Gravitational Wave Detection #2: How detectors work

1. Gravitational Wave Detection #2:How detectors work Peter Saulson Syracuse University Summer School on Gravitational Wave Astronomy

2. Outline • How does an interferometer respond to gravitational waves? • How does a resonant bar respond to gravitational waves? • A puzzle: If light waves are stretched by gravitational waves, how can we use light as a ruler to detect gravitational waves? Summer School on Gravitational Wave Astronomy

3. Three test masses Summer School on Gravitational Wave Astronomy

4. Solving for variation in light travel time: start with x arm First, assume h(t) is constant during light’s travel through ifo. Rearrange, and replace square root with 1st two terms of binomial expansion and integrate from x = 0 to x = L: Summer School on Gravitational Wave Astronomy

5. Solving for variation in light travel time (II) In doing this calculation, we choose coordinates that are marked by free masses. “Transverse-traceless (TT) gauge” Thus, end mirror is always at x = L. Round trip back to beam-splitter: y-arm (h22 = - h11 = -h): Difference between x and y round-trip times: Summer School on Gravitational Wave Astronomy

6. Multipass, phase diff To make the signal larger, we can arrange for N round trips through the arm instead of 1. More on this in a later lecture. It is useful to express this as a phase difference, dividing time difference by radian period of light in the ifo: Summer School on Gravitational Wave Astronomy

7. Sensing relative motions of distant free masses Michelsoninterferometer Summer School on Gravitational Wave Astronomy

8. How do we make the travel-time difference visible? In an ifo, we get a change in output power as a function of phase difference. At beamsplitter, light beams from the two arms are superposed. Thus, at the port away from laser and at the port through which light enters Summer School on Gravitational Wave Astronomy

9. Output power We actually measure the optical power at the output port Note that energy is conserved: Summer School on Gravitational Wave Astronomy

10. Interferometer output vs. arm length difference Summer School on Gravitational Wave Astronomy

11. Ifo response to h(t) Free masses are free to track time-varying h. As long as tstoris short compared to time scale of h(t), then output tracks h(t) faithfully. If not, then put time-dependent h into integral of slide 4 before carrying out the integral. Response “rolls off” for fast signals. This is what is meant by interferometers being broad-band detectors. But, noise is stronger at some frequencies than others. (More on this later.) This means some frequency bands have good sensitivity, others not. Summer School on Gravitational Wave Astronomy

12. Interpretation Recall grav wave’s effect on one-way travel time: Just as if the arm length is changed by a fraction In the TT gauge, we say that the masses didn’t move (they mark coordinates), but that the separation between them changed. Metric of the space between them changed. Summer School on Gravitational Wave Astronomy

13. Comparison with rigid ruler, force picture We can also interpret the same physics in a different picture, using different coordinates. Here, define coordinates with rigid rods, not free masses. With respect to a rigid rod, masses do move apart. In this picture, it is as if the grav wave exerts equal and opposite forces on the two masses. Summer School on Gravitational Wave Astronomy

14. Two test masses and a spring The effect of a grav wave on this system is as if it applied a force on each mass of Character of force: Tidal (prop to L,) and gravitational (prop to m) SLAC Summer Institute

15. Resonant-mass detector response Impulse excites high-Q resonance SLAC Summer Institute

16. Strategy behind resonant detectors Ideally, an interferometer responds to an impulse with an impulsive output. Not literally, but good approximation unless signal is very brief. Bar transforms a brief signal into a long-lasting signal. As long as the decay time of the resonant mode. Why? It is a strategy to make the signal stand out against broad-band noise. More on this in a later lecture. Summer School on Gravitational Wave Astronomy

17. First generation resonant-mass system Room temperature, hrms ~ 10-16 SLAC Summer Institute

18. Second generation resonant-mass system T = 4 K, hrms ~ 10-18 SLAC Summer Institute

19. How are tests masses realized? Interferometer: test masses are 10 kg (or so) cylinders of fused silica, suspended from fine wires as pendulums Effectively free for horizontal motions at frequencies above the resonant frequency of the pendulum (~1 Hz) This justifies modeling them as free masses. Bar: “test masses” are effectively the two ends of the bar. Resonant response, due to “spring” made up of the middle of the bar, is key. Summer School on Gravitational Wave Astronomy

20. How to find weak signals in strong noise Look for something that looks more like the signal than noise, at suspiciously large level. SLAC Summer Institute

21. The “rubber ruler puzzle” If a gravitational wave stretches space, doesn’t it also stretch the light traveling in that space? If so, the “ruler” is being stretched by the same amount as the system being measured. And if so, how can a gravitational wave be observed using light? How can interferometers possibly work? Summer School on Gravitational Wave Astronomy

22. Handy coordinate systems In GR, freely-falling masses play a special role, whose motion is especially simple.  It makes sense to use freely-falling masses to mark out coordinate systems to describe simple problems in gravitation.  In cosmology, we describe the expansion of the Universe using comoving coordinates, marked by freely-falling masses that only participate in the Hubble expansion.   For gravitational waves, the transverse-traceless gauge used on previous slide is marked by freely-falling masses as well. Summer School on Gravitational Wave Astronomy

23. Cosmological redshift Summer School on Gravitational Wave Astronomy

24. Heuristic interpretation Cosmological redshift:   Wavefronts of light are adjacent to different comoving masses. As Universe expands, each wavefront “thinks” it is near a non-moving mass. As separations between masses grow, separations between wavefronts must also grow. Hence, wavelength grows with cosmic scale factor R(t).  Especially vivid if we were to imagine the Universe expanded from R(t0) to R(t1) in a single step function.  All light suddenly grows in wavelength by factor R(t1)/ R(t0). Summer School on Gravitational Wave Astronomy

25. A gravitational wave stretches light Imagine many freely-falling masses along arms of interferometer.  Next, imagine that a step function gravitational wave, with polarization h+, encounters interferometer.  Along x arm, test masses suddenly farther apart by (1+h).  Wavefronts near each test mass stay near the mass. (No preferred frames in GR!) The wavelength of the light in an interferometer is stretched by a gravitational wave. Summer School on Gravitational Wave Astronomy

26. Are length changes real? Yes.  The Universe does actually expand.  Interferometer arms really do change length.  We could (in principle) compare arm lengths to rigid rods.  It is important not to confuse our coordinate choices with facts about dynamics. “Eppur si muove” Summer School on Gravitational Wave Astronomy

27. OK, so how do interferometers actually work? Argument above only proves that there is no instantaneous response to a gravitational wave.  Wait.  Arm was lengthened by grav wave. Light travels at c. So, light will start to arrive late, as it has to traverse longer distance than it did before the wave arrived.  Delay builds up until all light present at wave’s arrival is flushed out, then delay stays constant at Dt = h(2NL/c). Summer School on Gravitational Wave Astronomy

28. New light isn’t stretched, so it serves as the good ruler In the end, there is no puzzle. The time it takes for light to travel through stretched interferometer arms is still the key physical concept. Interferometers can work. Summer School on Gravitational Wave Astronomy