How do bar detectors work in gravitational wave detectors?

[Josiah]

TL;DR

How do bar detectors work

Hi, I was just wondering how bar detectors work. From what Ive read, the electrons act as one, thus accentuates the effect of a single gravitational wave.

 

[Josiah]

Or do the electrons in this state absorb more gravitational waves, thus the effect is magnified and can be detected

 

[PeterDonis]

From what Ive read

What have you read? Please give specific references.

 

[Ibix]

Don't they just change length when a gravitational wave passes through them? And you should be able to detect the induced strains with sufficiently sensitive strain gauges. Not sure where electrons come into this.

Unless there's some other kind of bar detector I've not heard of.

 

[AlexB23]

Don't they just change length when a gravitational wave passes through them? And you should be able to detect the induced strains with sufficiently sensitive strain gauges. Not sure where electrons come into this.

Unless there's some other kind of bar detector I've not heard of.

I think our guy is talking about Weber Bars. A precursor to LIGO, without lasers.

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

 

[renormalize]

I think our guy is talking about Weber Bars. A precursor to LIGO, without lasers.

Yes, and I think that's exactly what @Ibix is talking about as well. To quote his post #4, with some clarifications in brackets added by me:
"Don't they [Weber bars made of metal, e.g., aluminum] just change length when a gravitational wave passes through them? And you should be able to detect the induced strains [in the block of metal, caused by the passage of the wave] with sufficiently sensitive strain gauges. Not sure where electrons come into this [i.e., the block is just ordinary, room-temperature metal involving no special electron states other than standard conduction]."

 

[Ibix]

I think our guy is talking about Weber Bars. A precursor to LIGO, without lasers.

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

As @renormalize says, I know that. Interferometric detectors like LIGO have mirrors that are free to move independently, so they don't resist length changes and you can use interferometry to detect those changes as the gravitational wave passes through. Solid bars do resist length changes, so should ring (similar to the way they do when tapped by a hammer, but much, much weaker) when a gravitational wave passes through. But electrons don't really enter into the discussion any more than they do in any other material science topic.

Maybe you need to worry about electrons in terms of thermal noise. Or maybe the OP is talking about some other kind of detector. It's hard to tell. Hence the general requirement for references for discussion, and Peter's specific request for same.

 

[AlexB23]

Yes, and I think that's exactly what @Ibix is talking about as well. To quote his post #4, with some clarifications in brackets added by me:
"Don't they [Weber bars made of metal, e.g., aluminum] just change length when a gravitational wave passes through them? And you should be able to detect the induced strains [in the block of metal, caused by the passage of the wave] with sufficiently sensitive strain gauges. Not sure where electrons come into this [i.e., the block is just ordinary, room-temperature metal involving no special electron states other than standard conduction]."

What I do know is that the OP was vague. Bar detector does not return any results, unless one specifies Weber Bars.

 

[epiclifery]

The important mechanism is actually the mechanical resonance of the bar, rather than electrons collectively absorbing or amplifying the gravitational wave

A passing GW produces an extremely small tidal strain in the material, which can excite one of the bar’s resonant modes. Near its resonance frequency, the resulting displacement is much larger than the instantaneous static strain, making it possible to read out with a sensitive transducer. Thermal and mechanical noise are then major limitations

So the “amplification” is really an effect of the resonant mechanical response of the bulk material, not the electrons acting coherently as a separate GW detector

I was looking into the older resonant-bar approach recently because I noticed Nergis Mavalvala is giving a lecture in Karachi, Pakistan about her work in gravitational-wave detection, and it sent me down a bit of a rabbit hole on how different generations of detectors actually worked

 

[Paul Colby]

There is a coordinate system, referred to as the TT gauge, that I find makes visualization of bar detectors easier. Think of an isolated atom, a point mass. As the GW passes, the atom follows a geodesic. In the TT gauge, an atom at rest stays at rest. It’s X, Y and Z TT-coordinates don’t change with time. Now introduce a second atom bonded to the first. With no wave present, it stands off at an equilibrium distance determined by the atomic bond. As the GW passes, the proper distance between the atoms changes. It becomes time dependent. This change gives rise to a force between the atoms because the bond is being squished or stretched. This force causes the atoms to start moving relative to each other. Their coordinates become time dependent. Now, fill out this picture with an array of atoms at rest forming a solid.

Basically, the motion in the TT-gauge induced into the extended solid is vibrational energy. The bar begins to ring.

 

[marcusl]

The most sensitive Weber bar, designed at Stanford, used a 4800 kg aluminum cooled to 2 to 4K to reduce thermal noise. I forget its longitudinal resonance frequency--something in the neighborhood of 1 kHz. The amplitude of longitudinal ringing was amplified by coupling it to a light diaphragm tuned to the same mechanical resonance frequency. It acted like the classic undergrad physics lab demo where a cannonball is suspended by a wire with a golf ball suspended from its bottom with fishing line of a length such that they both have the nearly the same swinging frequency. A tiny displacement of the cannonball transfers energy at the beat frequency to the golf ball, which swings wildly. In the Stanford bar, displacements of the diapghram were detected inductively with a SQUID (superconducting quantum interference device). It was said that the system could detect changes in the bar's length at resonance equal to a small fraction of the diameter of an atomic nucleus.