Testing Our Fundamental Assumptions


Science is all about testing the things we take for granted — including some of the most fundamental aspects of how we understand our universe. Is the speed of light in a vacuum the same for all photons regardless of their energy? Is the rest mass of a photon actually zero? A series of recent studies explore the possibility of using transient astrophysical sources for tests!

Explaining Different Arrival Times


Artist’s illustration of a gamma-ray burst, another extragalactic transient, in a star-forming region. [NASA/Swift/Mary Pat Hrybyk-Keith and John Jones]

Suppose you observe a distant transient astrophysical source — like a gamma-ray burst, or a flare from an active nucleus — and two photons of different energies arrive at your telescope at different times. This difference in arrival times could be due to several different factors, depending on how deeply you want to question some of our fundamental assumptions about physics:

  1. Intrinsic delay
    The photons may simply have been emitted at two different times by the astrophysical source.
  2. Delay due to Lorentz invariance violation
    Perhaps the assumption that all massless particles (even two photons with different energies) move at the exact same velocity in a vacuum is incorrect.
  3. Special-relativistic delay
    Maybe there is a universal speed for massless particles, but the assumption that photons have zero rest mass is wrong. This, too, would cause photon velocities to be energy-dependent.
  4. Delay due to gravitational potential
    Perhaps our understanding of the gravitational potential that the photons experience as they travel is incorrect, also causing different flight times for photons of different energies. This would mean that Einstein’s equivalence principle, a fundamental tenet of general relativity (GR), is incorrect.

If we now turn this problem around, then by measuring the arrival time delay between photons of different energies from various astrophysical sources — the further away, the better — we can provide constraints on these fundamental assumptions.

A recent focus set in the Astrophysical Journal Letters, titled “Focus on Exploring Fundamental Physics with Extragalactic Transients,” consists of multiple published studies doing just that.

Testing General Relativity

Several of the articles focus on the 4th point above. By assuming that the delay in photon arrival times is only due to the gravitational potential of the Milky Way, these studies set constraints on the deviation of our galaxy’s gravitational potential from what GR would predict. The study by He Gao et al. uses the different photon arrival times from gamma-ray bursts to set constraints at eV–GeV energies, and the study by Jun-Jie Wei et al. complements this by setting constraints at keV-TeV energies using photons from high-energy blazar emission.

Tests of EEP

Photons or neutrinos from different extragalactic transients each set different upper limits on delta gamma, the post-Newtonian parameter, vs. particle energy or frequency. This is a test of Einstein’s equivalence principle: if the principle is correct, delta gamma would be exactly zero, meaning that photons of different energies move at the same velocity through a vacuum. [Tingay & Kaplan 2016]

S.J. Tingay & D.L. Kaplan make the case that measuring the time delay of photons from fast radio bursts (FRBs; transient radio pulses that last only a few milliseconds) will provide even tighter constraints — if we are able to accurately determine distances to these FRBs.

And Adi Musser argues that the large-scale structure of the universe plays an even greater role than the Milky Way gravitational potential, allowing for even stricter testing of Einstein’s equivalence principle.

The ever-narrower constraints from these studies all support GR as a correct set of rules through which to interpret our universe.

Other Tests of Fundamental Physics

In addition to the above tests, Xue-Feng Wu et al. show that FRBs can be used to provide severe constraints on the rest mass of the photon, and S. Croft et al. even touches on what we might learn from transients using multi-messenger astrophysics (astrophysics involving observations of particles besides photons, such as neutrinos or gravitational waves).

In general, extragalactic transients provide a rich prospect for better understanding the laws that govern the universe. Check out the entire focus set below to learn more about the tests of fundamental physics that can be done with observations of extragalactic transients!


Focus Set: Focus on Exploring Fundamental Physics With Extragalactic Transients

He Gao et al. 2015 ApJ 810 121. doi:10.1088/0004-637X/810/2/121
Jun-Jie Wei et al. 2016 ApJ 818 L2. doi:10.3847/2041-8205/818/1/L2
S. Croft et al. 2016 ApJ 820 L24. doi:10.3847/2041-8205/820/2/L24
S. J. Tingay and D. L. Kaplan 2016 ApJ 820 L31. doi:10.3847/2041-8205/820/2/L31
Adi Nusser 2016 ApJ 821 L2. doi:10.3847/2041-8205/821/1/L2
Xue-Feng Wu et al. 2016 ApJ 822 L15. doi:10.3847/2041-8205/822/1/L15