Normal space is filled with virtual photons that pop into and out of existence. These can be multiplied indefinitely by a time-loop, creating a huge gravitational field that will destroy your machine.

That never happened with mine.

People are setting their sights too high, going backwards in time. Why not go forward in time instead - it's much easier, and most, if not all, people manage to do it in their lifetime.
Just a few matters I wanted to clear up - first, the one about parallel universes - who's to say they exist? WHY should time traveling put you in one? Most models of time traveling I've seen (in real physics, not sci-fi) involve just the one universe.
Second, about the unforseeable consequences that pop up when you go to change something in the past - they're nothing special. There are unforseeable consequences for EVERYTHING someone does, or at least there could be. The problem here is people seeing the past as a different thing - and it may be, for a given value. But today will be the past eventualy, and someone may point to a decicion made today, no matter how large or insignificant, and say that IT had unforseeable consequenses. It's not a difficult thing to keep in mind.
The time paradoxes, with the universe failing. The definition of a paradox is that there is no clear answer - not that there is no answer. The universe does not have to blow up when it comes across one of these paradoxes.
Heh. Back to the question - time travel, I think, is not real and there are several reasons for that.
The first reason, like Majic said, is that we don't have time travelers from the future. Nevermind the other reasons.

Time travel is sort of possible.
See, even though I'm writing this post, I've already posted it.
You've already read it and replied to it.

Hell, I've already been banned.

Basically, You're just remembering this in the future, er... present. Time has already flown by so quickly, you're on your deathbed. You're just remembering this.

I mean, the idea of it is completely possible, and it can't really be disproven.

Seargentbig: people are discussing backwards time-travel as it is the more difficult and interesting direction. Travelling forwards is as simple as either (a) cryogenically freezing yourself and waking up in the future or (b) travelling away from earth at near the speed of light then returning. Before anyone says anything, I know these are impractical, if not impossible, but they're TV-Dinner compared to travelling backwards.

The "we haven't seen time travellers yet" proof against time travel is fatuous when you take into account that pretty much every theory regarding backwards travel necessitates that the time machine must exist in the past for you to go back to it. E.g. you must have one half of a linked wormhole-pair for you to enter the other half in the future.

Then again, a time traveller wouldn't exactly advertise his presence, knowing how messed up that would make the timeline. Sure, any effect he has will only create the future he is from, but revealling your identity in situation like that just just about the worst sort of cultural contamination possible.

I have a book with a few descriptions of theoretical time machines, unlike those we have mentioned here already. I haven't been able to bring myslef to transcribe it all yet, but I see that time is drawing near.

In the meantime, I refer Seargentbig to this old link from the 4Dthread.

Then again, there have been a lot of hobos saying they're from the future.

So it's all revealed now, the future has many hobos.

It starts with me.

And a fine example of cultural contamination you are too.

* Goes back to 1800's *

IM A TIME TRAVELER!

*returns to 2006 *

I just became the worlds first time traveller! Isn't that Amazing!

"People have been able to do that since the 1800's. Get with the times."

Keep in mind that time itself is an invention of man. A perception generated by living in a universe of limitations.

This conversation is confusing.

If time itself is a man-made invention, then this would mean that we could not choose where in time we could go. We could just push a buton and hope we find our century.

well, time is a kind of measurement. No high and mighty diety said "let there be time." people made time up to be able to measure said "time" (minutes, hours, months, years, decades, centuries, millenia, and so on.)
So time isn't an invention, it's a measurement. Kind of like how length isn't an invention, it's a measurement.
It's a really confusing aspect, but graspable none the less.

Time is a measurement as many people have said, so technically a time machine is the slider on the measuring scale. If time machines worked by slidy rulers it'd be cool.
I had a discussion on time travel tday with someone, very enjoyable. I still say that time is free-forming and free-flowing- the reason we don't know of an ytime travellers is because they do and don't exist at the same time- they have lived, are living and do not yet live all at once. Time reshapes itself to keep o in an even flow- if you go back in time and shoot Hitler as a child, you can't shoot him because he's already been shot by you so he's not alie to be shot, so you don't know who he ever was so you don't go back to kill him- basically time moudls itself around you. If you shoot Hitler as a child it makes no difference because he's already been shot, is being shot as we speak and is about to be shot.

It'd be easier to explain with some diagrams. But it's the same with the whole 'go back and kill your grandma' thing. It isn't a paradox because it physically can't happen- you go back in time and kill your grandma, and time merely erases you. It restructures itself and leaves you out. And you can't control how it is retructured, because it has already been restructured in your lifetime because the future you has already restructured it. So in effect you have the case of 'somebody has to alter the timeline because nby doing so, the timeline stays as it always has been. If you don't alter it, history is changed. For example say in fact you are the one who kills/killed JFK- you have to go back in time to shoot him because if you don't, he lives on and you have changed history. But again time flows aroundthe problem, in my theory.
I would say the timeline can only be damaged if you travel forward, because it's creating a change in time that hasn't yet happened- you know when the change to the timeline is coming so time again adapts, sending you down a different path anbd changing FUTURE, rather than HISTORY. as such time IS changed by being redirected, rather than restructured and repaired. Like taking a certain branching path on a complicated spider diagram. It's hard to explain fully without gestures and diagrams so I'll have to leave it at that.

All in all: it all sorts itself out in the end. Except for the congenital diseases you contract from the rampant inbreeding caused by being your own grandfather, but then, there's nothing you can do about that.

Indeed there isn't... except for going back in time to stop yourself from having a "relationship" with your grandmother. but then time would re-write itself to be that you never went back in time to not be your own grandpa because you already.......did?

If you are your own grandfather, then in order to exist at all, you must travel back in time to conceive your parent. Otherwise you won't actually exist in the first place.

Basically, don't do the nasty with your grandma.

Are we all agreed on that?

But it is sorta possible to assisinate someone in the past, without creating a paradox. After assisinating said person, go forward in time, around the same time the time machine was invented, and tell yourself to kill the person.

Unless of course, time immediately reverted to that time line, so you do not know who that person was. The only way you could kill someone was if it was by accident, and it is more than likely that, had they survived, their actions would have prevented you from accidentally killing them anyway.

Time doesn't flow anywhere. And paradoxes can't happen. The mere threat of a paradox is enough to cause quantum vacuum effects that stop the paradox. The paradox never happens. Period.

The paradox never happens because all the relavent events already happened, had always happened, and are part of the timeline. There's just a twist in the universal plot. You didn't kill your great-grandfather, because the person you are descended from the playboy your great-grandmother was seeing on the side. Or maybe you fail to kill him because you miss, or he pulls through afterwards, or another time traveller comes back and revives him, explaining the old family tale of the mysterious doctor who treated your ancestor. There's your quantum vacuum effect. It is simply observed as a mundane event.

Pretty much what everyone has been saying is:
Time travel is impossible, but if it were, we wouldn't really be able to change it massively because going back in time would only create the current world you live in instead of a completely different one because it has already happened.Certain holes and such may happen while doing stuff in the past, but ultimately it wouldn't change the present/future. and going forward in time hasn't been explained because it doesn't really need to be.

you should all read "a brief history of time" by stephen hawking. it´s all there.