I've got it all figured out.
That "third" dimension called "height" is simply a direction that's perpendicular to all the other dimensions you know of (length and width). Surprisingly, time is the same way. It's a fourth dimension (i.e. a fourth direction) that's perpendicular to length, width, and height. The only way time is different is, we can't move freely in it; we're moving along the line known as "time" with a mostly constant velocity that only changes when we change our velocity in the first three dimensions. But other than that, time is a line just like width is. Move three feet to your right, and the universe around you appears different; move ten seconds into the future, and the universe around you appears different.
Your range of motion in length, width, and height represents everywhere you can go in the first three dimensions; this is known by you as simply "the world": full movement along those three dimensions represents everywhere you can go in space--i.e. "the universe".
Tack on that fourth dimension: your range of motion in space
and time represents everywhere you can go in the world, in your lifetime--in other words, your entire history. True, you don't know how that history will unfold, because you haven't moved far enough in time. Just as you don't know what's a hundred miles to your left until you move over there and look.
Now add a fifth line--another direction perpendicular to length, width, height, and time. Basically you're now moving "sideways in time". Remember that the first three dimensions define a universe, and the fourth dimension defines that universe's history. So if you move sideways in time, you move to a different possible history. It's the same universe because you're still in the same first-three-dimensions, and for that reason the laws of physics are still the same, but you're at a different point in this fifth dimension. Maybe McCain won in 2008, or Saddam Hussein was a democratic President and all-around nice guy, or the Deepwater Horizon oil well never exploded. But this universe still contains John McCain, Saddam Hussein, and the Deepwater Horizon oil well. (Even though Saddam Hussein is dead, remember, he
does exist between the two points in spacetime known as
Al-Awja, Iraq, 28 April 1937 and
Kadhimiya, Iraq, 30 December 2006)
The universe is the same, and the physics are the same. And all those other possible histories don't actually exist; they're simply different viewpoints of the same spacetime. When you move three feet forwards, you're not walking into a parallel universe, and you're not moving "into another dimension". You're just moving to a different point in the same universe. Move sideways in time, you're simply seeing the same universe from a different angle.
Disclaimer: BasketCase is not responsible for your medical costs if your brain explodes. Though it sure would have been nice if he'd warned you
BEFORE you actually read the post that could cause your brain to explode.....
Edit: oh, and if you're wondering why time travel is impossible? No two objects can occupy the same point in space; the reason you can't go back to five minutes ago is, there's an object already there--you. Try to go back five minutes, and you'll bump into your past self same way as you bump into a wall if you try to occupy the space it does.
