What If Time Travel Was Possible?

Forward time travel is not science fiction. It is measured, confirmed physics, and someone has already done it on purpose, without calling it that. Cosmonaut Sergei Krikalev spent 803 cumulative days orbiting Earth at roughly 27,600 km/h (17,150 mph) and came home about 0.02 seconds younger than he would have been had he stayed on the ground. This piece asks a narrower question than "is time travel real": if a person could choose to jump decades into the future on purpose, using nothing but speed, what would that decision actually cost them?

Thought experiment. This article explores a hypothetical scenario for educational and speculative purposes. The events described are not predictions and should not be interpreted as warnings, advocacy, or instructions.

Forward time travel already happened, just in small doses

Every person who has ever flown on a commercial jet, orbited the planet, or driven a car has aged, in a strict physical sense, a vanishingly small amount slower than someone who stayed perfectly still. That is not a rounding error in the math. It is time dilation, the prediction from Einstein's 1905 special theory of relativity that a clock moving relative to an observer runs measurably slower than a clock at rest, and it has been directly confirmed since 1971 (Hafele & Keating, Science, 1972).

The clearest human example is astronaut Scott Kelly, who spent 340 consecutive days aboard the International Space Station between 2015 and 2016, orbiting at about 7.66 km/s (17,150 mph). When he landed, he had accumulated roughly five milliseconds less aging than his identical twin brother Mark, who stayed on Earth — meaning Mark, who used to be six minutes older than Scott, was afterward six minutes and five milliseconds older (Space.com, "Einstein's 'Time Dilation' Spread Age Gap for Astronaut Scott Kelly & His Twin"). Cosmonaut Sergei Krikalev, who logged more cumulative orbital time than almost anyone in history across six missions, is often described as the person who has traveled furthest into his own future: about 0.02 seconds, purely from velocity (HuffPost UK, "Russian Cosmonaut Sergei Krikalev Is The World's Time-Travel Record Holder"). As of 2026, cosmonaut Oleg Kononenko holds the record for cumulative time in orbit, having passed 1,111 days across five missions, which pushes his personal number even further into the future than Krikalev's (Spaceflight Now, "Cosmonaut Oleg Kononenko reaches 1,000 cumulative days in space").

None of these numbers are large enough for anyone to notice without an atomic clock. That is the entire point of this article: the physics that makes noticeable forward time travel possible already exists in miniature, and scaling it up is a question of engineering and consequence, not new laws of nature. Our sister piece on why aging slows down in space works through the full mechanism, including the muon experiments and GPS corrections, in detail. Here, the physics is the setup, not the subject. The subject is what happens to a person who decides to use it.

Kononenko's 1,111 cumulative days in orbit push his personal clock only a fraction of a second behind Earth's — proof that even humanity's most experienced space travelers are still, in relativistic terms, barely time travelers at all. Real forward time travel, the kind a novelist would recognize, would require velocities far beyond anything a chemical rocket can reach.

Why this is a different kind of "what if" than backward time travel

It is worth being precise about what makes forward time travel different from the backward kind, because the two get flattened together constantly in casual conversation. Backward time travel — visiting your own past, preventing an event, meeting a younger version of yourself — requires exotic, unproven physics: closed time-like curves, wormholes held open by matter with negative energy density, or rotating structures that cannot exist in a finite, buildable form. Our companion piece on whether time travel is physically possible goes through those mechanisms and their problems in depth, including Stephen Hawking's chronology protection conjecture.

Forward time travel requires none of that. It requires velocity, or a strong enough gravitational field, both of which are ordinary physics, confirmed daily by the GPS satellites correcting for exactly this effect. The open questions about forward time travel are not "is the physics real" — that was settled by 1971 — but "what would it be like to actually do it, on purpose, at a scale large enough to matter." That is a question about propulsion engineering, psychology, and law, not about General Relativity.

How fast would someone need to go to jump meaningfully into the future?

The relationship between speed and time dilation is described by the Lorentz factor, γ = 1 / √(1 − v²/c²), where v is velocity and c is the speed of light. At 50% of light speed, one year aboard a ship corresponds to about fourteen months on Earth — noticeable, but not dramatic. At 95% of light speed, the ratio jumps to roughly 3.2 to 1. A crew traveling to a star 20 light-years away at that speed would experience about 6.6 years passing on board, while about 21 years passed on Earth — a real, single jump forward of about a decade and a half relative to everyone they left behind, in exchange for a trip that, from the crew's own perspective, felt like six and a half years. Push velocity to 99.9% of light speed and the ratio grows to roughly 22 to 1: a century on Earth would compress into about 4.5 years for the traveler (time dilation mechanics, per the Lorentz transformation). No spacecraft built or even seriously designed today can reach a meaningful fraction of light speed; Voyager 1, humanity's fastest and farthest human-made object, travels at roughly 17 km/s, less than 0.006% of light speed.

What choosing forward time travel would actually mean for a person

Set the propulsion problem aside for a moment and assume it is solved. Someone straps into a ship, accelerates to a meaningful fraction of light speed, travels for what feels to them like a few years, decelerates, and returns, or simply continues onward, having jumped decades or centuries into Earth's future. What has that person actually chosen?

They have chosen a one-way ticket in every sense that matters, even if the ship itself can technically fly back to where it started. Everyone they knew when they left will be decades older, or dead, by the time they return to Earth's timeline. There is no return trip that undoes this: traveling back at the same speed only adds more elapsed Earth-time on top of what has already passed. Forward time travel, unlike the backward kind, has no grandfather paradox, no self-consistency puzzle, and no philosophical escape hatch. It is brutally, mechanically one-directional. The traveler does not get to go back and warn anyone, fix anything, or say goodbye a second time. Whatever life they had waits for them exactly as far in the past as the trip pushed them into the future.

Under Lorentz time dilation at 99.9% of light speed, a crew that experiences 4.5 subjective years would return to an Earth that has aged more than a century — meaning every person under roughly 50 years old that they knew before departure would likely be gone. The math has no exception for grief.

Does forward time travel change anything about the traveler's own experience?

No. This is the part science fiction gets consistently wrong. From inside the ship, nothing about the traveler's own experience of time speeds up, slows down, or feels unusual. Clocks tick normally, conversations happen at normal speed, and physics inside the cabin works exactly as it does on Earth. What changes is only the comparison between the traveler's elapsed time and everyone else's. The strangeness is entirely relational, not experiential, which is exactly why a person could complete a "century-long" journey and feel, subjectively, like only a few years had passed.

Real people who lived a version of this without a spaceship

Nobody has actually done relativistic forward time travel at a scale a person would notice. But history has produced a handful of situations that are eerily close analogues of what "returning to a changed future" would actually feel like, minus the physics.

The starkest is Japanese army officer Hiroo Onoda, who continued fighting World War II in the Philippine jungle for 29 years after Japan's 1945 surrender, refusing to believe the war had ended without a direct order from a superior officer. When his former commander was finally located and delivered the stand-down order in person in March 1974, Onoda emerged into a Japan he no longer recognized: modernized, transformed, decades removed from the imperial Japan he had left behind. He was greeted as a hero, but had, by multiple accounts, serious and lasting difficulty adjusting to the country he returned to (NPR, "Japanese Soldier Who Fought On For 29 Years After WWII Dies"). Onoda did not travel through time in any physical sense. He simply lost 29 years of social, cultural, and technological change all at once, then had to absorb it in a single re-entry, which is functionally the same experience a real forward time traveler would face on return.

The Japanese folktale of Urashima Tarō, a fisherman who spends what feels like a few days visiting an undersea palace and returns to find hundreds of years have passed on land, is old enough to predate relativity by centuries, but physicists have since borrowed its name: the "Urashima effect" is now used informally as another term for relativistic time dilation, precisely because the folktale's structure — a short subjective experience, a long objective one, and a jarring return — matches the physics so closely (discussion of the Urashima effect and its physics parallel). That a centuries-old story anticipated the emotional shape of a real physical phenomenon is a coincidence worth sitting with: cultures have been telling stories about the grief of forward time travel long before anyone could calculate it.

The rival technology: cryonics as forward time travel without the physics

If jumping into the future is genuinely appealing to someone, relativistic spaceflight is not the only proposal on the table, just the only one confirmed to work. Cryonics companies like Alcor, based in Scottsdale, Arizona, offer a very different mechanism: freezing a legally deceased person's body or brain at cryogenic temperatures immediately after death, in the hope that future medicine can revive and repair them. Alcor has cryopreserved roughly 200 to 248 people since 1972, at costs starting around $80,000 for the brain alone and roughly $200,000 for a whole body, typically funded through life insurance policies (Smithsonian Magazine, "200 Frozen Heads and Bodies Await Revival at This Arizona Cryonics Facility").

Cryonics and relativistic time travel are trying to solve the same emotional problem — skip ahead, leave the present behind, arrive in a future you could not otherwise reach — with completely different physics. Relativistic travel keeps the traveler biologically continuous and conscious the entire time; a subjective few years pass, and the body ages accordingly. Cryonics requires legal death first, and depends entirely on a future technology, tissue revival and repair at the cellular level, that does not currently exist and may never be developed (legal and scientific overview, cryopreservation). One is confirmed physics with an unsolved engineering problem. The other is unconfirmed biology with a confirmed preservation step. Neither currently gets anyone to a future they can act in.

Q: Is cryonics a form of time travel? A: In spirit, yes: both aim to move a person from the present into a future they would not otherwise live to see. Mechanically, they are unrelated. Relativistic time dilation is confirmed physics with an engineering barrier; cryonics is confirmed preservation technology attached to an unconfirmed revival technology.

What a decision like this would actually cost — a comparison

The following table lays out what a person would realistically be trading away at different scales of forward time travel, from the tiny, already-achieved version to the purely speculative extreme.

Scenario	Traveler's elapsed time	Earth's elapsed time	What is realistically lost
ISS orbit, 340 days (Scott Kelly, 2015-2016)	340 days	340 days + ~5 milliseconds	Nothing noticeable; confirms the physics is real
Cumulative orbital career (Oleg Kononenko, 1,111 days)	1,111 days	1,111 days + a fraction of a second	Nothing noticeable; largest human total to date
Hypothetical trip at 50% light speed, 1 year on board	1 year	~14 months	A few months of social and news catch-up
Hypothetical trip at 95% light speed, 20-light-year round distance	~6.6 years	~21 years	Most friendships and one full generation of family aging
Hypothetical trip at 99.9% light speed, "100-year" outside journey	~4.5 years	100+ years	Every person the traveler knew under about age 50; the entire social and technological world they left
Cryonic preservation (Alcor, indefinite)	0 (unconscious)	Unknown, dependent on future revival technology	Everything; requires legal death first, and revival is unproven

Table: What different real and hypothetical routes to "the future" would cost a traveler, compared against the physics-confirmed baseline of human orbital spaceflight.

What people get wrong about forward time travel

"It's basically the same as backward time travel, just easier." It is not just easier, it is a completely different category of claim. Forward time travel via time dilation is confirmed, repeatable, and already measured in real human beings. Backward time travel remains a theoretical possibility inside general relativity that has never been observed and may be forbidden entirely by deeper physics. Treating them as two flavors of the same idea obscures how solid the ground is under one and how speculative it is under the other.

"If we could do this, people would use it to escape debts, obligations, or crimes." Legally, this would be uncharted territory, but not in the way people assume. A person who leaves Earth for what is, to them, a few subjective years and returns after decades or centuries would not "escape" a debt or a criminal charge in most legal systems, since liability generally attaches to the person, not to elapsed calendar time on Earth specifically, though a bafflingly long list of statutes of limitations, inheritance laws, and identity-verification systems would need entirely new rules to handle someone who is, in every legal sense, still themselves but decades displaced. No such rules currently exist anywhere, because no one has ever needed them.

"You'd get to see the future and come back to tell people about it." Only the "see the future" half of that is true, and even it comes with an asterisk: the traveler would only see one specific future, the one that actually happened on Earth during the elapsed time, not a preview usable for prediction before departure. And "coming back to tell people" runs into the same problem as any one-way trip: the people they would want to tell are, in most realistic scenarios, no longer around to hear it, or radically changed by decades the traveler didn't experience.

"This would only ever be voluntary." Every documented case that approximates the experience — Onoda, and involuntary long-term isolation more broadly — happened to someone who did not choose the gap, only the return. A voluntary version, where someone consciously trades away decades of their social world for a stretch of subjective time, has genuinely never been attempted by anyone, which means every psychological assumption about how a person would cope with it is exactly that: an assumption.

The takeaway

Forward time travel is not a hypothetical waiting on new physics. It is a confirmed, measured phenomenon that astronauts like Scott Kelly and cosmonauts like Sergei Krikalev and Oleg Kononenko have already lived through in doses too small to notice without an atomic clock. Scaling it up to something a person would feel is entirely a propulsion problem, not a physics problem, but the honest reason nobody is racing to solve that problem is the part that gets skipped in most retellings: a large enough jump forward is not an adventure with a return leg. It is a one-way severance from everyone and everything a traveler currently knows, dressed up as a voyage. The closest real humans have come to living that severance, people like Hiroo Onoda, did not choose it, and even they found the return brutally disorienting. Anyone actually offered the deal would be agreeing to trade their entire present for a future they cannot preview and cannot undo.

FAQ

Q: Has anyone actually traveled forward in time? A: Yes, in small, measured amounts. Astronaut Scott Kelly returned from 340 days on the ISS about five milliseconds younger than his twin brother Mark, and cosmonauts Sergei Krikalev and Oleg Kononenko hold the largest cumulative totals, at roughly 0.02 seconds and a slightly larger fraction of a second respectively, purely from orbital velocity.

Q: How fast would you need to travel to jump years into the future? A: At 95% of light speed, a trip that takes about 21 years by Earth's clock would feel like about 6.6 years to the traveler. At 99.9% of light speed, a century on Earth compresses into roughly 4.5 subjective years. No propulsion technology exists today that can reach either speed.

Q: Is forward time travel the same kind of physics as backward time travel? A: No. Forward time travel is confirmed, repeatable physics under special and general relativity. Backward time travel depends on exotic, unproven mechanisms like closed time-like curves and wormholes held open by negative-energy matter, none of which have ever been observed.

Q: Could you go back and visit the people you left behind after traveling forward in time? A: Not in any way that undoes the trip. Traveling back to Earth at similar speeds only adds more elapsed time on top of what has already passed. There is no return trip in relativistic forward time travel that restores the world a traveler left.

Q: Is there a way to "jump to the future" without relativistic speed? A: Cryonics, offered by companies like Alcor, is the closest analogue: legally deceased patients are cryogenically preserved in hopes of future revival. It solves a different problem with different physics, requires legal death first, and depends on repair technology that does not currently exist.

Q: What is the closest thing to forward time travel that has actually happened to someone? A: Cases of extreme, involuntary isolation come closest. Japanese soldier Hiroo Onoda spent 29 years in the Philippine jungle believing World War II had not ended, and returned to a Japan transformed beyond recognition, facing the same disorientation a real time traveler would face, without ever leaving Earth's normal timeline.

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Read next:

• The Mysteries of Time: Why Aging Slows Down in Space
• What If Time Travel Were Physically Possible? The Real Physics Behind the Idea
• What If We Found a Time Machine?