**Ulvi Yurtsever and Steven Wilkinson**

While special relativity imposes an absolute speed limit at the speed of light, our Universe is not empty Minkowski spacetime. The constituents that fill the interstellar/intergalactic vacuum, including the cosmic microwave background photons, impose a lower speed limit on any object travelling at relativistic velocities. Scattering of cosmic microwave phtotons from an ultra-relativistic object may create radiation with a characteristic signature allowing the detection of such objects at large distances.(…)

**Summary**

We have explored the physical interaction of a relativistic spacecraft with the interstellar and intergalactic mediums. Our main discussion focused on the interaction with the CMB where very little information currently exists and seems to have been overlooked. Central to our discussion was not how to obtain relativistic speeds but the consequences of traveling that fast. We discussed the interaction with baryonic matter in terms of high speed collisions to present a complete picture. In general one can imagine the same interactions that occur in a particle accelerator to occur between relativistic spacecraft and interstellar matter. Our assumption that matter-matter interactions can be dealt with when civilization can build relativistic spacecraft may prove false and may be a barrier that will prevent space travel with a large γ.

We looked at two special reference frames, the spacecraft frame, and the rest frame of the CMB to understand how the CMB is distorted or acquires an berration from those view points. The scattering of the CMB from a relativistic spacecraft is very similar to the Sunyaev-Zedovich effect where the inverse Compton scattering instead occurs at a macroscopic level. Our calculation for what an observer on earth could detect predicts a very unusual signature that is unlikely to be caused by any naturally occurring object in the known universe.

This result is independent of propulsion technology, but the ability to detect the signal from Earth depends on available detector technologies. We are currently working to predict how far can we see this signature given our current capability.

Read more at http://arxiv.org/pdf/1503.05845v1.pdf

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