This morning I traveled into the future. So did you. Every person on this planet moves forward at one second per second. No one has to build anything for it.

That sounds like wordplay. It is physics. Since Einstein we have known that time does not pass at the same rate everywhere. The clocks in the navigation satellites above us run ahead by about 38 millionths of a second every day. The main reason is the weaker gravity at their altitude. Without correction, every position fix would be kilometers off after a single day. Anyone who spends half a year on the International Space Station returns a few thousandths of a second younger than their family on the ground.

So we have long been traveling through time. Only in one direction. That raises a question I have not been able to let go of for years. If people one day learn to travel backward as well, why has no one ever arrived here?

The Empty Party

Stephen Hawking once answered this question with champagne. On June 28, 2009, he hosted a reception for time travelers in Cambridge. He sent out the invitations only after the party was over. Anyone who showed up would therefore have had to come from the future. Hawking waited a long time. No one came.

A few years later, the astrophysicist Robert Nemiroff and his colleague Teresa Wilson at Michigan Technological University set out on a more thorough search. They combed Twitter, search queries and a heavily visited astronomy website for knowledge that no one could have had at the time it was posted. They even asked publicly for messages with an earlier date. They found nothing.

Both have something playful about them. Both are still a finding. From the direction of the future, there is silence.

There are obvious readings of this silence. Perhaps we will no longer exist in a thousand years. Perhaps our descendants made it and simply have no interest in us. Or nature does not allow the way back at all. I want to examine what physics has to say.

What Einstein Allows

The theory of relativity does not forbid travel into the past. That surprises many people. In 1949 the logician Kurt Gödel found a solution to Einstein's equations for a rotating universe. In it there are paths that lead back into one's own past. Physicists call them closed timelike curves. Einstein himself was troubled.

For a long time Gödel's universe was regarded as a mathematical curiosity, because our own does not rotate. That changed in 1988. Carl Sagan was looking for a credible way to send the heroine of his novel "Contact" to Vega. He asked Kip Thorne. Together with Michael Morris and Ulvi Yurtsever, Thorne worked out what a wormhole would have to look like for a human to travel through it. In the process they found that such a wormhole turns into a time machine almost by itself. One only has to move one of its mouths at high speed for a while. Less time then passes at that mouth. This creates a time offset between the two ends.

The price is high. Such a wormhole stays open only if it is propped up by matter with negative energy. Physicists call it exotic. Quantum physics knows such effects on small scales. Whether they can ever be gathered in the quantities required, no one knows.

The Limit of the First Day

Here lies a restriction that is missing from almost every public debate about time travel. For our question it is the most important one.

A wormhole connects two ends that must both exist. The time offset between them begins only once it is built. Anyone passing through can therefore go back no further than the day the machine went into operation. Before that, there is no exit.

For the silence from the future, this has a clear consequence. Even if a civilization builds such a machine a thousand years from now, it opens only the span between its own construction and its own present. Our year 2026 remains out of reach. Hawking's party and Nemiroff's search took place before any conceivable machine. No one could have come, even if the technology exists one day.

This leads to a thought with weight of its own. The day the first time machine starts up becomes the earliest moment the future can ever reach. Whoever builds it sets the beginning of visitable history.

What Quantum Gravity Suggests

It remains open whether nature permits such a machine at all. Einstein's theory alone cannot answer that. Once space and time are curved this strongly, quantum physics comes into play. A theory that unites both does not yet exist. There are, however, several serious candidates for this quantum gravity. I have examined what they say about time loops.

In 1992 Hawking calculated what happens at the boundary of a forming time loop. The energy of the quantum fields rises steeply there. This backreaction would likely destroy the loop before it closes. Hawking concluded that the laws of nature fundamentally prevent time machines. Later work showed that his calculation becomes uncertain at exactly this boundary. His conjecture remains unproven.

String theory has found a different route. When Gödel's rotating universes are examined with its tools, a screen forms around the regions containing the loops. No information reaches the rest of the universe from there. The loop is in the equation, but no one can use it. Another approach builds spacetime from an ordering of events in which "earlier than" is the fundamental law. There, a loop cannot even be written down.

There are also models that explicitly allow time travel. The Russian physicist Igor Novikov formulated a principle for this as early as the 1980s: a time traveler can influence the past but cannot change it. In 2020, Germain Tobar and Fabio Costa at the University of Queensland showed mathematically that such journeys are conceivable without contradiction. Events adjust so that every action of the traveler remains consistent with history. Whoever tries to stop their grandfather fails, without anyone forcing them to.

No candidate delivers a blueprint. Some forbid the loop. The others allow it only for processes that do not rewrite history.

Only What Changes Nothing Comes Back

What would that mean in practice? Picture a robot built a thousand years from now. It needs no food and no sleep. Its memory holds the history of the coming centuries. It steps through the machine and arrives in the past.

Physics has little to object to its arrival. A chip that merely stores data uses no energy to do so. Nor does the robot have to return to its own starting point. From its arrival on, it simply goes on existing.

It can only arrive where a machine stands, though, and so not before it was built. Even there, it is bound by something deeper than any technology. Everything it does must be consistent with the history that sent it.

Anyone thinking of "The Terminator" at this point is right to. The first film from 1984 is a remarkably clean example. A machine is sent back to kill the mother of the future resistance leader. The resistance sends a soldier after it. That soldier becomes the father of the very man the machine was meant to prevent. The attack on history produces the history it set out to prevent. Novikov could not have written the script more precisely. Only the sequels break with this logic. There, the future can be rewritten. That is exactly where cinema leaves physics behind.

In this picture, information can reach the past. It just cannot rewrite it. The robot could tell us about the future. It could not, however, tell us anything that prevents the future it comes from. Nature would not step in like a security guard. It would simply not permit a history in which such a thing happens.

An everyday example makes this tangible. A time traveler helps an elderly woman across the street. Without him, a distracted driver would have hit her. Has he changed history? According to Novikov, no. Changing requires two versions, one with the accident and one without. But there is only one history. In it, the traveler was always standing at that corner. The woman was never run over. In the archives of his own time, her rescue has long been recorded. He changes nothing. He fulfills what has already happened.

It only becomes difficult if the rescue would prevent his own journey. Suppose her grandson became the physicist who one day builds the machine only because of her death. Then the rescue cannot succeed. No one stops the traveler. He arrives a moment too late, or the traffic light changes differently. Circumstances arrange themselves so that everything fits together in the end. Small favors are possible, then, as long as they fit into the one history.

A second school among physicists sees it differently. For Barak Shoshany and others, every journey leads into a new timeline. There, the traveler really does save the woman. In the world he came from, she still died. History then changes, but never one's own.

Both models are mathematically free of contradiction. Which one nature chooses, no one knows today.

That closes the circle back to the beginning. We travel forward, one second per second. An object may find its way back. The chain of causes still runs in only one direction.

What the Silence Does Not Prove

This shifts the original question. The bleak reading was tempting: no one comes, so later there is no one left. That conclusion does not hold.

If nature closes the way back, the silence is the same whether humanity survives or not. If time machines reach back only to their own construction, the same applies. The empty party tells us nothing about our downfall. Nor does it tell us anything about our success.

I take that as good news. The future has not passed a verdict on us that we simply do not know yet. It is open. What happens in it will be decided on the only path we reliably master: forward, one second per second.

An Assumption to Close

There is one possibility I cannot rule out. Our physics is not the final word. A thousand years from now, what looks like a limit to us today may have been solved. Perhaps the machines will then reach further back than we consider possible today.

Let us assume that is the case. Let us assume the travelers have long been among us. They have imposed a rule on themselves never to reveal who they are. No lottery numbers, no warnings, no letter to their own grandmother.

Then this rule would have to achieve what no human order has ever achieved. Every traveler would have to keep it, across centuries, through crises and schisms. A single breach, and we would know. Who could sustain such an order? And what would it reveal about the people of the future if they succeeded?

| The Planet Futures Organization, the first multiplanetary NGO.

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