B Detecting Gravitational Waves: Earth-Like Planet Possibility

[roineust]

Is it theoretically possible that one day a gravitational wave detector will be developed, that is sensitive enough to detect gravitation at the order of magnitude that an Earth like planet has?

 

[Vanadium 50]

I predict that this will end up becoming an argument about what "theoretically possible" means.

This would be ~1000000000000000000000000000000000000 times weaker than a BH merger.

In the words of Jim Carey, "So you're telling me there's a chance!"

 

[roineust]

I predict that this will end up becoming an argument about what "theoretically possible" means.

This would be ~1000000000000000000000000000000000000 times weaker than a BH merger.

In the words of Jim Carey, "So you're telling me there's a chance!"

So it is correct to ask questions this way?

 

[berkeman]

So it is correct to ask questions this way?

As always at PhysicsForums, it's best to post a link to the reading you have been doing about gravitational waves, and do as much of the calculation as possible to show whether it would be possible/reasonable to do this detection.

You did post this in the technical forums, after all, not the General Discussion forum...

 

[roineust]

Should i ask the motivating question behind the initial question or is it already totally clear what i am about to ask?

 

[berkeman]

Should i ask the motivating question behind the initial question or is it already totally clear what i am about to ask?

I guess, but it would still be best if you addressed the missing parts of your OP that I already mentioned, and said quantitatively why what you want to discuss is superior to the current approaches in the search for Exoplanets. Thanks.

https://exoplanets.nasa.gov/

 

[roineust]

Oh, no it has nothing to do with exoplanets it has to do with general relativity equivalence principle:

If such a detector had existed, wouldn't it make the equivalence principle not correct?

 

[Stavros Kiri]

So it is correct to ask questions this way?

So after presenting links and calculations (as @berkeman said) I think the question becomes whether it is theoretically possible to practically detect ... (etc.)
I think theoretically it could be ... but ... practically? ... Not at this point.

Give it a try though (following suggested orders of magnitude and running some rough estimates of sensitivity etc. ...).

 

[Vanadium 50]

You got a quantitative answer, and somehow it's not good enough. I predicted that this will end up becoming an argument about what "theoretically possible" means. And we seem to be on that path, given that a quantitative answer is somehow unsatisfactory.

 

[roineust]

You got a quantitative answer, and somehow it's not good enough. I predicted that this will end up becoming an argument about what "theoretically possible" means. And we seem to be on that path, given that a quantitative answer is somehow unsatisfactory.

So i was supposed not to ask the "theoretically possible" continuation question, after you gave the quantitative answer to the initial question?

 

[Vanadium 50]

So i was supposed not to ask the "theoretically possible" continuation question

I predict that this will end up becoming an argument about what "theoretically possible" means.

And away we go!

 

[roineust]

And away we go!

So by writing "theoretically possible" you did not mean physics theory but argument theory?

 

[Dale]

Is it theoretically possible that one day a gravitational wave detector will be developed, that is sensitive enough to detect gravitation at the order of magnitude that an Earth like planet has?

Yes.

 

[Vanadium 50]

So by writing "theoretically possible"

Those are your words. Words #3 and #4 of this thread.

 

[pervect]

Oh, no it has nothing to do with exoplanets it has to do with general relativity equivalence principle:

If such a detector had existed, wouldn't it make the equivalence principle not correct?

I don't see why you say that. There are several versions of the equivalence principle. The one that causes the least misunderstanding and arguments is called the "weak equivalence principle". That says basically that if you drop small masses under identical conditions, they fall at the same rate. The composition of the masses doesn't matter. To put it another way, the equivalence principle is saying there is not inertial mass and gravitational mass - the two sorts of mass are equivalent.

Gravitational waves don't really come into the picture at all, they are basically an irrelevant distraction.

Anyway, what needs clarifying about your question is what version of the equivalence principle you are using, and why you think gravitational radiation is relevant to it.

 

[Dale]

If such a detector had existed, wouldn't it make the equivalence principle not correct?

No.

I am not sure what would make you think that

 

[roineust]

As much as i understand, if such a detector would have existed, then a claim that says that there is no experiment that can tell the difference between a gravitational field of a certain strength and non-gravitational constant acceleration, that causes equal acceleration to that gravitational field, will be a not correct claim.

 

[Dale]

As much as i understand, if such a detector would have existed, then a claim that says that there is no experiment that can tell the difference between a gravitational field of a certain strength and non-gravitational constant acceleration, that causes equal acceleration to that gravitational field, will be a not correct claim.

How is that different in principle from current gravitational wave detectors? So in your opinion a detector that can detect black holes and neutron stars is compatible with the equivalence principle, but one that detects planets is not?

This doesn’t make any sense. At what exact mass do gravitational waves suddenly go from validating general relativity to disproving it? How can the exact same phenomenon both be a prediction of a theory at low sensitivity and yet contradict it at higher sensitivity.

 

[Stavros Kiri]

Yes.

As always at PhysicsForums, it's best to post a link to the reading you have been doing about gravitational waves, and do as much of the calculation as possible to show whether it would be possible/reasonable to do this detection.

You did post this in the technical forums, after all, not the General Discussion forum...

Without him specifying like @berkeman suggested I think it's pointless to draw conclussions or even discussing equivalence principle etc. ...

 

[Nugatory]

there is no experiment that can tell the difference between a [constant] gravitational field of a certain strength and non-gravitational constant acceleration

That is not quite what the EP claims. The EP claims that “there is no experiment that can tell the difference between a [ constant] gravitational field of a certain strength and non-gravitational constant acceleration” within a sufficiently small region of spacetime, one that is small enough that the tidal effects that distinguish a gravitational field from non-gravitational constant acceleration are negligible.

None of this has any relevance to the detection of gravitational waves because if we’re detecting gravitational waves we aren’t working with a constant gravitational field.

 

[Janus]

As much as i understand, if such a detector would have existed, then a claim that says that there is no experiment that can tell the difference between a gravitational field of a certain strength and non-gravitational constant acceleration, that causes equal acceleration to that gravitational field, will be a not correct claim.

You do realize that gravitational waves are not simply the result of there being a gravitational field? You need something like a mass accelerating to produce them. With the BH detection, the GW radiation was produced by the holes spiraling in on each other. While an Earth-like planet can produce GWs, it would be due to its orbit around its star. In that case, an Earth-like planet orbiting a star like our Sun, at a similar distance, would generate GWs at a power of just 200 watts. These waves could have a frequency of 1 cycle per year, and a wavelength of 1 light year.

 

[DaveC426913]

At the the risk of speaking for the OP, I suspect what he is getting at is that in real-world practical scenarios, there are things that masses can do to gravity for which simple acceleration does not (normally) have an equivalent. Such as, say, "wobble".

And that this could be used to practically tell the difference between gravity and acceleration of a real-world event being measured.

roineust , I think you're missing the point. It's a principle - that term implies that you can (in principle) eliminate practical factors that might pollute it.

Detecting a "wobble" in the force you're measuring and being able to declare "it's gravity, not acceleration" doesn't invalidate the principle - any more than detecting tidal forces does.

Tidal forces and wobbles are examples of practical factors that pollute the ability to accurately test the principle; they don't invalidate it.

A spurious example:

Objects near Earth fall at 9.8m/s^2. That's hard to test that if you have no way to get rid of air resistance in your tests. That doesn't invalidate the known free fall acceleration near Earth - it doesn't mean it's falsified.

 

[Vanadium 50]

because if we’re detecting gravitational waves we aren’t working with a constant gravitational field.

And we are certainly not working with a region "small enough".

 

[Vanadium 50]

This whole thread started off on false pretenses, behind which is a gross misunderstanding of the equivalence principle. I guess we should be happy that this only took 17 messages to get to.

Personally, I think if the OP spent half as much time learning relativity as complaining about it, he'd be much happier.

 

[roineust]

Those are your words. Words #3 and #4 of this thread.

These are my words, which you have reused while changing their meaning, if the answer to the preliminary question was no, would it still be false pretense?

 

[PeterDonis]

If such a detector had existed, wouldn't it make the equivalence principle not correct?

No. The equivalence principle is only valid over regions of spacetime that are small enough that tidal gravity is not detectable. Gravitational waves are waves of tidal gravity, so if a gravitational wave is detected, then by definition the region of spacetime involved is not small enough that tidal gravity is not detectable.

 

[roineust]

No. The equivalence principle is only valid over regions of spacetime that are small enough that tidal gravity is not detectable. Gravitational waves are waves of tidal gravity, so if a gravitational wave is detected, then by definition the region of spacetime involved is not small enough that tidal gravity is not detectable.

Is the equivalence principle defined in such a way that it is experimentally non-falsifiable?

For example: the region is defined to always be smaller than the area that any future equipment will be able to measure?

 

[Ibix]

Is the equivalence principle defined in such a way that it is experimentally non-falsifiable?

For example: the region is defined to always be smaller than the area that any future equipment will be able to measure?

Of course it's falsifiable: drop two masses off the Leaning Tower of Pisa and have one hit the ground before the other because it's made of (hypothetical) material that accelerates twice as fast under gravity as normal matter. If you repeat the experiment in a closed box you could then tell whether you were accelerating in empty space (the masses "fall" at the same rate) or at rest on a planet (they fall at different rates).

 

[DaveC426913]

... it's made of (hypothetical) material that accelerates twice as fast under gravity as normal matter...

Not sure this helps the OP.

Is it falsifiable without invoking physics-defying materials?

 

[Nugatory]

Is the equivalence principle defined in such a way that it is experimentally non-falsifiable?

It's a heuristic to help form an intuition about how the theory works, not part of the theory itself, so it's not clear what it would mean to "falsify" it. We don't use it in those problems in which it does not help us form that intuition - which is another way of saying that it doesn't have much of anything to do with gravitational waves.

 

[PeterDonis]

Is the equivalence principle defined in such a way that it is experimentally non-falsifiable?

No. @Ibix described how it could be falsified.

Is it falsifiable without invoking physics-defying materials?

This is the wrong way to look at it. Falsifying the EP would mean falsifying our current theories of physics, so of course if our current theories of physics are correct the EP will not be falsified. But that doesn't mean the EP is not falsifiable. It is perfectly possible to test whether the EP is true. It just so happens that in our universe it passes the test.

 

[DaveC426913]

Is the equivalence principle defined in such a way that it is experimentally non-falsifiable?

For example: the region is defined to always be smaller than the area that any future equipment will be able to measure?

Have a look at an analogous principle:

"A sphere has every point on its surface equidistant from its center." (that is, in fact, the definition of a sphere)

What you are asking is: "If I look at the sphere with a powerful enough microscope, can I invalidate that principle?"

You have a real-world sphere that is not perfect, and a powerful microscope would indeed detect irregularities in a sphere made of atoms.

Does that invalidate the principle above? No.

 

[PeterDonis]

Have a look at an analogous principle I just made up.

It's not analogous. Your "principle" is a tautology, since part of the definition of a sphere is that it has the same radius at every point on it.

The EP is not a tautology; it is not a logically necessary proposition.

 

[DaveC426913]

It's not analogous. Your "principle" is a tautology, since part of the definition of a sphere is that it has the same radius at every point on it.

The EP is not a tautology; it is not a logically necessary proposition.

It's not the best analogy, granted. The point is that the Equivalence principle doesn't have a minimum resolution, below which it's not valid. That seems to be what the OP thinks.

 

[PeterDonis]

the Equivalence principle doesn't have a minimum resolution, below which it's not valid

Yes, agreed. It is a local proposition; the best way to formulate it is actually in terms of properties of worldlines and tetrads carried by observers on those worldlines, not properties of regions of spacetime.

 

[Ibix]

Is it falsifiable without invoking physics-defying materials?

I don't think it's all that outrageously physics-defying. In Newtonian physics, all we need to do is say that $F=kGMm/r^2$, where $k=1$ for normal matter and $k=2$ for this hypothetical matter. That $k$ is always 1 is merely something we happen to have observed - there's no other reason for it.

Relativity insists that there is a reason, that matter follows geodesics. This means that the coordinate acceleration of a free-falling body cannot depend on its mass, only on its position - i.e. the equivalence principle. So observing masses accelerating at different rates would falsify the equivalence principle.

 

[Dale]

Not sure this helps the OP.

Is it falsifiable without invoking physics-defying materials?

@Ibix is correct. Since our universe follows the equivalence principle of course a violation will be “physics defying”. But the point is that you can take any pair of materials and perform the experiment. If you ever find one that behaves as @Ibix described then the principle is falsified. Until we did similar experiments it was not a forgone conclusion.

 

[berkeman]

Thread closed temporarily for Moderation...

 

[Dale]

Since the OP is on vacation, this thread will remain closed. In short, gravitational waves are a prediction of GR so obviously they do not falsify GR nor any of the basic GR principles.