# Do gravitational waves affect our measurements of weight?

1. Aug 11, 2012

### Lelephant

Hi, it's my first post here, though I've long viewed these forums without a registered account.

I was thinking about gravitational waves, and what exactly they would do. From what I understand, these waves affect everything with mass in the universe. The force they exert is exponentially decreased the farther any specific object is from the point of the wave's propagation, but any given mass is still affected by these waves.

If this is so, why wouldn't supremely accurate (billion decimal point) weight measurements, if they existed, be affected by the pull or push of these gravitational waves?

Granted, the gravitational attraction exhibited by Earth would be the first and foremost force acting on any subject one could weigh, but wouldn't a near infinite amount of minute gravitational waves affect the force exhibited by Earth's gravitational attraction by pushing towards it or pulling against it? Couldn't a gravitational wave travel straight through Earth to push the subject being weighed away from the center of Earth, thus lightening its weight even minutely?

2. Aug 12, 2012

### Simon Bridge

They would be. Yes.

I don't think billion decimal point (lit. one part in 101,000,000,000) weight measurements can be done just yet ... iirc: for a big gravity wave you need to be accurate to one part in 1018 or so. So a 1 part in 109 (one part in a US billion) is still nine orders of magnitude too coarse. Mind you, I could recall incorrectly.

The same wave would also "push" the Earth in the same direction. The extra push is also not the same across the surface of the Earth - geometry an all.

Start to see the difficulty?
http://en.wikipedia.org/wiki/Gravitational-wave_detector

Last edited: Aug 12, 2012
3. Aug 12, 2012

### onomatomanic

In addition to the above, note that there is a theoretical limitation to how "supremely accurate" physical measurements can get: Eventually, you always run into the Planck constant. Never mind a billion decimal places, that restriction kicks in before you get to a hundred.

4. Aug 12, 2012

### Lelephant

I was thinking about that -- it seems impossible to realistically measure considering this is an omnipresent force.

If I may ask, couldn't this also be one reason why we can't achieve absolute zero? If space is constantly being warped to minuscule degrees and mass is actually converted into energy (for the waves) from the bodies which propagate the waves, and if the waves affect all mass, isn't it true that the waves affect the subject of our near-absolute zero cooling -- making it impossible to achieve a state of zero energy?

How could one prove this?

Huh, I didn't know that. I don't quite grasp what Planck's constant has to do with it, but it's kind of late and I haven't taken a very thorough look at the wikipedia page. An explanation would be helpful but if you don't want to explain, I understand.

5. Aug 12, 2012

### onomatomanic

How about you do take a more thorough look at that page first. Read the lede and the first three sections, with particular attention paid to the last subsection ("Uncertainty Principle"). If you still have questions then, I or another member will be happy to explain further. :)

6. Aug 12, 2012

### Staff: Mentor

The intensity just reduces with the square of the inverse distance.

Instead of weight measurements (which reach a precision of ~12 decimal places IIRC), length measurements are done, they reach a precision of ~20 decimal places for relative changes.

7. Aug 12, 2012

### onomatomanic

That's true regardless of the nature of the source, right? I was about to post a similar reply earlier, but while the bigger part of my brain told me that the intensity of all spherically expanding wave-fronts has to follow an inverse square, a small part kept insisting that it should be an inverse cube if the source is a dipole. To which the bigger part replied that that only applied to field strength, not radiative intensity, but the small part wouldn't quite acquiesce.

8. Aug 12, 2012

### Staff: Mentor

That is true for all radiation from (nearly) point-like sources in 3 spatial dimensions.
It is wrong (or a bad approximation) if the size of the source is not negligible, or if you look at non-radiating things like a static field from a dipole.

9. Aug 12, 2012

### onomatomanic

Thanks for the confirmation. :)

I believe I figured out in the meantime why I was confused - I was thinking about identical wavefronts coming from two neighbouring sources, which can give rise to other-than-inverse-square falloff along certain axes due to interference effects. It's only the intensity integrated over the whole shell which always obeys the fundamental relation in those cases.

But that's only superficially similar to a single dipole source, so has little relevance here. My bad.

10. Aug 12, 2012

### twofish-quant

The probably would be. Also, I don't think you'd been billion decimal point weight measurements to see the effect of gravitational waves. My gut sense is that it's more like eight or nine, but that's a "top of my head guess".

Gravity waves work through quadrapole radiation, there is a nice diagram in the wikipedia article of what that means

http://en.wikipedia.org/wiki/Gravitational_wave

But you would see minute changes in force due to gravity waves which is how we propose to see them.

11. Aug 12, 2012

### Lelephant

So do you all think it's possible that absolute zero is prevented by the discrepancies of these gravitational waves? Is it possible to prove that they are? How?

12. Aug 12, 2012

### Drakkith

Staff Emeritus
I think absolute zero is impossible because of the uncertainty principle.

13. Aug 12, 2012

### Lelephant

Wouldn't the uncertainty principle only mean it's impossible to detect absolute zero?

Also, I'm asking if gravitational waves could plausibly influence mass and be at least one of the reasons why it's impossible to achieve absolute zero. If this is so (if grav. waves do increase the energy of any given mass), how would one prove it?

14. Aug 14, 2012

### twofish-quant

As long as there is any energy transfer, then you can't get absolute zero. Gravity waves are one minor way of transferring energy, but as far as absolute zero goes, they aren't important.

One way of thinking about the problem is that the important number in most equations isn't temperature but rather 1/T, which people call beta. You can't get to absolute zero because that would result in infinite beta which you can't get to with finite energy. Curiously it's possible to get infinite and negative temperatures. A negative temperature corresponds to something that is hotter than infinity.