Why do we Experience a 'Flow' of Time?

[stevendaryl]

Moreover, the laws of physics are not sufficient to explain any experience:
http://philsci-archive.pitt.edu/12325/

I'm actually a long-time friend of David Chalmers (in the internet sense, although I did actually meet him in real life a couple of times, and he stayed at my house once). Consciousness is a fascinating topic, but part of what makes it a "hard problem" is that it's not clear what it would mean to explain it.

 

[stevendaryl]

I still don't agree. 4-Dimensional spacetime is a nice mathmatical concept dealing with events and Lorentz transformations between them, it's not existing of four spatial dimensions, though.

The theory of relativity in the modern formulation treats the four dimensions in the same way, though.

And now I couldn't disagree more! I know what you mean, but it simply isn't so. Saying that the other (not traveling) person is traveling with 2.29 hour per hour into the future with respect to the traveller is just as true, or untrue.

The same is true of traveling in space. You say that I'm traveling at 20 kilometers per hour. I say that you're traveling at 20 kilometers per hour. So why are you saying that it's more problematic when talking about travel through the time dimension?

 

[stevendaryl]

Only in case the traveler returns, there will turn out to be a time difference between the two; what I was trying to argue, is that they will have measured a different amount of time, not a different kind of time ('faster' or 'slower'). That's completely different from 'traveling into the future' or formulations alike.

I think you're completely wrong about that. Let's go back to Newtonian physics, and let's ask the simple problem:

An automobile is traveling at 80 kilometers per hour in the x-direction. His destination is 320 kilometers away in the x-direction. How long does it take for him to get there?

The answer is simple algebra: [itex]V^x = \frac{\delta x}{\delta t} \Rightarrow \delta t = \frac{\delta x}{V^x}[/itex]. So the answer is [itex]320/80 = 4[/itex] hours.

Now, let's switch to Special Relativity. A rocket is traveling at a proper velocity [itex]V^t[/itex] of 2.29 hours coordinate time per hour of proper time. His destination is the year 2068, which is 50 years in the future. How long does it take him to get there? Again, the answer is completely straight-forward:

[itex]V^t = \frac{\delta t}{\delta \tau} \Rightarrow \delta \tau = \frac{\delta t}{V^t}[/itex]. So the answer is 50/2.29 = 21.83 hours of proper time.

It takes the rocket only 21.83 hours of proper time to get to the destination, which is 50 years in the future.

I don't see how turning around is relevant to the question.

Now, the fact that the destination was 50 years in the future is of course a coordinate-dependent quantity. There are other coordinate systems in which it is more or less than 50 years. But that isn't essentially different from the problem for Newtonian physics. There, the destination being 320 kilometers away is a coordinate-dependent quantity, as well. But in both cases, the answer is independent of coordinate systems: No matter what coordinate system you use, the car will take 4 hours to get to its destination. The rocket will take 21.83 hours (proper time) to get to its destination. In neither case, is "turning around" relevant.

 

[pinball1970]

that time is flowing seems a subjective statement with no more meaning for physics than 'mass is increasing with mass' or 'temperature is increasing with temperature'. In fact: time is flowing with time.

I am trying my best as a layman to keep up with this.Biological systems/consciousness aside, surely velocity, acceleration, distance and the expansion of the universe would be meaningless if time was not a thing in itself?I have tried to describe the universe when it was much smaller and hotter compared to where we are now, without using time just using words, you cant.Every adjective you use has a reference to it. “Previously” “back to “ “When” “earlier” “then” “now”Usually you can represent statements like this using equations and a few posters have done this.Does not the fact you can distort time using near light speed particles in accelerators demonstrate time is a physical real thing just like mass?Mass has particles and a mechanism but you could still ask what it “is” and just end up in infinite regress.Perhaps when they work out what a quanta of space time is the philosophical part may fade away.As for consciousness? Smart creatures seem to have it and it is a function of the relative brain size, I don’t think it’s a big deal.

 

[PeterDonis]

that isn't essentially different from the problem for Newtonian physics. There, the destination being 320 kilometers away is a coordinate-dependent quantity

No, it isn't. In Newtonian physics, distance is invariant. The thing that corresponds in the SR case to distance in the Newtonian case is the spacetime interval, which in the scenario you described is 21.83 hours. So I don't think the analogy you are making in this post is really justified as you make it.

 

[PeterDonis]

I have tried to describe the universe when it was much smaller and hotter compared to where we are now, without using time just using words, you cant.

Yes, you can. You just did. You said it was "smaller and hotter". Those are descriptions that don't use time.

Every adjective you use has a reference to it.

But not every adjective has a time reference.

you can distort time using near light speed particles in accelerators

How is this "distorting time"? It's just geometry; the particles take different paths through spacetime than the observers, and those different paths have different lengths. Calling this "distorting time" is like saying that if you and I take two different routes with different lengths between New York and Los Angeles, we are somehow "distorting distance".

 

[Mister T]

Does not the fact you can distort time using near light speed particles in accelerators demonstrate time is a physical real thing just like mass?

Whether these things are real or not is not the question that seems to get to your point. The issue is whether or not these things are inventions of the human intellect. Some people claim they are, others claim they aren't. Either way, the physics is the same.

 

[stevendaryl]

No, it isn't. In Newtonian physics, distance is invariant.

The distance between two events that occur at the SAME time is invariant, but the distance between events that occur at DIFFERENT times is coordinate-dependent. If you're trying to figure out how long it takes for a car to travel to Chicago from New York City, you are dealing with events at different times.

The thing that corresponds in the SR case to distance in the Newtonian case is the spacetime interval, which in the scenario you described is 21.83 hours. So I don't think the analogy you are making in this post is really justified as you make it.

I don't see why not. If you want to know how long it takes a traveler to travel a distance in the x direction, it's [itex]\delta t = \frac{\delta x}{V^x}[/itex]. If you want to know how long it takes (in proper time) for a traveler to travel to an event in the future, it's [itex]\delta \tau = \frac{\delta t}{V^t}[/itex]. What's the disanalogy?

Note: In the Newtonian (actually, Galilean) case, the two quantities [itex]\delta x[/itex] and [itex]V^x[/itex] are frame-dependent. But the computed quantity [itex]\delta t[/itex] is frame-independent. In the SR case, the two quantities [itex]\delta t[/itex] and [itex]V^t[/itex] are frame-dependent, but the computed quantity [itex]\delta \tau[/itex] is frame-independent.

The two seem very analogous to me.

 

[PeterDonis]

The distance between two events that occur at the SAME time is invariant, but the distance between events that occur at DIFFERENT times is coordinate-dependent.

In the Newtonian (actually, Galilean) case, the two quantities ##\delta x## and ##V^x## are frame-dependent. But the computed quantity ##\delta t## is frame-independent. In the SR case, the two quantities ##\delta t## and ##V^t## are frame-dependent, but the computed quantity ##\delta \tau## is frame-independent.

Ah, I see.

 

[NoTe]

The same is true of traveling in space. You say that I'm traveling at 20 kilometers per hour. I say that you're traveling at 20 kilometers per hour. So why are you saying that it's more problematic when talking about travel through the time dimension?

You're hitting a point here (my point!), perhaps not on purpose: 'The same is true of traveling in space'. I was just trying to argue that time is not so different from distance (space) or any other quantity, as many people think it is. Just like space (or whatever), it is not flowing or moving nor has it a preferred direction - there is just more or less of it. My original point.

NB I'm not arguing with the existence of time dilation or whatever laws of physics, I'm trying to 'understand' the nature of a quantity like time, within these laws. There are a lot of possible different interpretations, as is shown in this thread.

 

[Fra]

[Saying that time is flowing seems a subjective statement with no more meaning for physics than...

I think this contains plenty of physics. The "subjectivity" at play here is simply observer depdendence.

We can get rid of time - and the observer - in Relativity by considering equivalence classes of observers. There is a tendency to consider this equivalence class as more "real" than the observer views, and seen an invariant description of things. Which is the basis for thinking time is not real, its just a necessary evil or nuisance you get form picking an observer.

But the physics(interactions) lies in the relations between these observers, we need the structure of the set, in order to appreciate its symmetry. The symmetry loose physical meaning if you start suggesting the structure has no meaning.

In the case of small subsystems, whose inteactions can be repeated we reach what Smolin calles the Newtonian schema (in his time reborn book). Here we effectively have another external observer, doing the scientific inference, but without itself beeing truly part of interactions. This external observer simply decodes the "observer equivalence" of the small subsystems.

But this all fails badly for cosmological perspectives, because there is not "context" for statistics. So no matter how you turn things around there is also a kind of cosmological time, which you can not explain away in the way you can with time in small subsystems. So what is experience "cosmological time" and what is "non-physical" "parameter time" is still a matter of perspective. And there is no, one "right" perspective. All of them must be valid. Now, what implications does this have for formulating the laws of physics?

Smolin argues at length in his book time reborn and "singular universe and reality of time" that the rational solution to this dilemma is to suppose the laws of physics the result of ongoing evolution.

Anyone who doesn't understand Smolins questions (wether you like them or not), i would recommend the books.

/Fredrik

 

[rede96]

There are a lot of possible different interpretations, as is shown in this thread.

In very layman’s terms, isn’t the flow of time at its most fundamental level simply due to the fact that all systems have energy and will therefore change?

From what I understand about QM, it’s not possible for any system to have zero energy. Therefore it must change and it’s our perception of ‘change’ that we view as the flow of time.

And as ‘change’ only has one direction, i.e. something either changes or it doesn’t, the flow of time only has one direction. And as I understand it entropy describes the natural state of that change.

Of course GR complicates that picture as time relative to other systems can change. But as the laws of physics are the same everywhere, locally we all perceive time in exactly the same way. E.g. if I measure with a clock the time for a candle to burn 1 inch, anyone else, anywhere in the universe will measure the same clock time given the same conditions.

 

[PeterDonis]

all systems have energy and will therefore change?

This does not follow. Systems in an energy eigenstate don't change.

From what I understand about QM, it’s not possible for any system to have zero energy.

Energy by itself has no meaning in QM; only energy differences do. You can set the "zero point" of energy wherever you like.

Therefore it must change

This doesn't follow. It's perfectly possible for a system to be in a state of definite energy that never changes. See above.

as ‘change’ only has one direction, i.e. something either changes or it doesn’t

This doesn't make sense. Even a scalar quantity can either go up or go down.

 

[rede96]

This does not follow. Systems in an energy eigenstate don't change.

Doesn’t that just mean the energy of that system doesn’t change?

Also, I thought all systems must have zero point energy as if the didn’t it would violate the uncertainty principle? So I didn’t think it was possible to have a system where there was simultaneously no change in all it’s constituent parts?

Just to be clear ‘no change’ to me means no change in anything. Momentum, position, energy etc.

This doesn't make sense. Even a scalar quantity can either go up or go down.

But going up or down is change.

 

[Paul Rubenstein]

I think what people always miss about this is that is time were going backward, everything would go backward: light would leave our eyes, and our neurons would "un-fire". We would never have an experience of the world going backward... the shattered cup re-forming and leaping back onto the table... because that would require our perception and cognitive processes to run forward as everything else went backward. We would not remember the future if time were going backward because everything happening in our brains would be occurring backward as well. I think people make this mistake because they cling to Decartes' mind-body split. There is no mind without a brain, and brain functions are causal and time-dependent. There is no reason to believe the arrow of time doesn't go in two directions. If time reversed, then reversed again, we'd never know it. This may happen all the time, so to speak, though from what perspective, I can't imagine.

 

[Sean Nelson]

It's hard to improve on Einstein's answer: Time is what a clock measures.

One of the nice properties of this definition is that it makes it clear that proper time, the thing that you experience as the flow of time, is what can be measured and is experienced as flowing.

Reading this forum is quite enlightening. Then I read the above answer from Einstein "Time is what a clock measures". I got a good laugh. All principles of physics. Involving countless hours of manpower discovery, theories, testing and evolving equations. All you need for time is a double "A" battery..

 

[PeroK]

Reading this forum is quite enlightening. Then I read the above answer from Einstein "Time is what a clock measures". I got a good laugh. All principles of physics. Involving countless hours of manpower discovery, theories, testing and evolving equations. All you need for time is a double "A" battery..

Physics is an empirical science. The philosophical and semi-philosophical arguments about the existence of time seem to me to be swept away by that one simple statement that a clock measures time; hence, time exists in physics in that role as a measurable quantity, if nothing else.

You can argue that time is "just another kind of distance". Okay, it's just a special kind of distance - one you measure with a clock.

 

[Tom Kunich]

You misunderstand. At any given event there is a clear future (formally, the future light cone of the event and its interior) and a clear past (ditto the past light cone). All relativity does is add a third region outside both future and past light cones that can be arbitrarily divided into past and future for that event.

The reason we experience time the way we do is that we compare what we can see now to what we now remember seeing in the past. And we remember comparing what we saw a secong ago to what we saw before that. For whatever reason our brains model that as a continuously changing world instead of a continuously accreting view of a static 4d world.

As to "why do we all experience the world the same way": do we? I wouldn't be surprised if there's a fair bit of variation although with a high degree of commonality. But in any case the answer is obvious - that's what happens if you use nearly identical hardware to perform a task.

Regarding the "direction" of time, I'd suggest watching Feynman's lecture on entropy.

Don't you think that is a bit too philosophical for the question? WE experience time simply because of biological processes.

 

[Sean Nelson]

Don't you think that is a bit too philosophical for the question? WE experience time simply because of biological processes.

No, That is why I said "I laughed" as I read it. Einstein place time in such a simplistic manner and I reacted to it.

 

[stevendaryl]

You can argue that time is "just another kind of distance". Okay, it's just a special kind of distance - one you measure with a clock.

I can't remember if I already made this comment about Einstein's remark: In some ways, it's a circular definition, because the definition of "clock" is something that accurately measures time.

 

[jbriggs444]

In some ways, it's a circular definition, because the definition of "clock" is something that accurately measures time.

Agreed. Though the fact that clocks, by and large, agree with each other is empirical evidence that whatever they measure has some basis in reality.

 

[stevendaryl]

Agreed. Though the fact that clocks, by and large, agree with each other is empirical evidence that whatever they measure has some basis in reality.

Yeah. A lot of definitions are in practice circular, or maybe helical. For clocks and time, we have a process such as this:

1. You start off with a rough, intuitive idea of the passage of time, as evidenced by cyclic processes such as sunrise/sunset, the phases of the moon, the change of the seasons, etc.
2. You invent devices clocks that measure the passage of time, in the intuitive sense.
3. The clocks then give us a more precise notion of time.
4. With this more precise notion of time, we develop physical theories that allow us to build more accurate clocks.
5. Etc.

 

[Sean Nelson]

Yeah. A lot of definitions are in practice circular, or maybe helical. For clocks and time, we have a process such as this:

1. You start off with a rough, intuitive idea of the passage of time, as evidenced by cyclic processes such as sunrise/sunset, the phases of the moon, the change of the seasons, etc.
2. You invent devices clocks that measure the passage of time, in the intuitive sense.
3. The clocks then give us a more precise notion of time.
4. With this more precise notion of time, we develop physical theories that allow us to build more accurate clocks.
5. Etc.

Well put

 

[PeroK]

I can't remember if I already made this comment about Einstein's remark: In some ways, it's a circular definition, because the definition of "clock" is something that accurately measures time.

Isn't everything in physics circular in some respect? ##F = ma##, for example. ##F## and ##m## define each other. And what is ##a## if you don't have a definition of time?

 

[Arman777]

Time or measuring time is the same as measuring the distance. There's no difference in the logic. Time is just about measuring cycles. In general, I can make a clock from everything that cycles.

The other thing that we should be careful about is that we cannot define time without defining the length and vice versa.

So asking What is time? or Why time flows is a missing question. We should ask these questions for space-time.

I think there's no "flow of time" the only thing that we can say objects are moving in space-time relative to someone and that's all.

To say flow we need ##Δ(t)## and to measure that ##Δ(t)## we need a clock that cycles but for "cycle" we also need to define the length. So what we see as "flow of time" is again just a motion of an object in space-time. Think a normal clock it cycles and makes a motion in space-time and we observe or record it.

The other interesting thing I guess to understand "flow of time" we need "memory" so that we can understand that "##Δ##" part.But it's a biological thing

 

[PeterDonis]

Doesn’t that just mean the energy of that system doesn’t change?

Yes.

I thought all systems must have zero point energy as if the didn’t it would violate the uncertainty principle?

Sort of. The idea of "zero point energy" originally came from the fact that, if you solve the most basic QM harmonic oscillator, there is a term in the Hamiltonian that is present even when the oscillator is not oscillating at all (i.e., where classically we would say that it had exactly zero energy). But that term in the oscillator Hamiltonian does not "vary"; it isn't uncertain at all. It's a definite value. So all it really means is that the vacuum--the state with zero oscillations--is also an eigenstate of the Hamiltonian. It doesn't mean there is any "uncertainty" that drives the vacuum to have "zero point energy". (This also means that the common idea of "vacuum fluctuations" is, at best, somewhat misleading. I believe there is a series of PF Insights articles on this.)

More generally, any physically reasonable Hamiltonian must have a ground state, i.e., an eigenstate of lowest energy. Often, if you just solve the equations and don't make any adjustments, the energy of this eigenstate won't be zero, just as for the harmonic oscillator. But, as I said before, absolute values of energy in QM have no physical meaning; only energy differences do. So you can always just subtract the ground state energy from the Hamiltonian to get a Hamiltonian whose lowest energy state has exactly zero energy. That's what is normally done.

A better reason for thinking of real systems as having "zero point energy" is that, for any real system, it's impossible to get it exactly into the vacuum state--because that state would be a state with nothing at all present, and if nothing at all is present, how can you have any equipment that prepares anything in any particular state? So what actually happens when you try to get some particular system into its vacuum state is that it is in a state which is "close" to the vacuum, but also has nonzero amplitude to be in some non-vacuum state--i.e., its state is not an exact eigenstate of the Hamiltonian. That means that, when you measure the system's energy, it will not always be the same, and some people interpret this in terms like "vacuum fluctuations". (IIRC, the Insights series I referred to above talks about this.)

Just to be clear ‘no change’ to me means no change in anything. Momentum, position, energy etc.

That's not possible, because, for example, momentum and position can't both have definite values in any state. An energy eigenstate will generally be a momentum eigenstate, but certainly not a position eigenstate. So it is impossible to have any quantum state that does not "change" by your definition. Which means your definition is unhelpful, since it doesn't pick out any particular states at all.

going up or down is change

Change in two different directions (up vs. down), yes. Not "just one direction of change", which is what you were claiming.

 

[PeroK]

I think there's no "flow of time" the only thing that we can say objects are moving in space-time relative to someone and that's all.

If there is no flow of time, what do you mean by "moving"?

 

[Sean Nelson]

If there is no flow of time, what do you mean by "moving"?

Time is part of the uniform larger fabric of the universe, not something moving around inside it

 

[Arman777]

Time is part of the uniform larger fabric of the universe, not something moving around inside it

Yes,

If there is no flow of time, what do you mean by "moving"?

By moving I mean velocity of the object (or motion) in space-time. And this is the point where everything should be clear. We can describe an object that only moves in the "time" axis or only on the "space". But that doesn't reflect the reality due to SR. All objects move in space-time hence all motions can be desrcibed as velocity. We define time using the motion. Thats what I am thinking at least.

 

[Dale]

All objects move in space-time hence all motions can be desrcibed as velocity.

Hmm, the common “block universe” view of spacetime would probably say that no objects move in spacetime. In that view, instead of “motion” you have tangent vectors to worldlines.

 

[Arman777]

Hmm, the common “block universe” view of spacetime would probably say that no objects move in spacetime. In that view, instead of “motion” you have tangent vectors to worldlines.

I see well okay then.

 

[PeroK]

I see well okay then.

It's not okay. If someone can be really clever and describe physics without using the notion that time "flows", then all well and good. But, all you're doing is declaring that you don't need this concept, but that leaves you with no way to describe physics. Put simply, we use a time parameter to put events into a sequence. You can study 1D, 2D or 3D problems, but you always need a time dimension. You can't drop the time dimension and study physics in two spatial dimensions.

If you want to move away from this view, you need something more sophisticated than a wave of the hand and a vague reference to "spacetime".

 

[Arman777]

It's not okay. If someone can be really clever and describe physics without using the notion that time "flows", then all well and good. But, all you're doing is declaring that you don't need this concept, but that leaves you with no way to describe physics. Put simply, we use a time parameter to put events into a sequence. You can study 1D, 2D or 3D problems, but you always need a time dimension. You can't drop the time dimension and study physics in two spatial dimensions.

If you want to move away from this view, you need something more sophisticated than a wave of the hand and a vague reference to "spacetime".

I am not saying we don't need time or time dimension. I am saying that there's no such thing as just "time" or "space" there's only spacetime

You cannot separate space and time and treat them as different things.

Or in general, objects "flow" in spacetime not just in time.

 

[PeroK]

I am not saying we don't need time. I am saying that there's no such thing as just "time" or "space" there's only spacetime

You cannot separate space and time and treat them as different things.

Or in general objects flows in spacetime not in "time".

Time and space are different. For example, as soon as you start to do physics it's the time derivatives (of spatial coordinates among other things) that appear. You don't have equivalent formulations of the laws of motion in terms derivatives with respect to the spatial coordinates.

 

[rede96]

Change in two different directions (up vs. down), yes. Not "just one direction of change", which is what you were claiming.

Ah ok. No that’s not what I was claiming at all. What I was trying to say was along the lines that change, as in the evolution of the system is absolute. Something either changes or it doesn’t. So in that sense evolution is one directional. Always moving forward, never static. And if it can never be static then it can’t reverse as it can’t go through zero change. Hence why the flow of time is always forward.

That's not possible, because, for example, momentum and position can't both have definite values in any state.

Again, that was my point. As it’s impossible for a system not to change all systems are constantly evolving forward. Which is how I understood that the flow of time was always forward. And our perception of time is just how we perceive those changes. In very layman’s terms.