# Arrow of Time? I don't understand.

"Arrow of Time?" I don't understand.

I don't understand, and I'm not kidding about that, TOO many people that are a hell of a lot smarter than me ponder the question of why there appears to be an "arrow" of time, i.e. "why doesn't time run in reverse every now and then".

So I have to figure that the problem isn't in asking the question, the problem has to be that I've got something screwed up in MY head.

If you could bare with me for a time, I'd like to explain exactly why the entire question seems to me to be - literally - an inherently , a forever untestable, and therefore moot question.

Now mind you, I'm NOT trying to put forward an argument or theory here. My intent is to make clear my own fouled up thinking, so that exactly where I'm getting all of this tangled up can be more clearly seen, and explained to me.

That said - my own answer to the question of "Why is there an 'arrow' to time?" would (at the moment) be "Well, there isn't".

Time probably DOES run backwards, all the time, but - due to the nature of human perception, there's just no way in hell it's EVER going to be possible to detect - ever.

A thought experiment:

Suppose for a moment that you could take a snapshots of the state of every impulse within a human brain. Each of these snapshots is represented by a playing card.

So now, as you take one snapshot of the brain for each passing instant (say, of a "teacup" falling and breaking), you start laying them in a neat stack.

As time moves forward , your memory of passing events is represented by the stack of accumulating playing cards, where the last card laid down is always what you perceive to be "the present".

So now look at what the stack (i.e. your memory) is comprised of from moment to moment as time moves forward ...

duce, (a teacup teeters on the edge of a table)<<<--perceived as "the present"

duce, (a teacup teeters on the edge of a table)
three, (it tips off the edge and starts to fall)<<<--perceived as "the present"

duce, (a teacup teeters on the edge of a table)
three, (it tips off the edge and starts to fall)
four, (the teacup is in mid-air falling)<<<--perceived as "the present"

duce, (a teacup teeters on the edge of a table)
three, (it tips off the edge and starts to fall)
four, (the teacup is in mid-air falling)
five, (it hits the floor and shatters)<<<--perceived as "the present"

duce, (a teacup teeters on the edge of a table)
three, (it tips off the edge and starts to fall)
four, (the teacup is in mid-air falling)
five, (it hits the floor and shatters)
six, (the pieces come to rest)<<<--perceived as "the present"

Now suppose that time DID IN FACT begin to run backward. Well, regardless of HOW the hell your memory actually works, it doesn't matter, because that process would be getting undone from moment to moment as time ran in reverse. It would HAVE to be.

As a result, even though time IS IN FACT running backwards, you wouldn't be able to sense that. To you, time would still appear to be running forward, with "the present" always perceived as the card on the top of the pile, and "the past" consisting of the cards beneath it.

In the first instant time begins to run backwards your memory of events would be comprised of...

duce, (a teacup teeters on the edge of a table)
three, (it tips off the edge and starts to fall)
four, (the teacup is in mid-air falling)
five, (it hits the floor and shatters)
six, (the pieces come to rest)<<<--perceived as "the present"

... and so the pieces coming to rest is what you'd perceive as "the present".

In the second instant of reverse time, as the teacup pieces jump back towards the point of impact, your memory of those pieces having spread from the point of impact would be de-constructed as the chemical processes that formed that memory also ran in reverse.

As a result you'd only possess memories up to the moment of impact, and now that moment of impact becomes what you perceive as "the present", since in the second instant of reverse time, your memory now consists of...

duce, (a teacup teeters on the edge of a table)
three, (it tips off the edge and starts to fall)
four, (the teacup is in mid-air falling)
five, (it hits the floor and shatters)<<<--perceived as "the present"

In the third instant of reverse time, your memory of the teacup hitting the floor would be de-constructed (again, because the very chemical processes that formed that memory would be running backwards), and now "the present" would appear to be those moments of the cup in mid-air falling.

duce, (a teacup teeters on the edge of a table)
three, (it tips off the edge and starts to fall)
four, (the teacup is in mid-air falling)<<<--perceived as "the present"

An instant of reverse time later...

duce, (a teacup teeters on the edge of a table)
three, (it tips off the edge and starts to fall)<<<--perceived as "the present"

and finally...

duce, (a teacup teeters on the edge of a table)<<<--perceived as "the present"

At this point, time could go back to running forward again, and you'd have no idea at all that it ever happened, or any means at all of detecting that it had actually occurred.

I'm hoping that at this point someone can see exactly where I've got this all tangled up. The problem I have is that I don't see an arrow of time, I see an "arrow" inherent in human memory, due to the chemical processes that produce memory running in reverse, and "undoing" memories from moment to moment if time where to run in reverse.

And just to be clear - I do NOT believe I've got this right and everybody else has got it wrong. Lets face it, when a guy like Steven Hawking starts talking about an "arrow of time", I've got to figure he, and probably one heck of a lot of other people, have already thought of what I'm pointing out here, and that I'm the one lost in my own undies so to speak.

I'd also suspect that anyone teaching has probably also already had to untangle the thoughts of a student or two making the same point semester, after semester, after semester...

So... what am I missing here?

You're right in a way: according to (classical) physics time doesn't "run", since the entire past and future are completely determined by any single moment. And it's completely symmetric. But that misses the point of the question:

Why do we always remember one side (the past) and not the other (future)? Ignoring that, why is it that in all our records, intact porcelain cups always *precede* broken shards and dust (but never the other way around)? There is a pronounced asymmetry, despite your argument that no asymmetry should exist.

Claude Bile
Our perception of time is aligned with the thermodynamic arrow of time, since it takes energy (and a production of entropy) to create a memory (we therefore "remember" a low-entropy history). Our perception of time is not an illusion, it is very real, since it is aligned with thermodynamics, which is also very real.

(The thermodynamic arrow of time is the overwhelming probability that entropy increases in a certain direction of time - what we call forward).

The next question is, do you accept a thermodynamic arrow of time to be a "true" arrow of time?

Claude.

The problem is that time does not 'flow' like a river from the past into the future- since there's nothing to judge the flow against. How can you say that time is going forward if you haven't specified what you're taking the direction relative to?

Block time is one philosophical way out of the dilemma- but it feels uncomfortable for psychological reasons.

You're right in a way: according to (classical) physics time doesn't "run", since the entire past and future are completely determined by any single moment. And it's completely symmetric. But that misses the point of the question:

Why do we always remember one side (the past) and not the other (future)? Ignoring that, why is it that in all our records, intact porcelain cups always *precede* broken shards and dust (but never the other way around)? There is a pronounced asymmetry, despite your argument that no asymmetry should exist.
As Penrose pointed out- in a non-equilibrium state- entropy should increase in both forward and backward directions from the present time. i.e. it should also increase as you wind the clock backwards. The only reason it doesn't is because the origin of the universe was a period of comparatively low entropy. It's like taking a random walk from the top of a hill- you will always go down.

Edit: An example: Run a simulation of a tottering pencil precariously on its end. If you run the simulation forward or backwards in time the pencil will fall over. There's nothing in that simulation which tells you what the 'forwards' direction of time should be. Thus- there's a time symmetry to the laws of physics.

Last edited:
As Penrose pointed out [..]
Can that really be attributed to Penrose?? Anyway, you are clearly aware that in our universe an arrow of time does exist, contrary to:
The problem I have is that I don't see an arrow of time
And since an objective time-arrow does exist (despite seeming contrary to a fact which we are all aware of, that everything is really time-reversible), the explanation of the time-arrow is a worthy topic for study (and no doubt Penrose could tell you that one state of the universe had a vastly lower probability than would seem necessary for us, which only raises further questions).

I actually plan to discuss this in my dissertation.

Short and Horribly Butchered Almost To The Point Of Nonsense Version:

The Cosmological Topography of the universe has a "swivel" toward the beginning. In a forward time arrow this swivel widens into a universe. In a backward time arrow this swivel compacts into a singularity. Do we exist? Sure. Must have been a forward arrow then.

So far the expanding/compacting force on the "swivel" has only been dubbed Time-Torque. Mostly because of those bad 50's sci-fi movies where Time as a prefix made something cool and edgy.

You're right in a way: according to (classical) physics time doesn't "run", since the entire past and future are completely determined by any single moment. And it's completely symmetric. But that misses the point of the question:

Why do we always remember one side (the past) and not the other (future)? Ignoring that, why is it that in all our records, intact porcelain cups always *precede* broken shards and dust (but never the other way around)? There is a pronounced asymmetry, despite your argument that no asymmetry should exist.
That's what confuses me Ces. The thought that the reason we always remember the past and not the future, is that our memories of the future would have to be deconstructed as time ran in reverse.

Think of it this way - imagine making pencil strokes on a piece of paper. You know, like you're counting something and making one small vertical line for each item counted.

As you make each stroke the "lead" (graphite, whatever) is deposited on the paper as the pencil moves over it.

Now time starts to run backwards.

As the the pencil retraces its motion in reverse, if time really WAS running in reverse, then the graphite would be returning from the paper back to the pencil lead (with each and every atom of graphite winding right back where it had been before), and thus, not only would there be no marks on the paper, there wouldn't even be any evidence that the lead had ever been deposited on the paper in the first place. So in the end, you be back to a virgin and unmarked piece of paper.

What I'm saying is that the same thing would have to happen with your memory. As time ran in reverse, every single sub-atomic particle, every electrical impulse of every single synapse, would be going right back to where it had been before, just like the lead returning to the pencil, and again, you'd wind up with "a blank and untouched sheet of paper", i.e. no memory of time having reversed itself.

In that case the "arrow" of time wouldn't really exist. Time could be reversing itself all the time, you'd just have no way of remembering, or even detecting that it had.
----------------------------------------

Claude Bile - (sorry I don't know how to quote more than one post at a time)

"The next question is, do you accept a thermodynamic arrow of time to be a "true" arrow of time?"

Well, here's the thing, it seems to me that for time to truly run in reverse, entropy, pretty much by definition, would HAVE to also run in reverse. Things would HAVE to go from cold to hot. Broken teacups could ONLY go from broken to whole. Things would have to migrate from the more likely to less likely. Otherwise what you're talking about wouldn't count as a true reversal of time.

I.e. "The total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value as time moves forward, however the total entropy of any isolated thermodynamic system decreasing over time, approaching a minimum value as time moves in reverse, can not be recorded or observed since any such recordings or memories of such observations would be obliterated by a reversal of time?"

A very rough and imperfect analogy. As you approach the speed of light, time really does in fact slow down, but you can't see it, or feel it, or measure it with a clock within your own inertial reference frame, since the clock slows down as well.
-------------------------------------------------
christianjb

"As Penrose pointed out- in a non-equilibrium state- entropy should increase in both forward and backward directions from the present time. i.e. it should also increase as you wind the clock backwards."

That I can't agree with Chris. That wouldn't count as true time reversal. To count as true time reversal - EVERYTHING - including entropic states, would have to return to where they were previously, other wise you're not talking about time backing up to an earlier moment. At best you're talking about time moving forward, along a remarkably similar mirror image path, to a later moment in time, where things are NOT as they were in the past, and therefore time hasn't reversed at all, it's continued to move forward.
---------------------------------------
GleefulNihilism

"The Cosmological Topography of the universe has a "swivel" toward the beginning. In a forward time arrow this swivel widens into a universe. In a backward time arrow this swivel compacts into a singularity. Do we exist? Sure. Must have been a forward arrow then."

Not sure I follow. Could you expand a bit?

Time could be reversing itself all the time, you'd just have no way of remembering, or even detecting that it had.
When people talk about the arrow of time, they are not disagreeing with that statement, in fact they are not saying that time "runs"/"flows" at all.

They are merely observing that it obviously is possible to distinguish events in one direction of time from events in the other direction of time (for example, since you only remember in one direction). This is contrary to directions in space (for example, there is nothing universally different between events occurring to the left and events occurring to the right).

To avoid getting confused by your imagined "flow" of time, think back to when the world seemed flat. The four horizontal directions would have seemed equivalent, since no local experiment could distinguish them. But up and down *are* distinguishable (a bead inside a globe always points in the down direction). It doesn't matter what height we observe from, the distinguish-ability still persists even regardless of whether we are moving up or down in an elevator. We can validly ask ourselves why this arrow of height exists.

---------------------------------------
GleefulNihilism

"The Cosmological Topography of the universe has a "swivel" toward the beginning. In a forward time arrow this swivel widens into a universe. In a backward time arrow this swivel compacts into a singularity. Do we exist? Sure. Must have been a forward arrow then."

Not sure I follow. Could you expand a bit?
Sure, least I could do. The project is in an early enough stage that I can make up words so it's no surprise I lose people now and again.

Picture a Basic 3-D Minkowski Diagram where the x and y axis are space and the z axis is time. You, on your computer reading this article while checking your email or youtubibg or whatever it is you kids do in this day and age, are the co-ordinates (0,0,0)- because we say you can thanks to the infinite nature of the universe.

Take this diagram back toward the beginning of the universe and how can we represent the moments after the Big Bang? I'm putting forth the idea that cosmologically it would be a twisted parabola. Time becomes a mathimatically open set with an implicit Time's Arrow because backwords going energy and information would be forced into a singularity by the shape. A forward moving Time's Arrow expands out, allowing for a universe where a backwards one does not.

Have I done the calculations yet? Hell no. This is a hypothesis at best. The project is barely started and will probably take years. I've been calling the twisting "Time-Torque" solely because those 50's movies where the hero dons his "Time-Suit", "Space-Goggles", "Jet-Pack", "Radar-Boots", and the like amuse me.

I hope that helps.

I don't think there's a such thing as 'time', at all.

Outside of describing a rate of change, that is. Relative to another rate of change.

Beyond that?

There's no 'arrow of time'. There's no reversal of time. Time is what we use to measure one form of motion (change) against a yardstick of motion/change (a clock).

Which is a bummer to time travel theorists, I suppose. Sorry.

ZapperZ
Staff Emeritus
I don't think there's a such thing as 'time', at all.

Outside of describing a rate of change, that is. Relative to another rate of change.

Beyond that?

There's no 'arrow of time'. There's no reversal of time. Time is what we use to measure one form of motion (change) against a yardstick of motion/change (a clock).

Which is a bummer to time travel theorists, I suppose. Sorry.
Then would you care to offer an explanation on why "T" in CPT symmetry is one of THE most fundamental symmetry principle that we have today, on par with the parity and charge conjugation? Do you also think that there is no such thing as the principle of conservation of energy? Look at Noether theorem to figure out the connection.

Zz.

Then would you care to offer an explanation on why "T" in CPT symmetry is one of THE most fundamental symmetry principle that we have today, on par with the parity and charge conjugation?
It's still simply a quantification of a rate of change from a frame of reference.

ZapperZ
Staff Emeritus
It's still simply a quantification of a rate of change from a frame of reference.
You missed the significance of the fact that the "T" symmetry is on par with both C and P. It means that if you "demoted" T to being non-existent, then you also demoted both space and charge to the same level. That's why I asked for how you would care to explain such a thing.

You also have ignored the fact that broken time-reversal symmetry is a fundamental signature of many phenomena. Don't you find it rather strange that something you claim to not exist is such a fundamental property of many phenomena?

Zz.

I'm not dismissing its significance, but rather the commonly-held conception that time is a 'thing' that has flow and can be reversed. The motion and energies of particles can be reversed, but that is no more an example of 'time flowing backwards' than a clock that runs backwards. I feel that concepts such as an 'arrow of time' are meaningless.

ZapperZ
Staff Emeritus
I'm not dismissing its significance, but rather the commonly-held conception that time is a 'thing' that has flow and can be reversed. The motion and energies of particles can be reversed, but that is no more an example of 'time flowing backwards' than a clock that runs backwards. I feel that concepts such as an 'arrow of time' are meaningless.
But you just SAID earlier that there's no such thing as "time", or did you forget?

Note that I can also play this game. I can say that there's no such thing as "space", since it is merely the time it takes for light to travel in a certain period. It has the SAME logic as what you applied. So how come I don't hear anyone trumpeting that space doesn't exist?

The concept of arrow of time is NOT meaningless, unless you think you can trump over Boltzmann. There's a very nice article that appeared in Physics Today a while back regarding the concept of the http://www.math.rutgers.edu/~lebowitz/PUBLIST/lebowitz_370.pdf" [Broken] using nothing more than pure statistical mechanics. Have you read it before making these statements?

If the concept of "arrow of time" is meaningless, then you need to explain to me why broken time reversal symmetry is such a fundamental concept in superconductivity, anti-ferromagnetism, etc.. etc... You need to stop throwing out all of these meaningless, handwaving argument that simply are based on a matter of tastes, and provide concrete examples in PHYSICS to support such claims. If not, this does not belong in the physics forums.

Zz.

Last edited by a moderator:

GleefulNihilism - "I hope that helps." Well, umm,.. not really. At least not yet. I looked up "minkowski diagram" on the 'ol Wikipedia. There's a lot there to read before I've got the very slightest chance of understanding where you're goin'. I do however like the term "time-torque".

;-)
I feel quite certain that Exeter could have spent many hours explaining the more deleterious effects of Time-Torque on Metalunan Mutants to Drs. Ruth Adams & Cal Meacham.
;-)

I see what you're saying. However, I think you are incorrect.

[You assume] if time suddenly started moving backwards for 5 minutes, we wouldn't be able to tell. However, how much time does it take for time to move backwards? Let's assume that it takes the same amount of time for time to move backwards as it does to move forwards. Hence, if it took the tea cup 3 seconds to fall and shatter, it would take 3 seconds from that point for the tea cup to come back together again. At the end of this period, it would have been 6 seconds, and thus we have a contradiction, because 0 seconds should have passed.

Now, I know you will want to respond by saying that "if time truly moved backwards, the clock would also move backwards", as if you had 3 seconds for the cup to shatter, then -3 seconds for the cup to come back together again. However, the concept of negative time is meaningless, so again, this doesn't sustain your argument.

Last edited:

You missed the significance of the fact that the "T" symmetry is on par with both C and P. It means that if you "demoted" T to being non-existent, then you also demoted both space and charge to the same level. That's why I asked for how you would care to explain such a thing..
What you seem to miss is the point that you are using symbols as proof that time is a thing. If time is a physical thing then it would be perceivable in some way. This symbol "T" that you are using to describe time, what exactly is this symbol supposed to represent?

Do you think time is a physical thing or a concept?

We can use the letters C. A. T. to represent the actual thing that is perceivable and exists. We can all agree that the letters C. A. T. are not a living breathing thing that we know as a cat. So this letter "T", what real thing is it representing?

Note that I can also play this game. I can say that there's no such thing as "space", since it is merely the time it takes for light to travel in a certain period. It has the SAME logic as what you applied. So how come I don't hear anyone trumpeting that space doesn't exist?.

This thread is dedicated to the discussion of time, having said that, I will tell you that space is not a physical thing, it is a concept.

The term space is used to describe that area of nothing between objects. That area between you and what you are observing, that is space. Space is caused by looking out from a point. The concept of space comes about from the idea that one perceives through something when looking out from our point of view. There are objects that exist other than where we are viewing from, and by looking out to these items we create the idea of space. If space were a real physical thing don’t you think that reference books would state that space is a real thing. If space were a real physical thing it would have to exist in a location in space, that would not be logical. This thing called space if it were real would have an atomic structure, yet there is no such structure called space.

If anyone here thinks that space is a physical thing please give a reference or observation that provides evidence of this.

Thank You

This thing called space if it were real would have an atomic structure[...]
If anyone here thinks that space is a physical thing please give a reference or observation that provides evidence of this.
One reason why your post sounds nonsensical is that your use of the term "physical" is not aligned with the meaning used by physicists.

...This is contrary to directions in space (for example, there is nothing universally different between events occurring to the left and events occurring to the right).
Excellent cesium! An excellent point to consider.

This is true of empty space, but there are uncountable physical arrangments where one direction in space is not the same as the opposite: a smear or mud thicker at one end than the other, a gravitational gradient... So how could this apply to time?

Last edited:

As Penrose pointed out- in a non-equilibrium state- entropy should increase in both forward and backward directions from the present time. i.e. it should also increase as you wind the clock backwards. The only reason it doesn't is because the origin of the universe was a period of comparatively low entropy. It's like taking a random walk from the top of a hill- you will always go down.
How could this account or be related to CPT symmetry? Not a easy question, of course. Just wondering.

If the concept of "arrow of time" is meaningless, then you need to explain to me why broken time reversal symmetry is such a fundamental concept in superconductivity, anti-ferromagnetism, etc.. etc...
Zz.
I had no idea that time assymetry showed up in superconductivity and I'm not sure what anti-ferromagnetism is. Could you explain where it enters in?

One reason why your post sounds nonsensical is that your use of the term "physical" is not aligned with the meaning used by physicists.

Alright, tell me the meaning of physical that is used by physicists. I just thought that physical as it is used in all of the physics books and physics references and all the standard dictionaries that I have seen was the meaning of physical that is used by physicists.

Please correct my mistake and give me the standard definition that physicist use to describe the term physical.