What’s time?

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What’s time?

In: Physics

4 Answers

Anonymous 0 Comments

Time is our way of deciding the order of events. It may or may not be part of the fundamental laws of nature, but we need it to understand order, cause and effect, and how things change.

Anonymous 0 Comments

Time is defined as the indefinite continued progress of existence and events in the past, present, and future regarded as a whole.

In simpler words, Time is the construct by which we measure the speed at which matter and energy moves through space. Before clocks were used to measure time, early civilization used constant repeated natural phenomena to describe this (such as how long it took for the sun to appear back on the horizon (a day) or how long it took for the moon to appear as a full circle in the sky again (a month).

Anonymous 0 Comments

Time is one of the four dimensions of the spacetime containing our Universe. We perceive that we are moving at one year per year in that direction.

Anonymous 0 Comments

Most of the laws of the physics are the same backwards and forwards. If you take a movie of a few seconds of one billiard ball hitting another [1], you can play it forwards or backwards. Either forwards or backwards looks physically reasonable, even if you carefully track each ball’s position and velocity and check all the math.

But obviously there’s something *different* about the past and future — we experience time “flowing”, we get older over time, lots of things in the natural world change over time, etc. Scientists have a couple different explanations for why this is (called the “arrow of time”).

Imagine putting something with color into water, like when making tea or coffee. Then play the movie forwards and backwards. You can easily tell which is the backwards one, because the color un-dissolving from the water looks super-unnatural — stuff dissolves all the time, but we never see it un-dissolve on its own [2].

This seems reasonable. But there’s a bit of a tricky problem, if the tea and the cup are all made of atoms, and atoms are like billiards, and billiards look the same backwards and forwards, how does it happen the tea looks different?

Think about it for a minute.

It turns out the answer has to do with probability. The *most probable* situation is for the tea to be totally mixed. The billiards analogy is like the triangle you put the balls in when you start the game — if you play a movie of someone hitting the *triangle* backwards and forwards, you can tell which one is backwards. Since while each collision is individually physically plausible, having the balls fly into a triangle and come to complete rest right next to each other is so unlikely, it just won’t happen.

[1] In space, so there’s no surface friction or air resistance to complicate things.

[2] Of course you can un-dissolve stuff by changing the conditions, which always boils down to using energy — for example boiling all the liquid away.