The Devastating Communication Problem Facing Light-Speed Travel

Communication is key. That’s true on Earth and it’s true in space. But in space, you need to contend with a crucial fact of life. The speed of light is finite and distances between worlds are pretty big; between star systems, they are enormous. An analysis envisions what it would be like to communicate with a spacecraft traveling close to the speed of light. And it is certainly not good news.

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Researchers David Messerschmitt, Ian Morrison, Thomas Mozdzen, and Philip Lubin envision two scenarios with a spaceship traveling close to the speed of light. This vehicle doesn’t exist (yet) but nothing in physics denies that it might be possible to build. It’s a vehicle that never goes out of thrust and moves with an acceleration of 1g, the same as the pull of gravity while we stand on the Earth.

The first scenario sees a spaceship maintaining this acceleration as it moves away from Earth. At first, the communication will work, although with a lag due to the finiteness of the speed of light, but after a while, messages from Earth won’t be able to reach the spacecraft anymore. As the craft gets nearer and nearer the speed of light, it will always be a step ahead of the message. The spacecraft will then no longer be in contact with Earth.

There is also another peculiar effect but this is on board of the craft. An object moving close to the speed of light experiences time dilation. Its clock slows down. So for someone on board, the spaceship accelerating at 1g would take just 20 years to reach the center of the galaxy (which is 26,000 light-years away). And just 45 years to reach the edge of the visible universe (tens of billions of light-years away).

The second scenario sees the spacecraft accelerating at 1g for a while before decelerating at 1g as it approaches a destination. The communication from Earth would be affected in the same way as the first case, until during the deceleration phase when all the messages caught up with the spaceship. The destination instead could communicate with the spacecraft but the messages will tend to accumulate as the spaceship gets closer. You won’t be getting a nicely spread-out set of messages announcing their arrival long before they are very close.

“Interstellar spacecraft and their crews must accept highly autonomous operations, and abandon notions of maintaining operational and social interactions with those at the origin or destination throughout the mission, with the exception of a short period following launch or prior to landing,” the authors wrote in the paper.

The work looked at some classical and relativistic effects but there are even more that have not been considered that would affect the communications. The signals from a moving ship will experience a Doppler effect like an ambulance siren’s pitch changing if it’s approaching or driving away from you. So you need antennas that can detect light whose frequency will change over time. And there’s relativistic aberration: the light of a moving object is concentrated conically towards the direction of motion.

So if we ever build a spacecraft, its crew will be on their own after a while.

A paper describing this research is available in Elsevier.

An earlier version of this article was first published in December 2023.