Now let me start off by easing the rants of the haters out there that would be willing to send me forward to meet the Morlocks, or back 65 million years to meet a gruesome fate at the (small) hands of a T-Rex- I love me some time travel in my Science Fiction. Additionally, I am a super huge geek fan of the Back to the Future trilogy. So don't mistake this as saying time travel as it relates to fiction is pointless, because it isn't. It has its place, and it would be a sad day if we never spoke of it.
But let's talk about reality here, folks. Time travel just isn't possible. Why, you ask? I'll lay it out for you in three simple points: 1) Time does not exist in nature, 2) Time is not a physical entity, and 3) What time represents is linear in nature, not a location.
So what does this all mean?
First, let's talk about what time really is. As much as time is second nature to all of us starting at a very early age, time is a construct of humans. That's to say that it doesn't exist in nature – you can't go out and dig up a bucket of seconds, and there are no trees deep in the Amazonian Rain Forest just waiting to have their month fruit harvested.
What is time then? Simple. It's a measurement - but a necessary creation as we could not measure what it represents with any device or standardized unit. For example, I can't tell you that I had a hamburger 4 feet ago, nor could someone tell you that you have a doctor's appointment 6 liters from now – neither make sense. So time is a construct (more on the fallacy of this term in my next point) created by humans – seconds, minutes, hours, and so on – in order to measure the interval between two (or more) events.
As I mentioned above, time is a construct. But that term implies that it's something physical that we created. That's simply not true – time is a logical non-entity; that is to say that I cannot reach into my pocket and pull out a handful of hours, and I can't go to my local hardware store and buy a year or two (as much as I would love to). So time is not physical.
My final point is this – time does not represent a location. As explained above, time is a logical, non-physical, man-made construct that is only linear in nature. Time never cycles backward, nor in any cyclical sense – ever – it only moves forward. Sure, we can represent what we call the "past" on a timeline, but time itself only moves forward.
So back to my original question – what does this all mean?
Well, let's put it together. If time is something that we created, is not physical, does not represent a location and is uni-directionally linear – how exactly would we be able to traverse a construct such as this in any manner except forward in the interval that naturally occurs? The simple answer is: you can't. If it doesn't really exist, except in our minds (and physically represented on devices that we have created to do such), and it's not somewhere we can physically go – then it's not possible to travel to that destination.
Now, there are theoretical physicists that will tell you that if we had access to dimensions outside of the ones we all know and love, that we could achieve some sort of temporal displacement (thanks for that cool terminology, Doc!) aka time travel. In essence, that is (to me) the equivalent of saying "hey, let's suspend reality for a moment and say that 2+2 is actually not equal to 4". I can say anything I want, but that doesn't make it true, and that certainly is the case here. While we can talk extra dimensions all we want, there simply hasn't been any evidence that any exist outside of our own 3 (and some nay-sayers would say that we don't even have 3 of them).
There is also the well-known theory by a Mr. Albert Einstein that says that time slows as we approach the speed of light – but only for the object near the speed of light.
But is that truly time travel?
Not hardly. Sure, we've managed to slow our "time" due to our relativity to the speed of light, but we haven't really traversed the timeline to reach a point in time without moving forward linearly – nor do we have the possibility of moving backward to our previous point on that timeline.
So while the dreamers will dream of visiting famous events and people of the past, and SciFi authors will concoct wild stories of time traveling heroes - time travel will remain just that – the stuff of dreams and fiction.