Interstellar Dreams Dashed: Why Light-Speed Ships Face Communication Challenges

Effective communication is essential. This holds true on our planet as well as in the cosmos. However, in space, one must grapple with a vital aspect of existence. The speed of light is limited and the spaces between celestial bodies are quite vast; between star systems, they are colossal. A recent assessment imagines the implications of communicating with a spacecraft maneuvering near the speed of light. Unfortunately, the implications are far from favorable.

In their assessment, which is available on a preprint server and has yet to undergo peer review, researchers David Messerschmitt, Ian Morrison, Thomas Mozdzen, and Philip Lubin propose two scenarios with a spacecraft moving near the speed of light. This vehicle does not exist (as of now) but there is nothing in the realm of physics that rules out the possibility of its construction. It is a vessel that maintains constant thrust and accelerates at 1g, identical to the force of gravity felt while standing on Earth.

The first scenario involves a spacecraft sustaining this acceleration as it departs from Earth. Initially, communication will function, albeit with a delay owing to the finite speed of light; however, after some time, messages from Earth will cease to be received by the spacecraft. As the craft approaches the speed of light, it will always be ahead of the message. Consequently, the spacecraft will lose contact with Earth.

Additionally, there is another intriguing phenomenon that occurs onboard the vessel. An object traveling at speeds near that of light experiences time dilation. Its clock runs slower. Thus, for individuals onboard, the spacecraft accelerating at 1g would take merely 20 years to reach the center of the galaxy (which is 26,000 light-years distant). And it would require just 45 years to reach the limits of the observable universe (tens of billions of light-years away).

The second scenario depicts the spacecraft accelerating at 1g for some time before decelerating at 1g as it nears a destination. The communication from Earth would experience similar effects as in the first scenario, until the deceleration phase where all messages would catch up with the spacecraft. The destination, on the other hand, could converse with the spacecraft, but the messages would tend to pile up as the spaceship approaches. You will not receive a well-timed assortment of messages announcing their arrival well before they become extremely close.

“Interstellar spacecraft and their crews must accept highly autonomous operations, and relinquish the idea of sustaining operational and social interactions with those at the origin or destination during the mission, except for a brief window following launch or prior to landing,” the authors noted in their paper.

The study examined some classical and relativistic phenomena, but there are even more effects that have not been accounted for that would influence communications. The signals from a moving vessel will undergo a Doppler effect similar to the changing pitch of an ambulance siren as it approaches or moves away from you. Therefore, antennas that can detect light of changing frequency over time are necessary. Additionally, there is relativistic aberration: the light from a moving object is focused conically toward the direction of motion.

Thus, if we ever create a spacecraft, its crew will find themselves isolated after some time.

The document has been uploaded to the preprint server arXiv.

An earlier iteration of this article was published in December 2023.