[time-nuts] LIGO detects gravitational waves

Paul Boven p.boven at xs4all.nl
Sun Feb 14 20:26:21 UTC 2016


Hi PHK, everyone,

On 2016-02-14 0:30:22, Poul-Henning Kamp wrote:
> --------
> In message <1E75A9592178425ABD11390EB725D060 at pc52>, "Tom Van Baak" writes:
>
>> Yes, the interferometer is 4 km in length but they bounce the beam back
>> and forth 400 times so the effective length is more like 1600 km. They
>> keep the mirrors stationary to "picometers". They use hundreds of clever
>> tricks to pull this off.
>
> It's actually more amazing than that, each arm is a resonant cavity,
> so while the actual laser is only about 10W, they have about 20 kW of
> photons inflight at any one time.
>
> With 20 kiloWatt of light safety-glasses are not _that_ important any more.

This number keeps getting repeated, but I have some doubts there.

The 'finesse' of the cavity is about 1000. The view that the photons 
keep bouncing back and forth seems a bit simplistic, wouldn't it be more 
like a standing wave?

The cavity acts as a resonator, and although the instantaneous power 
would indeed be 20kW, as soon as you load that cavity, its stored energy 
would be dissipated in whoever was unlucky enough to end up in the beam. 
Given a length of 4km, it would take no more than 13 us to empty the 
cavity. And 13us times 20kW gives an energy of only 0.27 J.

The part that I am still having trouble understanding is why the two 
cavities in the arms of the interferometer help increase the 
sensitivity. Are they modulating the reflectivity of the mirrors on the 
inner testmasses so they can 'dump' both beams at the same time back 
into the half-silvered mirror?

Cheers, Paul - 73 de PEaNUT.






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