<p>In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect, eliminating some approaches to a new theory of gravity. The animation link below shows the delay scientists had expected to observe. 
</p><p>Credit: NASA/Sonoma State University/Aurore Simonnet
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<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/gsfc/4058451581/">Quantum Gravity Photon Race</a>