> There have been alleged time travelers like John Titor
John Titor is the only one I've heard of, are there others? And Titor's predictions, even the really short-term ones, were so off the mark that there's no reason to think it's anything other than a prank. Note that even the many-worlds hypothesis would have the short-term predictions coming true and only the longer-term predictions becoming more and more inaccurate (more importantly, any differences should be due to his presence, and his presence did not e.g. cause CERN to fail to discover the basis for time travel in 2001).
> Additionally, we don't actually have any way of detecting time travelers if they decide not to do silly things like leak future information on Twitter.
We don't have any way of detecting time travelers if they behave in a fashion indistinguishable from people who aren't time travelers. But it's pretty safe to assume that's not the case. They are, after all, from the future, and had a reason for coming to the past. That reason alone should cause observable differences in behavior. In addition, knowledge about future events should be detectable in some fashion, their lack of a real fleshed-out personal identity. Heck, even just their manner of speech should give them away.
> For example, perhaps time traveling technology is legally controlled and extremely expensive
We'd still see people breaking the rules, and such people would in fact be far more likely to attract attention. We live in a world where teenagers have successfully built nuclear reactors; no matter how legally controlled it is, someone is going to figure out how to do it anyway. If it's a matter of cost, some billionaire would figure out how to build it even without government approval. If you had billions of dollars to your name, and time travel was real, wouldn't you be tempted to build a time machine no matter what the government says?
---
As far as I'm concerned, the only two really valid reasons why we haven't seen time travelers are:
1. Time travel is impossible, or
2. Time travel is possible but you can only go back as far as the point where the machine is first switched on.
> We live in a world where teenagers have successfully built nuclear reactors; no matter how legally controlled it is, someone is going to figure out how to do it anyway. If it's a matter of cost, some billionaire would figure out how to build it even without government approval.
How many people build nuclear bombs? Or supercolliders? Nuclear reactors are an altogether different thing, particularly if you're not concerned about producing surplus energy. Despite the fact that the principles behind nuclear weaponry are known, even few governments have (independently) built them and they are very strictly controlled. I can easily imagine the engineering required for time machines is even more complex, the materials as hard to acquire, the energy expenditures prohibitive, etc. And if time travelers could change history, then they would surely be regarded as incredibly dangerous superweapons, not something Elon Musk-types can just build and hop into on a lark. Indeed one could even imagine a future in which time machines are feasible but there's a "MAD" principle at work keeping anyone from building one - so if any were built, they would be kept top secret and time travelers would act with as much secrecy and delicacy as possible.
To me the idea "we have not seen time travelers; ergo this class of time machine cannot exist" is a failure of imagination. Other reasons I can imagine: human civilizations never devote the necessary resources to engineering a time machine; some social collapse or stagnation prevents technology from progressing beyond some given point; humans or human civilization is wiped out before the invention of time machines by any number of social or natural disasters; it is more difficult to travel further back in time, so nobody goes this far back, though they can go before the creation of the time machine; the singularity happens and post-singularity transhumans don't care about traveling to pre-singularity times; etc.
> And Titor's predictions, even the really short-term ones, were so off the mark that there's no reason to think it's anything other than a prank. Note that even the many-worlds hypothesis would have the short-term predictions coming true and only the longer-term predictions becoming more and more inaccurate (more importantly, any differences should be due to his presence, and his presence did not e.g. cause CERN to fail to discover the basis for time travel in 2001).
I don't believe Titor was a time traveler either, but you can make reasonable arguments about why his predictions were incorrect: his presence changed the future in unanticipated ways, the MWI model of time travel doesn't actually send you to a world that branches off from your arrival time, CERN actually did discover time travel, etc. I don't find any of these plausible but I am also not willing to categorically assert we have no evidence of time travelers or that an analysis of Twitter is even weak evidence that time travel is not possible.
> How many people build nuclear bombs? Or supercolliders?
How useful are those to individual people? Nuclear bombs are basically only useful as a deterrent (or as leverage, e.g. making demands in return for halting your nuclear program), and that only works for countries.
Supercolliders are also incredibly specialized and only useful to a small group of scientists, and there's zero incentive for an individual person to attempt to build one. What use would a private person have with a supercollider?
> Despite the fact that the principles behind nuclear weaponry are known, even few governments have (independently) built them and they are very strictly controlled.
Sure, because of how the world reacts to countries attempting to run their own nuclear bomb programs. And because of the actual low utility value in having nuclear bombs, as covered earlier.
> I can easily imagine the engineering required for time machines is even more complex, the materials as hard to acquire, the energy expenditures prohibitive, etc.
Ah, but the upside is so much higher! Not only is it vastly more useful than a nuclear bomb, but it's also useful to individual people, as opposed to just useful to countries.
> so if any were built, they would be kept top secret and time travelers would act with as much secrecy and delicacy as possible.
They only need to act with secrecy in their home time.
> They only need to act with secrecy in their home time.
How would the USA react if, say, it discovered Russian time-travel agents from the future in our time?
> How useful are those to individual people? Nuclear bombs are basically only useful as a deterrent (or as leverage, e.g. making demands in return for halting your nuclear program), and that only works for countries.
The only reason it only works for countries is because only countries have the resources to build nuclear bombs. There are certainly other groups and organizations that wouldn't mind using them as deterrents, or companies that wouldn't mind selling them to the highest bidder.
Even if they were easy to build though, they'd still be dangerous superweapons. Like a time machine.
> Supercolliders are also incredibly specialized and only useful to a small group of scientists, and there's zero incentive for an individual person to attempt to build one. What use would a private person have with a supercollider?
Perhaps they're a component of a time machine. :) My point is even if supercolliders were not useful only to a few people, they are incredibly difficult to design and build regardless. Bill Gates or Elon Musk can't just tspend some money and get one. They would have to invest large sums of money and hire legions of engineers and scientists. We don't know that a time machine isn't similarly extremely difficult (indeed it seems more likely that it is extremely difficult to build.)
I can't emphasize the "time machines are superweapons" enough. A time machine might be useful to average individuals but you'd be insane to ever let them have their hands on one. It would effectively be suicide for everyone involved. Why would you ever let anyone have one?
John Titor is the only one I've heard of, are there others? And Titor's predictions, even the really short-term ones, were so off the mark that there's no reason to think it's anything other than a prank. Note that even the many-worlds hypothesis would have the short-term predictions coming true and only the longer-term predictions becoming more and more inaccurate (more importantly, any differences should be due to his presence, and his presence did not e.g. cause CERN to fail to discover the basis for time travel in 2001).
> Additionally, we don't actually have any way of detecting time travelers if they decide not to do silly things like leak future information on Twitter.
We don't have any way of detecting time travelers if they behave in a fashion indistinguishable from people who aren't time travelers. But it's pretty safe to assume that's not the case. They are, after all, from the future, and had a reason for coming to the past. That reason alone should cause observable differences in behavior. In addition, knowledge about future events should be detectable in some fashion, their lack of a real fleshed-out personal identity. Heck, even just their manner of speech should give them away.
> For example, perhaps time traveling technology is legally controlled and extremely expensive
We'd still see people breaking the rules, and such people would in fact be far more likely to attract attention. We live in a world where teenagers have successfully built nuclear reactors; no matter how legally controlled it is, someone is going to figure out how to do it anyway. If it's a matter of cost, some billionaire would figure out how to build it even without government approval. If you had billions of dollars to your name, and time travel was real, wouldn't you be tempted to build a time machine no matter what the government says?
---
As far as I'm concerned, the only two really valid reasons why we haven't seen time travelers are:
1. Time travel is impossible, or 2. Time travel is possible but you can only go back as far as the point where the machine is first switched on.