According to Einstein’s Special Theory of Relativity — time progresses differently for a moving clock, and according to Einstein’s General Theory of Relativity — time advances more slowly for a clock embedded in a gravitational potential. The rate by which we age biologically is affected by both effects.
Because of our circular motion under the Sun’s gravity, our average time dilation relative to a distant stationary observer is obtained by summing the contributions of the Sun’s gravitational potential and the transverse Doppler effect. Their sum equals half of the square of the Earth’s circular speed around the Sun in units of the speed of light. The Earth’s speed of 30 kilometers per second is ten thousand times smaller than the speed of light, yielding a net time dilation of 23 years over the age of the solar system, 4.6 billion years.
However, the longitudinal Doppler effect is much larger if astronauts were to depart the solar system on a spacecraft. For existing rockets which can move at 35 kilometers per second away from Earth, each year in the receding astronauts’ frame would appear longer by an hour for us on Earth. According to the principle of relativity, the same is true in reverse since the Earth would be receding from the spacecraft; hence, each year on Earth would appear longer by an hour for the astronauts. This time dilation by the longitudinal Doppler effect is twenty thousand times larger than the gravitational time dilation for earthlings relative to a stationary distant observer.
The longitudinal Doppler effect has the opposite sign of shortening the apparent time on the return trip of the astronauts or for aliens that are approaching Earth from interstellar space.
However, gravitational time dilation always makes aging slower near to the source of gravity than far away from it. For interstellar relocation, the rate of aging is dominated by the gravitational potential of the Milky Way galaxy. Doubling the distance from the Galactic center with no change in velocity would cause faster aging by 20 seconds every year.
Of course, traveling near the speed of light makes the time change much more significant. However, it poses major practical hurdles. The primary challenge is to supply the needed energy for propelling the passengers. For example, launching an astronaut in a spacecraft with a total mass of 100 kilograms to 80% the speed of light would consume the global electricity generation on Earth for about 20 days. This much energy must be supplied slowly over a nine-month period if the acceleration is set to the comfortable gravitational value on the surface of Earth, 1g=9.8 meters per second squared. A trip that would last a terrestrial decade to the nearest star, Proxima Centauri, would return astronauts who aged merely six years after accelerating at 1g to 80% the speed of light followed by 1g deceleration back to Earth.
Another major challenge is repairing the damage from impacts by interplanetary and interstellar dust and gas. At 80% the speed of light, the energy released by impactors is 6,000 larger than for a thermonuclear weapon of the same mass. Even a tiny dust particle, a millionth of a meter in size, would release as much energy as five light bulbs of 75 Watts do within a second in a well-lit room. Interstellar space carries the dust mass equivalent of a few such particles per cubic kilometer. A spacecraft with a frontal size of a meter would have roughly one collision with such a dust particle every second and a hundred times more heating from collisions with gas particles. This implies that the spacecraft’s frontal surface would have to sustain a steady heating equivalent to the power of 500 light bulbs per square meter and the resulting ablation from craters on its surface for the duration of the trip. Given the relativistic speed at impact, the interstellar gas particles will all appear as cosmic-rays and a thick shield will be required to protect astronauts from saying goodbye through “death by a thousand cuts”, in the words of Taylor Swift.
This is why it is unlikely for “life-as-we-know-it” to survive interstellar travel based on “physics as we know it”. Either the journey would be too long for a human to live through it, or the relativistic trip would be deadly because of the bombardment by gas and dust particles.
Given our current technologies, it makes most sense for us to venture out of the solar system by launching small technological payloads at rocket speeds. Artificial Intelligence (AI) would substitute natural intelligence, enabling the autonomous travel required by the large interstellar distances. With this perspective in mind, AI should be tested in space soon.
Personally, I am content to stay on Earth and benefit from the minute gravitational time dilations of a sixth of a second per year contributed by the Sun and ten seconds per year contributed by the Milky-Way. Earthlings could proudly monitor AI astronauts venture towards interstellar space as their technological ambassadors. As we grow older, we must learn to enjoy the little perks that terrestrial life offers us, irrespective of how small they appear on the cosmic scale. With a proper dose of cosmic humility, we can hope that our technological gadgets will represent us well among the stars.
But new scientific knowledge might come to our salvation. If probes from alien civilizations will be discovered by the Galileo Project, they might teach us a few tricks that we had not imagined based on the limited 120 years of modern science that elapsed since Einstein formulated relativity.
If there are any shortcuts to interstellar travel, we better find them sooner rather than later by studying the technological products of extraterrestrial civilizations that preceded us by billions of years. It is not much fun to run the Marathon, like the Greek messenger Fidippides did to announce the invasion of the Persians in the Battle of Marathon at 490 BC, and realize 2,500 years later that it only takes a fraction of a second for the same information to travel through satellite communication.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
