My favorite "layman's" argument for why time travel, as it's been classically described in science fiction, can't exist is that we haven't been overrun with tourists from the future.
I'm sorry I cannot give a better cite than this, but sometime in the last 20 years there was a terrific short story, probably in Analog (but possibly in Asimov's) about the first time traveller.
His first shock was when he found that the person in the past they selected to visit, Shakespeare, did not seem at all surprised when visited by a time traveller. In fact, he seemed to find it routine. The traveler at one point says he doesn't understand how Shakespeare can already know about time travelers, since he is the first. Shakespeare tells him that he may have been the first to leave, but he wasn't the first to arrive.
Shortly after, many time travelers arrive...but not to see Shakespeare. They are reporters, from all throughout the timeline, coming to try to interview the first time traveller, and Shakespeare, an old hand at dealing with time travelers, steps in to protect the first time traveller and prevent him from being overwhelmed.
Looks like this thread is gathering the list of good Time Travel stories. So here's one I like: "Dinosaur Beach" by Keith Laumer. A delightfully twisty and multi-layered novella.
Sure, but CTCs only allow one to travel back to a time when the CTC existed, so I don't think that's a good argument unless there was a CTC-generating device in our neighborhood. Of course, as far as I know, all CTCs require infinities (Tipler cylinders, for example: http://en.wikipedia.org/wiki/Tipler_cylinder ), which is an argument against constructing one...
I've always found this argument a bit bizarre, because in my mind any time travelling device would work via a pair of machines: a sender and a receiver. So nobody could come to the present simply because there's no receiver yet.
The reason we haven't been overrun by tourists because there's no stop in this time period.
>I've always found this argument a bit bizarre, because in my mind any time travelling device would work via a pair of machines: a sender and a receiver.
Were did you get that impression from? Not most Science Fiction or Physics, because neither postulates such a thing.
Actually the only vaguely plausible-ish (though by no means convincing) designs I've heard of for physically realisable time machines work like this. You do some horrendous things with spacetime curvature and you get something that can send you back in time, but no further back than the point at which you constructed your horrendously curved spacetime thingy.
Paul Davis has a short book How To Build A Time Machine which does it this way, if I recall correctly (it's been a while since I read it).
As for physics, you are mistaken. The only kind of "time travel" that has any basis in physics is the one using Closed Timelike Curves (which is what the article is about) and those are limited to the time the CTC was created in the past.
So, in fact, this is a very good reason to reject the argument of "If it is possible, where are the time travelers?".
I hate this argument, not because it isn't correct as far as it goes, but because it's very often represented as much more broad than it really is.
That is, yes, we can be reasonably sure that time travel mechanisms whereby tourists definitely could and would become known in our time period will not exist (even this isn't the same as can't exist). But this is a very small sampling of time travel scenarios you could imagine, and I think it's far from the only thing described 'classically' by science fiction.
And my favourite "layman" counter argument to that is that plenty of time travel theories only allow travel as far back as to when the time travel machine was first created.
It strains credulity to believe that we do in fact have time travelers living among us and 100% of them have managed to completely escape detection. For what it's worth, there actually have been serious attempts at determining whether there are time travelers living among us, including at least one study[1] looking at Twitter for evidence of time travelers tweeting
Who says they have escaped detection? There have been alleged time travelers like John Titor. Additionally, Titor and some other claimed time travelers have said the many-worlds hypothesis is correct, meaning that own history and experience does not necessarily provide them with the ability to make accurate predictions about our future.
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.
To be clear, I don't actually believe there are time travelers living amongst us, but studies of Twitter posts are very far from being conclusive, and there might be very good reasons why time travelers do not or cannot predict the future or just aren't very common anyway. For example, perhaps time traveling technology is legally controlled and extremely expensive, like nuclear weapons. The future of humanity is not infinite, so there's no guarantee that even if time travelers exist they should amount to any substantial number of people visiting any substantial number of times for trivial reasons like time travel conventions.
> 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?
Consensus seems to be that the photo is actually real, just not so out of place. From the Wikipedia page "Time travel urban legends" [1]:
> Further research suggests that the modern appearance of the man may not have been so modern. The style of sunglasses first appeared in the 1920s. On first glance the man is taken by many to be wearing a modern printed T-shirt, but on closer inspection it seems to be a sweater with a sewn-on emblem, the kind of clothing often worn by sports teams of the period.
Not sure how one can confirm the photo is actually real, it's so easy to fabricate picture nowadays and to add noise to make it seem like it belongs to the original. It's be interesting to actually have an in-depth article on the methods to detect fake pictures, I have seen a couple on HN but they only focused on a few examples.
