[Pure Neutronium (OP)]

...is it reasonable to assume time-travelers must now be amongst us?

[WiredScientist]

We have create time machine since 2001, or 2000. Since the scientist make the mini black hole on SPEARS, the particle accelerator.

In interdimensional means, time travel are possible through Astral plane. Just google it, astral plane/astral projection. Im too lazy to explain it here, it will be miles of typos [xx(]

[BenV]

We have create time machine since 2001, or 2000. Since the scientist make the mini black hole on SPEARS, the particle accelerator.

Prove it.

In interdimensional means, time travel are possible through Astral plane. Just google it, astral plane/astral projection. Im too lazy to explain it here, it will be miles of typos [xx(]

Also, it's not really appropriate for a science forum.

[Chemistry4me]

This should be interesting...

[graham.d]

In the spirit you ask the question I would suppose the answer would be "maybe". There are a lot of assumptions though. Time travel (at least back in time) is not possible as far as we know and would either lead to logical paradoxes or need some concept of multiple parallel universes. Some recent ideas suggest that going back in time maybe be possible, but only a very small amount and that, even then, it would take an unreasonably huge amount of energy. Then you have the mundane problem that if you maintain your spacial position (relative to something) and go back in time, would you be on the earth or somewhere floating in space.

I guess, even if these problems were soluble, how many people in the distant future would have access to the equipment and the interest in going back in time, and would they be interested in this period? It might be unlikely just on the basis of statistics.

[neilep]

Isn't there a theory that states that you can not go back in time earlier than the time you activate the time machine ?  ie: You activate the machine Sunday Aug 9th but you can not travel back to Saturday August 8th ?...only in the future can you travel back to Sunday Aug 9th !!?

[graham.d]

I seem to remember that concept appearing in a sci-fi book but I can't think what reason there would be for such a restriction. I mean there are plenty of huge reasons why backward time travel is unlikely, but this seems a bit artificial. I think the reason may have had something to do with going back in time to a point where the time machine had not been invented but then publicising "your" time machine invention. But I don't think this is any more of a paradox than numerous others that can occur.

[neilep]

Could it be something to do with ' how do you get back ?'...unless you can travel with your time machine how can you activate it in the past (before it was invented) to bring you back ?

[graham.d]

I assumed that the machine was portable and travelled back with you. But if it is a sort of "transporter" with a send and receive station or a fixed installation that just moved its contents through time, then you could be right. Maybe that is the reason for the idea.

[LeeE]

Going back in time within our time 'stream' doesn't necessarily have to result in paradoxes as it could simply be viewed as a 're-winding' of the sequence of all events that occurred from the point where you return to, up to the point where you started going back.  Everything thus remains consistent.

What most people think of with respect to time travel requires the journey back to be made outside our time stream, so that locally, the time traveling person's stream either just halts or continues going forward whilst, most importantly, without un-winding their history.

[graham.d]

But if no paradox then you have parallel universes don't you? If you go back in time and kill your father before you were conceived, then either there is a paradox or, the future is from that point on, a different one from whence you came. Given there seems no limit to the extent whereby the future could be changed, it means that there must exist many co-existent universes. The concept of going back a very small amount of time (if time quanta ruly exist) may be possible, but this is not the spirit of the enquiry.

[lyner]

But there are plenty of paths,worth considering. which could involve backwards time travel without killing your grandfather or other paradoxes.

[DoctorBeaver]

There is no need for paradoxes nor alternative universes. The past has happened & is immutable. If you did go back, you didn't kill your grandad. Even if you went back with that intention, something had stopped you. Where's the problem?

[graham.d]

You would be suggesting a complex set of criteria that would prevent you from doing anything that would alter the future in any way you could subsequently recognise. And, what is more, these protection systems must have been put in place in advance of your possible return and at every possible time that you may return to. This would be unlikely to the extent of being impossible. I know Dr Who manages it but then he does not have real physics to contend with :-)

[DoctorBeaver]

graham - not at all. There is no need for anything to have "been put in place". What happened in the past has happened. It is cast in stone. Nothing can alter it. The very fact that a time traveller is alive and able to travel back in time is enough to show that he didn't manage to kill his grandfather.

[graham.d]

Yes, but you are saying that, no matter how much he tried, he could not kill his grandfather. Now whilst you may think that it is just about possible that there could be events that prevent him from doing this no matter how much he tried, it seems unlikely that all such future changing events (perhaps much less dramatic than murder) from any arbitrary number of time travellers could be so restricted.

[DoctorBeaver]

You say it seems unlikely, but if he is alive to travel back then that is what happened. I don't see that there can be any argument about it.

[lyner]

You are using the term "what's happened" but, if this guy goes back, for him there's an element of "what's going to happen"  AND "what's happened", too, about the same event.. If he's desperately looking for ways to kill his grandfather, whilst back in 1943, he's thinking that he has good old freewill. But he clearly hasn't, because circumstances are channeling his actions away from certain paths and towards others.

There's a memorable short story in the Spectrum series (Kingsly Amis) called "By his own bootstraps" (or somesuch). In the story, a guy meets himself on the way through the adventure and it all hangs together very well - he is 'channeled' into certain choices, whilst behaving quite normlly.

I don't actually have a particular problem with this because I don't think you need to treat time as a steadily increasing quantity any more than distance. I have an idea that the only reason that we appreciate time with an 'arrow' is so that we can cope with the complexity of things. 'Experiencing' things one at a time makes life much easier.

[DoctorBeaver]

You are using the term "what's happened" but, if this guy goes back, for him there's an element of "what's going to happen"  AND "what's happened", too, about the same event.

No there isn't. Although he is now experiencing it as part of his present, it is really a part of the past. If we accept that was has happened is now immutable then it is obvious that nothing can be done to alter it. Paradoxes then become irrelevant as they cannot exist.

[graham.d]

Think about this mathematically and in extrema if you like. Assuming there is no restrictions on who or how many people can travel back in time and to when they can travel back, then extrapolate this to everyone at every conceivable time. This gives an infinite number of possible events that could be changed at an arbitrary large number of occasions in history. The only way to avoid such events being tampered with would be if such time travel were prevented by nature, otherwise I would say that the result of going back in time would be accompanied by an infinite set of caveats to prevent an individual having any free will whatever. I would therefore conclude that the likelyhood of such a system is confined to the less plausible end of science fiction. The parallel universe system is, at least, logically self consistant, although there are some difficulties regarding the physics.

What you are saying is logical to a point. If I had a time machine I could never go back and meet myself in the past because I have no memory of having met my future self, which would be a paradox. But the mechanism by which nature should prevent such actions, when taken to every possible case, is simply that it cannot be possible to go back in time. The reason being that a paradox results. To say that you can go back in time but that nature somehow intervenes to avoid specific paradoxical situations seems, at least to me, impossible.