If UFOs were time machines, what would we expect to find?

On 28 June 2009, Stephen Hawking held a reception at Gonville and Caius College in Cambridge. There were balloons, champagne and a banner reading "Welcome Time Travellers". He posted the invitations the following day.

Nobody came. He sat in the room for a while and then left, and afterwards described the empty party as experimental evidence that time travel is not possible.

It is a weak experiment and he knew it. It is also more than almost anybody else in this argument has done, because it did the one thing the subject almost never does: it made a prediction in advance and then went and looked.

So take the idea properly. Suppose the occupants reported in the UFO literature are our own descendants, arriving from later. What follows from that, and how would we tell?

Three hypotheses, and why they are not interchangeable

Three explanations circulate for the same body of reports, and they are usually treated as flavours of one idea. They are not. They predict different things.

The extraterrestrial hypothesis puts the origin at another star. It predicts an independent biology: separate origin, separate chemistry, no shared ancestry with anything on Earth, and an enormous unsolved engineering problem in getting here.

The interdimensional hypothesis puts the origin alongside us rather than across from us. It is the hardest of the three to test, because it does not clearly predict anything at all — which is a weakness and not a strength.

The future-human hypothesis puts the origin here, later. It is the only one of the three that predicts a shared biology, and shared biology is measurable.

What general relativity actually allows

The physics is worth stating accurately, because both sides of this argument tend to overstate it.

General relativity contains solutions with closed timelike curves — paths through spacetime that return to their own past. Kurt Gödel found one in 1949: a rotating universe in which such paths exist for every observer. Frank Tipler described another in 1974 involving an infinitely long, extremely dense, rapidly rotating cylinder. In 1988, Michael Morris, Kip Thorne and Ulvi Yurtsever showed that a traversable wormhole with its two mouths at different gravitational potentials or velocities becomes a time machine.

These are real mathematics. They are not hand-waving, and anyone who tells you physics forbids time travel outright is going further than the physics does.

None of them has been shown to be buildable. Gödel's universe is not the one we live in. Tipler's cylinder has to be infinite. The wormhole route requires exotic matter with negative energy density, in quantities that quantum field theory's own inequalities appear to rule out. In 1992 Hawking proposed the chronology protection conjecture — that the laws of physics conspire to prevent closed timelike curves on macroscopic scales. It remains a conjecture. Nobody has proved it, and nobody has disproved it.

The date the sightings should start

Here is the first genuine prediction, and it is a strange one.

In the wormhole family of solutions, closed timelike curves only exist to the future of a boundary that forms when the machine is made. Before that boundary there are no such paths. A machine of that kind cannot reach back to a time before it existed — the earliest destination is its own construction.

If that constraint holds for whatever a future civilisation actually builds, then visits from the future have a start date, and it is the date of the first machine. Before it, nothing. After it, potentially everything, from every subsequent century, all arriving at once.

That is a testable shape. It predicts that reports should begin abruptly and then persist, rather than trailing back through history at a steady rate.

The handedness test

Now the forensic arguments, which are much stronger than the physics, and which nobody seems to run.

Life on Earth is built almost exclusively from left-handed amino acids and right-handed sugars. Both molecules exist in mirror-image forms, both forms work equally well chemically, and life here uses one of each. As far as anyone can tell, the choice was arbitrary — frozen in once, at the origin, and inherited by everything since.

A descendant of ours would share it, because it is not the sort of thing evolution can reverse. An independently originated organism would have no reason to match, and a fifty-fifty chance of not matching.

This is a laboratory measurement. Polarimetry and chromatography. It is undergraduate work, and it distinguishes the two hypotheses on a tissue sample the size of a grain of rice.

The genome would answer it outright

Roughly eight per cent of the human genome consists of retroviral sequence — the remains of infections that got into the germ line of our ancestors and have been copied down ever since. Each insertion sits at a specific address in the genome, and it sits there because a particular ancestor was infected at a particular time.

A human descendant would carry those same insertions at those same addresses. Along with the same chromosome architecture, the same pseudogenes, and the same broken vitamin C gene we share with the other apes and monkeys.

Nothing from an independent biosphere would have any of it, and nothing could fake it without deliberately constructing a genome to look like ours — which is a hypothesis you can entertain but not one you can build an argument on.

What a tooth would say

This is the sharpest of the three, and it is the one we have not seen anybody propose.

Atmospheric nuclear testing in the 1950s and early 1960s roughly doubled the amount of carbon-14 in the atmosphere. Since the test ban it has been falling steadily. Everything alive takes carbon up from that atmosphere, and dental enamel, once formed in childhood, does not turn over — it locks in the value of the year it was laid down.

In 2005 a group led by Kirsty Spalding published a method in Nature that uses this to determine a person's year of birth from a tooth, to within about a year and a half. It is used in forensic identification now.

So a body has a date written into it. And a person born after our present would carry an enamel carbon-14 value from an atmosphere that has not happened yet — a value below the current curve, continuing a decline we have not lived through. That is not an interpretation. It is a number, measured against a calibration curve, and it either sits on the known curve or it sits off the end of it.

The things that would be harder

Not everything predicted is so clean.

Microbes. A descendant would carry gut flora descended from ours, and would be vulnerable to pathogens descended from ours — with, presumably, no current immunity to the specific strains circulating in whichever year they arrived in. That is a real prediction and it is also the sort of thing that would make casual visiting extremely dangerous in both directions, which argues against the frequent-visits picture rather than for it.

Anatomy. Our bodies are full of inherited compromises: the recurrent laryngeal nerve that loops down into the chest and back up for no reason other than fish ancestry, the retina wired backwards so that the optic nerve punches a blind spot through it, a pelvis shaped by an argument between walking upright and giving birth. Some would persist in a descendant. Some might be engineered out, and once you allow deliberate modification the anatomical argument loses most of its force.

Language. Any linguistic relationship to a modern language would be extraordinary evidence. The absence of one would prove nothing, since twelve thousand years is enough to erase relationship entirely and we are talking about longer.

The problem with unfalsifiable answers

Every objection to this hypothesis has a ready reply, and that is precisely what should worry its supporters.

Why no proof? Because interference has consequences. Why no contact? Because contamination of the timeline. Why does nothing ever get recovered? Because they are careful. Why did nobody come to Hawking's party? Because they knew not to.

Each of those is coherent. Collectively they form a structure that no observation can disturb, and a structure no observation can disturb is not a scientific hypothesis, whatever else it may be. The Novikov self-consistency principle — the proposal that only self-consistent histories are physically possible, so paradoxes simply cannot be created — is a serious idea in physics and a conversation-ending one in this argument, because it can absorb any absence of evidence you like.

The way out is to insist on the predictions that cannot be absorbed. A tooth does not care about the timeline.

What we know and what we do not

We know that closed timelike curves are permitted by the equations of general relativity, that nobody has demonstrated a physically realisable one, and that the leading argument against them is an unproven conjecture.

We know that biochemical handedness, retroviral insertion sites and enamel radiocarbon are measurable properties that would distinguish a human descendant from an independently evolved organism, and that all three techniques already exist and are in routine use.

We do not know whether anything has ever visited. We have no sample of anything, from anywhere, that has survived independent analysis — which means the entire debate is being conducted about the properties of an object nobody has.

That is worth saying plainly. The interesting result here is not an answer. It is that the question has three cheap decisive tests waiting for it, and has had for twenty years.