# Define time.

#### Northern

##### Hall of Fame
You need to understand what happened and what can happen. The Universe can be replayed back and it is called the Big Crunch. If it cannot, it due to MACRO causes related to entropy, which I mentioned before, like breaking an egg. And even that is possible to reverse with a very LOW probability.

All fundamental physics can run forwards and backwards without violating any Physics laws.

And causality can go backward in time. Read about Feynman diagrams in which events in the future can affect the past.

It is even possible to see that in classical Physics. Write Newton's second law in terms of second derivative of time. Replace t by -t. What happens? The sign inverts on the first derivative, but inverts again on the second, keeping the equation the same. That is what is meant by saying that fundamental Physics is the same with time reversal.
I don't think this is correct. The example of the egg breaking doesn't take into account other physical phenomena. For example. If you look at exponential decay, the quantity of the radioactive isotope always decreases with time. it never augments or generates spontaneously. There is absolutely no chance it might happen that way, because it would violate physical laws beyond what you see with the egg. Also, what you say about reverting the sign of t makes no sense. There is no negative time, time can only increase, it can never decrease. A different story is if you are looking at time shift in different frames of reference, like what happens in clocks in satellites running slower than on Earth. But that doesn't mean that time runs backwards, it means it simply runs slower. If you think of time shift, then it can be negative. But time itself never runs backwards.

#### sureshs

##### Bionic Poster
I don't think this is correct. The example of the egg breaking doesn't take into account other physical phenomena. For example. If you look at exponential decay, the quantity of the radioactive isotope always decreases with time. it never augments or generates spontaneously. There is absolutely no chance it might happen that way, because it would violate physical laws beyond what you see with the egg. Also, what you say about reverting the sign of t makes no sense. There is no negative time, time can only increase, it can never decrease. A different story is if you are looking at time shift in different frames of reference, like what happens in clocks in satellites running slower than on Earth. But that doesn't mean that time runs backwards, it means it simply runs slower. If you think of time shift, then it can be negative. But time itself never runs backwards.
Like I said, processes like radioactive decay can happen backwards, with a very LOW probability, so low that it might not happen in the lifetime of a universe. That is called the thermodynamic arrow of time and some believe that it is the only basis for time to exist. But it is NOT a feature which is shown in the fundamental equations (radioactive decay is a complex quantum phenomenon), all of which are reversible with time.

Anyways, you need to pick it up from here, because arguing here won't help you.

#### Northern

##### Hall of Fame
Like I said, processes like radioactive decay can happen backwards, with a very LOW probability, so low that it might not happen in the lifetime of a universe. That is called the thermodynamic arrow of time and some believe that it is the only basis for time to exist. But it is NOT a feature which is shown in the fundamental equations (radioactive decay is a complex quantum phenomenon), all of which are reversible with time.

Anyways, you need to pick it up from here, because arguing here won't help you.
What are the fundamental equations?

Maybe I'm not understanding what you are trying to communicate. Can you tell me if there is any possibility at all that an apple might shoot upwards instead of falling down to the ground according to what you are saying?

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#### FedFosterWallace

##### G.O.A.T.
Thank you.

No time to reply at length just now.

Will attempt to carve out a moment later. What's a "moment"? How much of whatever "time" is just elapsed? And who's keeping track? And for what purpose?

That all took way too long.

#### TahoeTennis

##### Legend
Time is What Happens To You While You’re Busy Making Other Plans

#### sureshs

##### Bionic Poster
What are the fundamental equations?

Maybe I'm not understanding what you are trying to communicate. Can you tell me if there is any possibility at all that an apple might shoot upwards instead of falling down to the ground according to what you are saying?
Very good question. The answer is subtle. Usually the question is posed like this: if 2 particles are attracted by gravity, then they must come closer. So how can this be reconciled with time reversal which implies that the movie run backwards is also true, i.e. they are separating?

First, it is true, because if you express force of gravity (the Newton law with1/ r^2) with the same second derivative of time, time reversal results in the same equation.

So then how do you say that the movie run backwards, which will show the two particles moving away from each other after being together, be valid?

Answer is as follows and requires careful attention. When the movie is run backwards, the apple does not start from the earth with 0 speed- it starts at the same speed it would have had in the forward running movie at impact. Read this carefully.

Then, you ask yourself what can cause an apple to shoot out with that velocity? One answer which could be reasonable if it was some other object is that it was actually a mini rocket which can take off by itself. But you say no, it is only an apple. An apple could still have a missile inside it which you cannot see, or the other explanation is that the apple bounced off the earth and was recoiling back to where it came from. In fact, if two shiny billiards balls were involved, then instead of gravity, the force was coming from a cue, and you saw two balls separate from each other rapidly in the beginning of the movie, you would say that they must have collided before and are recoiling now, but the movie did not have the first part of their coming together. Understand this carefully.

The reason you would find it hard to accept the apple recoiling from the earth explanation is because you are accustomed to the apple getting squashed in what is called an inelastic collision. In such a collision, kinetic and potential energy are transformed into heat - the system entropy is increased as energy goes into the statistical motion of air molecules surrounding the apple and in vibrations inside the apple and the ground, and what you observe as natural is the thermodynamic arrow of time working for macroscopic objects.

As for the probability that for the inelastic collision case that it will be reversed, as I said it is a very very very very very very small probability.

Now I will make it even simpler. I show you a movie of a tennis ball which shows it moving vertically up from the ground. Do you go around yelling that it is not possible? Rather you would assume that the part of the movie when the ball was dropped was not shown in the clip. But if that part was shown to you but run backwards, you will accept it in the same way as the first case, assuming that there was a prior event. But it was just the movie of the ball falling that was run backwards.

Both forward and backward directions satisfy the laws of Physics, as long as the entropy of macroscopic objects is not involved. If there is a physical explanation for the forward movie, there is also one for the reverse movie.

#### donquijote

##### Legend
That implies that time has an axis, like space. That is not clear at all because all fundamental equations are time-reversible - the backwards-running movie is as fully compatible with Physics as the forward one. It is only at a macro scale that there is an arrow, like breaking an egg. So if there is no arrow at a fundamental level, the comparison to spatial axes collapses. That is why I said it is a circular argument.

What he probably meant was that time is needed to enforce causality.
Can you run this movie backwards?

#### Sudacafan

##### G.O.A.T.
It’s what umpires call before players should start.

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#### Sysyphus

##### G.O.A.T.
an illusion, an elaborate conspiracy concocted by the The Swatch Group Ltd and the rest of Big Horology in order to sell needlessly expensive timepieces.

#### BarNotchky

##### Semi-Pro
The simplest and most elegant definition came to me in a dream.

Time is a natural resource providing order.

#### mmk

##### Hall of Fame
In eight words:
A way to keep everything from happening simultaneously

#### tennytive

##### Professional
Time can be measured by daylight vs darkness, heat vs cold, full moon vs new moon, rain vs drought… but the fastest measure of time today is the time it takes for a traffic light to turn green before the driver behind you honks her horn.

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#### sureshs

##### Bionic Poster
In eight words:
A way to keep everything from happening simultaneously
LOL that is what Einstein said. Why are you quoting him without reference?

#### mmk

##### Hall of Fame
LOL that is what Einstein said. Why are you quoting him without reference?
Didn't know that, I guess he must have used an Einstein-Rosen bridge, looked into the future, and quoted me without reference.

#### Poisoned Slice

##### Bionic Poster
One of the great lessons of time travel is that many things cannot be changed. Time wants to happen.

#### BetaServe

##### Semi-Pro
Time, a victim periodically abused and stretched by Rafael Nadal before executing his serves

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#### Bagumbawalla

##### Hall of Fame
Time is not a thing in itself, just as "speed" is not one thing, but- rather, part of a formula or equation to help measure or locate some physical object in relation to something else.

The "perception of time" may be something completely different- especially as the human entity (and others) perceives such things as time, color, temperature, smell and so forth- through senses which (as in the famous parable of the cave) do as much to separate us from "reality" as to reveal it.

#### stringertom

##### Bionic Poster
Time can be measured by daylight vs darkness, heat vs cold, full moon vs new moon, rain vs draught… but the fastest measure of time today is the time it takes for a traffic light to turn green before the driver behind you honks her horn.
I like my draughts and they taste better during severe drought conditions but I’ll drink a few of them in the rain as well.

#### vive le beau jeu !

##### G.O.A.T.
Time, a victim periodically abused and stretched by Rafael Nadal before executing his serves
the same also applies to his underwear, by the way ewww...

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#### tennytive

##### Professional
I like my draughts and they taste better during severe drought conditions but I’ll drink a few of them in the rain as well.
Noted and fixed, thanks. I'll have a draft myself to toast your proofreading prowess.

#### sureshs

##### Bionic Poster
Didn't know that, I guess he must have used an Einstein-Rosen bridge, looked into the future, and quoted me without reference.
Or more likely, you are more intelligent than him

#### SystemicAnomaly

##### Talk Tennis Guru
Another 8 word definition:
An often wasted, non-renewable, non-reusable resource
There we go. If not for the presence of a 4th dimension, time, then your statement here and my posting of virtually the same thing 4 days ago (post #2), would have happened at the very same moment. Proof positive of its existence perhaps?

#### SystemicAnomaly

##### Talk Tennis Guru
@mmk
LOL that is what Einstein said. Why are you quoting him without reference?
One very good reason is that Einstein did not originate this clever statement. It has also been wrongly attributed to Richard Feyman, Woody Allen and others. Several sources, seemingly reliable, attribute it to the writings of Ray Cummings in the 1920s. He used the phrase in several different stories of his during that decade.