What is the true nature of time?

[Passionflower]

OK. So what distinguishes a 4D pseudo-Riemannian manifold from a 4D Riemannian manifold?

The metric has a different signature.

While a Riemannian manifold has a positive definite metric a pseudo-Riemannian (or Lorentzian) manifold does not. Due to this, distance exists in three classes, timelike, spacelike and nulllike.

But the key interest wrt dimensions is the comparison between the classical Galilean spacetime, where both the space and time dimensions physically relate to the observer's measure of space and time, and the Minkowski spacetime (and also a Lorentzian spacetime) where this direct mapping is lost. What consists of physical time and physical space depends on the observer's orientation in spacetime, e.g. how the observer is oriented wrt the dimensions of spacetime. In other words what an observer measures as space and time is not universally true, each observer could in principle have a unique view of what consists of space and time. Now one could build a coordinate system around each individual observer with three spatial and one time dimension but obviously this coordinate system is not the same as the spacetime itself.

 

[Dale]

The metric has a different signature.

Exactly. Specifically four dimensions of space (++++) vs one dimension of time and three of space (-+++).

While a Riemannian manifold has a positive definite metric a pseudo-Riemannian (or Lorentzian) manifold does not. Due to this, distance exists in three classes, timelike, spacelike and nulllike.

Right, and this cannot happen unless the manifold has at least one dimension of time.

It is fine if you choose to avoid this terminology in your writing (just as it is ok to avoid "relativistic mass"), but because of what you describe here it is standard terminology to say that time is a dimension. What you have described is the essence of that statement.

 

[Passionflower]

Exactly. Specifically four dimensions of space (++++) vs one dimension of time and three of space (-+++).

At the manifold level we have no need to distinguish between physical space and time, all we need to do is have four dimensions organized as +--- or -+++ (they are equivalent). Then we add a metric that satisfies the EFE. Then if we want the time between two events on an observer's clock we have to take the length of a path that varies potentially over all four dimensions of spacetime.

Right, and this cannot happen unless the manifold has at least one dimension of time.

All that is required for something to be a pseudo-Riemannian manifold is the signature (well actually there are a few other conditions, which I am sure you are familiar with, but they are not a distinguishing factor wrt a vanilla Riemannian manifold), what physically constitutes space or time is not answered at this level it is answered after we introduce a valid instance of the EFE and then, physically, time is a length of a path not a dimension.

It is fine if you choose to avoid this terminology in your writing (just as it is ok to avoid "relativistic mass"), but because of what you describe here it is standard terminology to say that time is a dimension. What you have described is the essence of that statement.

I have no trouble finding time a dimension in the coordinate system of an observer. Of course an observer observes three spatial dimensions and a time dimension orthogonal to this but this is not the same as calling time a dimension of spacetime, time is a dimension of an observer yes but not of spacetime. Also note that this observer effectively has a Galilean coordinate system, even an accelerating observer can have such a coordinate system, for instance one that is Fermi-Walker transported. But wrt spacetime such a coordinate system constantly (pseudo) rotates. Thus the time dimension of the coordinate chart constantly (pseudo) rotates wrt to the "timelike" dimension of the manifold.

 

[Dale]

all we need to do is have four dimensions organized as +---

Excellent.

I am going to disengage at this point. Not because I believe that I have convinced you, but because I realize that we agree on the important physics/math and the remaining disagreement is only a matter of semantics. I try to avoid semantic arguments once I have identified them as such since they are frustrating and unimportant.

 

[TheAlkemist]

No, I wasn't going to introduce time into the discussion, at least, not yet.

What I wanted to point out is that if you have two different ways to determine a distance between two points (diagonally opposite corners of a brick), one where you actually made a measurement, which is what I thought you were suggesting, something along the lines of placing the brick between two objects and then measuring the distance between the objects and the other where you measure some other components, the three dimensions of the brick and then calculate the distance, they both should yield the same result. In other words, any meaningful determination of a parameter that we want to discuss, like the "distance" between two points, should always get the same answer, don't you agree?

Yes, if the metric (measuring device, ruler, what ever) used to calculate the distance is same.

Can you cite any mainstream reference for this definition?

Any dictionary. Here's one: http://www.merriam-webster.com/dictionary/dimension
(definition "b")

Your first statement is simply false. Distance and duration are both quantitative. A brick is e.g. 7 cm in height by 10 cm in width by 20 cm in length by 100 years in duration. All quantitative.

I meant length by the way, which is not synonymous with distance or else their definitions would be circular). Anyway, an object has the quality of length before you make a measurement with a ruler or whatever. For the purposes of math, length (and distance) are actually itineraries. But I think I'm done with this issue. I will respectfully bow out.

Exactly the same with time, except that we have fewer words for 4D shapes so we make up new ones like "light cone" and "helical worldline", etc.

But you can't point to a light cone or a helical worldline.

With a clock. Btw, your phrasing is wrong here. You are confusing e.g. length with position.

So I have a static brick on a table, exactly what would I be measuring using a clock? And how am I confusing length and position?

Length, width and height are quantitative. I can specify the brick's x, y and z extent as arbitrarily precisely as I want. I can also specify its extent in time. It came into existence on Aug 23, 2003, and ceased to exist *WHAM* now.

So by "extent in time" you mean from when you first observe it, t1, to when it ceases to exist, t2?

 

[Dale]

Any dictionary. Here's one: http://www.merriam-webster.com/dictionary/dimension
(definition "b")

Excellent reference. See definition 1a. Time is specifically listed as a dimension.

Anyway, an object has the quality of length before you make a measurement with a ruler or whatever.

Same with the quality of duration.

But you can't point to a light cone or a helical worldline.

Sure you can, although I am not sure what relevance "pointability" has in this conversation.

 

[ghwellsjr]

No, I wasn't going to introduce time into the discussion, at least, not yet.

What I wanted to point out is that if you have two different ways to determine a distance between two points (diagonally opposite corners of a brick), one where you actually made a measurement, which is what I thought you were suggesting, something along the lines of placing the brick between two objects and then measuring the distance between the objects and the other where you measure some other components, the three dimensions of the brick and then calculate the distance, they both should yield the same result. In other words, any meaningful determination of a parameter that we want to discuss, like the "distance" between two points, should always get the same answer, don't you agree?

Yes, if the metric (measuring device, ruler, what ever) used to calculate the distance is same.

Nowadays, you can buy measuring devices that use laser beams. Do you consider them to give the same results as rulers do?

 

[DaveC426913]

So I have a static brick on a table, exactly what would I be measuring using a clock?

We cannot freely move through the t dimension; that's what makes it timelike. We are doomed to pass through it eternally at a constant rate.

If we could move thorugh it freely, we would move to the beginning of the object's existence in time and place a t1 (just like I could move to the beginning of the object's existence in the x dimension and place an x1).

So by "extent in time" you mean from when you first observe it, t1, to when it ceases to exist, t2?

t1 is when the brick first becomes definably a brick. Its existence has nothing to do with my observation of it.

In the x dimension, it is not a brick at x0 (an inch to its left), it is a brick at x1 through x2 and is no longer a brick at x3 (an inch to its right).

In the t dimension, it is not a brick at t0 (a minute before it is formed), it is a brick from t1 through t2, and is no longer a brick at t3 (one minute after it is destroyed).

 

[nismaratwork]

We cannot freely move through the t dimension;that's what makes it timelike. We are doomed to pass through it eternally at a constant rate.

I add a vote (and I voted for you getting humor again!) for you as the most able to depress me with a single sentence. It's true, and it's well said, but ouch.

If we could move thorugh it freely, we would move to the beginning of the object's existence in time and place a t1 (just like I could move to the beginning of the object's existence in the x dimension and place an x1).



t1 is when the brick first becomes definably a brick. Its existence has nothing to do with my observation of it.

In the x dimension, it is not a brick at x0 (an inch to its left), it is a brick at x1 through x2 and is no longer a brick at x3 (an inch to its right).

In the t dimension, it is not a brick at t0 (a minute before it is formed), it is a brick from t1 through t2, and is no longer a brick at t3 (one minute after it is destroyed).

I wonder if the some of the confusion here comes from the ability to use distance and time interchangeably in some circumstances, because as you say it, this seems like the clearest of points to make about time.

 

[TheAlkemist]

Excellent reference. See definition 1a. Time is specifically listed as a dimension.

ok.

Same with the quality of duration.

What if the object isn't moving?

Sure you can, although I am not sure what relevance "pointability" has in this conversation.

It's relevant because there's a difference between objects and concepts and the two shouldn't be conflated. Don't you agree?

 

[Ich]

It's relevant because there's a difference between objects and concepts and the two shouldn't be conflated.

A brick is most certainly a 4-dimensional object.
As opposed to the concept of 3-dimensional bodies, which itself stems from the concept of simultaneity at a distance, which is derived from an errorneous generalization of slow-speed observations.

 

[DaveC426913]

What if the object isn't moving?

No such thing.

What you can say is that it is not moving wrt some frame of reference (like your own).

 

[Dale]

What if the object isn't moving?

Then it is at rest.

 

[TheAlkemist]

Nowadays, you can buy measuring devices that use laser beams. Do you consider them to give the same results as rulers do?

But isn't a laser beam 'ruler' is an indirect measuring device that infers distance from the speed of light? I think I see where you are going with this but I'm really curious to see so i'll say yes.

We cannot freely move through the t dimension; that's what makes it timelike. We are doomed to pass through it eternally at a constant rate.

If we could move thorugh it freely, we would move to the beginning of the object's existence in time and place a t1 (just like I could move to the beginning of the object's existence in the x dimension and place an x1).

From what I understand here, you are saying you can't measure the brick's time with a clock because you can't go back in time to the beginning of the brick's existence and make the t1 measurement? Please correct me if I'm wrong.

t1 is when the brick first becomes definably a brick. Its existence has nothing to do with my observation of it.

I wasn't suggesting that your observation brings the brink into existence. I'm not Deepak Chopra. Fine. I'll rephrase the question: So by "extent in time" you mean from when you first measure it, t1, to when it ceases to exist, t2?

In the x dimension, it is not a brick at x0 (an inch to its left), it is a brick at x1 through x2 and is no longer a brick at x3 (an inch to its right).

In the t dimension, it is not a brick at t0 (a minute before it is formed), it is a brick from t1 through t2, and is no longer a brick at t3 (one minute after it is destroyed).

Ok. So does everyone get to chose their own t1 and t2 as they deem fit?

I wonder if the some of the confusion here comes from the ability to use distance and time interchangeably in some circumstances...

I think so.

 

[TheAlkemist]

A brick is most certainly a 4-dimensional object.
As opposed to the concept of 3-dimensional bodies, which itself stems from the concept of simultaneity at a distance, which is derived from an errorneous generalization of slow-speed observations.

Wait what?

No such thing.

What you can say is that it is not moving wrt some frame of reference (like your own).

Ok. I'm not going to argue with you about this any more. But I have one last question. So is it safe to say that physics does not deal with static concepts at all?

Then it is at rest.

Why are you rolling your eyes? Did I ask a stupid question? If an object is at rest and time requires motion then... you know what, never mind.

I'm going to withdraw from this discussion and just casually observe from the sidelines from now.

 

[cshum00]

Ok. I'm not going to argue with you about this any more. But I have one last question. So is it safe to say that physics does not deal with static concepts at all?

-The reason why dimensions doesn't have an absolute frame of reference does not have to do with physics but the mathematical concept of it.
-In mathematics, dimensions are special vectors that become a coordinate system.
-Just like when mapping a vector onto a coordinate system, there is no fixed position we have to place it on. Similarly, when mapping the dimensions onto the real world; there is no fixed origin but we happen to choose the frame of reference.

 

[Dale]

So is it safe to say that physics does not deal with static concepts at all?

There is a whole branch of classical mechanics called statics. And equilibria and conserved quantities are very important precisely because they are static.

 

[slider123]

Can anyone help me with this one!,i don't think its a very original question but keeps recurring in my head..
If time existed before the big bang,then it seems to me that time in the past may be infinite ie :That if there is no beginning of time the past therefore must be infinite,..If this is correct and the past is infinite ,then how could we reach the present time ,,ie have we waited an eternity to be born ,,which then leads me to conclude a contridictve answer that maybe by logic that time does not exist at all ..Or am i just talking a load of

 

[bobc2]

Ok, fair enough then what kind of time is it? Coordinate time perhaps? You call a coordinate dependent entity a dimension of spacetime the spacetime we live in?

Spacetime is certainly a 4D pseudo-Riemannnian manifold I completely agree with that. No single dimension of this manifold can be attributed to time and space for all observers, in fact in non-stationary spacetimes no single observer will measure the, what you call, timelike dimension as time and no single observer will measure the spacelike dimensions as space as GR is basically a background independent theory.

Unlike in the case of Galilean spacetime time for any observer is the path length between two events not the amount traveled in the timelike dimension.

Passionflower, I've been browsing through your posts on this thread. You seem to have an interesting concept of time vs. spacetime. But, I'm having a little difficulty having a clear understanding of your conept. Are you regarding the universe as a 4-dimensional space populated by 4-dimensional spatial objects (in the context of a fundamental physical description)? Time would then just be an available parameter useful in computing changes in position along a spatial 4th dimension (DX4 = c(Dt)? The more fundamental understanding of time would then be outside of a discussion of physics, i.e., a metaphysical discussion involving memory, consciousness, etc.(more appropriate for the philosopy forum)?

 

[King Wildog]

We talk about the scale of time, which should help one to realize it only exists in concept. Even an inch is a concept- an object is actually exactly that long, but we concieved an inch to more easily percieve it in respect with other objects. So time then is absolutely constant as is an inch. An object can be acted on however it may- completely changing its lifespan, but never did time change. Perception of time is so easily confused with actual time which is more of a duration or measure of how much energy must be applied to an object to speed up happenings.

If I were to kick a clock across the lot, should I then be confused to read the time and find it off calibration?-- King Wildog

I use this absolutely unplagiarized self-quote to kick start conversations on Time and my notion of an object's endurance to its environment that changed the read on your brilliant timepiece. If I stick a clock in the oven I am also unsurprized by the results- but if I run really, really fast- well I must have changed time and not anyway have I affected my clock. I'm not OK with that. Our clocks have always been awesome attempts at guaging time for us. In fact, we make them better and better in an attempt to make them more rugged or resistent to our environment. In fact, I have a water resistant watch, for example.

A clock is just a mechanical repeating device invented to produce our best at constance, because our brain loves to compare things. So if you abuse or misuse your clock you will void its warrantee, and you will affect its ability to do its job.

 

[DaveC426913]

We talk about the scale of time, which should help one to realize it only exists in concept. Even an inch is a concept- an object is actually exactly that long, but we concieved an inch to more easily percieve it in respect with other objects. So time then is absolutely constant as is an inch. An object can be acted on however it may- completely changing its lifespan, but never did time change. Perception of time is so easily confused with actual time which is more of a duration or measure of how much energy must be applied to an object to speed up happenings.

If I were to kick a clock across the lot, should I then be confused to read the time and find it off calibration?-- King Wildog

I use this absolutely unplagiarized self-quote to kick start conversations on Time and my notion of an object's endurance to its environment that changed the read on your brilliant timepiece. If I stick a clock in the oven I am also unsurprized by the results- but if I run really, really fast- well I must have changed time and not anyway have I affected my clock. I'm not OK with that. Our clocks have always been awesome attempts at guaging time for us. In fact, we make them better and better in an attempt to make them more rugged or resistent to our environment. In fact, I have a water resistant watch, for example.

A clock is just a mechanical repeating device invented to produce our best at constance, because our brain loves to compare things. So if you abuse or misuse your clock you will void its warrantee, and you will affect its ability to do its job.

I am not really sure what your point is. Are you asking a question? Or are you purporting a new theory of your own?

Regardless, you should probably start a new thread.

 

[King Wildog]

Thanks for replying. I felt I was on topic. Sorry, are we not talking about time? I am brand new to this Forum, so I'm not quite ready to start a new thread. I was hoping to actually find a thread to reply to which would give me some experience talking about this subject. In my everyday life I have no one to discuss this with. I don't think I make up theories. I think I iterate points which may be less than mainstream, but it does seem some share my views. I haven't been here long enough to be disagreeing with anyone here. My credentials are modest at best- High school physics and a lot of Discovery and Science channel. I am an Ammonia refrigeration maintenance supervisor- so I am a novice dabbler in many subjects such as refrigeration, chemistry, thermodynamics, meteorology, electricity, mechanics, educational instruction, management- I am certainly a layman, so I am not touting myself as a professor. I disagree with relativity theory, though, and any notions of time travel. I say time travel is impossible- and I don't mean physically impossible- I mean absolutely impossible. The present tense is the only real tense. The past is only a memory or telling of what occurred in a past presence. The future is only a prediction of a future presence. I watched Hawking hoping his latest appearance was to shed new light on time travel and time itself to those of learned background- but no- I was left still shaking my fist at the TV. You see most are left to accept time for what someone tells them because most don't understand time enough to even have an opinion on it. I know I understand time as well as- in fact, obviously better than even Hawking himself. I truly did laugh out loud when I imagined a full grown man acting as a child would running around really fast trying to timetravel.

Hopefully my two cents weren't ill spent.

 

[Dale]

I don't think I make up theories. I think I iterate points which may be less than mainstream, but it does seem some share my views.

This forum is for discussing mainstream physics only. Please read the rules about overly speculative posts. There are many other forums on the internet for speculation, this one is for education.

I disagree with relativity theory, though.

But nature disagrees with you, and only her vote counts:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

I would encourage you to go elsewhere if you just want to expound an anti-relativity viewpoint and pat yourself on the back for being so visionary. On the other hand, if you want to learn some actual physics then you are welcome to stay, but you must abide by the rules which you agreed to when you signed up.

 

[bobc2]

...In relativity, we have four coordinates that are used in order to specify an event. It doesn't make sense in relativity to treat a time coordinate differently from a spatial coordinate, because when one observer is in motion relative to another observer, each observer's measurements of time and distance are related to the other observer's measurements by equations that don't break apart cleanly into time and space equations.

That is one of the most efficient and insightful comments about spacetime I've seen on this forum--and worth a repost at this point. Thanks, bcrowell.

 

[khemist]

Thanks for replying. I felt I was on topic. Sorry, are we not talking about time? I am brand new to this Forum, so I'm not quite ready to start a new thread. I was hoping to actually find a thread to reply to which would give me some experience talking about this subject. In my everyday life I have no one to discuss this with. I don't think I make up theories. I think I iterate points which may be less than mainstream, but it does seem some share my views. I haven't been here long enough to be disagreeing with anyone here. My credentials are modest at best- High school physics and a lot of Discovery and Science channel. I am an Ammonia refrigeration maintenance supervisor- so I am a novice dabbler in many subjects such as refrigeration, chemistry, thermodynamics, meteorology, electricity, mechanics, educational instruction, management- I am certainly a layman, so I am not touting myself as a professor. I disagree with relativity theory, though, and any notions of time travel. I say time travel is impossible- and I don't mean physically impossible- I mean absolutely impossible. The present tense is the only real tense. The past is only a memory or telling of what occurred in a past presence. The future is only a prediction of a future presence. I watched Hawking hoping his latest appearance was to shed new light on time travel and time itself to those of learned background- but no- I was left still shaking my fist at the TV. You see most are left to accept time for what someone tells them because most don't understand time enough to even have an opinion on it. I know I understand time as well as- in fact, obviously better than even Hawking himself. I truly did laugh out loud when I imagined a full grown man acting as a child would running around really fast trying to timetravel.

Hopefully my two cents weren't ill spent.

It would do you some good to read up on material relating to relativity and how time is dilated. You will also find that relativity does nothing to predict time travel, just a change in the rate of clocks ticking at in two different reference frames, and it has nothing to do with the fact that clocks are past their warranty date...

Your approach to time is not a physical question but a philosophical question.

 

[jtbell]

I am brand new to this Forum

In that case I suggest you click on the "Rules" link at the top of every page here, and note particularly the section on Overly Speculative Posts.

 

[Zephyr777]

I thought Pyotr Ouspenskii was quite concise in his explanation of time as it pertains to our understanding of the proposed dimensional structure of the universe. Events that occurred have equal relevance with respect to measurements as events that are still occurring, and those that have yet to occur. That is why we keep records and make predictions about the outcome of the experiments we are performing.

Perhaps the reason why there seems to be so much difficulty understanding time dilation is because the four-vector model is incomplete and fails to account for all three dimensions of time?

In that case I suggest you click on the "Rules" link at the top of every page here, and note particularly the section on Overly Speculative Posts.

I believe you are also being a little too stringent in your interpretation of the rules. The section you refer to clearly states that only, "Poorly formulated personal theories, unfounded challenges of mainstream science..." will not be tolerated. According to the rules, as long as his statements are well formulated and/or founded challenges to mainstream science, then he is well within his rights to question this stuff. After all, up until 1914, 'mainstream science' was rather convinced that electrons resided in the nucleus of the atom, and until the 1940's that it was not possible to travel faster than the speed of sound.

 

[jtbell]

Perhaps the reason why there seems to be so much difficulty understanding time dilation is because the four-vector model is incomplete and fails to account for all three dimensions of time?

Can you provide a reference to a peer-reviewed article in a professional physics journal that discusses three-dimensional time? I didn't find anything with a quick Google search.

According to the rules, as long as his statements are well formulated and/or founded challenges to mainstream science, then he is well within his rights to question this stuff.

If someone in the professional physics community has taken up an idea and published it in a peer-reviewed journal, then it is fair game here. Sometimes we accept articles that have been "published" only on arxiv.org, but this is on a case-by-case basis because that site is not peer-reviewed.

If it is the poster's own personal unpublished theory, then it can be discussed (here on PF) only in the Independent Research forum.

The physics forums are for physics topics, i.e. related to things that we can actually do experiments about, at least in principle. We have a philosophy forum for philosophical and metaphysical discussions.

 

[Dale]

I believe you are also being a little too stringent in your interpretation of the rules.

Hi Zephyr777, welcome to PF?

jtbell is correct. This forum is not for philosophy and it is not for speculation and it is not even for the advancement of physics as a whole. All of those have their value and their place, but the purpose of this form is for education. There are many forums without moderation where you can go if you want to speculate and philosophize, and there are many scientific conferences that you can attend if you want to advance physics. This is not the place for those, and to permit that on this forum would be to lose the thing that makes it unique and valuable.

 

[bobc2]

Perhaps the reason why there seems to be so much difficulty understanding time dilation is because the four-vector model is incomplete and fails to account for all three dimensions of time?

If you would look at some of the many posts on this forum dealing with time dilation you would see that there are many experienced physicists and students on here who demonstrate a good understanding of time dilation. I don't think they are having any difficulty at all.

I am a relatively new-comer and have been very impressed with the presentations on this forum (unlike those of some of the less disciplined forums). I'm sure the standards maintained here account in some measure for the quality of the contributers attracted to this site.