How does a time traveler prove he or she is from the near past?

Question

Assume a college student from 2018 accidentally traveled to 2023. He is accidentally taken by an alien spaceship that travels close to the speed of light away from earth and then travels back. He is put back to the place where he was taken.

The question is how can he prove to the people in 2023 that he has time traveled, against other possibilities, such as running away from school.

More relevant details/premises are given in the following:

• Assume apart from the alien spaceship part, the setting is totally realistic.
• There is no drastic technological advance happened in the 5 years nor new outbreak of a global disease/disaster. Assume that there is also no total elimination of any current disease.
• The incidence happened in a camping trip that he goes alone during the semester, and has no witness.
• The spaceship that took him was advanced enough to not to be detected using current technology. It is well sheltered against radiation in space, and it is not made by radioactive material.
• From his point of view, only a few minutes have passed. He does not have a coherent understanding of what happened.

Answer 1

Radioactive isotope dating.

Each of us born in the late 20th (second half) or early 21st Century have in us a small portion of the calcium in our bones replaced by an Isotope of Strontium 90 that is a by-product of above-ground Nuclear testing, common in the early 1960s. See reference here.

As with all radioisotopes, there is a very predictable rate of decay, in this case 28.8 years. For this young person, the rate of decay would be offset by the time dilation produced by spending 5 Calendar years at near-C velocities.

The testing would have no way to prove this person had been abducted, or why he's 5 years behind, but it would produce proof that 5 years was missing from his history.

Answer 2

To give him a chance of people believing his story in the absence of proof, you (as the author) are going to have to set things up ahead of time. Make him a newly-turned 18 y.o. college freshman with an identical twin brother attending the same school. If both of the brothers appear young for their age prior to the abduction, the difference between them upon his return could be pretty obvious.

Then to kick it up a notch, give him a girlfriend who gave him a four-leaf clover hours before his abduction. Finding that rare little leaf, still fresh in his pocket might go a long way to convincing her.

Answer 3

If there happened to be a photograph taken of him just before his disappearance, then he could point out any blemishes or minor injuries on him that perfectly matched the photograph. This could be minor scrapes or cuts, acne, a hangnail, mosquito bites, or a peeling sunburn. Bonus points if he has a recent tattoo or piercing that hasn't fully healed. No single one of these things is likely to convince anyone, but five or six of them in combination might.

Note that this only works if the photograph was taken by one of the people he is trying to convince, and the photograph has remained under the exclusive control of that person for the entire time. Otherwise our time traveler would have no way to prove the photo wasn't photoshopped.

This does not necessarily need to stand on its own as absolute proof either. This could be the thing that convinces people that it's worth the time and expense of (for example) trying radioisotope dating, as described in Joe's answer.

Answer 4

Guinea Pigs

It's 2018. Our college student, let's call him Edward, was getting ready for his camping trip. He would be gone for a few days, and he realized he couldn't just leave his new friend, Bartholomew, behind. Bart was just a baby guinea pig, his cute new pet, this little thing of only 8 centimeters, that he was gifted from a friend, Rayleigh.

Bartholomew had a very peculiar set of spots and fur colors, which made him stand out among his siblings. Edward took a lot of photos of little Bart the day they first met, sent to a lot of friends, and all that.

So Edward took Bart to the camp with him. His shirt had a good pocket for him, so that was easy.

When Edward finally showed up again in 2023, his friends, especially Rayleigh, couldn't believe their eyes. Not only Edward was suddenly back, but little Bart was there with the same 8 centimeters!! How could that be? It's common knowledge that not only guinea pigs live at most 4 years, but they also double in size after just one week... It couldn't be Bart... But it had to be, because how on earth there could be another baby guinea pig with the exact same spots and fur colors??

Source for Guinea Pig numbers: https://www.guineapigcorner.com/size

Edit addressing comment: if this is not enough, an extra possibility is to justify getting a DNA sample from Bart before going to the camp. Perhaps Rayleigh is a researcher and he took DNA samples of Bartholomew and his siblings to conduct some sort of long-term experiment - sending one guinea pig for each of 10 friends maybe, and planning to check on them in the future to compare their development for some reason...

Answer 5

He hasn't actually "time traveled"

There is no FTL involved. There is no going backwards in time. So this is simply time dilation and not time travel. That doesn't change the problem, just clarifying that this is different - and therefore much more plausible to those investigating - than actual time travel.

FWIW, I wouldn't be concerned about identity - DNA + more traditional identity confirmation such as dental records will take care of that.

Electronics

If he has an up-to-date iPhone or Android device with him and has not turned it off at all then a bit of analysis could show that the device only experienced 5 minutes of elapsed time instead of 5 years. This would likely not work if the device was turned off at some point during the trip - and he might have done just that to try and figure out why he wasn't getting a good signal, which of course he wouldn't get while on the alien spaceship.

Detailed explanation I can't say that any current device definitely works this way, but it is plausible:

• Like almost any computer, device has an elapsed time counter - number of seconds since it was turned on. Reset to 0 every time the device is turned on. This shows the device was turned on 8 hours + 5 minutes ago.

• The device, typically for cell phones and optionally (but quite commonly) for regular desktop/laptop computers (Windows and Linux), gets the "real" time via a web service, updated periodically. This avoids the need to have a super-accurate clock in every little device.

• The device logs the results of "time checks" and other network calls for a limited amount of time, with the local elapsed time logged as part of the information.

• The device would show that 5 minutes before the student left Earth, the network time check got a 2018 time. And that right after he got back - but only 15 minutes later in local elapsed time - it got a 2023 time.

The reason that the phone must stay on through the trip for this to work is that otherwise there is no simple way to distinguish between a 5-minute/5-year time dilation and simply turning the phone off for 5 years. The fact that the phone turned on and had to "correct" the time by 5 years is meaningless because the internal clock that runs while the phone is off might be broken (as can happen on a computer with a low battery).

Medical

As a young college student, he will likely be pretty healthy. But if, for example, he had his wisdom teeth taken out 2 weeks before the trip, a dentist could examine his mouth and determine that the amount of scar tissue/healing matches 2 weeks (or perhaps "within a range from a week to a couple of months") and not 5 years.

Answer 6

The process called DNA methylation can pinpoint individual's age with +/- 4 years precision. If your student skipped 5 years, his DNA test will look unusual, but not as much as to suggest things like alien abduction. However, if he had a blood sample taken and frozen right before the abduction, this will cause scientists to do a lot of head scratching.

Answer 7

Assume a college student from 2018 accidentally traveled to 2023. He is accidentally taken by an alien spaceship that travels close to the speed of light away from earth and then travels back. He is put back to the place where he was taken.

The question is how can he prove to the people in 2023 that he has time traveled, against other possibilities, such as running away from school.

First, necessary step - establish identity

He's identical to his document pictures from five years before, so that plus DNA matching with family members can establish he's who he's saying he is. So, we know that he should be around 25 years old.

Now how to prove he's actually 20?

Standard radioisotope dating (e.g. ¹⁴C) would not work. Radioisotope dating only measures time since loss of equilibrium, which usually happens at death. But even if the guy had died on the ship, that time would be his local elapsed time, so just a few minutes; there would be no clue as to how long he had lived before meeting his end.

If something had happened outside in those five years, and the guy did not show the signs, then yes, it would work - but the premise is that nothing of the kind happened.

• update: it might work, because something did happen outside in that time. There are environmental isotopes that decay naturally and, of course, most living beings are at equilibrium with them. But our guy isn't - he's got in his body five years of undecayed radioisotopes. This could be another factor to take into account, even if it would be probably explained away as natural variation or contamination by those same isotopes.

Immediate examination

Chances are that his clothes and appearance were recorded in selfies, videos and memories from five years back. He's still almost identical. While possible, this is really unlikely - a 25-years old is quite different from a 20-years old.

Interrogation

It is possible to question him on everything that went on in the weeks before his disappearance, five years ago for the interrogators, a few weeks for him. Were he an impostor, he might have studied again whatever examinations he gave last, and might have painstakingly reconstructed all the facts from those days and learned them by heart. He wouldn't be able to fool police detectives, whose job it is to see through alibis. If he were simulating, he would remember too little, or too much, in ways that are obvious to an expert. So either he's telling the truth or he's been coached by an expert on how to lie.

Medical examination

Growth in the interval 15..25 years can be analyzed quite effectively and will say that the guy is now 20 years old, give or take three years. The possibility he's actually 25 is still open, but not very likely.

After he disappeared, almost surely some kind of DNA sampling of the area was done; and it's very likely that some DNA samples from five years back can be recovered anyway.

That, plus PCR amplification, would show that either he's being time-shifted five years in the future, or something did a really weird number on his telomeres.

Conclusion

Probably nobody would believe in a starship, and people would rather assume some strange time-warp effect - the time equivalent of Tay al-Arz - or some "Bermuda Triangle" time-stasis, or ultra-black government experiments. I bet lots of people would start combing the disappearance area looking for clues, measuring radioactivity, magnetic fields, and wearing tinfoil hats and dowsing rods...

But the fact that he somehow skipped five years of his life would be quite hard to deny.

Answer 8

He took his dog with him on the camping trip, and the the dog experienced the same time dilation that he did. The dog happens to be AKC registered with DNA on file. It's also about one year old, has some distinctive markings, and is microchipped.

A picture of the dog pre-abduction and a quick trip to the vet would establish that it's likely the same dog. Dogs at one year old still look and act like puppies. The difference between a one-year-old dog and the same dog at six years would be striking and obvious to anyone who works with dogs. Most vets and animal shelters will have microchip readers on hand.

If a greater degree of proof is needed, someone could contact the AKC to do a DNA test based on the sample that they have on file.

In principle, the student's pet could be a cat instead of a dog. Or a horse which has been registered (horse camping is a real thing).

Answer 9

The question is how can he prove to the people in 2023 that he has time traveled, against other possibilities, such as running away from school.

His family will know he time traveled. He won't show any signs of aging and will match the missing person photo from years ago, but his family will know it's him.

It will be so obvious that people will have a harder time accepting the truth then accepting it's a lie.

It's also the same plot from the movie "Flight of the Navigator". A boy disappears in 1978 and returns home in 1986. His parents and brother have all aged but he hasn't.

https://www.imdb.com/title/tt0091059/

Answer 10

DNA Telomers, or a (still) fresh wound

This requires a tiny bit of handwavium, and one less fantastic assumption. The assumption is simply that it is known that this person has been unexpectedly missing for 5 years.

Telomers are your cells' countdown timer

DNA Telomers are...

...a region of repetitive nucleotide sequences at each end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Its name is derived from the Greek nouns telos (τέλος) "end" and merοs (μέρος, root: μερ-) "part". For vertebrates, the sequence of nucleotides in telomeres is TTAGGG, with the complementary DNA strand being AATCCC, with a single-stranded TTAGGG overhang.1 This sequence of TTAGGG is repeated approximately 2,500 times in humans.2

In humans, average telomere length declines from about 11 kilobases at birth to less than 4 kilobases in old age, with the average rate of decline being greater in men than in women.

The handwaving is that we can measure the length of telomeres.

From this is is easy: we get a hair brush, old clothes, dust and similar from your protagonists's old apartment and/or items that were stored after their disappearance. Best of all maybe an adhesive plaster that was discarded not long before their disappearance.

Then we do a comparative analysis, measure telomer length from the old items, and compare to samples we pick freshly from the protagonist. And from that we will then find that the telomers of the person before us are not five years shorter, as they should be.

The really boring, author ex machina way...

Your protagonist had an accident a couple of weeks before disappearing, and either broke something, or hurt themselves in a way that left a very clear wound somewhere that needed hospital work. This is noted in their medical journal.

When they then make their reappearance, their physician concludes: no way that this wound — that I know I treated — is five years old. Look, we can even see the last remains of the polyglycolide sutures. Or the cracks in the bones that have not fully healed. Either someone went through extreme measures to mimic this wound five years later, or the wound matches a timeline that suggests a five year lap forward in time.

Answer 11

He had a box of individually packaged twinkies - or any other individually packaged snacks, especially those that decay quickly - with him, and some of them are still left over. Since they are still fresh, still originally packaged but by expiry date should have decayed years ago, something timey-wimey must have happened.

Of course it's not a definite proof, a very determined person might have managed to acquire old twinkie packaging, put fresh twinkies into them and resealed them somehow, or just removed the original expiry date from a fresh package and added a new label. It could still suffice to convince friends and family.

The good thing about this solution is that it's absolutely non-technical, i.e. you can understand the proof just by looking at it, and it's plausible, because, well, students tend to eat a lot of snacks, so it's pretty much guaranteed that he brought some on his camping trip.

Answer 12

Scenario:

It was not he, but she. And she was pregnant with quadruplet. All of them had extremely rare genetic mutation. She was pregnant for such and such time.

When she returned her pregnancy was still on the same stage, she still had quadruplet inside her and said quadruplet had the same extremely rare genetic mutation.

Answer 13

@Kenster brings up a good point with pets, but most of those animals are simply too large.

Rodents, such as rats, have short lifespans - Somewhere along the age of 2-ish years if I remember correctly. Rats, in particular, are prone to getting tumors when they are older. They're generally benign, and with some a vet will recommend against removal - Surgery would be worse for the rat than simply monitoring the tumor.

A pet rat is also easy to bring along wherever, and easy to bring along on an accidental spaceflight. People carry them in their pockets all the time, while a dog or a horse is far less likely to be able to be accidentally brought along on an alien spaceship.

To prove that the rat is the same rat, we'd need to have a rat with distinctive markings, a distinctive tumor, Vet records of said tumor, and some pictures of the rat from the time of leaving.

At this point, it's fairly simple to go "Hey, this rat was 18 months old 5 years ago, and was taken to the vet about this tumor on this date. Here's a picture taken at that vet's office on that day, showing the vet, the rat, and the tumor. We've tracked down this vet and they say it's the same rat - And no rat lives 6 1/2 years." It proves something funky is going on, for certain.

Answer 14

Two really basic questions to address are:

Does he have any living relatives who would remember and recognize him?

Has he ever been fingerprinted or have he or any close relatives been DNA tested?

Also the hospital where he was born should have his footprints on file.

The OP didn't spell it out but I'm assuming he wasn't carrying any form of legal identification. Since his phone, if he has one won't be working anymore due to nonpayment, I'm not convinced that the time will be synched to the network.

Answer 15

Dental records will show that his body hasn't aged.