I'm kind of curious as to how close we are to being able to use them for Gravitational-Wave astronomy giving LIGO[1] a run for its money. Anyone know if this is even possible?
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Short answer - not anytime soon.
Longer answer: there might be a clever way to use clocks to pick off gravitational waves, particularly at low frequency, but I doubt that they'll be competitive with pulsar timing/LISA.
There are proposals to use atom interferometers for gravitational wave detection. The community is quite divided on the chances for success (I'm in the pessimistic camp at the moment). Barring any surprises, an atom interferometer is much more likely than a timing-based measurement to see any gravitational wave signal. On the LIGO front, so long as the source estimates for gravitational wave signals are correct and as long as Advanced LIGO hits its sensitivity goals, a LIGO GW detection is imminent.
There's always room for cleverness.
Could you explain why you're pessimistic on GW atom interferometers, or point me to an argument elsewhere? I'm especially interested in the AGIS proposal by the group at Stanford ( http://arxiv.org/abs/0806.2125 ).
Also, is it obvious how one could use an ultra precise clock to see evidence for GWs? Such clock can reveal spatial gradients in the rate of ticks (and so reveal things like local mass densities), but GWs should be spatially homogeneous, no?
Thanks for any help!
There are some rebuttals by Peter Bender (his abstract for the 2013 April APS meeting may point in useful directions). For the space-based experiment, barring concerns about the technique itself, the right question may be, "Even if the atom interferometer might work, is it a better bet than the existing plans for LISA?"
I haven't studied the atom interferometer GW stuff in great detail, but the measurements look challenging from a feasibility and systematic perspective. Any one objection can be addressed, but there are a lot of them, making it challenging in the aggregate. Continued work on the ground in the atom interferometer field over longer baselines will explore both. If that work can show that it works as proposed, then pessimism will turn to optimism; experiment is the arbiter of truth.
As for the second point, regarding clocks, my initial response of "no" came from exactly your argument. I hesitated when considering a global array of clocks. For wavelengths comparable to the size of Earth and smaller, there's enough phase difference that perhaps a sufficiently precise clock, read out sufficiently fast, might pick off a signal. Additionally, if there are signals in the micro-to-nanohertz regimes, it might appear as an unexplained fit residual to an overall model of the gravitational potential at any one clock. My GR-fu is insufficient to make reliable estimates of the sizes of these effects (I bet they're small!).
Thanks so much for the thoughtful reply. I think the argument for these atom interferometers as GW detectors is that they are sensitive to a different frequency range and so would compliment rather than compete with LISA (or LIGO). (See http://meetings.aps.org/Meeting/DAMOP12/Event/172056) But I suppose the funding is pretty limited, so in that sense they compete. I'm actually rooting for them because the cool quantum superpositions they would need to produce (and possible sensitivity to low-mass dark matter!) rather than gravitational waves.
This project is the first thing I thought of, but LIGO is attempting to detect changes at the subatomic level. [1]
> Based on current models of astronomical events, and the predictions of the general theory of relativity, gravitational waves that originate tens of millions of light years from Earth are expected to distort the 4 kilometer mirror spacing by about 10−18 m, less than one-thousandth the charge diameter of a proton.