A pair of engineers say it's possible to detect the signatures of spacecraft traveling at relativistic speeds, and we can do so using current technologies. The trouble is, their new analysis also suggests that moving through space at ludicrous speed is more hazardous than previously thought.
Illustration: Sins of the Prophets/Annihilater102/MODDB
A new paper by Raytheon engineers Ulvi Yurtsever and Steven Wilkinson suggests that spaceships traveling at speeds approaching the speed of light must interact with the cosmic microwave background (CMB) and subsequently produce detectable and distinguishable light signatures. At the same time, however, the ensuing drag from the collisions imposes an upper constraint on the speeds at which spaceships can travel.
What a Drag
The CMB is the "afterglow" of the Big Bang — a lingering remnant that cosmologists use to peer back into the Universe's primordial age. The CMB has stretched across the entire cosmos, but its energy can still be detected in the microwave region. So even if a spaceship could travel through matter-clear space, it would still have to contend with collisions with cosmic microwave photons, which would appear as highly energetic gamma rays at relativistic speeds.
According to Yurtsever and Wilkinson's new analysis, each cubic centimeter of space contains over 400 microwave photons. A ship traveling through space, say, with a hull made from ordinary baryonic matter, would collide with thousands of billions of these photons every second — collisions that should create electron-positron pairs. This would produce considerable drag on a spaceship. MIT's Technology Review explains:
[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.
"In general one can imagine the same interactions that occur in a particle accelerator to occur between relativistic spacecraft and interstellar matter," write the authors in their study.
In addition to this drag, the effect of an an object hitting the baryon matter hull at high speeds would be calamitous. For a ship traveling near the speed of light, the collision with a single cosmic dust grain with a mass of 10^-(14) grams would release about 10,000 megajoules worth of energy. That's the same amount of energy released by 2,400 kilograms of exploding TNT. And that's just a single grain of dust.
"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 [at relativistic speeds]," write the engineers.
That's a rather unfortunate conclusion.
Signatures of Speed
Okay, that's the bad news. The good news is that this same effect should allow us to detect alien ships moving near the speed of light.
Yes, scientists have speculated about detecting alien ships before — like detecting radiation from spacecraft engines or light from nearby stars reflecting off spacecraft. But Yurtsever and Wilkinson's approach is a bit different.
When traveling this fast, a ship would scatter the CMB to produce a unique signature in the form of a frequency shift. This shift would be detectable in the terahertz to infrared regions of the spectrum as an object moves relative to the background. Remarkably, the researchers say the scattering would "cause a frequency shift that could be detectable on Earth with current technology."
Yurtsever and Wilkinson, 2015.
But given the problems of drag and the energy produced by collisions, alien ships may not be able to move this fast, thus diminishing our hopes of ever detecting such tantalizing signatures.
You can read the entire study at arXiv pre-print journal: "Limits and Signatures of Relativistic Spaceflight".