Is time a dimension the way the three dimensions of space are? Are the past and the future places we could travel to if we had the technical means of getting there?
I don't believe so. I believe all we can retain of our past is our memories and recordings of events that once took place and have now vanished forever. Likewise, the future consists only of our mental models, our anticipation of events that may one day occur. We can rationally anticipate the arrival of tomorrow morning. But it does not exist now.
We live in the Eternal Now.
The Eternal Now is a tiny sliver, a standing wave, an event that occurs and vanishes, and is then replaced by new events in a linear fashion. We anticipate it, it's here, and then we remember it.
If this is true, there is no question of traveling backwards into the past because there is nothing left to travel back to. There's no there there. And the only time travel we can engage in is the one we've always engaged in: To the future one second at a time as each succeeding second obliterates its predecessor.
Einstein says that space and time are inextricably linked in our vast universe of space-time. Some physicists believe that if we enter a region of the universe in which the space-time continuum is strongly warped, perhaps near a black hole, we can then travel far into the future or even into the past in apparently little time.
Traveling near the speed of light definitely affects the flow of events, slowing them down, nearly stopping them, in comparison to the flow of events in normal space-time. However, this effect still forbids any information from traveling faster than the speed of light. Why is light speed so important?
The speed of light is nature's ultimate enforcer of limits. It prohibits any bit of information to travel faster than it does. As such, it delimits events. No event can be perceived outside of its "light cone." No event can affect anything outside of those limits.
If a star goes nova 2,000 light years away from Earth, no one will be able to perceive it, not even 1,999.999 years from now. We must wait until the 2,000th year for astronomers to pick up that light wave. Just so. No future grandfather as a young man could ever receive any information about his future grandson well before he is conceived. The grandfather paradox can never occur because no homicidal grandson could ever breach that light cone before its time.
Seen as a series of unreplicable, effervescent events, periodic, resembling one another, but not identical-time dissolves under close scrutiny. It isn't a dimension at all, the way height, width, and breadth are. As such, there is no question of "traveling" along this jumble of eventful causes and effects. There is no highway there. Nothing upon which or against which we may move.
We are misled by the regularity we perceive in events. The ancients believed time was cyclic, that we would "travel" back to where we started in a cyclic age. We now know this isn't true. Events promulgate novelty, not sameness.
We use clocks to tell time. They actually create tiny, reasonably regular events, in the form of hand movements caused by the loosening of springs; the regular movement of water or sand through a small aperture, the sweep of shadow on a sundial, the swing of a pendulum, or the changing of light patterns forming numbers on a small screen at predetermined intervals. When they "tell time," we simply note what event is taking place by looking at the clock. We learn that it is 1:47:33 pm on August 21, 2009, for example. But none of those events measured by a clock are ever actually the same event.
When we repeat the old maxim about not being able to turn back the hands of time, we don't mean we can't fiddle with the inner or outer workings of a clock. We actually mean we can't travel back to that point in which the clock reported that it was 12:00 noon on August 21, 2009.
We are embedded in our own space-time event. At any given micro-second each of us will be found in a very specific three-dimensional place. Location + Duration = Event. We are events. We have a specific physical size and we last for a specific amount of time. Remove us from our space-time event by means magical or technological, and you will disrupt the space-time continuum. At the very least, this would violate the law of conservation of energy.
Will our advancing technology ever permit this to happen? Perhaps. But it seems much more likely that after we leave our universe we will enter some parallel universe at some specific point in time congruent to our own. This particular event may appear to be "somewhen else" in our own past or future, but it really won't be.
Or perhaps when we attempt to travel through time, we would actually cause some sort of space-time shockwave that will generate a new universe, branching off from our old one at that instant.
Did our own universe result from a similar attempt to tamper with time? I doubt if we could ever figure out how to answer such a question.