Every time we look into space, we are looking into the past. The Sun appears to us as it was about eight minutes ago, because that’s how long its light takes to reach Earth. Look towards the Andromeda Galaxy, and you’re seeing it as it was roughly 2.5 million years ago. Astronomy gives us a remarkable window into history. But seeing the past isn’t the same as visiting it. The light made the journey. We didn’t.
Astronomy lets us see the past. Time travel would mean actually visiting it. Can we?
No one has ever been shown to travel into their own past, and physics gives us no known way to do it. But the answer isn’t quite a flat no. Some solutions to Einstein’s equations contain paths through spacetime that loop back to earlier events. Wormholes, rotating spacetime and faster-than-light travel have all appeared in serious theoretical discussions of how such paths might arise.
The equations leave the door slightly open. The question is whether nature does too.
Forward in Time Is the Easy Direction
You’re travelling into the future right now, at the rate of one second per second. But Einstein’s theory of relativity tells us that two people don’t necessarily experience the same amount of elapsed time. Someone travelling at extremely high speed could return to Earth decades later while ageing only a little themselves — not because they reversed time, but because less time passed for them.
This time dilation is established physics and has been measured experimentally. Getting back to an event that has already happened is a different problem entirely. So what would that actually require?
What Travelling Back in Time Would Actually Require
Imagine your life as a path through spacetime. Breakfast this morning. Somewhere else this afternoon. Somewhere else again tonight. Join those moments together and you get your worldline — your path through spacetime. Normally, that path carries you towards later events. But some solutions of general relativity allow something stranger — a path that loops back to an earlier event.
Physicists call this a closed timelike curve, or CTC — “closed” because the path eventually loops back to the same event in spacetime. Crucially, you wouldn’t experience your own time running backwards. Your watch would still tick forwards and your heart would beat normally. It’s your path through spacetime that loops back into the past. That gives us the basic requirement for a relativistic time machine: somehow creating a CTC.
What could possibly create such a path?
Possible Routes Back in Time
Physicists have found several situations where spacetime could theoretically allow paths back toward the past. None gives us anything we could build today — but each keeps the question alive in serious physics.
Wormholes
A wormhole would be the ultimate shortcut — if it existed. Imagine a wormhole with two entrances. Normally, it would simply provide a shortcut through space. But suppose we leave one entrance behind and take the other on a near-light-speed journey. Because of time dilation, less time passes for the travelling entrance. Bring the travelling entrance back and the two ends could once again sit side by side in space — while one clock might read 2pm and the other 1pm. If the wormhole still connected them internally, stepping through it could potentially take you from the later end to the earlier one. A shortcut through space has effectively become a shortcut through time.
Physicists Michael Morris, Kip Thorne and Ulvi Yurtsever famously explored how a traversable wormhole could be turned into a time machine — provided such a wormhole could exist at all. That’s the enormous problem: none has ever been observed, and theoretical models suggest keeping one open would require negative-energy conditions we don’t know how to produce at any usable scale.
There’s a catch, though: this kind of time machine wouldn’t let you dial up any date you liked. You couldn’t travel back to before the wormhole had been turned into a time machine. Turn one into a time machine in 2200, and no future traveller could use it to reach a date earlier than 2200. Ancient Rome and Julius Caesar would stay forever out of reach. Which offers a neat answer to an old question: if time travel is ever invented, why isn’t anyone from the future visiting us already? Maybe their road into the past just doesn’t reach back this far.
Rotating Black Holes and Time Travel
Most black holes are expected to rotate. And according to general relativity, a rotating mass doesn’t just spin — it can drag the surrounding spacetime around with it, an effect known as frame-dragging. Think very roughly of a spoon stirring thick honey: as the spoon turns, it pulls the honey around with it. A rotating mass similarly affects the spacetime surrounding it. Push that effect far enough and something unexpected shows up. In the simplified mathematical description of a rotating black hole — known as a Kerr black hole — closed timelike curves can appear deep inside: paths that loop back towards the past.
But that doesn’t mean you could fly into a rotating black hole and emerge last Thursday. These strange paths appear deep inside that mathematical model. We don’t know whether they could exist inside a real black hole — let alone whether a traveller could reach them and survive. So black holes give us another theoretical road into the past. They don’t give us a road we know can actually be travelled.
Black holes aren’t the only place where rotation can twist spacetime this way. In 1974, physicist Frank Tipler showed how an idealised, infinitely long, extremely dense cylinder, spinning at enormous speed, could drag spacetime around with it strongly enough to produce closed timelike curves. It’s another theoretical route into the past — but not a time machine we’re remotely capable of building.
Faster Than Light — and Arriving Before You Left
Perhaps there’s an easier solution. Forget wormholes and black holes. What if we simply travelled faster than light? Even at 99.999% of light speed, relativity only gives you time dilation — not a trip into the past. Go faster than light, and the trouble really starts. Relativity allows observers moving relative to each other to disagree about the order of certain distant events. Normally that can’t reverse cause and effect, because no information can travel between those events faster than light. An FTL signal could change that.
Imagine sending a faster-than-light message to a distant friend moving relative to you. Their reply — DON’T SEND THE MESSAGE — could, under the right conditions, reach you before you sent the original one.
So you don’t send it.
Then where did the warning come from?
That’s the problem with faster-than-light travel: it doesn’t just get you somewhere quicker. It can scramble which came first, cause or effect. This is also why theoretical warp drives come up in time-travel discussions. Instead of accelerating a ship through space past light speed, the idea is to contract space ahead of it and expand space behind it — moving the ship by moving spacetime around it. That doesn’t dodge the causality problem, though. Under the right theoretical conditions, a warp drive could also create paths that loop back through time, much like faster-than-light signals can.
Breaking the light-speed barrier, in other words, wouldn’t just get you somewhere faster.
It could get you there before you left.
Other Ways Spacetime Can Loop
Wormholes, rotating black holes and FTL aren’t the only ways Einstein’s equations can open paths towards the past. In 1949, mathematician Kurt Gödel discovered a solution to Einstein’s equations describing an entire universe that rotates — not galaxies spinning within space, but the universe itself rotating. In this strange universe, spacetime behaves in such a way that a traveller’s path could loop all the way back to their own past. Our universe doesn’t appear to work that way. But Gödel had demonstrated something remarkable: general relativity itself could accommodate closed paths through time.
Physicist J. Richard Gott later found another possibility involving cosmic strings — incredibly thin, incredibly dense structures that may have formed in the early universe. If two passed each other at close to light speed, their combined effect on spacetime could theoretically produce a closed timelike curve. We don’t know whether cosmic strings even exist, let alone whether they could be used this way. But by now a pattern has emerged. Einstein’s equations contain several apparent roads into the past.
So why can’t we simply follow one?
Mathematical Time Machines vs. Reality
By now, every route we’ve explored has run into the same catch. There are really three different questions:
Can mathematics describe it?
Can it physically exist in our universe?
Could we actually build or use it?
A proposed time machine can pass the first test and still fail spectacularly when confronted with physical reality. A wormhole may require conditions nature doesn’t provide. An idealised spacetime may not behave the same way once the complications of the real universe are included. And quantum physics may introduce problems that general relativity alone doesn’t reveal. Even if a route into the past could physically exist, that’s still a long way from knowing how to create or control one. Mathematically possible doesn’t necessarily mean physically possible — and physically possible doesn’t mean buildable.
But there may be an even deeper problem. Nature might prevent time machines from forming in the first place.
Does Physics Protect the Past?
Physics may have a defence of its own. Stephen Hawking wondered whether nature somehow prevents time machines from ever forming — an idea he called the chronology protection conjecture. The basic idea is that as a time machine gets close to forming, quantum fields could behave in extreme ways around the developing time loop, producing enormous amounts of energy. That energy would itself affect spacetime and might disrupt the time machine before it could fully form. Put simply:
General relativity may open the door. Quantum physics may slam it shut.
But we don’t know that it does. Chronology protection remains a conjecture, not a proven law. Answering the question properly may require a theory of quantum gravity — a theory capable of describing gravity and quantum physics together — which physicists don’t yet have. So perhaps backward time travel is merely unimaginably difficult. Or perhaps some deeper law of nature forbids it outright. Either way, physics hasn’t given us the final word. Suppose nature doesn’t stop us. What happens if we actually reach the past?
What Happens to Cause and Effect?
Imagine that tomorrow you discover a time machine. You travel back to today and destroy it before your earlier self can use it.
So how did you travel back to destroy it?
That’s the fundamental difficulty created when the future can influence its own past — the kind of contradiction illustrated by the Grandfather Paradox.
The Bootstrap Paradox creates a different puzzle. Imagine your future self gives you the plans for a time machine. Years later, you travel back and give those same plans to your younger self. Who originally designed them? The plans simply circulate from future to past and back again, with no obvious point of origin.
Various solutions have been proposed. Perhaps events involving time travel would have to remain self-consistent. Perhaps changing the past would create a different history. Or perhaps paradoxes are another clue that backward time travel simply can’t occur. Those questions deserve their own discussion. For our purposes, the important point is simpler: Travelling into the past isn’t only a problem of getting there. It’s also a problem of what happens to cause and effect once you arrive.
So, Could We Ever Travel Back in Time?
At present, we know of no way to travel into our own past. No person, object or message has been demonstrated to do so. We have no observed traversable wormhole, no working warp drive and no known route through a black hole into yesterday.
Yet relativity doesn’t leave us with a simple no. Einstein’s equations contain possible paths through spacetime that loop back towards earlier events. Wormholes and other theoretical constructions show how such paths might arise. What we don’t know is whether nature actually allows any of them to exist in a usable form. Perhaps quantum physics prevents time machines from forming. Perhaps some deeper law makes backward time travel impossible. Or perhaps there are possibilities we haven’t yet discovered.
For now, we can say three remarkable things:
We can travel towards the future at different rates.
We can look millions — even billions — of years into the past.
And we can write equations containing roads that lead back there.
What physics has yet to show us is whether any of those roads can actually be travelled.
Frequently Asked Questions
Is it actually possible to travel back in time?
We don’t currently know of any physically possible way to travel into the past. General relativity does allow unusual spacetime paths called closed timelike curves, which could theoretically return a traveller to an earlier event. But no usable CTC, traversable wormhole or other backward time machine has ever been demonstrated.
If time travel is possible, why haven’t we seen any time travellers?
As the wormhole section explained, a time machine might not be able to reach back to before it was built. So even if backward time travel is someday achieved, its road into the past may simply not stretch back far enough to reach us.
Does travelling faster than light mean travelling back in time?
Not by itself. But relativity shows that faster-than-light travel or communication could, under the right conditions, allow a traveller or a message to arrive before they left or were sent — a genuine violation of cause and effect. That’s why FTL and backward time travel are so closely linked in physics.
Could you change the past if backward time travel were possible?
We don’t know. Some proposals say the past would have to stay self-consistent — anything a time traveller did would always have been part of history. Others suggest branching timelines, where changing the past creates a new history rather than rewriting your own. Which idea — if either — actually applies remains unknown.
Can a black hole take you back in time?
Not as far as we know. A black hole’s gravity can cause extreme time dilation, meaning time could pass differently for you than for someone farther away — but that isn’t the same as travelling into the past. Some idealised rotating black-hole models contain paths that loop backwards through time — but we have no evidence that such a route exists inside a real black hole, let alone that it could actually be travelled.
