Interstellar travel may be the stuff of science fiction, but it’s straightforward to calculate that it should be possible given the ability to travel at a significant fraction of the speed of light. These kinds of speeds may even be achievable with near future technologies and the tax dollars to make them work.

There are significant challenges, of course. And today, Ulvi Yurtsever and Steven Wilkinson at the defense contractor Raytheon in El Segundo, California, outline another that seems to have been overlooked until now.

These guys point out that any object traveling at relativistic speeds will interact with photons in the cosmic microwave background. This interaction should create a drag that imposes specific limits on how fast spacecraft can travel, they say.

But it should also produce a unique signature of relativistic spaceflight that ought to be visible with today’s technology should any vehicles of this type be zipping through our galactic neighborhood.

The cosmic microwave background is the echo of the Big Bang. This is light left over from the earliest moments of creation that has been stretched as the universe expanded. So although it started off as much higher energy, shorter wavelength radiation, it is now in the microwave region.

This radiation fills the universe. Each cubic centimeter of the cosmos contains over 400 cosmic microwave photons so a spacecraft crossing interstellar space would collide with thousands of billions of them each second.

These collisions can be thought of on the microscopic level as each photon hitting a nucleus at high-energy. Particle physicists well know that if the energy in these collisions is high enough, they should create electron-positron pairs.

Yurtsever and Wilkinson point out that in the rest frame of the spacecraft travelling close to the speed of light, these photons will appear as highly energetic gamma rays. If these gamma rays have an energy greater than the rest mass of an electron and a positron, then the collision will create an electron-positron pair.

They go on to show that this process will dissipate huge amounts of energy. The creation of each electron-positron pair dissipates 1.6 x 10^(-13) Joules. “Assuming an effective cross-sectional area of say 100 square meters, the dissipative effect is about 2 million Joules per second,” say Yurtsever and Wilkinson.

In the spacecraft’s rest frame, the dissipation is even higher because of time dilation. Seconds effectively last longer when travelling at high speed so the energy dissipation is significantly higher, of the order of 10^14 Joules per second.

That’s a significant drag for the spacecraft’s engines to overcome, just to keep it at a constant velocity, say Yurtsever and Wilkinson. They argue that this is a good reason to keep the spacecraft’s velocity below the threshold for electron-positron pair creation and thereby reduce the drag to a negligible level of just a few joules per second. This threshold occurs when the spacecraft reaches a velocity that is 1 – 3.3 x10^-(17) of the speed of light.

The movement of a relativistic spacecraft will have another effect. It should scatter the cosmic microwave background in a way that produces a unique signature. “As a baryonic spacecraft travels at relativistic speeds it will interact with the CMB through scattering to cause a frequency shift that could be detectable on Earth with current technology,” say Yurtsever and Wilkinson.

They go on to calculate the properties of this signature. They say the scattering should generate radiation in the terahertz to infrared regions of the spectrum and that this signal should move relative to the background. “The salient features of the signal are a rapid drop in temperature accompanied by a rapid rise in intensity, along with the motion of the source with respect to a reference frame fixed to distant quasars, which should be observable,” say Yurtsever and Wilkinson.

In other words, if relativistic spacecraft are zipping across interstellar space, this kind of signature should be visible using the current generation of astrophysical observatories.

That’s an interesting piece of work that takes the analysis of relativistic space travel to a new level. Other researchers have explored the possibility of observing relativistic spacecraft using the optical emissions that their engines must generate. But Yurtsever and Wilkinson go further.

Of course, they make a number of assumptions, not least of which is that relativistic space travel is possible at all. Indeed, should some advanced civilization make this kind of jump into the cosmos, the interaction with the cosmic photons is likely to be the least of their problems since a collision with matter would be much more damaging.

Yurtsever and Wilkinson provide some numbers to put this in context. For a spacecraft travelling close to the speed of light, the impact with a single cosmic dust grain with a mass of 10^-(14) grams would have an impact energy close to 10,000 megajoules.

Intergalactic space is relatively clear of debris but even still, any relativistic spacecraft would need a way of clearing its path.

Food for thought for potential cosmonauts.

Ref: arxiv.org/abs/1503.05845 : Limits and Signatures of Relativistic Spaceflight