What is time?


Registered Member
Forgive is this subject has already been brought up. I could find no thread that pertained to my question. I was wondering if anyone could help to explain, what is time. I am not talking about the man made time, A way to know when it is lunch time or how to determine velocities. I am wondering about time in terms of physics. Like does time have to "exist"? Is there any other way to get the leading theorys for explaining the universe to work without using a time component? Also, if for some reason time does have to "exist", Wouldn't there need to be some sort of quantum unit to transmit is "force" to other particles? I have very little background in physics, but this is a question that has bugged me for a while. I appreciate any insights anyone can give me.
Hi MikeD,

As far as I know, there is to date no physical theory that can explain where time comes from, or why it "evolves". Time is introduced as a variable parameter.

What probably comes closest to a "description" of time is the spacetime structure introduced in General Relativity.


Short answer no one knows. Long answer it is the something that gives an order to events. When two people (and other objects) are in the same place they can agree on the order things happen. This is what is meant by time. There is also a concept that certain things (clocks) repeat in a consistant way so that we can compare two (or more or many) devices and observe that the give a predictable ordering to events, and this is how we measure time.

One unanswered question is a chicken and egg situation. Is the speed of light dependant on time or is time dependant on the speed of light ? Put another way the question is "Is the way we measure time always dependant on the speed of light in vacuo" or is "Time separate and the speed of light fixed relative to time" ? The maths for SR, GR etc all work out the same whichever assumption you start with.
Originally posted by MikeD22
I am wondering about time in terms of physics. Like does time have to "exist"? Is there any other way to get the leading theorys for explaining the universe to work without using a time component?


Yes mike it would be possible to explain the universe without using the time component as being responsible for the causality of events as relativity does. Relativity defines the causality of the force of gravity in terms of a curvature of space-time. However if one defines time is only as a measure of <a href=http://home.attbi.com/~jeffocal/chapter16.htm> the sequential ordering of the causality of events </a> one can explain the workings of a universe where time does not contribute to the physical causality of events.

Welcome MikeD.

What I have understood of the matter (not much, maybe ;)) time is dependent of the speed of light in vacuum (c).
Also, there are those who say time is the direction of processes caused by the increase of entropy. We experience time as going a certain direction, because our brains obey the same law of therodynamics, so that our memory 'forces' us to perceive time as we do.

However, how the two relate (if at all) is alltogether unclear to me at this moment (whatever that is).

Hi Merlijn, all others

There are several possible definitions of a time arrow. I believe some of them are discussed in "A Brief History of Time" (dutch translation was entitled: "Het Heelal") by Stephen Hawking.

First of all there is what you could call a psychological arrow of time, which is what you refer to as how our brain perceives events. Some events just "feel right" when they happen in a certain order (a cup breaks after it hits the floor and not in mid-air). Secondly - more into physics - there is the thermodynamical time arrow, i.e. that time flows forward in the direction of increasing entropy. Unfortunately the increase of entropy is more or less a postulate in thermodynamics, and it happens to be a hot topic in current research to explain how the increase of entropy can be explained in terms of particles and their interactions. I should add that the increase of entropy is also not an absolute law, research suggests that it does not always hold (I think I've mentioned this before on the forums, there is going to be a forum on this somewhere in the summer: quantummechanical calculations seem to somewhat contradict this, but I guess we'll have to wait a while before some results are published).

Note also that "a direction of time" is totally unexpected from a physical point of view: after all, all fundamental equations of motion in physics are time reversible (Newton's third law, Schrodinger equation, relativity, ... ). I've also mentioned this in the "emergent properties" thread.

So what is time and what about a direction of time ? I am not entirely sure if the scientific community has already a good idea what time is exactly. We're working hard on it though :)


Time, as you've probably heard, is another coordinate in the 3 dimentional space we know, thus turning it into a 4D space.
I'll try to explain why is that so and by doing it I'll answer on some of your questions.

Some imagination and basic algebra lead to the following formula:

(Delta)t = (delta)t' / sqrt(1 - v^2 / c^2)

sqrt = square root.

I won't explain how to derive the formula. I can if you want, but right now I don't have the patience for that.

According to the above formula, (Delta)t (the time) depends on the speed of a moving frame (v), the speed of light (c) and a (delta)t', which I won't explain, and even if I did, you would not understand, because it can only be understood if I derive the formula step by step.

Note: I'm not sure what's the exact word for what I mean by "frame" in english, but the word in russian is sistema otchota. I'll just use the word "frame".
It's simply put a coordinate system to which things are related.
For instance, a ball thrown forward at a speed of 7 m/sec from a car moving at a speed of 25 m/sec will seem to be moving at a speed of 25+7=32 m/sec to a still observer outside the car. The person who threw the ball from inside the car will see the ball as if it was moving at the speed of 7 m/sec.
In other words, the car is another frame, moving relatively to the frame of the earth at the speed of 25 m/sec. the observer outside the car is in the frame of the earth and that is why the ball seems to be moving faster.
whenever you move, you are a frame, or a coordinate system and every part of you is in that coodinate system. Your liver, your brain, your heart, everything.
If my English vocabulary was slightly wider, perhaps I could explain better... sorry about that...

Now, take two coordinates. X and Y. Coordinates are not absolute, but relative.
The geometrical place of point A (see attachment) can be described using X and Y and also using X' an Y'.

from now on, I'll use 'D' for 'delta'.

When turning the coordinate system clockwise, from (X,Y) to (X',Y'), one coordinate (Y) becomes shorter and the other one (X) -longer. Thus, there should exist a combination of the coordinates which always remains constant.
In our case, it's:

(Dr)^2 = (DX)^2 + (DY)^2 = (DX')^2 + (DY')^2 = ... = const.

because a^2 + b^2 = c^2, the pithagorian theorem (dunno how to spell it).

Now, lets get back to our formula.
According to the formula, the higher the velocity (v), the larger the values of Dt and DX (=Dt*v). :eek: Ok, I guess I'll have to explain how to derive the formula... but not now. Maybe tomorrow...
Anyway, now a question arises. What if there is a combination of Dx and Dt which, as in the case with the coordinates, remains constant?
There is a little problem. You can't combine time and distance.
luckily, this problem has a solution. All you have to do is to simply multiply Dt by c, which is the speed of light ~ 300000 km/sec.

The next stage requires a full understanding of the first formula plus knowing how to derive it plus it's geometrical representation.
Just in case you're familiar with it, I'll continue, but probably it won't make any sense...
Don't worry. When I have time, I'll post the explanation.

Dx/2 and c*Dt/2 are two sides of a triangle. Its' height remains constant in all frames:

DS=sqrt[(c*Dt)^2 / (Dx)^2]

If you raise the above expression to the power of two, you'll have:

(DS)^2 = (c*Dt)^2 - (DX)^2 = (c*Dt')^2 - (DX')^2 = ... = const.

Wow, this looks just like the coordinates formula!!!

And thus we have proved that time is the fourth coordinate of space.

Now, back to your questions.
Does time have to exist? Yes. It's a part of space-time, it's the fourth coordinate. Assuming that space will exist without time is like assuming that a man can live without his brain.
Your second question is basically the same as the first one...
The third question I didn't quite understand. Maybe it's my English... What do you mean by "force"? As long as I know, time has no "force"...

All written above are the basics of the special theory of relativity
and it's not guaranteed to be 100% correct. I'm not a professional, I just buy books and read them. I think it's much better than learning it in a class... dunno.

Anyway, if there are any mistakes, please let me know...
Thank you all for the great explainations. They have really helped me to better understand time.


If time is a fundemental component of the universe, would in not need a unit to carry or transmit it's "force"? For example, the strong nuclear force is carried by gluons and the electromagnetic force is carried by photons. Wouldn't time need a "timeon"? My thinking my be wrong on this issue, but it seems to me that time would need some way to make its effects felt by the universe.

Time is a coordinate. It's the same as an x axis or a y axis or a z axis. It is a 't' axis.
The examples you gave are the fundamental forces in the universe. Well, time is not a force and and it doesn't possess any energy either.
To imagine what time is think of an infinitely long table with an infinite number of columns and each column contains a still image of a 3 dimentional space. If you load this table into a cine projector and run it, you'll see the real 4D space.
This is just another way to imagine a 4D space coz it's hard to imagine a 4th coordinate. Think of the table as being the 4th coordinate, the time. It gives order to events. These events are the images of the 3D space in the table.
Time has no force, just like space. It just exists. It sustains everything, including the fundamental forces.

P.S. Mike, do you want me to post the derivation to that equation or not? :confused:
I just now saw a program in which the ideas of Julian Barbour about time were discussed. (See http://www.platonia.com/ for his website.) His idea is that time does not exist but is only percieved by us: it only appears to exist. Change is what matters, not time. Time is perceived by the changes in the world around us.

Time in Einstein's theory is flexible whereas in quantum physics it is inflexible. The Wheeler-De Witt theorem unified these two theories. This theorem doesn't use time. So the way to unify these two perceptions about time is to leave the time out!

He explained his idea using a three particle universe. In this universe every instance of the universe can be represented by three numbers: the distance between (A and B), (B and C) and (C and A). So every instance can be represented by a point in three-dimensional space. Not every point in this three dimensional space represents a valid instance of the universe: if A is close to B and B is close to C then A and C cannot be very far apart). Imposing these restrictions in the three dimensional space gives a section of this space representing all the possible configurations of this universe. What we perceive as time is a certain sequence of instances in the universe (in the model this is represented by a one-dimensional curve connecting all those instances). In classical mechanics there are certain laws connecting those particular instances. This also explains why mechanics has no direction of time.

Interesting view...

Looks like the view of the beings in the wormhole in Star Trek Deep Space Nine: they cannot perceive time and don't understand why we would use such a concept.


Hi Han,
Welcome to Sciforums. I am glad you desided to join. (Heb je me opgegeven als referral? :))

Can you please give us some more insoght on the Wheeler-De Witt theorem? It sounds very interesting!

The problem with Julian Barbour is that he fails to understand that there is real time and there is recorded time, both of which exist in two different forms. An analogy: Take a video recorder and record a clock so that you have a recording of a clock showing time. Take the video out and place the video on a table, the video looks timeless, yet does not change the fact that in the video, there is time, but of a different nature than the time outside the video.

Barbour visualizes our universe from a point standing outside of the space-time model where it appears time stands still and therefore, according to Barbour, time is an illusion. He fails to understand that it will always look like that no matter what the real nature of time is, thus it is a lack of time that is the illusion.
Q, thank you for pointing this out. You are right: Barbour does not explain anything about the nature of time. However, as you also pointed out: his model is independent of what the nature of time is. This is I think the real strength of the model: freed from any particularities of time we can think about time in the model. The question of what time is then becomes in the model (without loss of generality): what relation is there between the different points in Platonia (different "nows"). (Platonia is the name Barbour gives to his model.) The classical vision of time would then say that for every history (any given path in Platonia) the next point would be a function of the points before. The restrictions that come from Einstein in this model would be the light conus: nothing travels faster than light: so no history is possible where particles travel faster than light.


I've always preferred Wheelers statement on the nature of Time; "Time is natures way of ensuring everything does not happen at once".

It's either that or the immortal line from Ford Prefect in Hitch Hikers, "Time is an illusion, lunchtime doubly so".

I have a problem with these existentialist arguments that time is some how an illusion or a human construct. It seems obvious to me that time is a very real phenomena. I'll freely admit I can't describe what time actually is but it's there.
Originally posted by thed

I have a problem with these existentialist arguments that time is some how an illusion or a human construct. It seems obvious to me that time is a very real phenomena.

Does time have a physical property that can be experimental observed or is its “physical” existence or "what it is" based only on abstract mathematical equations?

No, I don't want you to waste you time deriving your formula for me. It will just go over my head. Thank you though.

I would like to thank everyone who posted for trying to clear up this question for me. From what I gather from your posts and other reading the nature of time is not well understood by many in the scientific community.

I read an article in Discover Magazine(vol.21,No.12,Dec.2000) discussing Julian Barbour's ideas on time and that is what started me thinking about the nature of time. He has some really fascinating ideas on the subject.

The question of what time is then becomes in the model (without loss of generality): what relation is there between the different points in Platonia (different "nows").

Since we live in a 4 dimensional universe, 3 spatial and 1 temporal, these dimensions are not completely separable as Einstein shows how distance and time can to some degree be exchanged. The universe can be divided into spacelike and timelike directions and is something that can be measured precisely with clocks and distance measurements according to Minkowski. This is the relationship between the points or 'nows' in Platonia, and is no illusion.

The classical vision of time would then say that for every history (any given path in Platonia) the next point would be a function of the points before.

The most fundamental aspect of time is the effect that makes a cold beverage warm and a warm soup cold, the same effect that causes us to remember as well as forget the past but not the future, the effect that is the direction in which computers work, the effect which is the direction of increase in entropy. This would be the function of points before the next.

Therefore if both forms of time, the direction of time, as shown by Einstein and Minkowski, and the direction of increase in entropy, can be measured, they exist.
"Time is an illusion, lunchtime doubly so". Those books are cool.

"The problem with Julian Barbour is that he fails to understand that..."

I wouldn't be so quick at saying that. Are you implying that you understand more than this Barbour. I think instead of insulting you might give some proof for your part.

"Since we live in a 4 dimensional universe, 3 spatial and 1 temporal"

It is not very prudent to take this kind of thing as a fact. Just becuase Einstein said it, does not make it true.

Apart from this id just like to give a little food for thought. Time doesn't make a hot soup cold or a cold soup hot, directly. Time only lets the mechanisms for the universe work their magic.

The only problem i have with the idea of frames of time that are that our brains don't just preceive time. Our brains are a construct of matter in the univers just like everything else is. So The only way our minds work is through the passing of time.

There is no reason to suppose that time does not exist. The only thing we do know is that there is a very many direct patterns to how one time frame looks and how the next one looks. So it makes sence to suppose that the previous frame was derived from the last.
Actually I can see what the original subject was aimed at.

I've discussed many times about the subject of parallel worlds, and the usage of a timeloop that proves the universe itself repeats over the same ground using it as a kind of guide rail.

(You might see this as some form of trace if you move your hand infront a particular frequency of light. You see you arm in front and the trace behind, but your hand at some point exists as that trace.)

This pretty much means that the whole universe is in time, some form of universal time, although some very small changes can occur with the use of mutliworlds. In a visual sense a bit like a pattern continually evolving and morphing in an animate fractal.

If you could continue keeping an observable position though (and move it when it needs to be moved to keep it relative) time would repeat itself with no change what so ever. (Since your changing the observation point)

The main problem here is never being able to be at that observation vantage point, at least for no longer than an instance.