Einstein’s special theory of relativity, published in 1905, made a number of strange predictions about how time and motion interact, several of which sounded so counterintuitive that for the next half-century even physicists who accepted the underlying mathematics found themselves uncomfortable with the practical implications. One of those predictions was that time itself does not run at the same rate for two observers moving at different velocities relative to each other. Specifically, a clock moving at high velocity, observed from a stationary frame of reference, would appear to run slower than an identical clock at rest. The faster the moving clock travels, the more pronounced the effect becomes. At ordinary human velocities — walking, driving, flying in commercial aircraft — the effect is so small as to be undetectable without extraordinarily precise instruments. At velocities approaching the speed of light, the effect becomes substantial enough to dominate the experience of the moving observer. For a hypothetical traveller moving at 99 percent of the speed of light, time would appear to pass approximately seven times more slowly than for a stationary observer. For a traveller at 99.9 percent of the speed of light, the factor would be approximately 22. The faster you move, the slower your clock runs, from the perspective of anyone you have left behind.
According to a Universe Today calculation of the cumulative time-dilation effect from Sergei Krikalev’s six orbital missions, the orbital velocity of the International Space Station — approximately 7.66 to 7.71 kilometres per second, or roughly 27,600 kilometres per hour — is a vanishingly small fraction of the speed of light. The fraction is approximately 0.00003 (three parts in a hundred thousand). At such velocities, the predicted time-dilation effect is approximately 28 microseconds per day. An astronaut spending six months on the International Space Station accumulates approximately 0.007 seconds of time dilation — seven milliseconds, roughly an order of magnitude smaller than the duration of a single human blink. The effect is too small to be perceived by the astronaut, too small to be detected by any clock the astronaut might carry, and too small to affect any biological process that could matter for the astronaut’s experience. It is, however, real. It has been measured. It accumulates over time.
How the calculation works
Krikalev’s 803 days in orbit, spread across six missions over a 17-year career, accumulated approximately 0.02 seconds of relativistic time dilation by the time he retired from spaceflight in 2007. The arithmetic is direct: 803 days multiplied by approximately 28 microseconds per day equals approximately 22 milliseconds, conventionally rounded to 20 milliseconds or 0.02 seconds in popular accounts. The figure has, since approximately 2013, become one of the most widely cited individual physics facts in popular science journalism — partly because it is the only documented case of a human being for whom relativistic time dilation has produced a cumulative effect large enough to be communicated in non-scientific notation, and partly because the framing as “time travel into the future” has obvious appeal to readers who have grown up with the science-fiction version of the concept. Krikalev has not, in any meaningful sense, time-travelled. He has aged slightly less than he would have if he had spent the same period on the surface of the Earth. The two formulations describe the same physical phenomenon. The first sounds more dramatic.
As described in the Institute of Physics’s educational resource on Krikalev’s time-travelling status, the calculation is, mathematically, an application of the standard Lorentz factor — the equation Einstein derived in 1905 that gives the ratio between time as experienced by a moving observer and time as experienced by a stationary one. The factor is essentially 1 for ordinary velocities, increasing slowly as velocity rises, and approaching infinity as velocity approaches the speed of light. At the ISS’s orbital velocity of approximately 7.66 km/s, the factor is approximately 1 + 3.27 × 10^-10 — a number indistinguishable from 1 in any practical context, but multiplied across 803 days, sufficient to produce the 0.02-second effect that has made Krikalev a recurring fixture in physics-education curricula around the world.
The complication from gravity
The full picture is, technically, more complicated than the special-relativity-only version that produces the 0.02-second figure. Einstein’s general theory of relativity, published in 1915, established that gravity also affects the rate at which time passes — specifically, that clocks closer to a massive body run more slowly than clocks farther away. An astronaut at the altitude of the International Space Station, approximately 400 kilometres above Earth’s surface, is in a slightly weaker gravitational field than a person at sea level. The astronaut’s clock therefore runs slightly faster than the ground-based clock, by general-relativistic effects, even as it runs slightly slower by special-relativistic effects. The two phenomena work in opposite directions. At the ISS’s altitude, the special-relativity effect from orbital velocity is approximately five to six times larger than the general-relativity effect from reduced gravity. The net result is that the astronaut ages slightly less than the ground-based observer, with the net rate of time dilation coming out to approximately 22 to 23 microseconds per day rather than the special-relativity-only figure of 28.
The combined effect is what produces Krikalev’s cumulative 0.02 seconds. As documented by the World Air Sports Federation’s official record citation for Krikalev’s cumulative-time achievement, the figure became official in October 2005 when Krikalev completed his sixth mission (Expedition 11) aboard the ISS and surpassed the previous cumulative-time record holder. Krikalev held the absolute cumulative record for approximately a decade, until Gennady Padalka exceeded his total in 2015. The current record holder, as of 2026, is the Russian cosmonaut Oleg Kononenko, who has accumulated over 1,110 days across five spaceflights and who is now, by the same calculation, approximately 0.025 seconds younger than people born at the same time as him. The cumulative-time record is no longer Krikalev’s. The famous 0.02-second figure remains his, by historical association rather than current standing.
What this confirms about physics
Per a comprehensive reference summary of Krikalev’s full career and the broader significance of his time-dilation calculation, the substantive point of the Krikalev figure is not the time-travel framing but the experimental confirmation it provides for one of the more counterintuitive predictions of modern physics. Einstein’s special theory of relativity has been tested in many ways across the past century — by particle accelerators that produce velocity-induced time dilation in subatomic particles, by atomic-clock experiments aboard commercial aircraft, by the day-to-day operation of GPS satellites that must continuously correct for relativistic effects to maintain navigational accuracy. Krikalev’s 0.02 seconds is, in the strict sense, a smaller-precision result than several of these other experiments. What makes it useful as a teaching example is that it applies the theory to a single identifiable human being who has spent more than two cumulative years of his life moving at the velocities at which the theory becomes relevant. The man is approximately 0.02 seconds younger than he should be. The figure is real. The implication, that time itself is not a fixed background against which events unfold but a quantity that depends on the relative motion of the observer, is one of the deeper claims about the structure of physical reality that the 20th century introduced — and Krikalev, by simply going to work in orbit six times across a 17-year career, became one of the human beings whose biographies most directly demonstrate the claim.
