning black hole of the same mass, no. To the extent the actual Earth deviates from this, the yes GW would be generated but extremely small even compared to the extremely small production from B1. One could say, that on the scale of the B1 waves, the B2 waves are insignificant.

I don't know the quantitative ratio between B1 an B2 waves, but my guess would be many orders of magnitude smaller for B2, noting that B1 is likely too small to ever be detected, in practice.

A question:

If a LIGO device was taken to outer space, where there are no planets and stars near by, still calibrated to sense gravitational waves as if it was on Earth and a gravitational wave will pass by, will the LIGO operators be able to detect that it is not on Earth anymore, only by looking at the gravitational wave measurement results?

 

[jbriggs444]

A question:

If a LIGO device was taken to outer space, where there are no planets and stars near by, still calibrated to sense gravitational waves as if it was on Earth and a gravitational wave will pass by, will the LIGO operators be able to detect that it is not on Earth anymore, only by looking at the gravitational wave measurement results?

Why would one want to do such a daft thing? What question are you really trying to ask?

 

[roineust]

If the answer to the previous question is yes, then my next question is:

Now, if that Earth calibrated LIGO machine in outer space will be accelerated at constant 9.8 m/s^2, will the LIGO operators be fooled to think it is back on Earth again when detecting gravitational waves?

 

[jbriggs444]

If the answer to the previous question is yes, then my next question is:

Now, if that Earth calibrated LIGO machine in outer space will be accelerated at constant 9.8 m/s^2, will the LIGO operators be fooled to think it is back on Earth again when detecting gravitational waves?

Again, what is it that you are trying to get at with these nonsense questions?

 

[roineust]

I prefer not to elaborate without having some answers, if the answers are no to the first question or yes to the first question and yes to the second, then i prefer not to go on asking questions which are based on wrong assumptions and wrong understanding.

 

[jbriggs444]

I prefer not to elaborate without having some answers, if the answers are no to the first question or yes to the first question and no to the second, then i prefer not to go on asking questions which are based on wrong assumptions and wrong understanding.

Must not respond... must not respond... *runs away quietly*.

 

[roineust]

Why is it not possible to answer these questions first?

 

[Ibix]

Why is it not possible to answer these questions first?

Because asking a series of questions without context like this typically ends up with us saying something that may be generally true but isn't correct in the specific case it turns out you are thinking of. Then threads end up with dozens of "but you said..." and "yeah, but I didn't know you were thinking about..." posts.

What's the context for your questions?

 

[roineust]

Because asking a series of questions without context like this typically ends up with us saying something that may be generally true but isn't correct in the specific case it turns out you are thinking of. Then threads end up with dozens of "but you said..." and "yeah, but I didn't know you were thinking about..." posts.

What's the context for your questions?

The context is if a LIGO like device can tell the difference between gravity and acceleration of the same magnitude, by measuring difference in results pattern, when gravitational waves pass thorugh it.

 

[jbriggs444]

The context is if a LIGO like device can tell the difference between gravity and acceleration of the same magnitude, by measuring difference in results pattern, when gravitational waves pass thorugh it.

But the acceleration of LIGO on Earth is not due to gravity. It is due to electromagnetic interactions with the Earth's crust. So what is it that you are really trying to ask?

 

[roineust]

But the acceleration of LIGO on Earth is not due to gravity. It is due to electromagnetic interactions with the Earth's crust. So what is it that you are really trying to ask?

What i am trying to ask is exactly what i replied above, i don't understand what electromagnetic interactions with Earth crust have to do with my question, which is about the fact that LIGO is exposed to a gravitational pull of 9.8 m/s^2 like any object on Earth ground and i am trying to find out if while detecting gravitational waves, it can tell the difference between being on Earth ground and being accelerated in outer space at 9.8 m/s^2.

 

[jbriggs444]

What i am trying to ask is exactly what i replied above, i don't understand what electromagnetic interactions with Earth crust have to do with my question, which is about the fact that LIGO is exposed to a gravitational pull of 9.8 m/s^2 like any object on Earth ground and i am trying to find out if while detecting gravitational waves, it can tell the difference between being on Earth ground and being accelerated in outer space at 9.8 m/s^2.

We are talking general relativity here. There is no such thing as the pull of gravity. LIGO is being pushed upward by the Earth's surface at 9.8 m/s^2. The fact that you choose to use an accelerating coordinate system when describing the motion is irrelevant.

We are talking about a thought experiment. In a thought experiment, the theory is taken as true. The equivalence principle is upheld automatically.

 

[roineust]

We are talking general relativity here. There is no such thing as the pull of gravity.

We are talking about a thought experiment. In a thought experiment, the theory is taken as true. The equivalence principle is upheld automatically.

I will take you answer as:

The non-theoretical, material instrument that is called LIGO, will not be able to tell the difference between being on Earth ground and being accelerated in outer space at an equivalent 9.8 m/s^2, while detecting gravitational waves.

 

[jbriggs444]

I will take you answer as:

The non-theoretical, material instrument that is called LIGO, will not be able to tell the difference between being on Earth ground and being accelerated at an equivalent 9.8 m/s^2, while detecting gravitational waves.

You are talking about a non-existent hypothetical instrument floating in space similar to the existent physical instrument named LIGO. Accordingly, you are talking about a thought experiment.

You are still playing coy about your reasons for asking.

And you have still not comprehended that both hypothetical and real LIGOs are being accelerated at an actual 9.8 m/s^2 by physical forces.

 

[roineust]

You are talking about a non-existent hypothetical instrument floating in space similar to the existent physical instrument named LIGO. Accordingly, you are talking about a thought experiment.

You are still playing coy about your reasons for asking.

There is a big difference as much as i understand, between describing an instrument which is currently not possible to build, because there is a lack of physics theoretical knowledge of how to build it and an instrument that is not only possible to build but already exists, and what is asked about that instrument is lacking in that that existing instrument, was not yet put into outer space and accelerated there.

It is not a thought experiment, but a question if LIGO can be considered to be ignoring 9.8 m/s^2 acceleration and Earth gravity to the same degree, be it because it was never even calibrated to ignore them, since they have nothing to do with its gravitational wave detection goal or because it is calibrated to ignore them, but both (acceleration and gravity) influence the instrument, exactly the same.

 

[Nugatory]

... a question if LIGO can be considered to be ignoring 9.8 m/s^2 acceleration and Earth gravity to the same degree.

It is still not clear what you're asking. The different parts of a LIGO observatory on the Earth are being accelerated in slightly different directions by electromagnetic interactions with the atoms that make up the surface of the earth. You haven't specified the mechanism that's accelerating your hypothetical LIGO in space, but clearly it's going to be something different, but that's not enough to say that it is detectably different.

But if you are asking whether LIGO is sensitive enough to detect tidal effects... The answer is yes.

 

[jbriggs444]

It is not a thought experiment,

It is not a real experiment because it has not really been run. You are asking what would happen if it were run. That is precisely what a thought experiment is.

 

[vanhees71]

Why is it not possible to answer these questions first?

This I also don't understand. I think the answer is that the gravity of the Earth is way too weak to have a significant effect, because over the extension of the LIGO detectors the gravitational field is sufficiently homogeneous and thus compensated by mounting the mirrors with the precision they are mounted. So this compensates for the homogeneous gravitational field. So I'd expect indeed that if you could put the LIGO detector to outer space it would just work the same there as it does on Earth. In fact such a thing is planned:

https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna

A first test mission was:

https://en.wikipedia.org/wiki/LISA_Pathfinder

 

[PeterDonis]

If a LIGO device was taken to outer space, where there are no planets and stars near by, still calibrated to sense gravitational waves as if it was on earth

This cannot be done, because if the device is in free fall, it has to be calibrated differently to detect gravitational waves at all, since zero proper acceleration is not the same as 1 g proper acceleration plus tiny differences in direction of acceleration because of the sphericity of the Earth.

In other words, you can't test the equivalence principle this way, because to apply the EP at all, the objects in the two cases you're comparing have to have the same proper acceleration.

if that Earth calibrated LIGO machine in outer space will be accelerated at constant 9.8 m/s^2, will the LIGO operators be fooled to think it is back on Earth again when detecting gravitational waves?

No, because LIGO is large enough that the sphericity of the Earth affects its calibration on Earth. A rocket in deep space accelerating at 1 g will not have that sphericity.

In other words, you can't test the equivalence principle this way either, because the EP only applies over a small enough region of spacetime that tidal effects (such as the effects caused by the Earth's sphericity) are not detectable.

if a LIGO like device can tell the difference between gravity and acceleration of the same magnitude, by measuring difference in results pattern, when gravitational waves pass thorugh it

In other words, you are asking if gravitational wave detectors can violate the equivalence principle. The answer is no, they can't. See above.

 

[PeterDonis]

over the extension of the LIGO detectors the gravitational field is sufficiently homogeneous

I'm not sure this is exactly true given LIGO's sensitivity. I think the calibration has to make some allowance for the sphericity of the Earth. But I have not looked deeply into the details.