Can gravitational waves tell us if space is continuous vs. discrete?

Question

Along the lines of this question posted here: Does the speed of a wave travelling through a chain vary based on the size of the links?

...it seems that the speed an momentum of a gravitational wave would vary based on the size or continuous nature of space itself? If so, would further refinements in the accuracy of gravitational wave detectors help us answer the question of whether space is continuous or discrete?

Would the gravitational wave measurements be enough? Or would it require additional information, like also measuring the electromagnetic waves from the same event (i.e. the EM waves from the same black hole collision event)?

Answer 1

Gravitational waves are the best ways to probe any spacetime anomalies or structure, for sizes where general relativity ((GR) applies. That means sizes significantly larger than the Planck,length, where it won't apply. We don't know what spacetime will mean at those scales, and we think it won't be continuous nor identifiable as space time. Maybe it'll be strings, or loops or foamy, we don't know. A gravitational wave at those scales might just be no different than the other fluctuations happening. There is no spacetime.

But at cosmological scales or other macroscopic scales, gravitational waves could see large changes. In fact large changes are likely to emit gravitational waves - for instance large, so called cosmic strings, if they exist, would be likely strong emitters of gravitational radiation. See the wiki article on cosmic strings. https://en.m.wikipedia.org/wiki/Cosmic_string

In fact the simplest effects of spacetime composition, at macroscopic cosmological scales, is the cumulative Wolfe Sachs effect which accumulates redshifts from spacetime voids. This is observed in the CMB (yes, electromagnetic, not gravitationa waves). The same could be observed by gravitational waves (GWs), but we need bigger GW observatories (interferometers) that come closer to matching the wavelengths involved which will be very large.

LIGO has 3 Kms legs, is able to see a few hundred Kms wavelengths (maybe more). A longer leg interferometer would be that much more sensitive and be able to see longer wavelengths from bigger objects. LISA or eLISA, space based and planned for the 2020's, would have a million Kms legs. LIGO and similar and better but earth based interferometers will be able to see inside neutron stars, supernova, and other stellar type objects, and map out their insides some (where electromagnetic waves cannot get through, and see for instance the quark cores of some neutron stars, and other features inside.

As for cosmological features, LISA/eLISA would be able to see some that electromagnetic waves cannot. That would include those cosmic strings and other large features, anomalies or discontinuities.

As far as calculational capabilities, yes we need them to be able to know what to look for. Numerical techniques are used to calculate in the fully nonlinear regime of GR (general relativity), where the gravitational field is too strong and can't be approximated well. That happened, for instance, in the Black Holes (BHs) merging where they got close to each other's horizons, and the full nonlinear numerical calculations. Before it got to that regime they were able to do what is called a PPN (parametrized post Newtonian) approximation, with various levels beyond the Newtonian approximation (they went to 2.5 or 3). As the gravitational field gets stronger it will need the numerical calculations. As @Countto10 implied, the question is whether those can be done for more complex problems.

For the BHs merging the main problem was finding a way to do numerical calculations at different time scales, as time went slower near the horizon, and it needed more steps to do. That was solved about 10 or more years ago, by constructing a different coordinate frame as it got closer, and actually extending it inside the horizon without a coordinate singularity. The calculations are done and are able to compute a large number of posible waveforms that could be generated (the GWs), so they'd know what to look for and use it to do matched filter detection, and parameter estimation to be able to get the BH parameters. A similar type of calculation, with different coordinate systems and issues will need to be done for other cases, but there is no clear showstopper. A summary and description of the methods used in numerical relativity can be read at https://www.classe.cornell.edu/rsrc/Home/Research/GradTheses/Mroue_Abdul.pdf

Answer 2

I don't think that the waves will imply spacetime is discrete. Einsteins equations (which predicted the waves) are based off of classical physics, which says that space is continuous. So if space is discrete, it will most likely be predicted by something else.

Answer 3

My (totally amateur) guess would be no to using gravitional waves as probes for spacetime composition. So far, we have seen no evidence of discrete spacetime using electromagnetic based technology, based on e/m signals travelling across large distance

Quite apart from the extraordinary amount of technical precision needed, (which may never be achieved). I think / guess that there may be another possible limitation in our techology.

As far as I know, the predictions made regarding gravitational waves are based on linearized versions of the non linear, coupled Einstein equations and are (very good) approximations to what the full equations may tell us if we could solve them analytically, (which we can't), rather than numerically, involving significant time on supercomputers.

My point is that if the possible discrete nature of space is at a smaller scale that the level of precision we use in numerical models, then any possible discreteness will still be hidden.

I don't think this answer is fully correct, (because I don't know how much computer power it took to do the math related to prediction and verification of the LIGO results) but I think it makes enough sense to post it as a possible hurdle to overcome:) If it's wrong, then I will delete it.