Hypothetical: How would spot a time traveller?

You live in a world where time machines exist. Any criminal or state with sufficient resources can travel back in time and screw up history. You are an investigator tasked to identify disruptions in history that would indicate malicious time travel. There exist no objective instruments that can identify why a time travel event has taken place.

The question: How would you know?

The easiest thing would be to scrutinize records of past events and look for instances of obvious anachronisms… Like a train pushing a DeLorean off a cliff. So let’s assume the time traveller was smart enough to conceal himself and his technology. What else would you look for? A motiveless murder? An inexplicable technological leap? A series of stunning coincidences?

Somebody attempting to pass counterfeit money that looked real convincing until they noticed that the date on it wasn’t right.

He will look like a longshoreman.

I don’t think you have a chance unless you suspect someone is a time traveler. At that point you would have to look into their background. Oh, they were born in a hospital that burned down, so no records. How suspicious. Oh, they have a job that requires extensive travel where no one sees them or pretends they’re an intelligence agent or something like that. How suspicious.

Problem is, there are people with jobs like that in real life.

Or build a tachyon detector, and just start mass-scanning.

I would start by looking at a trace analysis of the person’s body or possessions. People who grew up in an era of leaded paint and gas would have higher levels than those who came before or after. There’s things like PCBs and mercury are also possible indicators. I’m told you can tell whether steel was manufactured after WWII because nuclear testing introduced traces of radiation. And we’re just now seeing nanoparticles in things like cosmetics, so I assume those will become increasingly common. Maybe there are traces of carbon nanotubes all over his clothes?

While it’s not impossible to spoof/remove these things with advanced technology, someone who routinely traveled between multiple time periods would have a lot of work to do trying to make sure they always looked like a native.

I’m not quite sure what you intend to exclude here, but carbon dating with a resolution of around a year for recent history is possible. Take a tissue sample from the suspect, and you can determine if he was born in the future, provided no more nuclear detonations that mimic recent history occur again.

Applying Carbon-14 Dating to Recent Human Remains

ETA: If there were a global agreement to police criminal time travel, I wonder how feasible it would be to release distinct (but harmless) levels/types of various non-natural isotopes into the atmosphere on an agreed future schedule, so that people’s time of origin would be clearly identifiable by isotope measurement? And would there be ways to do this that would be difficult to spoof?

I was just trying to rule out some kind of Star Trek device that alerts you to an incident of time travel, so that the investigator has to use nothing but reason and his investigative skills to deduce that a strange event in history must have been caused by time travel.

The method you propose for confirming the suspect is perfectly legitimate.

When some bonehead is stupid-lucky at betting on sports and amasses a fortune, or plays the stock market like a wiz with no education or apprenticeship in managing finances. Or somebody has a really super-awesome coin collection with no provenance. Or is selling lots of curious artifacts on eBay. Or has a Renoir of himself.

So I’m guessing you would have to start with a list of wealthy and powerful people, and then investigate their backgrounds until you found one that looked like pure dumb luck.

Then there’s the other direction. Look at people who mysteriously disappeared, and then see if they resemble anyone from another point in history who did something unusual. Does Alexander Fleming resemble Judge Crater? Jimmy Hoffa look like The dude who invented Coca-Cola (John Pemberton, IIRC)?

Both the immune system and the pathogen population carried by someone born ~30 years or more in the future would look significantly different. We’re in a constant arms race with pathogens, and non-pathogenic immunogenic substances in the environment are also constantly changing. One could purge one’s body externally before traveling, but it would be extremely hard to spoof the entire immune system and pathogen population of somebody born in a different era.

Current biotech could pretty much do this. We would sequence the suspect’s entire pathogen population. For the immune system, I think we’d need high-throughput testing for the presence of antibodies to a large array of potential antigens. Not something do right now, because there’s no reason for it, but if there were motivation it’s within reach of current tech. Of course, we’d also need a lot of control data from the current population.

It also occurs to me that for time travel from beyond ~50 years we can simply ask the suspect to tell us who he claims his parents or relatives are, and sequence them. There’s currently no way around meiotic recombination once per generation. He could claim to not know who his parents are, but this narrows the suspect pool dramatically. Even if we can’t rule out more advanced biotech in the future, it would be a trivial way to massively raise the bar for criminal time travel: it would require cloning or genetic manipulation that’s way beyond anything we can currently do.

How far in the past or future could someone travel and still expect one’s immune system not to be overwhelmed by sudden contact with unfamiliar pathogens? Would there just be something like “time-traveler syndrome,” where you have a runny nose, the trots, and be a little run-down for a couple of days, while you caught up, or could you catch something really bad? One assumes that travel to fourteenth century Europe would require a little fibbing at the doctor in the 21st century beforehand to procure some antibiotics to have in hand, and lots of hand sanitizer and a flea collar probably wouldn’t hurt.

First I’d check all the lottery winners.

Then everyone who got in on SDMB on the ground floor - they are all multmillionaires now.

Do I get to question the suspected time traveler, to try to trip him up? That would be fairly easy. Ask him about popular songs from a year ago, or the plots to last year’s movies or sitcoms. Or who won sports championships last year. A person who was present last year would remember quite a few of those details, but a recently-arrived time traveler would be unlikely to have researched them.

Apparently, I’m a time traveler. (From somewhere around the late nineties, would be my best guess.)

I think this would do it, but in a slightly subtler way; interview someone using a lot of contemporary cultural references and language, and sprinkle in a few deliberate mistakes.
I think with a well-designed test you could tell the difference between someone who’s lived up to 2016 but is not up to date on the latest fads, and someone who’s just arrived and is trying to blend in.

Obviously this method doesn’t work for time travellers who are not trying to blend in (e.g. they say they are from some country that few people in the current country have even heard of, they don’t speak the local language very well etc). So the interview technique needs to be combined with a healthy suspicion of foreigners :wink:

Casually take the suspect out shopping and dining. Watch for evidence that he/she was expecting something to be automated that we commonly accept to be fully manual.