Near-light speed travel increasingly impossible, according to maths

Travelling at close to the speed of light may be necessary for humans to colonise the galaxy, but the maths show it'd be like flying through a cloud of bombs - but also that we should notice the explosions here on Earth, if any other civilisation has managed the feat.

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The sci-fi blockbuster Interstellar depicts a dilapidated Earth too tired to support life anymore. To survive, mankind sends a team of researchers on a spaceship through a wormhole to find a new home in another galaxy, thus preserving possibly one of the rarest things in the universe – intelligence. This sounds fanciful, but it could happen in real life  engineers say that future technologies may make a spacecraft capable of interstellar travel possible.

No matter how badly we treat the planet, the Sun will be responsible for its ultimate destruction. After roughly one billion years the Sun will have grown into a red giant, and the gradual increase in temperature will have wiped clean the Earth’s surface. There are three options (assuming our species isn’t extinct before then): 1) sit and twiddle our thumbs until we are fried to death; 2) move the Earth far away from the Sun (which would bring a different set of problems); or 3), find a new home.

It seems the third option is the only ideal choice we have. We'll have to start building interstellar spacecraft eventually, and the lenient deadline of one billion years should give us enough time to do so. Fairly straightforward calculations tell us spacecraft capable of travelling at a significant fraction of the speed of light is possible - in so-called "relativistic spacecraft"  with enough time for technological advancement, and, of course, money.

There are of course plenty of challenges, though, and Ulvi Yurtsever and Steven Wilkinson from defence contractor Raytheon outline one which until now has been overlooked. In a paper published in arXiv, they say that any object travelling at relativistic speeds will interact with photons in the cosmic microwave background (CMB), creating a drag that results in slower travel.   

The CMB is the afterglow of the Big Bang, present in every direction that we point our telescopes as a faint light occupying the microwave part of the electromagnetic spectrum. Each cubic centimetre of the cosmos has over 400 microwave CMB photons, so any spacecraft moving through these would have a difficult time avoiding them. It would be like trying to dodge a swarm of flies on the driveway; you will get some goo on your windshield. And in this case plenty – trillions per second for a spacecraft moving at significant fractions of the speed of light.

Particle physics dictates that if the energy involved in a collision between an atom's nucleus and a microwave photon is high enough, electron-positron pairs can be created. An electron-positron pair is when a high energy photon (a packet of energy) interacts with a heavy nucleus to form a positively charged electron – a positron.

Yurtsever and Wilkinson describe how CMB photons will appear, from the perspective of the spacecraft travelling close to the speed of light (known as its "rest frame"), as highly energetic gamma rays that have a range of effects. If those photons interact with the material of the spacecraft hull, the effects will range from ionisation to "Compton scattering"  the scattering of high energy photons from a charged particle at rest, which in this case means further gamma rays, creating electron-positron pairs. Each time one of these pairs is formed, it creates a massive amount of energy - as much as 1.6 x 10-13 joules per pair. This doesn’t seem like a lot, but a spacecraft can collide with trillions of CMB photons per second. Assuming an effective cross-sectional area of, say 100 square metres, the effect is about 2 million joules per second across the face of the ship. That's roughly equivalent to the energy released when half a kilo of TNT explodes, every second.

Things also get more complicated when taking time dilation into account. Seconds last longer when something travels nearly as fast as the speed of light, relative to something travelling at a slower speed, so our theoretical spaceship will take longer to disspiate the energy that builds up on its front  increasing the effective energy hitting it per second to somewhere in the order of 1014 joules, or a little bit more energy than that released by the atomic bomb which fell on Hiroshima.

So, travelling at almost the speed of light will obviously have a huge drag effect. Yurtsever and Wilkinson write that a way to overcome the issue would be to keep the spacecraft’s velocity below the threshold for electron-positron pair creation, thus reducing drag and energy dissipation. That threshold is crossed as the spacecraft reaches 99.9999999999999967 per cent of the speed of light, so it's still a relatively high velocity.

There's an interesting side effect to all this, though  any relativistic spacecraft like this will bounce into so much of the CMB, it'll scatter it in a way that produces a unique light 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," write Yurtsever and Wilkinson. In other words, if we know what to look for, we should be able to spot the interstellar contrails of near-light speed spaceships.

They actually calculate the properties of this signature  it should take the form of radiation in the terahertz to infrared regions of the electromagnetic spectrum, and it should also be moving relative to the rest of the CMB. If relativistic spacecraft are darting through the cosmos, this kind of signature should be visible using current astrophysical observatories.

However, Yurtsever and Wilkinson also look at what would happen to such a ship if it hit anything bigger than a photon  like, say, a grain of dust. Collision with an object as tiny as 10-14 grams would have an impact energy close to 10,000 megajoules, which makes it clear that any relativistic spacecraft would need a clear runway before it can take-off to a new land for the sake of humanity. Or perhaps this is just yet more evidence that, like the crew in Interstellar, jumping between wormholes is a better bet.

Tosin Thompson writes about science and was the New Statesman's 2015 Wellcome Trust Scholar.