ISS crew member
7.66 km/s for 6 months (182.5 days)
Speed part only. In orbit gravity is weaker, which makes clocks run a little fast and cancels roughly a tenth of this; that gravity part (general relativity) is not computed here.
Relativity Calculator
Leave Earth the same age as your twin, fly to a star near light speed and back - who comes home older, and by how much?
Your twin on Earth
You, the traveller
Range 0.01–0.999999999
Departure age
Result ▸
- Lorentz factor γ (how many times slower time runs)
- One-way distance, contracted
- Round trip on Earth
- Round trip for you
Real speeds do it too, just tiny
It is real, just far too small to feel. The same formula, applied to speeds humans have actually reached. A slower clock means coming back younger. Each card says how far behind the moving clock ends up, in its real unit, never rounded to zero.
Parker Solar Probe
If it held its peak 192 km/s for 24 hours
Speed part only. Near the Sun its gravity also slows the clock, in the same direction and by a comparable amount, so the real total is larger; that gravity part (general relativity) is not computed here. The probe is near peak speed only briefly, so a full day is a what-if.
Airliner passenger
900 km/h for 12 hours
Speed part only. At cruising altitude the weaker gravity speeds clocks up by more than this, in the opposite direction, and Earth's rotation matters too, so a real flight's net can go either way; none of that is computed here.
About the numbers
Why the traveller comes back younger. The twins are not symmetric: the traveller turns around at the destination and so changes reference frame (the point of view the clocks and distances are measured from), the twin on Earth does not. Special relativity then says the traveller's clock has run less. This tool assumes the turnaround is instantaneous and does not count any time spent accelerating. At speed, distances along the flight also shorten (length contraction); the contracted one-way distance in the readouts is that shortened distance.
For those who want the formulas
With speed β as a fraction of c and distance d in light-years, γ = 1/√(1 − β²), Earth round-trip time = 2d/β, traveller time = Earth time ÷ γ, and the contracted distance = d ÷ γ. A year is 365.25 days, as in the light-year. Close to c the tool builds 1 − β² from the exact 1 − β, because squaring a number that close to 1 throws digits away.
Why the slider counts nines. Time dilation only becomes dramatic very close to c: γ is 2.294 at 0.9c and 7.089 at 0.99c, and it reaches 22,361 at 0.999999999c. On an ordinary slider almost every notch would sit on a speed where nothing visibly changes, so here every extra 9 costs the same thumb travel. The bars are not squeezed like that: they share one plain scale of the time since leaving, and its top is rescaled to a round number just above the longer time, so the ratio of the two bars is the ratio of the two times (1/γ) and no bar is ever cut off. When the traveller's bar is too small to see it is drawn as a thin sliver, and the line under the bars says the ratio in words. Whichever twin would be older than 120 at the reunion is flagged as one who would not live to see it, and the big number for that twin then says how much time passes on their own clock instead of an impossible age.
Sources and limits. Distances (Earth frame, one way): Proxima Centauri 4.24 ly (NASA); Sirius 8.60 ly (Hipparcos, van Leeuwen 2007); TRAPPIST-1 (a star outside the solar system) 40.7 ly (Gaia DR2); the galactic centre 26,700 ly (GRAVITY Collaboration 2019); Andromeda about 2.5 million ly (measurements run from roughly 2.4 to 2.6 million). The real-world cards use the ISS orbital speed and NASA's reported Parker Solar Probe peak speed. Special relativity only: gravity changes clocks too. GPS satellites show how much: their speed slows the clocks, but the weaker gravity speeds them up several times more, so gravity wins and the net correction has the opposite sign. This tool does not model gravity. Proper motion of the stars and Earth's own motion are ignored.
Quick guide
Use cases, answers, and nearby tools
Compact below-tool notes that help first-run users and repeated visitors move faster without changing the main interface.
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How to use
Run a clean first pass
- Pick a destination (Proxima, Sirius, TRAPPIST-1, the galactic centre or Andromeda), then drag the speed slider right. The slider counts the nines after the decimal point, so every extra 9 costs the same thumb travel. For an exact speed, type it in the box beside it, for example 0.9, 0.9c or 99.9%.
- The two bars are the answer: your twin who stayed on Earth on the left, you on the right, both leaving at the same age. The big number above each bar is the age on return, the label on top of each bar is the time elapsed since leaving (bar height is proportional to it, on a scale whose top adjusts to the scenario; a line under the bars says what fraction of Earth's time yours is), and the box between them says how many years apart. Change the departure age with the − and + buttons.
- The result line under the slider writes the answer as one sentence, departure age included; Copy, at the top left of the bars, puts it on your clipboard with a link to the same scenario, and Reset goes back to TRAPPIST-1 at 0.999c, age 30. The scenario buttons below jump straight to the galactic centre or Andromeda and leave your departure age alone.
Examples
Real jobs this page helps with
- The textbook case: Proxima at 0.9cPick Proxima and type 0.9. Proxima is about 4.24 light-years away, the round trip takes 9.42 years on Earth, γ = 2.294 and you age 4.11 years. Leaving at 30, you come back 34.1 and your twin is 39.4, 5.32 years apart.
- Sirius at 0.99cPick Sirius and type 0.99. γ = 7.089; you come back at 32.5, your twin is 47.4, 14.9 years apart.
- TRAPPIST-1 at 0.999cThis is the default. Pick TRAPPIST-1 and type 0.999. γ = 22.37; you come back at 33.6 and your twin is already 111.5.
- The galactic centre: seven ninesPick the galactic centre and type 0.9999999. γ = 2,236; your round trip takes 23.9 years and you come back at 53.9, while 53,400 years pass on Earth.
- Andromeda: nine ninesPick Andromeda and type 0.999999999. γ = 22,361 and 5.0 million years pass on Earth; your own round trip takes 224 years, more than a lifetime, so the page shows "224 years on your clock" instead of an age and flags that you would not live to see it.
- Real speeds: the ISS and an airlinerA line directly under the bars, above the speed box, takes you to "Real speeds do it too, just tiny": after half a year on the ISS the clock is about 5.15 milliseconds behind one on the ground, after a 12-hour airliner flight about 15.0 nanoseconds. Same formula, real units, never rounded to zero.
FAQ
What people usually want to know
How far apart are the twins after a round trip to Proxima at 0.9c?
Proxima is about 4.24 light-years away and the round trip takes 9.42 years on Earth; γ = 2.294, so you age only 4.11 years. Leaving at 30, you come back at 34.1 and your twin is 39.4, 5.32 years apart.
Why is the traveller the younger one, instead of it being symmetric?
The twins are not symmetric: the traveller turns around at the destination and changes reference frame, the twin on Earth does not. Special relativity then says the traveller's clock has run less. This tool assumes the turnaround is instantaneous and does not count time spent accelerating.
How much younger does an astronaut get from six months on the ISS?
It is real, just far too small to feel. Counting speed only (7.66 km/s), six months (182.5 days) leaves an astronaut's clock about 5.15 milliseconds (about 0.005 s) behind one on the ground. Weaker gravity in orbit makes clocks run a little fast, which cancels roughly a tenth of that; this tool does not model gravity.
Is the GPS clock correction this same effect?
Only part of it, and the smaller part. Speed slows a GPS satellite's clock, but the weaker gravity up there speeds it up several times more, so gravity wins and the net correction has the opposite sign. This tool computes only the speed part and does not model gravity.
Can I enter the speed of light, or more?
No. Anything with mass cannot reach c, so the top speed here is 0.999999999c (nine 9s). Enter 1, 100%, c or "light speed" and the result line turns into that explanation and the bars fade until you enter a valid speed. 0.9, 0.9c and 99.9% all work; a bare whole number with no % (for example 90) is refused, because it could mean 90% or 90 times c, and the message tells you to type 90% or 0.9 instead.
How long does a trip to the galactic centre take at seven nines?
At 0.9999999c your round trip to the galactic centre takes 23.9 years while about 53,400 years pass on Earth. This is a constant-speed round trip with acceleration and turnaround ignored; a real ship would not fly like this.