That’s why the past is only in your head, in your memory, in your records. It isn’t a place you can travel to. It’s just the places where things were. All those places that are still here in the universe. And while the past is the sum of all nows, now lasts for no time at all. Because there’s no time like the present, and time needs events, and when you take away the events, you take away the time. A second isn’t some slice of spacetime. It’s just nine billion motions of light from a caesium atom. Accelerate to half the speed of light and a second is still nine billion motions of light from a caesium atom. But there’s only half the local motion there used to be, because the other half is already doing the travelling motion through space. That’s why time dilates.
It’s easy to understand time dilation. Imagine yourself as a metronome. Each tick is a thought in your head, a beat in your heart, a second of your time. If you’re motionless with respect to me I see you ticking like this: |||. If you flash by in a spaceship, I see you ticking like this: ///. If you could reach c and we know you can’t, you wouldn’t tick at all. Your time would flatline like this ______ because any transverse motion would cause c to be exceeded. You wouldn’t tick for me, you wouldn’t tick for you, and you wouldn’t tick for anybody else in the universe.
That’s the thing we’re interested in. The universe. That’s the thing that’s out there, the thing we’re a part of, the thing we’re trying to understand. It’s full of motion, and this is what it’s like:
What can you see? What can you measure? You can measure the height. You can measure the width. And if it wasn’t just a picture you could measure the depth. That’s three Dimensions, with a capital D because we have freedom of movement in those dimensions. What else can you see? What else can you measure? You might imagine a fourth dimension, a time dimension. But the picture comes from the wikipedia temperature page. It’s a gif, a moving image, and in that image, those red and blue dots are moving. The thing you can measure is temperature.
Temperature is an aspect of heat, an emergent property, a derived effect of atomic and molecular motion. When you measure the temperature, you are measuring an aggregate motion. If you were one of those dots, you would not talk of climbing to a “higher temperature”. There is no real height. You can’t literally climb to a higher temperature. Hence we don’t call temperature a dimension. But people did. Temperature used to be called a dimension, but the word has gradually changed from its original meaning of “measure”, and is now assumed to be something that offers a degree of freedom, something you can move through.
We are immersed in time like the dots are immersed in temperature. It’s a different measure, but just as we cannot travel in temperature because there is no real height, we cannot travel in time because there is no real length. Because time is a dimension with a small d. There is no degree of freedom. I can hop backwards a metre but not backwards a second. Because time is a measure of change rather than a measure of place, and it has no absolute units, because you can only measure one change of place against another. It’s a relative measure of motion. The units are relative, and that’s what Special Relativity was telling us all along.
Special Relativity tells us that your relative velocity alters your measurement of space and time compared to everybody else. You increase your relative velocity and space appears to contract while time dilates by a factor of √(1-v²/c²). If you travel at .99c, space appears to contract to one seventh of its former size. So your trip to a star seven light years away only takes you a year. But physics is about the universe, and in that universe it took you seven years. The star didn’t become a disc because you flashed by. The space in the universe didn’t really contract because you travelled through it. But your time did.
Einstein didn’t quite understand the full meaning of relativity until later in life. He started off by saying there is no absolute time, using a postulate that says the the speed of light is always measured to be the same. But when he did general relativity, he said the speed of light varies with position. It it wasn’t until he was with Godel in Princeton that he really got it:
“It is a widely known but insufficiently appreciated fact that Albert Einstein and Kurt Godel were best friends for the last decade and a half of Einstein’s life. They walked home together from Princeton’s Institute for Advanced Study every day; they shared ideas about physics, philosophy, politics, and the lost world of German-Austrian science in which they had grown up. What is not widely known is that in 1949 Godel made a remarkable discovery: there exist possible worlds described by the theory of relativity in which time, as we ordinarily understand it, does not exist. He added a philosophical argument that demonstrates, by Godel’s lights, that as a consequence, time does not exist in our world either. If Godel is right, Einstein has not just explained time; he has explained it away… (Palle Yourgrau, A World Without Time)”.
And what he got was this: time exists like heat exists. It’s real because it does things to us. But just like heat it’s an emergent property, a derived effect of motion. It means time is not fundamental. It isn’t a dimension like the dimensions of space. We don’t see four dimensions. We see space and motion through it. The thing called c is a conversion factor, between the measure of distance and the measure we call time, and both are derived from the motion of light. It’s the motion that’s king, the universe is not a block universe, it is a world in motion. The worldlines are only in mathematical space, and in your head. There’s no place that’s the future, and no place that’s the past. There’s only this place, and the time is always now. We don’t travel in time at one second per second. We don’t travel in time at all. To travel backwards in time we’d need to unevent events, we’d need negative motion. But motion is motion whichever way it goes. You can’t have negative motion, just as you can’t have negative distance. Just as you can’t have negative carpets. So you can’t travel in time. There are no time travel paradoxes, because there is no time travel, and there is no time travel because time is just a relative measure of motion. And motion is travel. You can’t travel through travel.
So those celebrity physicists who talk earnestly of time machines are wrong. Dead wrong. Not even wrong. And all those folk who puzzle about the beginning of time are chasing the wrong horse. There never was any beginning of time. Time didn’t start thirteen point seven billion years ago. Because time didn’t start in the first place. It was motion that started in the first place. It was a place, not a time. And it’s this place, the place we call the universe, marked out by every light path you can track through timeless space. That’s how far we’ve come. A long long way, in no time at all.
But now we can move on. Because now we’ve got the key that unlocks all the doors in physics.