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What is Time?


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Hi

 

What is time?

 

Some say it is just measure of movement or an aggregation of events. Time is not a constant and I would like you guys to put forward your own ideas on the topic before adding my peace to the story.

 

Alan

 

Well, we recently had a thread which tackled this question. I can tell you, that time does not mean change. Things can be unchanging while time trucks on, so they are not synonymous. Time is certainly not a constant as you say, it's a variable.

 

But time may not even exist, through the eyes of the Wheeler de Witt equation, so time is a very complicated subject. It has no flow and if it has any precedence in quantum mechanics, it can be seen rather as stops and starts.

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I can tell you, that time does not mean change. Things can be unchanging while time trucks on, so they are not synonymous.

I disagree. The very essense of time is directly related to change or "happenings".

 

A friend of mine wrote an article on this topic. See http://www.wfu.edu/~brehme/time.htm. I agree with everything in that page.

 

If there were a universe in which nothing happened and nothing changed then time would have no meaning.

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I disagree. The very essense of time is directly related to change or "happenings".

 

A friend of mine wrote an article on this topic. See http://www.wfu.edu/~brehme/time.htm.'>http://www.wfu.edu/~brehme/time.htm. I agree with everything in that page.

 

If there were a universe in which nothing happened and nothing changed then time would have no meaning.

 

You put a period at the end of your link so it gives a "Page not found error". This is it I think;

 

http://www.wfu.edu/~brehme/time.htm

 

My attempt at a definition: a book-keeping or counting system used for sequencing and measuring the period between events.

Edited by StringJunky
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You put a period at the end of your link so it gives a "Page not found error". This is it I think;

 

http://www.wfu.edu/~brehme/time.htm

 

My attempt at a definition: a book-keeping or counting system used for sequencing and measuring the period between events.

I agree. In fact that is the essence of what appears in that link.

Edited by pmb
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Bit confused. Is time a scalar or vector quantity?

Time is a number, not a vector. The term scalar refers to a number which remains unchanged by a change in cordinates. A 4-scalar in relativity refers to a number which is independant on the choice of spacetime coordinates. A 3-scalar is a number which remains unchanged by a change in spatial coordinates. Time is a 3-scalar.

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I disagree. The very essense of time is directly related to change or "happenings".

 

A friend of mine wrote an article on this topic. See http://www.wfu.edu/~brehme/time.htm. I agree with everything in that page.

 

If there were a universe in which nothing happened and nothing changed then time would have no meaning.

 

I can give you a perfect example of time not equaling change.

 

Make momentary observations on a system ready to up its energy in the form of radiation, then you will effectively freeze the quantum evolution of that system (the zeno effect). Does time stop because your system no longer changes?

 

Time does not mean change at all... time can truck on without there needing to be any change in your system.

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I can give you a perfect example of time not equaling change.

 

Make momentary observations on a system ready to up its energy in the form of radiation, then you will effectively freeze the quantum evolution of that system (the zeno effect). Does time stop because your system no longer changes?

 

Time does not mean change at all... time can truck on without there needing to be any change in your system.

Time does not refer to a particular system but to the universe as a whole. That means that somewhere in the universe something is changing. The sun is giving off energy, galaxies a moving away frmo each other, water pours over waterfalls, hearts are beating, cars race down the highway, the lab clock ticks away, radioactive material is decaying, paint is oxidizing, people fall in love, they work, pay bills, go food shopping, watch tv etc. The list goes on ad infinitum.

 

When a quantum system is stationary it means that when you compare it to the ticking of a clock the quantum state is independant of time. That doesn't mean that time dosn't apply to the system. And it also doesn't mean that the particle which the system my be describing isn't moving. The truth is that you can't even speak about what the system is doing unless you make a measurement.

Edited by pmb
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When a quantum system is stationary it means that when you compare it to the ticking of a clock nothing changes in the system. That doesn't mean that time dosn't apply to the system.

 

Of course it doesn't, but I didn't say time didn't apply to the system. I am giving you a perfect example of how a quantum system does not change even when time does. That's the point, the two are certainly not equal.

 

On another note, systems which are measurable are physical. Time is not physical, it's not even an observable; the kind of things we can measure by observation.

 

Time does not refer to a particular system but to the universe as a whole.

 

That would be a global time and that doesn't exist in GR. Global time vanishes when you quantize the EFE equations.

Edited by Aethelwulf
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Of course it doesn't, but I didn't say time didn't apply to the system. I am giving you a perfect example of how a quantum system does not change even when time does.

True, but that doesn't mean that time doesn't pertain to changes, You can only speak of a stationary state when you have a clock to compare it to.

 

A.P. French speaks about time in his text Newtonian Mechanics[/i]. On page 61-62 he writes

The sense of the pssage of time is deeply embedded in every one of us. We know, in some elemental sense, what time is. But can we say what it is. The distinguished Dutch physicist H.A. Kramers once remarked: "My own pet notion is that in the world of human thought generally, and in physical science particularly, the most important and most fruitful concepts are those which it is impossible to attach a well defined meaning." To nothing, perhaps, doe this apply more cogently than to time. Nevertheless, if one tries to analyze the problem, one can perhaps begin to see that it is not entirely elusive. Even though a definition of time may be hard to com by, one can recognize that our concept of the psage of time is tied very directly to the fact that things change. In particular we are aware of certain recure events or situations-the beat of our pulse, the daily passage of of the sun, the seasons, and so on.

 

On another note, systems which are measurable are physical. Time is not physical, it's not even an observable; the kind of things we can measure by observation.

Of course that depends on how the term physical is defined. Different people have different ideas of what it means to be physical. When I use the term it means that its something that is directly related to something in nature that has an existance. In this case the chaning of the universe as a whole. If i was talking about velocity then that's a mathematical construct which cannot be measured directly but can only be calculated from other measurements such as the change in positioin of an object when compared to a clock. I think of velocity asbeintg physical. Energy pertains to a sum of terms for which the sum is constant of motion. Only the individual portions can be measured. E.g. we can't measure kinetic energy directly. What we can do is to measure its velocity and then calculate it from that an the body's mass. Or you cn let it do work on a system and then measure the variables which define the system. E.g. let the body hit a paddel which is emersed in water and let it stir it. The water will then heat up and you can measure that with observing the level of mrcury in a thermometer which is submerged in the water. Same with potential energy. It can't be measured directly. You can only measure the body's position in a field and then calculate its energy from that and the formula for potential energy. Then its the mathematical sum that is constant and I think of that as a physical quantity.

 

That would be a global time and that doesn't exist in GR. Global time vanishes when you quantize the EFE equations.

That doesn't mean that time doesn't pass around the universe. It just means that you have to be careful when you assign a number to it.

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True, but that doesn't mean that time doesn't pertain to changes, You can only speak of a stationary state when you have a clock to compare it to.

 

A.P. French speaks about time in his text Newtonian Mechanics[/i]. On page 61-62 he writes

 

 

 

Of course that depends on how the term physical is defined. Different people have different ideas of what it means to be physical. When I use the term it means that its something that is directly related to something in nature that has an existance. In this case the chaning of the universe as a whole. If i was talking about velocity then that's a mathematical construct which cannot be measured directly but can only be calculated from other measurements such as the change in positioin of an object when compared to a clock. I think of velocity asbeintg physical. Energy pertains to a sum of terms for which the sum is constant of motion. Only the individual portions can be measured. E.g. we can't measure kinetic energy directly. What we can do is to measure its velocity and then calculate it from that an the body's mass. Or you cn let it do work on a system and then measure the variables which define the system. E.g. let the body hit a paddel which is emersed in water and let it stir it. The water will then heat up and you can measure that with observing the level of mrcury in a thermometer which is submerged in the water. Same with potential energy. It can't be measured directly. You can only measure the body's position in a field and then calculate its energy from that and the formula for potential energy. Then its the mathematical sum that is constant and I think of that as a physical quantity.

 

 

That doesn't mean that time doesn't pass around the universe. It just means that you have to be careful when you assign a number to it.

 

 

The problem with Newtonian Mechanics views on time, is that there is no such thing as a flux (aka passage) to time. Time does not flow, as was once believed in the Newtonian train of thought.

 

If by what you are saying ''does not mean that time does not pertain to changes'' is really meant to mean ''we can't have a change without some kind of freedom in time'' I'd agree with it in the sense that time is often treated as a degree of freedom, just another space dimension. But setting time directly equal with change is problematic because obviously a system does not need to change while time trucks on.

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If time is change and a system reaches equilibrium, would it be reasonable to say that time has stopped in the frame of that system so time only actually exists within systems that are in disequilibrium?

No. Time is not simply that which pertains to a particular system. Time pertains to the universe in its entirety. As Aethelwulf pointed out a system can be static but that doesn't mean that time has stopped in that frame. Frames are infinite in size. Just because a box in intergalactic space is static and thus unchaning in time it doesn't mean that time has stopped. There's an entire universe of change going on around it.

 

... in the sense that time is often treated as a degree of freedom, just another space dimension.

I disagree. Time is not a spatial dimension. Its a dimension in spacetime. Spacetime is a space in the mathematical sense of the term but time is not a spatial dimension.

 

But setting time directly equal with change is problematic because obviously a system does not need to change while time trucks on.

This would be so much easier if people just agreed with everything I said. :lol:

 

You keep saying that but I consider that to be a straw argument since time is about change in the entire universe. It doesn't just refer to whether a particular system is static or not. Even for a static system one can place a clock near the system and it would tick thus measureing the passage of time. That's like saying that an electric field is the force that would be exerted per unit charge if a charged were placed in the field. But with the charge gone we still consider the field to be present. Same with time. Just because a clock isn't in the room its still said that time passes in the room because if one places a clock in the room it would tick thus noting the passage of time.

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The problem with Newtonian Mechanics views on time, is that there is no such thing as a flux (aka passage) to time. Time does not flow, as was once believed in the Newtonian train of thought.

 

 

Genuinely curious - what makes you say that?

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No. Time is not simply that which pertains to a particular system. Time pertains to the universe in its entirety. As Aethelwulf pointed out a system can be static but that doesn't mean that time has stopped in that frame. Frames are infinite in size. Just because a box in intergalactic space is static and thus unchaning in time it doesn't mean that time has stopped. There's an entire universe of change going on around it.

 

Yes ok. My thought was that time is inextricably tied to process(es) and if there is none there is no time. If all the processes in all your hypothetical boxes-within-boxes stopped time would be no more?

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I disagree. Time is not a spatial dimension. Its a dimension in spacetime. Spacetime is a space in the mathematical sense of the term but time is not a spatial dimension.

 

 

This would be so much easier if people just agreed with everything I said. :lol:

 

You keep saying that but I consider that to be a straw argument since time is about change in the entire universe. It doesn't just refer to whether a particular system is static or not. Even for a static system one can place a clock near the system and it would tick thus measureing the passage of time. That's like saying that an electric field is the force that would be exerted per unit charge if a charged were placed in the field. But with the charge gone we still consider the field to be present. Same with time. Just because a clock isn't in the room its still said that time passes in the room because if one places a clock in the room it would tick thus noting the passage of time.

 

Of course time is a spatial dimension, this is why in relativity it was given a special name, called the imaginary space dimension. The idea was to treat it as a space dimension which was 90 degree's off of the real space legs on the spacetime triangle.

 

 

 

You keep saying that but I consider that to be a straw argument since time is about change in the entire universe. It doesn't just refer to whether a particular system is static or not.

 

Time is indeed about changes in the universe - whether one can describe that as an entire change for the universe in a global sense is heavily under debate, as I showed you. But if time is change, it better be able to refer whether a particular system is static or not simply because time is local in every sense of the word.

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Of course time is a spatial dimension, this is why in relativity it was given a special name, called the imaginary space dimension.

Space refers to where things are. A time parameter doesn't tell you anything about where something is. That's why its not a spatial dimension. Be careful not to confuse "space" in the mathematical space from "space" in the physical sense. People have a tendancy to confuse these two usages

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Space refers to where things are. A time parameter doesn't tell you anything about where something is. That's why its not a spatial dimension. Be careful not to confuse "space" in the mathematical space from "space" in the physical sense. People have a tendancy to confuse these two usages

 

All I can suggest to you is actually investigate this for yourself. Try searching for the Minkowski spacetime triangle because nothing I have told you is false.

 

Time and space are dimensions of the same manifold - this is as profound as saying that time is another space dimension - a special kind of one called an imaginary space dimension.

 

Genuinely curious - what makes you say that?

 

We just don't believe this is the case any more. One reason comes from relativity, if it is flowing, what is it flowing relative to? I will find you a paper which would shed some light on this perhaps.

 

Here

 

http://arxiv.org/abs/0812.0240

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We just don't believe this is the case any more. One reason comes from relativity, if it is flowing, what is it flowing relative to? I will find you a paper which would shed some light on this perhaps.

 

Here

 

http://arxiv.org/abs/0812.0240

 

Thank you - I'll let you know what I think after I have had time to read it and digest the information. I appreciate the information immensely.

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All I can suggest to you is actually investigate this for yourself. Try searching for the Minkowski spacetime triangle because nothing I have told you is false.

I believe that you believe exactly what you say. And I've researched this many many years ago. Eintein had something to say about this as does Tolman.

 

From A Brief Outline of the Developement of the Theory of Relativity, by Albert Einstein, Nature, No. 2677, Feb 17, 1921.

From this it follows that, in respect of its role in the equations of physics, though not with regard to its physical significance, time is equivalent to space-coordinates (apart from the relations of reality.

 

From Relativity, Thermodynamics and Cosmology by Richard C. Tolman, page 29

In using this language it is important to gaurd against the fallacy of assming that all directions in the hyper-space are equivalent, and of assuming that extension in time is of the same nature as extention in space merely because it may be convenient to think of then plotted along perpendicular axes. ... That there must be a difference between the spatial and temporal axes in our hyper-space is made evident, by contrasting the physical possibility of rotating a metre stick from an orientation where it measures distances in the x-direction to one where it measures distances in the y-direction, with the impossibility of rotating a meter stick to a clock.

These points eloquently state my position on this subject.

Edited by pmb
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I just find it a bit deceiving saying that time ''isn't a space dimensions'' when clearly this is what it is when you read top texts on the subject.

I disagree. A point in Minkowski space is denoted as follows X = (time, space). This is an element in spacetime. It is not an element in space. Its only mathematically that the temporal variable is in some ways treated like the spatial variables. But in no way can it be called a spatial dimension.

 

Please eleborate on why you believe that time is a spatial variable. Thanks.

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I disagree. A point in Minkowski space is denoted as follows X = (time, space). This is an element in spacetime. It is not an element in space. Its only mathematically that the temporal variable is in some ways treated like the spatial variables. But in no way can it be called a spatial dimension.

 

Please eleborate on why you believe that time is a spatial variable. Thanks.

Of course it is a type of spatial dimension, I've told you more than once now, physicists call it the imaginary leg of the space triangle. Why do you think time makes up the fourth dimension of space?

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Of course time is a spatial dimension, this is why in relativity it was given a special name, called the imaginary space dimension. The idea was to treat it as a space dimension which was 90 degree's off of the real space legs on the spacetime triangle.

 

 

There are consequences to this view that you may not have appreciated.

 

Take a 12 inch ruler.

 

Choose a coordinate system so that the ruler extends from a to a+12 inches along one (spatial) axis.

 

We say that the extent of the ruler is 12 inches.

 

If we look at the endpoints with sufficient magnification we see that they are less well defined.

 

Now let us consider the time axis.

 

The ruler comes into existance at time t and is consumed by fire 1 year later so the ruler has a duration in time of t+1 years.

 

The temporal end points will again be more blurred under closer examination.

 

Would you agree with this?

Edited by studiot
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