I’ve enjoyed a lot of time machine movies, especially of late, and have finally put some thought into this idea of time travel. I’ll try to keep this concise so please stick around. I’m pretty sure you’ll find it gets interesting:
The standard Hollywood idea is that scientist shall invent a vehicle for traveling through time; literally transporting an object (such as a human) from one point in time to another. Now, we don’t even need to understand Einstein’s or Sagan’s explanations of the nature of time to understand that this particular scenario is never going to happen. We need only be observant and contemplative in our lives, exercise our brains enough to access powers of logic, and be honest enough to want the truth instead of the easy half-logical routes to the answers we want to hear.
Given all of that, you would understand that a “person” is not precisely a factual physical solitary thing. A person, of course, is a giant community of interacting microorganisms, organs, tissues, chemicals, liquids and whatnot in a non-closed environment which is constantly interacting with its surrounding environment and not only physically (such as intake and outtake of oxygen and other atmospheric components and exchange of energies) but causally. As a human I am exchanging with my environment, a combination of materials and cause-and-effect transactions averaging a rate of – I actually have no idea – let me guess, oh – a combined thousand of various unit types per second maybe? Ten thousand? A hundred thousand per second? More? Everything tiny thing we do is irrevocably connected to every other imaginable minuscule component in the universe either directly or by any number of degrees – even if some exponential trillions of degrees.
The universe at any given time cannot be what it is without “you” being in your place and doing what you do physically and causally.
To transport a person to a different time, Hollywood style, is to make enormous calculations to gather up all of these constantly-interacting and constantly-interchanging materials and energies in the precise configuration which we think of as a person and remove them all simultaneously from the equation of the universe at a given moment and re-insert them all into the equation of the universe at a different moment, which is, in effect, a different universe.
The problem is that, understanding causality (and oh – what a criminally negligent job they did of teaching causality at my Catholic grade school and presumably all others – if we could time travel I’d go back and ring their necks), the number of changes you are actually making to the universe(s) is for all intents and purposes… infinite.
You are literally, upon sending “someone” to a different time period, dismantling a universe and reconstructing another one. And calling that time-travel is no different than tearing down an office building, building a new office building in its place and calling it time travel. It would be just as absurd.
Also, because the younger universe in this scenario is basically responsible for the older universe (through a number of causal transactions with probably more zeros in it than would fit in all the computers on planet Earth combined), you’re creating a circular reference loop like the error you get on your excel spreadsheet when you do something that can’t be done.
Now here’s the awesome news in case you haven’t pondered this before:
What about, not traveling through time, but just seeing through time?
It seems to me that looking into the future will be extremely hard to accomplish because you need a computer or network of computers (or some future calculating/reporting device) that understands every component of reality, and of causality, and is capable of tracking all such components at a minimum speed just ever so slightly faster than things actually happen, and it will be able to accurately predict and fully display future events. There you go!
The problem is that the faster the fancy computer gets the faster it needs to get because it can not possibly help but be an element of the very reality it monitors and thus it must be faster than itself. So maybe that’s a serious problem! I'd love to hear from someone who sees a way out of that.
But get this! Looking into the past is much less a problem. In fact it's a breeze. You’re doing it now. What you see is the result of light that has travelled from the objects of concern into your eyes and that process took time. Everything you ever see has always happened in the past.
Are you disappointed? Are you thinking, “Screw that. I want to see back to when Kennedy got shot and see if I can glimpse any clues as to all these conspiracy theories!”
Well, that is not necessarily impossible. As long as it was not too cloudy a day. It's just a matter of scale. You just need to get really far away from Dallas Texas so that the light hitting your eyes at your viewing place would have originated on November 22,1963. In other words you need a telescope with a 52 light-year range and a vehicle that can take it 52 light-years into space, in an appropriate direction (there's plenty of wiggle room) which was free of obstacles (including clouds) on that November day, and you will witness Kennedy's assassination first hand.