Time is not universal: how speed and gravity bend it

There is no single clock for the universe. Moving fast makes less time pass for you, and so does sitting deeper in a gravitational field. Both effects have been measured, from a clock raised two feet to the satellites that run GPS. None of the three physicists here claims to know what time actually is.

Michelle Thaller, an astrophysicist who spent years at NASA, treats it as everyday engineering. Clocks are now accurate enough to detect time running differently two feet higher, so, she says, "your head is in a different time frame than your feet right now." Two things change the rate: speed, and distance from a mass like the Earth. GPS satellites face both. Moving fast slows their clocks, and being farther from Earth's gravity speeds them up, which is the larger effect. Without correction, positions would be off by roughly six miles within a day. For astronauts on the space station speed dominates, and a year aboard leaves them about a hundredth of a second younger. She explains the speed limit by energy. As anything with mass approaches the speed of light, it takes more and more energy to accelerate it even a little.

Brian Cox, who teaches relativity at Manchester, draws the consequences for travel. From a fast ship's view, distances shrink. Protons in the Large Hadron Collider see the 27-kilometer ring as about four meters across. A ship close enough to light speed could in principle cross to the Andromeda galaxy, two million light years away, in a minute of its own time. On return, at least four million years would have passed on Earth. That is time travel into the future, which physics allows. Of the past he says, "You can't go back as far as we can tell." That prohibition is suspected, not proven. Warp drives work on paper but need matter or energy that does not appear to exist.

Sean Carroll gives the structure underneath. Space and time are one four-dimensional thing, with no fixed division between them. Different paths between the same two events take different amounts of time. The straight, unaccelerated path takes the longest, which is why the twin who travels out and back comes home younger. Gravity is the curvature of spacetime, so clocks at different depths in a gravitational field are on different paths.

The three agree on the facts and the evidence. Thaller's six miles and Cox's 30,000 nanoseconds a day are the same GPS error in different units. They explain it at different levels: that it is real, what it permits, and why. They differ on how far to go about the nature of time, and the idea that past and future exist like the present goes beyond the measurements.

What to do with it: drop the idea of a universal now. Remember the two causes. Speed slows your clock relative to others, and being deeper in gravity slows it too. Use GPS as the proof. Keep the scale in view, since the effects are tiny outside extreme speeds and gravity. Use the twin picture. Sort the fiction: a one-way jump to the future is allowed, and a return to the past appears not to be. Accept that physicists can measure how time behaves without knowing what it is.