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SCIENTIFIC DISSEMINATION · PHYSICS AND COSMOLOGY

Can we travel back in time?

What relativity, wormholes, and quantum physics really say about crossing time

🕒 Estimated reading time: 11-12 minutes     · ✍️ Writing for popular science

📅 Updated: September 2026

From H. G. Wells to Back to the Future, the idea of getting on a machine and scrolling through the calendar has fascinated entire generations. But far from being just a narrative device, time travel has been, for more than a century, an object of serious study within theoretical physics. The question that is the title of this article does not admit a simple yes or no: the answer depends on which direction of time one wants to travel, forward or backward, and what nature is willing to admit.

To understand this, it is necessary to abandon the idea of time as a river that flows equally for everyone. Albert Einstein demonstrated, first with special relativity (1905) and then with general relativity (1915), that time is one more dimension, intertwined with space in a single fabric called space-time. And that fabric can be stretched, curved and, according to some mathematical solutions, even knotted in on itself.

If the goal is to get ahead in time, the scientific answer is blunt: yes, it is possible, and it has already happened. The phenomenon is called time dilation and has two variants that relativity predicts with mathematical precision.

Speed: The faster you move, the slower your watch moves forward

Special relativity states that time passes slower for a moving object than for an observer at rest. The effect is minuscule at everyday speeds, but it becomes measurable in orbit. Astronauts on the International Space Station (ISS), traveling at about 28,000 km/h, age a fraction of a second slower than those who remain on Earth.

🛰️  Cosmonaut Gennady Padalka, with more than 878 days accumulated in space, is today the person who has "travelled" the most into the future due to this effect: a fraction of a second ahead of the rest of humanity.

👬  The case of twins Scott and Mark Kelly — one of whom spent 340 consecutive days on the ISS — allowed NASA to directly compare the biological and temporal effects of long-duration spaceflight.

Gravity: the stronger the gravitational field, the slower the clock

General relativity adds a second ingredient: clocks located in a stronger gravitational field—near a massive planet or black hole—move more slowly than those farther away. That's why GPS satellites must correct their atomic clocks several times a day: without that adjustment, the positioning system would accumulate errors of several kilometers in a matter of hours.

The real challenge—and the one that fuels all science fiction—is the backward journey. Here, physics doesn't say "no" outright, but neither does it offer a practicable "yes." Einstein's equations admit, in theory, exotic solutions known as closed time curves (CTCs): trajectories in space-time that, always moving into the local future, end up returning to their own starting point.

Wormholes: shortcuts between two points in space-time

The best-known proposal is the Einstein-Rosen bridges, described mathematically in 1935 and renamed wormholes. In 1988, physicists Kip Thorne and Michael Morris refined the model and showed that if one of these tunnels could be kept open and traversed, and if its two ends were also moving relative to each other at different speeds or gravities, the mathematical result would be a real time machine.

The problem is not geometric, but physical: keeping a wormhole open would require exotic matter, a form of energy with negative density that has never been observed in useful quantities. As astrophysicist Eric Davis explained, although these tunnels could connect different moments in time, turning them into a functional travel machine would require a technological and energetic effort that today is far ahead of any human capacity. Kip Thorne himself has pointed out that the laws of physics will probably end up prohibiting a human being from physically crossing one of these structures.

At the beginning of 2025, a team from Conicet and the National University of La Plata (Argentina) published a theoretical model – in the journal The European Physical Journal C – that proposes a way to distinguish a rotating wormhole from a common black hole, based on the electromagnetic pattern it would generate. It would be the first method capable of providing indirect observational evidence on whether these objects really exist in the universe, although for the moment it remains a hypothesis pending data from telescopes.

Warp motors: the other side of the same coin

In 1994, the Mexican physicist Miguel Alcubierre proposed another solution to Einstein's equations: a ship could, in theory, move faster than light without violating relativity if, instead of accelerating itself, it contracts the space in front of it and expands it behind, wrapped in a bubble of curvature. Alcubierre himself and other physicists have pointed out that this type of configuration, combined with certain geometries, could also give rise to closed time curves, that is, they would open an indirect door to the past.

The obstacle, again, is energetic: Alcubierre's metric requires the same exotic matter of negative energy as wormholes, in quantities that in the original calculations were equivalent to the mass-energy of an entire planet. Subsequent studies, such as those of the physicist Harold White, have explored geometric variations that would drastically reduce this requirement, although they remain, for now, mathematical exercises without experimental verification.

Even if the technology to open a closed time curve existed one day, a deeper problem would remain: causality. The most cited example is the grandfather paradox, formulated by the French writer René Barjavel in 1943: if someone travels to the past and prevents the meeting of his own grandparents, how could he be born to undertake that journey?

🔁  Novikov's principle of self-consistency: proposes that the universe only allows trips to the past that are logically consistent; any attempt to alter history would end, by the laws of physics themselves, generating the same result that has already occurred.

🌐  Interpretation of branching universes: suggests that a trip to the past does not modify "your" timeline, but creates a different parallel reality, thus avoiding logical contradiction.

🛡️  Chronological protection conjecture: the most cited position among physicists, proposed by Stephen Hawking in 1992.

Hawking argued that even if general relativity mathematically allows closed time curves, some additional physical mechanism—probably tied to quantum effects, such as vacuum polarization—would prevent them from forming on a macroscopic scale, thus preventing travel into the past and its paradoxes. His way of summarizing it became famous: he joked about the existence of a kind of "chronological protection agency" that keeps the universe safe from historians. As informal evidence in favor of his own conjecture, he pointed out that humanity has never been "overrun by hordes of tourists from the future."

The honest and up-to-date answer is twofold. Into the future, yes: physics allows it, predicts it with accuracy and has already been verified in astronauts, satellites and experiments with high-precision atomic clocks. Into the past, the door remains ajar only on paper: equations do not prohibit it absolutely, but each proposed path – wormholes, warp engines, closed time curves – runs into the same wall, the need for exotic matter or energy that no one has ever detected, and with the suspicion, increasingly consolidated among theoretical physicists, that the universe itself has mechanisms to prevent paradoxes before they even form.

While science continues to polish these models—with new tools such as next-generation telescopes and quantum laboratory simulations—the truth is that we already live, in a literal sense, surrounded by time travelers: every GPS satellite and every astronaut who returns to Earth brings with it, on its internal clock, a few seconds stolen from the future.

Is there any evidence that you can travel to the past?

No experiment has ever detected a closed time curve or matter with enough negative energy to sustain it; for now it is a purely mathematical and theoretical field.

Do astronauts really travel in time?

Yes, in the strict sense of time dilation: they return to Earth a fraction of a second younger than they would have been if they had stayed on the ground, an effect measured with atomic clocks.

Why is "exotic matter" needed and what exactly is it?

It is a hypothetical type of matter with negative energy density, needed in wormhole and warp motor models to keep space-time stable. It has not been observed in nature in appreciable quantities.

What do physicists think today about the real possibility of traveling to the past?

The majority position remains aligned with Hawking's chronological protection conjecture: even if general relativity does not mathematically prohibit it, some additional physical principle—probably quantum—would prevent these structures from forming in practice.

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🔗  Infobae — What are wormholes and why are they linked to time travel?

🔗  National Geographic — What is a wormhole

🔗  La República — South American Theoretical Model for Detecting Wormholes (Conicet / UNLP)

🔗  Wikipedia (EN) — Chronology protection conjecture (Stephen Hawking, 1992)

🔗  Physical Review D — Hawking, S. W., "Chronology protection conjecture" (1992)

🔗  arXiv — Martín-Vázquez & Sabín, "Closed timelike curves and chronology protection in quantum and classical simulators"

🔗  Sky at Night Magazine — How Time Dilation Affects ISS Astronauts

🔗  PMC/NCBI — NASA Twins Study: Effects of Long-Duration Spaceflight

🔗  Hibridosyelectricos.com — Advances and challenges of the Alcubierre curvature motor

🔗  Meer — The Grandfather Paradox, Origin and Physical Explanation

A popular science article for information purposes. The theories described—wormholes, curvature motors, and closed time curves—correspond to mathematical models valid within general relativity, but none have been experimentally verified as of the date of this publication.

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