Ding Dong, ALIENS CALLING
We already saw your infrared ‘signature’ suckers!
Sorry, sci-fi fans: pretty much anyone who's imagined what a near-light-speed spacecraft would look like has got it wrong, because they've forgotten its interaction with photons.
Not only that, but according to a couple of scientists working for Raytheon, it doesn't matter whether Einstein's proposition that you'll never accelerate matter beyond light-speed is right or wrong: collisions with matter will probably rip your spaceship apart anyway, and photons will slow you down.
The upside is that if there are aliens out there approaching us at near light-speed, the radiation they'll give off will tell us, loud and clear, that they're on the way.
This paper at Arxiv, by Ulvi Yurtsever and Steven Wilkinson of Raytheon in California, explains that Einstein's universal speed limit applies to a vacuum – but travellers in both interstellar and intergalactic space will at the very least encounter the photons of the cosmic background radiation to slow them down.
At relativistic speeds, the boffins note, cosmic microwave background (CMB) photons will “appear in the spacecraft frame as highly energetic gamma rays”, which, striking the spacecraft's hull would cause “effects ranging from ionization and Compton scattering to pair production” (the creation of matter/antimatter pairs).
A relativistic spacecraft that actually struck something the mass of a baseball would generate an impact energy of 30 megatons, but the researchers presume anyone building such a machine would also work out how to avoid such catastrophic collisions before they tried.
That still leaves things such as the interaction with the CMB to deal with: “one can imagine the same interactions that occur in a particle accelerator to occur between relativistic spacecraft and interstellar matter”, the scientists write.
“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," they conclude.
So if you're worried that planet Earth could be destroyed by the wavefront from an incoming spaceship, have a quick read of this.