The Physics of Time Travel: Wormholes, Paradoxes, and Black Holes

Time travel is one of humanity’s favorite bad ideas.

We have spent decades imagining phone booths, modified DeLoreans, glowing portals, mysterious watches, magical stones, and machines with far too many blinking lights. Apparently, no fictional scientist can bend spacetime unless their laboratory looks like an electrical fire waiting to happen.

But behind all the Hollywood nonsense lies a legitimate scientific question: Do the laws of physics actually allow us to time travel into the past?

The irritating answer is that we still do not know for certain.

Einstein’s theory of general relativity allows physicists to describe situations in which a person could follow a path through spacetime and eventually return to an earlier moment. However, every plausible attempt to create such a path appears to encounter the same small technical difficulty: the entire system tends to collapse into a black hole.

Nature, it seems, may tolerate theoretical discussions about time travel. It simply becomes rather hostile when someone tries to build one.

Note: This article is a summary and interpretation of a public lecture by physicist Sean Carroll at the Linda Hall Library. It simplifies several ideas discussed in the presentation, so it should not be mistaken for a complete technical treatment of relativity, quantum mechanics, or time travel. The full lecture can be watched in the video embedded below.

The Past Is Not an Editable Document

The first major problem with travelling into the past is not engineering. It is logic.

Suppose you build a time machine, return to a time before your parents met, and prevent them from getting together. You are never born. But if you were never born, you could not have built the time machine. And if you did not travel back, your parents would meet after all.

Congratulations. You have created the grandfather paradox, damaged causality, and made every screenwriter in the room extremely excited.

The traditional response is simple: time travel must be impossible because it creates contradictions.

Carroll offers a more subtle possibility. Perhaps travelling into the past is theoretically possible, but changing the past is not.

Whatever happened, happened.

Under this interpretation, you might return to the past and attempt to prevent your own birth. But something would always stop you. Your machine might fail. You might arrive at the wrong location. You might accidentally introduce your parents to each other while trying to separate them.

The universe would not necessarily stop you with a dramatic lightning bolt. It would simply require that everything you do remain consistent with the history that already produced you.

This means time travel would come with an uncomfortable form of predestination. You could make choices, but only choices compatible with events that have already occurred.

Your trip to the past would not rewrite history. It would become part of history.

Free Will Meets an Annoying Little Kid

This idea bothers us because we like to believe we have free will. If we travelled into the past and stood in front of our younger parents, surely we would remain free to do whatever we wanted.

Physics is less emotionally supportive.

At a fundamental level, many classical laws of physics are deterministic. If we knew the exact state of a physical system and all the relevant laws governing it, we could theoretically calculate what the system would do next.

That sounds terrifying, but Carroll provides a useful analogy. Determinism is not like a wise prophet revealing your destiny. It is more like an annoying child who says:

“I know what you’re going to do.”

You ask what you are going to do.

The child refuses to tell you.

Then, after you do something, the child says, “See? I knew it.”

The universe may contain enough information to determine what happens next, but that does not mean we possess that information. We do not know the precise condition of every particle, every brain, every object, or every badly maintained office printer.

From our perspective, the future remains uncertain.

The past, however, is different. We have records, memories, evidence, fossils, photographs, and embarrassing social-media posts. The past appears settled, while the future appears open.

Time travel threatens that distinction because your personal future could take place inside the universe’s past. Suddenly, something you have not experienced yet would already be part of established history.

That is where causality begins developing a headache.

Quantum Mechanics Offers a Loophole—Sort Of

Quantum mechanics complicates the story.

In one interpretation of quantum mechanics, commonly associated with the “many-worlds” view, the universe branches into multiple versions whenever different outcomes occur.

In one branch, a particle is observed in one location. In another, it appears somewhere else. Both outcomes exist, but in separate versions of reality.

This opens an intriguing possibility. Perhaps a time traveller could return not to their own past, but to the past of another branch of the universe.

In that alternate timeline, you might prevent your parents from meeting without erasing yourself, because you were born in a different branch. Your original history would continue to exist. You would simply have moved to another cosmic address.

This avoids the classic paradox, although it produces several new personal problems.

You have not actually changed your own past. Your original parents are still together in your original universe. You have merely arrived in another reality and caused trouble for a different version of your family.

It is logically conceivable, but no one knows how to travel between branches of the quantum wave function. At present, “use another universe” is less an engineering blueprint and more the physicist’s version of adding “somehow” to the middle of a plan.

Newton’s Universe: One Clock for Everyone

To understand why relativity makes time travel imaginable, we first need to understand how physics treated time before Einstein.

In Newtonian physics, space and time are separate and absolute.

Space is the arena in which objects exist and move. Time is a universal sequence of moments through which everything progresses at the same rate. Every sufficiently accurate clock in the universe should agree with every other sufficiently accurate clock.

Under this view, the universe resembles a film strip. Each frame represents one moment, and every observer agrees about which events belong in each frame.

You can travel through time in Newton’s universe, but only in the boring direction. Sit in a chair for 24 hours and you will successfully travel to tomorrow.

No plutonium required.

Everyone moves forward at one second per second. You may feel that time passes more slowly during a tedious meeting, but this is a failure of human perception, not a rebellion by the fundamental structure of reality.

Then Einstein arrived and ruined the simplicity.

Einstein Makes Time Personal

Einstein’s special theory of relativity abolished universal time.

Two perfectly accurate clocks can begin together, separate, follow different paths through the universe, and disagree when they meet again. Neither clock is broken. They have genuinely experienced different amounts of time.

This effect has been repeatedly confirmed experimentally.

The key insight is that time behaves somewhat like distance. Two people can travel between the same locations along different routes and cover different distances. Similarly, two observers can move between the same events along different paths through spacetime and experience different durations.

There is one important difference. In ordinary space, the straight path between two points is the shortest. In spacetime, the unaccelerated path generally produces the longest elapsed time.

Imagine that one person remains on Earth while another flies away in a spacecraft at nearly the speed of light and later returns. The traveller may experience only a few years while decades pass on Earth.

This is genuine travel into the future.

The astronaut does not feel time slowing down. Their heart still beats normally. Their watch still advances one second per second. The difference becomes apparent only when they reunite with someone who followed another path through spacetime.

So physics already allows one-way time travel. Unfortunately, it is the least useful kind. You can reach the distant future, but you cannot return and brag about it to people in the present.

Light Cones: The Universe’s Traffic Rules

Special relativity also introduces a universal speed limit: the speed of light.

To understand its significance, physicists use the concept of a light cone.

Imagine switching on a light bulb for an instant. Light travels outward in every direction. As time passes, the expanding sphere of light covers more space. When represented on a spacetime diagram, its complete history forms a cone.

Everything you can influence in the future must lie inside your future light cone. Everything that could have influenced you lies inside your past light cone.

Events outside the cone are inaccessible because reaching them would require moving faster than light.

A party on Alpha Centauri next Friday may be excellent. It is also physically impossible for you to attend, since Alpha Centauri is more than four light-years away. Even light would arrive several years late, by which point the snacks would presumably be disappointing.

In special relativity, you may move along different paths and experience different amounts of time, but you must always remain inside your light cone. You still move toward the future.

The door to the past remains closed.

General Relativity Warps the Doorframe

Einstein’s general theory of relativity changes the situation again.

Special relativity describes spacetime without gravity. General relativity explains gravity by treating it not as a traditional force, but as the curvature of spacetime itself.

Mass and energy distort spacetime. Objects then move through that curved geometry.

Earth orbits the Sun not because the Sun reaches across space with an invisible gravitational rope, but because the Sun changes the geometry around it. Earth is attempting to follow the straightest possible path through a spacetime that is no longer straight.

Gravity also affects time. A clock in a strong gravitational field ticks at a different rate from a clock in a weaker gravitational field.

This is not merely theoretical. GPS satellites experience time differently from clocks on Earth because they move at different speeds and occupy a different gravitational environment. Navigation systems must correct for relativity. Without those corrections, GPS positions would quickly become wildly inaccurate.

In other words, Einstein’s strange ideas about curved spacetime are partly responsible for preventing your phone from directing you into a lake.

More importantly for time travel, general relativity allows light cones to tilt.

Near a black hole, spacetime becomes so distorted that all future-directed paths inside the event horizon lead toward the singularity. Escaping would require moving outside the light cone, which means moving faster than light.

Inside a black hole, the singularity is not merely a location sitting somewhere ahead of you. It is part of your future. Reaching it is as unavoidable as reaching tomorrow—except tomorrow is usually less destructive.

If gravity can tilt light cones that dramatically, physicists can ask a dangerous question:

Could spacetime be warped so severely that the light cones eventually loop back toward the past?

Mathematically, the answer appears to be yes.

Closed Timelike Curves: Walking Forward Into the Past

A path that always moves locally toward the future but eventually returns to an earlier event is called a closed timelike curve.

A traveller following such a curve would never feel themselves moving backward. They would age normally. Their clock would always move forward.

Yet the overall shape of their journey through curved spacetime would bring them back to their own past.

They might leave a room, travel through an extreme gravitational configuration, and eventually re-enter the room moments before their younger self departed.

At the beginning of the journey, the younger traveller would already see their older self arriving. Otherwise, the history would be inconsistent.

This is not the cinematic version of time travel. There would be no sudden disappearance, no swirling portal, and ideally no vehicle leaving trails of fire across a parking lot.

A realistic relativistic time machine would work more like a spacecraft following a particular route through an exceptionally warped region of spacetime.

In that sense, time travel would be a special form of space travel.

The difficulty is creating the route.

Cosmic Strings: A Time Machine Made from Hypothetical Space Cracks

One proposed method involves cosmic strings.

Cosmic strings are hypothetical, extremely thin concentrations of energy that may have formed during the early universe. They have never been confirmed, but they are not automatically forbidden by known physics.

Physicist J. Richard Gott showed that two infinitely long, perfectly straight cosmic strings passing each other at enormous speeds could distort spacetime in a way that creates closed timelike curves.

A spacecraft travelling around the strings along the correct path could theoretically return to the past.

Naturally, there are minor complications.

We have not found suitable cosmic strings. They would need to be infinitely long in the simplified calculation. They would need to remain precisely arranged. They would also need to move past each other at extraordinary speeds.

Perhaps we could start with slower strings and accelerate them.

Physics has considered your suggestion and would apparently prefer not to.

The energy required to accelerate the strings into the necessary configuration would become so concentrated that the system would likely collapse into a black hole before functioning as a time machine.

This will become a recurring theme.

Wormholes: Shortcuts Through Space and Possibly Time

Another famous proposal involves wormholes.

A wormhole is a hypothetical tunnel connecting two distant regions of spacetime. Enter one mouth and you could emerge from the other without travelling through all the normal space between them.

Imagine travelling from Kansas City to St. Louis through a tunnel only a few kilometres long. The tunnel would not merely run underneath the ordinary landscape. It would exploit a shortcut through the geometry of spacetime itself.

Wormholes are highly speculative. Keeping one open may require exotic matter with negative energy, and it is not clear whether stable, traversable wormholes can exist.

Still, suppose we had one.

Place a clock next to each mouth. Leave one mouth stationary while sending the other on a high-speed journey. Because of special relativity, less time would pass for the travelling mouth.

When the mouths reunite, their clocks would no longer agree.

If the internal connection between the two mouths remained intact, entering one side could cause you to emerge from the other at an earlier external time.

The wormhole would have become a time machine.

Unfortunately, quantum fluctuations may destroy the arrangement just as it begins to permit backward travel. Energy circulating through the wormhole could increase dramatically, destabilising the structure and causing it to collapse.

Possibly into a black hole.

Again.

At this point, black holes are starting to feel less like astronomical objects and more like the universe’s error message.

Does Nature Protect History?

The repeated failure of proposed time machines inspired Stephen Hawking to suggest the chronology protection conjecture: perhaps the laws of physics prevent closed timelike curves from forming.

Under this idea, quantum effects would always become strong enough to destroy a time machine before anyone could use it.

However, chronology protection has not been conclusively proven.

Physicists currently occupy an awkward position. General relativity allows mathematical solutions containing time loops. Yet every attempt to produce a physically realistic version seems to require impossible materials, absurd amounts of energy, infinitely idealised objects, or configurations that collapse before becoming useful.

We cannot demonstrate that a working time machine can exist.

We also cannot yet produce a complete theorem proving that no time machine could ever exist.

Carroll’s own expectation is that the final laws of nature will prohibit travel into the past. But expectation is not proof, and physics has a long history of humiliating confident expectations.

Even a Working Time Machine Would Have Limits

Suppose future physicists solve every problem. They stabilise a wormhole, control the quantum fluctuations, avoid the black hole, and attach enough warning labels to satisfy the legal department.

They still could not travel to any arbitrary point in history.

A wormhole time machine could only send travellers back to a period after the time machine’s relevant configuration had been created. It would not provide access to ancient Rome, the dinosaurs, or the meeting where someone approved the final season of your least-favourite television show.

This explains why the absence of tourists from the future does not prove time travel is impossible.

Perhaps no one has visited us because the doorway has not been built yet.

A future civilisation might be able to return to the day its first time machine became operational, but no earlier. History before that moment would remain inaccessible.

So even successful time travel would come with strict terms and conditions.

The Real Answer Is Still “We Don’t Know”

The physics of time travel produces three broad conclusions.

First, paradoxes cannot be allowed. If a journey into the past occurs, everything the traveller does must fit consistently into history—or perhaps take place in another branch of the quantum universe.

Second, relativity shows that time is not universal. Different observers genuinely experience different amounts of time, and travelling into the future faster than everyone else is already permitted by known physics.

Third, general relativity allows us to imagine geometries containing paths into the past. Cosmic strings, wormholes, and other exotic structures provide mathematical possibilities.

But mathematical possibility is not the same as physical reality.

Every known attempt to turn those possibilities into an actual machine encounters severe problems. The required objects may not exist. The energy demands may be impossible. Quantum effects may destabilise the system. Spacetime may collapse into a black hole before causality can be violated.

The universe seems willing to let physicists write equations about time machines, as long as nobody tries anything funny.

For now, the safest conclusion is that travel into the past is probably impossible—but “probably” is doing a great deal of work.

And perhaps that is appropriate. If physicists eventually discover that time machines are possible, someone from the future can always come back and correct this article.

Assuming, of course, that whatever happened has already happened.