Science › Space, time & gravity
Faster than light and time travel
· Open question
SpeculativeA serious idea with no experimental proof yet.
Whether anything can travel faster than light, or travel back in time, and whether the two are linked.
See it on the Science map →How it works
In relativity, a signal faster than light would arrive before it was sent for some observers, so effects could precede causes. That is why faster-than-light travel and backward time travel stand or fall together. Travel into the future is real: fast or high-gravity clocks age less. Travel to the past is allowed only in exotic solutions of general relativity, like rotating universes or wormholes, which may not exist.
Why it matters
It is where physics meets paradox. Testing whether the past can be changed probes how consistent the laws of nature really are.
How we know
No faster-than-light particle has ever been found. A 2011 claim that neutrinos beat light was traced to a loose fiber-optic cable. Astronaut clocks age slightly less, confirming travel into the future in a tiny way.
Still a mystery
Hawking conjectured in 1992 that physics forbids backward time travel, but nobody has proven it. A theory of quantum gravity may settle the question.
Connections (2)
▸Causality and light cones forbids Faster than light and time travelProven
The relationship
If a signal could go faster than light, some moving observer would see it arrive before it was sent. Combined with relative motion, that permits messages into one's own past, creating paradoxes. Keeping causes before effects therefore rules out faster-than-light signals and backward time travel in ordinary spacetime.
How the link was found
Recognized soon after 1905; Einstein discussed the problem with superluminal signals in 1907.
Why it's proven
No faster-than-light signal has been observed. The ban is a logical result of relativity plus causality, not a direct measurement.
Open questions and mysteries
General relativity permits exotic spacetimes with time loops. Whether nature forbids them, as Hawking conjectured in 1992, is unproven.
▸Wormholes would in theory allow Faster than light and time travelSpeculative
The relationship
A wormhole would be a shortcut: you would travel slower than light inside it, yet arrive somewhere faster than light could go the long way. Thorne's group showed that if one mouth were moved at high speed and brought back, time dilation would leave the two mouths at different times, making it a time machine.
How the link was found
Morris, Thorne and Yurtsever published the time-machine argument in 1988.
Is it proven?
Purely theoretical. The reasoning follows from accepted relativity, but it depends on wormholes existing, which has never been shown.
Open questions and mysteries
Hawking's 1992 "chronology protection" conjecture suggests quantum effects would destroy any such time machine as it formed. It is unproven.