# What is time? Simply & Factually

1. Dec 6, 2011

### JackYoung

Dear All,

My quest for an answer began a few days ago when I was at my mates house and we were watching a documentary on time travel and such.

My friend voiced a gripe with Theoretical Physicists because them "seem to accept crazy, far fetch ideas as a basis to theorise further, even crazier ideas" (such as worm holes, time travel, quantum mechanics (specifically that something can exist and not exist at the same time).

While I consider this view acceptable for a person who knows nothing of the topic, my argument was that it is difficult to believe that scientists are out there just making things up for the fun of it with no real evidence to support their theories.

So, the challenge was set out. My friend stated that if something cannot be explained to him in a way that he can understand then it does not exist (to him). His example is of time. My friend does not believe that time exists (watches and clocks exist) but time does not exist, time is something we have invented to explain change, change exists because he can see it, but he cannot see time.

Does this make sense? Is there anyway to prove to my friend that time is real?

What I am hoping is that you will all share with me your knowledge on time or any topic you feel relevant that would easily explain some of these concepts. Specifically, what we know, how we know it and what we don't know.

As I assume most members of this community are qualified to discuss such matters I am hoping that the information you provide is accurate, well presented and you have a sound understanding of the topic. If you can only explain how time works without explaining what it is then you are as good as I am at getting shut down by my friend. He will not accept "time" into the conversation until someone can prove to him that it exists in the first place.

.... this is getting a bit long now but I hope some of you will find this an opportunity to defend your science to people who do not understand it and therefore deny it's credibility (I mean that in the nicest possible way) as opposed to some homework to help someone win a silly argument. I am extremely interested in hearing your responses and truly appreciate any time (no pun intended) you devote to helping me.

Best Regards,

Jack.

2. Dec 6, 2011

### Staff: Mentor

Probably not, since you and he appear to agree on the observable facts (things change), but you think the word "time" is an appropriate one to use to describe these facts, while he does not. Such debates, IMO, have to do with personal preference about the use of words, not with physics.

However, he has left you an opening:

You might ask him what his definition of "exists" is. How does saying that something "exists", or saying what it "is", differ from explaining how it works? If he responds as I am thinking he will, he will provide a good illustration of why scientists do not try to explain what "is" or what "exists" but confine themselves to explaining how things work: the latter can be defined objectively, whereas the former leads to an endless rabbit hole of subjective speculation.

3. Dec 6, 2011

### ghwellsjr

Einstein said, "Time is what a clock measures". Since your friend believes that watches and clocks exist, ask him what they measure or display or are used for.

4. Dec 6, 2011

### jewbinson

Basically, time is what you think it is.

We can spend all day and night trying to come up with the perfect definition of time, but at the end of the day we all know what it "really is", so what is the point?

If you're going to debate whether or not things exist... well then this is philosophy, not physics.

5. Dec 6, 2011

Time is just simultaneous events.

If my watch points to 7 the moment a train arrives that is the time of that event.

This was Einstein's thinking anyways.

"If we wish to describe the motion of a material point, we give the values of its co-ordinates as functions of the time. Now we must bear carefully in mind that a mathematical description of this kind has no physical meaning unless we are quite clear as to what we understand by “time.” We have to take into account that all our judgments in which time plays a part are always judgments of simultaneous events. If, for instance, I say, “That train arrives here at 7 o'clock,” I mean something like this: “The pointing of the small hand of my watch to 7 and the arrival of the train are simultaneous events.”3"

http://www.fourmilab.ch/etexts/einstein/specrel/www/

Also I am thinking in my head that Time can just be referred to the location of two bodies relative to each other. As two bodies move relative to each other (earth and sun) the time for the Earth is only in comparison to the Sun, specifically a location a certain point on the Earth is corresponding to a certain point on the Sun.

I have trouble with these questions myself.

6. Dec 6, 2011

7. Dec 6, 2011

8. Dec 6, 2011

### atyy

Almost: time (and space and matter) is something we have invented so that both of you can describe changes you see to each other.

An example of this: some say spacetime is curved, others say spacetime is flat and the curvature an illusion due to a type of matter called a spin-2 field, yet both are equally consistent and thorough descriptions of all the changes we have seen so far.

9. Dec 7, 2011

### PatrickPowers

"It depends on what the definition of 'is' is" -- Bill Clinton

"Time is what a clock measures." --- Albert Einstein

I can't think of a better definition. Albert went on to describe exactly what clocks do in all manner of situations, which is remarkably complicated. Few people are able to use this description, so you could say that few know what time really does. Let's just say that the surface of a planet is one of the few (only?) places in which clocks tend to stay synchronized.

Myself, I can't claim to know what time is. I've spent a fair amount of time looking at the math and now I really know that I don't know. Which is progress.

10. Dec 7, 2011

### atyy

But what is a clock? A sundial? A pendulum? A heart? A Swiss watch?

11. Dec 7, 2011

### ghwellsjr

It's whatever Jack's friend calls "watches and clocks" because he believes they exist:

12. Dec 7, 2011

### Dickfore

I think what the biggest problem with people accepting time as a 4th coordinate is that you can most certainly change your direction of motion and turn back towards where you started from, but you can never turn back in the 4th dimension.

But, if you look carefully at the metric of a Minkowski space-time:
$$s^2 = (c \, t)^2 - x^2 - y^2 - z^2$$
you can see that time (multiplied by the limit speed c) enters with a positive sign, while spatial coordinates enter with a negative sign. Thus, this quadratic form is not necessarily positive definite and describes quite a different geometry than a 4 dimensional Euclidean space with a positive definite metric.

13. Dec 7, 2011

### dacruick

Love it.

I as well think that this is more of a question of what your friend defines as existence. This is a pretty trivial argument. We define time to be what is useful and relevant to us.

14. Dec 7, 2011

### Elroch

Time cannot be extricated from space, so you instead need to convince your friend that space-time exists. If he says it doesn't, ask him where the heck he is.

15. Dec 7, 2011

### nitsuj

I assume it is the definition of time that your friend says was "invented".

Look at Robpys post where he quotes Wheeler;

"time is what prevents everything from happening at once."

Time isn't the event (change), it's a component of the interval inbetween. That's one way of many to look at it.

Time is a measurement, just as real as length. Do meters exist? More specifically does the space between the measurement exist?

Wheeler quote continued;

"Space is what prevents everything from happening to me!"

These Wheeler quotes are great!

Your friend may "see" time in a spacetime diagram. Perhaps he could be convinced there. And time doesn't have to be mentioned. For example it could be 299,792,458 meters instead of one second.

16. Dec 8, 2011

### PatrickPowers

All of the above. Anything you like. That's why it's a good definition.

17. Dec 8, 2011

### boogiewithstu

I am not entirely sure what you mean when you say that we invented time to explain change. If your friend agrees that change occurs and that we perceive change, he must agree that time is real as this is the only way we could perceive change. Without time Kant argues that change would many times lead to contradictions as it would lead to contradictory qualities both being possessed by the same object. Claiming that it possessed one quality at one time and another quality at a second time seems to be the only way to resolve the contradiction. However, this does not show that time is something that actually exists in the world itself, it could be a property of our perception, that we perceive everything existing in time despite the fact that it is not a property of the world itself. It is still real in the sense that it must a least be a property of our perceptions and all of our experiences must therefore exist in time. If this is what your friend meant, then he could be correct.