If you've ever watched an ice cube melt or stirred creamer into a pool of black coffee, you've witnessed a fundamental truth about the universe: The "arrow of time" always marches forward.
Time flows from past to future. What we call the present is a never-ending series of inflection points, where the collection of events that have already happened (the past) meets the collection of events that have yet to happen (the future). And while time doesn’t ever stop, it can slow down.
Slowing Down Time
Einstein’s theory of special relativity gave us a completely new way to view the cosmos. Prior to Einstein, we had movement through space, and we had passage through time. Special relativity unties those theories into a single, unified framework called spacetime. In this new framework, it’s impossible to move through space or time separately; instead, every object in the universe is constantly moving through both simultaneously.
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This means that it’s (theoretically) possible to slow down your progression into the future. The faster you move in space, the slower you move in time. So if you jump on a rocket ship and accelerate yourself to 99% of the speed of light, you’ll travel through time about 7 times slower than stationary observers. (Actually reaching that velocity, however, is another matter entirely.)
Can Humans Feel Time Slowing Down?
There’s a caveat here, though: You will never experience this time dilation yourself — your heart will beat at the same rate; your hairs will grey after the same amount of time; and your wristwatch will tick at the same speed as always. But outside observers looking in will see you move and live in slow motion, seven times slower than they are.
It sounds like a contradiction, but this is what puts the relative in the special theory of relativity: Our perceptions of distance and time depend on our perspective, and no two observers are ever guaranteed to agree.
Is It Possible to Stop Time?
Relativity allows you to (again, theoretically) skip forward into the future, as well. If you travel fast enough, what may only be a handful of years to you can translate to hundreds, thousands, or even millions of years for everybody else. By the time you came back to the Earth, it, or the people inhabiting it, could be completely unrecognizable — a major conceit in (spoiler alert) sci-fi films like Planet of the Apes and Interstellar.
Still, no matter what, you can’t stop time, because it's not possible to travel at 100% the speed of light. You’ll always move a tiny fraction below that ultimate speed limit, and so you’ll always experience at least some progression of time.
The End of Time
As for time ending, as far as we can tell time will have no end in the future. Our universe is expanding every day, and it appears that it will continue to do so for eternity. The future is wide open, and there will always be another tomorrow. Sure, in the far far future, all the stuff in our universe will be spread out to incomprehensibly thin dust, but time still won’t come to an end.
However, by the same token, it appears as if time does have a beginning. Our universe is expanding, which means that in the past it was smaller. In the distant past, billions of years ago, it was much smaller. In the extremely distant past, around 13.77 billion years ago, our universe was the size of a peach and had a temperature of over a quadrillion degrees.
Beyond that point, all of our physics knowledge says that the universe started out in such a tiny space that it was a singularity, a point of infinite density. Strictly speaking, that’s also a point where time and space originate, meaning that time has an endpoint in our own past.
Nonetheless, our knowledge of the extremely early universe is rather hazy; we don’t really know what’s going on in those earliest moments, so we can’t say much about it with anything resembling confidence.
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