You're still missing what I noted in my final paragraph.  Having the coordinates alone is simply not good enough.  You'd also need to travel to that new location immediately.    That kind of immediate travel would require a form of worm hole. 

So we don't just need the ability to travel in time but the ability to generate a worm hole at will and between any two points.   To really compolicate this, you'd have to generate the receiving end of the worm hole in the new time period.

Let's really look at this.  You need to begin a time travelling session.  Then you open a moving worm hole entrance (it must be in perfect sync with Earths own vector or it would immediately get left behind).  Now you generate the receiving end worm hole in that new period all moving at the same vector of the receiving end.    If your calculations are off even by even a plancks length, you're dead.

Travel 1 year into the future or past and you'll need to offset your wormhold location by 7.2 billion miles and the receiving wormhole must be moving in sync with Earth at ~820,000 miles per hour with perfect rotation to the axis, sun, galaxy, etc... Want to travel 1,000 years into the past, 7.2 trillion miles of displacement from origin.  To say nothing of the fluctations in the vectors as affected by trillions of gravitational sources plus the rate of universal expansion caused by dark matter.  And ALL frames of reference are also moving so you'd have nothing to calculate your new location against.

Some of the responses on here are awesome! As far as a true answer, Troll Whisperer has it down pretty good. Some concepts to consider in a discussion such as this are a) humans don't truly understand time b) time is not necessarily constant and c) we probably can't time travel back to our own past. It's a pretty far out concept that often becomes more philosophical than scientific.

What is time exactly?

Time travel, eh?

It's possible in the forward direction for sure. We have measurements confirming that much. Backwards? It's theoretically impossible, at least for macroscopic objects (larger than a molecule) traveling large stretches of time (longer than a nanosecond).

That doesn't necessarily mean that it's impossible, but that our theories about how the universe works need to be wrong for us to do it. Personally, I expect that time travel isn't possible. Time viewing might be (particularly in the past direction) but the universe has been largely self-consistent in our experiences and time travel to the past would challenge that.

That's not to say I think all the classic "time travel paradoxes" hold water. Remember the grandfather paradox, where you travel back in time and kill your own grandfather, thus making yourself invalid? How is that supposed to work? The universe does not concern itself with the future of an object or it's past, so when you travelled back in time you became a valid object in the past. Killing your own grandfather means a parallel you won't be born, but you won't suddenly become an invalid object when your grandfather dies.

I don't think time travel is possible but it needs 2 things to work.

We would first need to be able to build a transporter. That is theoretically possible (by way of quantum teleportation) but it needs a receptor on the end. We would need to build the receptor first before we can receive anything from the future.

Secondly we need to be able to send information to the past. That might be theoretically possible using quantum postselection. http://www.telegraph.co.uk/science/7904712/Quantum-time-machine-allows-paradox-free-time-travel.html It sounds to me a bit like building a quantum computer that can predict the future state of itself, and thus pull future information to the past.

Quantum teleportation doesn't seem compatible with qauntum postselection though from what I can tell, plus it's up to semantics whether accurate prediction is the same as pulling information to the past.



Once we have transporters and we send a receptor to Mars, and we find a method for them to work by way of quantum entanglement (which is not bound by the speed of light) we could in theory travel there instantly. Then from there you can watch your own actions from 20 minutes ago by use of a powerful telescope. You can see youserlf step in the transporter and leave. This leads to a paradox instantly, what happens if you decide to travel back within those 20 minutes. You would see yourself arrive back (since you're looking at past events) before you have actually left. Yet then you still have time to change your mind and not got back.

Edit: nvm on the paradox part, by the time you can actually see your decision through the telescope you would have had to have left already. It's all a matter of what your reference point of 'now' is. You can look into the past, not alter it.

"People like us who believe in physics know that the distinction between the past,
the present and the future is only a stubbornly persistent illusion" Albert Einstein

I don't know almost anything about this subject but to our mind, isn't it time travelling to the future everytime we sleep or are unconscious?

Of course it is. The dinosaurs did it and traveled to Alexander the Great's time as well as a few others. The art from those times proves it.

One of the things I've heard about time travel is that even if it's possible, you can't go back in time beyond the point of the invention of the time machine. So yeah, that's one thing.

"why have we never encountered time travellers"

there needs to be a future for them to "go back" in time. if we haven't reached that point yet, we can't see time travelers.