LIGO experiment Gravity waves

[carl susumu]

I read that they found gravity waves that have the frequency of 35 to 250 hz but are not these frequencies that of sound waves? I thought sound does not propagate in vacuum or is there a new kind of gravity vacuum that contains a gravity ether or dark matter?

 

[PeterDonis]

are not these frequencies that of sound waves?

Sound waves can be in that frequency range, yes. But that does not mean any wave in that frequency range must be a sound wave.

is there a new kind of gravity vacuum that contains a gravity ether or dark matter?

No. Gravitational waves are waves of spacetime curvature.

 

[carl susumu]

Do these gravity waves that have a frequency of 35 to 250 Hz make a sound, like a Chirp or something like that?

 

[PeterDonis]

Do these gravity waves that have a frequency of 35 to 250 Hz make a sound, like a Chirp or something like that?

No. They're not sound waves.

 

[carl susumu]

were Weber gravity wave sound waves?

 

[PeterDonis]

were Weber gravity wave sound waves?

No. The vibrations in Weber's bar detector that were triggered by gravitational waves, if indeed they were (most physicists believe they were actually not gravitational waves but measurement error), could have been considered "sound waves" (although they were much lower frequency than the frequency range you mentioned before). But those are not the same as the gravitational waves themselves (again, assuming his detector was actually detecting them, which it probably wasn't).

 

[secur]

It's generally agreed that Weber didn't detect gravitational waves, he only thought he did. They wouldn't have been sound waves.

By the way the correct term is gravitational wave. A "gravity wave" is one which gets its energy from gravity. Best example, waves on the surface of the sea.

You probably saw some video where they played a "chirp" and said that was the sound made by LIGO's detected waves, right? No. The chirp was sound waves, generated by a conventional sound card, of the same frequency and duration as the gravitational waves.

 

[carl susumu]

And what are the wavelengths of these so called gravity waves?

 

[PeterDonis]

what are the wavelengths of these so called gravity waves?

Do you mean the gravitational waves that LIGO detected? The wavelengths of those waves are not given in the reports of the detection, probably because they are too long to be measured by the detector. The usual expectation for the wavelength of gravitational waves is that they will be comparable to the size of the source; the "size" in this case would be roughly the horizon radius of the black holes involved, which would be at least tens of kilometers.

the structure.. EM?

No, gravitational waves are waves of spacetime curvature, as I said before.

the force (pounds) that are being detected.

Gravitational wave detectors like LIGO don't detect force, they detect strain, i.e., the fractional change in size of the detector. The strain of the waves that LIGO detected was around $10^{-21}$, i.e., the length of the arms of LIGO was changed by a fraction of roughly $10^{-21}$.

 

[carl susumu]

A wavelength of five thousand miles! And what are the units of the strain, meters? Ha. Ha. Physicists are extremely funny bunnies.

 

[secur]

And what are the wavelengths of these so called gravity waves?

Strictly speaking they're so-called gravitational waves, but that's alright.

You mentioned the frequencies were 35 to 250 hz.
Speed of light is 3* 10^5 km / sec.
So wavelengths are 8600 to 1200 km.

But the sound wavelengths are much smaller at the same frequency.
The ratio: speed of sound / speed of light = 1.14 * 10^-6 (assuming dry air, 1 atmosphere pressure).
So sound wavelengths would be 7.54 to 2.2 meters.

The strain has no units, it's a dimensionless number, a ratio of lengths

Physicists are extremely funny bunnies.

- Hard to argue with that!

 

[carl susumu]

What are the lengths used to determine the strain?

 

[pervect]

What are the lengths used to determine the strain?

The strain is a dimensionless quantity. Wiki has a a rather unclear definition, unfortunately, so I won't bother quoting it. If you consider that the base distance between the Ligo test masses is L, and that this distance changes by an amount $\Delta L$ due to the passing gravitational wave, the strain is $\frac{\Delta L}{L}$ which is a dimensionless quantity. For instance you might measure both L and $\Delta L$ in meters, or in centimeter, or in inches. Regardless of which units you use, the ratio, or strain, will be the same.

 

[carl susumu]

Just one more question before I get back to the strain, is the length L decreasing or increasing?

 

[PeterDonis]

s the length L decreasing or increasing?

It oscillates; the strain is the amplitude of the oscillation.