What is Time?

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“Now” is essentially everything that happens in a single moment in time. Most people think everywhere has the same “now” at the same time. Since light has a finite speed, you can never actually see the “now” of any object.

This means we need an external way to measure if a weather an event happens at the same time. We can not do this by measuring the passage of time, because any movement has an effect on time. We need to use the only thing that is constant in relativity, light.

For the speed of light to stay constant, space and time must warp. How space and time is warped depends on the observer. Imagine space-time like a loaf of bread, where the ends are both arbitrary points in the past and in the future. Not moving at all will have you going straight down the bread, full speed through time. Anything moving relative to you will be sacrificing some of this speed through time to move through space. In other words, the other object will be moving at an angle. To the object the reverse is true, you would be moving at an angle while it would be going straight through time.

To any object in space-time, now is just a straight cut across the bread. The problem is, they will disagree on what is straight. Image one clock on a train and somebody holding one on the ground next to the track. Due to time dilation, the person sitting off the track would see the clock on the train move just a bit slower than his. He would see it hit twelve just a fraction of a second before his does. The main on the train would see the opposite, his own clock would strike twelve first while the guy on the ground would be moving slower.

When these two people met up later, they would disagree on the order of events, even when the amount of time it takes light to get there is taken into account. This means “now” is not really set in stone, and changes based on the observer.

(Acceleration does actually cause a change in the amount of time elapsed, so that is not relative.)

How does time change? That is a relatively simple question, but with a complicated answer. Some people think of it as a continual flow from one moment to the next, each one exists or is highlighted for the smallest period of time, a moment, and then reality moves onto the next.

This is fundamentally flawed. There is no known mechanism to change all of reality from one moment to the next. In fact, figuring out which moment is the next is hard. Imagine a video of a ball flying through space, now imagine running the video backwards. Both scenarios are entirely possible. For example a ball could be thrown from a planet, decelerate, and fall back down to the ground. As time is reversed, the ground could push up on the ball, launching it into the air and eventually into the hand of the thrower. The physical process is possible, there is just one problem, it isn’t likely.

Entropy is the measure of disorder within a system. The more ways something could be arranged without any significant change, the higher the entropy. This can be shown with a new deck of cards. New decks of cards are often in a specific order (low entropy), when shuffled that order is changed. After a few shufflings, to get the cards fully mixed, shuffling more probably won’t do anything. This means it has higher entropy. Entropy is now a law, but more of a guideline. Things have a tendency to get disordered, but they can become more ordered. With a 52 card deck there are around 8 x 10^ 67 different combinations. One of those is ordered, another that can appear in 2652 different ways is a set where two cards are switched, another with 132600 different combinations has three interchanged, but the vast majority of possibilities is the cards out of any recognizable order at all.

This situation is the same with particles, but they are many orders of magnitude bigger than just the 52 playing cards. This means the particles managing to randomly work together to give the ball enough velocity to launch off the ground is basically zero.

This raises another problem, entropy still hold when looking backward in time. A deck with two or three cards in order is far more likely than a deck with all cards in order. If we knew nothing about how or when the deck was created, we would say it's far more likely that is is a cause of random shuffling of the more disordered states than the leftovers of a more ordered state. This applies to the universe too, taking entropy into account its far more likely the universe came into an ordered state this instant instead of a far more ordered state sometime in the past. Most people just ignore this because that means nothing we do can be trusted and no arguments or learning can take place about any field of physics. Instead, we assume the origin of the universe was for some reason a state with extremely low entropy. The increase of entropy is what gives us an arrow of time.

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(Much of my knowledge came from a book I read titled: The Fabric of the Cosmos by: Brian Greene)