B Why do we Experience a 'Flow' of Time?

[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.

 

[timmdeeg]

Yes,

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.

If we talk about the flow of proper time how can you "model" it using motion? Proper time is invariant but motion isn't.

 

[Arman777]

If we talk about the flow of proper time how can you "model" it using motion? Proper time is invariant but motion isn't.

For a light clock it can be defined as $\frac {2d}{c}$ where the $d$ is distance between two mirrors.

Here again there's $c$ which is motion of a light.

I don't think its possible to measure/define time using motion.

Time and motion are linked together. We can't define motion without time but also vice versa

 

[PeroK]

For a light clock it can be defined as $\frac {2d}{c}$ where the $d$ is distance between two mirrors.

Here again there's $c$ which is motion of a light.

I don't think its possible to measure/define time using motion.

Time and motion are linked together. We can't define motion without time but also vice versa

What about studying, say, the posts on PF? The number of posts per hour, average, maximum etc. There you clearly have the concept of time, but no immediate concept of motion through space.

Or, it would seem artificial to consider the concept of carbon dating, say, in the context of four dimensional spacetime.

 

[Stavros Kiri]

why do we experience a flow of time? Why is everything in the 'now' flowing toward the future, and not the past? Also, why do we all experience this flow the exact same way?

From the practical standpoint, the flow and arrow of time is connected to motion, processes and the 2nd law of thermodynamics. Especially irreversible processes "leave their mark" and they cannot but be (potentially) observed, especially since we also take part. I think that explains (partially [?]) of what we feel etc. ...

 

[Arman777]

What about studying, say, the posts on PF? The number of posts per hour, average, maximum etc. There you clearly have the concept of time, but no immediate concept of motion through space.

Or, it would seem artificial to consider the concept of carbon dating, say, in the context of four dimensional spacetime.

To post something you need to write something that's motion. Also making a clock using PF posts per hour is not a great idea. Anyway I don't think this example is a proper example ??

For carbon dating well, for decay we have also some motions that we can describe for atoms etc.

 

[Dale]

the fact that we remember the past and not the future has to do with entropy. And this is what gives us a sense of "flow" of time.

I think this actually answers the title question. Our mental experience of time is based on this.

It has always struck me that we walk through time backwards, “looking” at where we have been and unable to “see” what is right in front of us.

 

[Stavros Kiri]

I think this actually answers the title question. Our mental experience of time is based on this.

It has always struck me that we walk through time backwards, “looking” at where we have been and unable to “see” what is right in front of us.

Cf.
https://www.physicsforums.com/threads/why-do-we-experience-a-flow-of-time.941255/page-4#post-5954695

 

[Dale]

For carbon dating well, for decay we have also some motions that we can describe for atoms etc.

Radioactive decay, and other state transitions (like the hyperfine transition) do not appear to require thermal motion. In fact, it is a confounder that we try to eliminate or compensate.

 

[Arman777]

Radioactive decay, and other state transitions (like the hyperfine transition) do not appear to require thermal motion. In fact, it is a confounder that we try to eliminate or compensate.

"Carbon-14 dating, also called radiocarbon dating, the method of age determination that depends upon the decay of the nitrogen of radiocarbon (carbon-14). Carbon-14 is continually formed in nature by the interaction of neutrons with nitrogen-14 in the Earth’s atmosphere; the neutrons required for this reaction are produced by cosmic rays interacting with the atmosphere."

https://www.britannica.com/science/carbon-14-dating

I mean this a type of motion. Carbon 14 just don't produce by itself and also to understand the decay part, again we need the motion. "Decay" itself is the motion.

 

[jbriggs444]

"Decay" itself is the motion.

Decay is not a "motion" in any relevant sense.

 

[Arman777]

Decay is not a "motion" in any relevant sense.

The process of decay is motion. In decay, it emits an electron and an electron antineutrino. So there's "motion".

I mean really any of you think that we can measure time without using "motion" or in other sense "velocity". Is this really how universe works or you guys just want to think that way?

Even when you measure the time you are measuring the cycles of a motion of a thing.

I am asking this seriously without motion how can you even define time? If I am wrong tell me a way to measure time without motion in any sense.

If any of you can prove this with math, I ll stop arguing. Otherwise I can assume that I am right and you guys are wrong.

And I ll be really happy If I am wrong because I want to see a way that its possible.

 

[Sorcerer]

What about studying, say, the posts on PF? The number of posts per hour, average, maximum etc. There you clearly have the concept of time, but no immediate concept of motion through space.

Or, it would seem artificial to consider the concept of carbon dating, say, in the context of four dimensional spacetime.

But wouldn’t someone in motion with respect to Earth measure those same coordinates as mixed between space and time as described by the Lorentz transformation?

 

[rede96]

I am asking this seriously without motion how can you even define time? If I am wrong tell me a way to measure time without motion in any sense.

Imagine a universe where you are the only thing that exits. You are floating in space, you have no sight, hearing, touch, taste or smell. Only your thoughts. It is impossible for you to define ‘motion’ in the way you have used the term ‘motion’ in your posts. Yet you can still count the seconds in your mind.

 

[Arman777]

Imagine a universe where you are the only thing that exits. You are floating in space, you have no sight, hearing, touch, taste or smell. Only your thoughts.

A problematic description. First thing is how can you define only yourself in the entire space and nothing else?

Yet you can still count the seconds in your mind.

I don't see any math in your description. I don't think this argument will be valid.
How you defined time? by just counting seconds in your mind? Whats seconds for you ? just counting numbers? Even you count numbers there's "speed" (motion). I can count 10 number faster than you.
In this sense there's also speed of how fast you count.

In any case I don't think this is valid

 

[jbriggs444]

A problematic description. First thing is how can you define only yourself in the entire space and nothing else?

I don't see any math in your description. I don't think this argument will be valid.
How you defined time? by just counting seconds in your mind? Whats seconds for you ? just counting numbers? Even you count numbers there's "speed" (motion). I can count 10 number faster than you.
In this sense there's also speed of how fast you count.

In any case I don't think this is valid

None of those objections are substantive.

 

[Arman777]

None of those objections are substantive.

What about his objection. Do you think its valid?

Is it solves my question? I don't think so

I still didn't get any mathematical answer to my question rather then I get 1 philosophical answer. Which doesn't make any sense at all

 

[stevendaryl]

I don't see any math in your description. I don't think this argument will be valid.
How you defined time? by just counting seconds in your mind? Whats seconds for you ? just counting numbers? Even you count numbers there's "speed" (motion). I can count 10 number faster than you.

Well, you're now at the point of defining any change between moments of time to be motion. Obviously, the phrase "between moments of time" makes it circular if you're using "motion" to define "time". If you eliminate "between moments of time" in your definition of motion, then it no longer has to do with time. The change in temperature between the North Pole and the equator doesn't imply time or motion, unless you assume that there is an observer who observed the temperature at one place at one time and observed the temperature of the other place at another time.

So, that might be the key to the notion of time: When two different objects with different properties are described as the same object at different times. There's one object, which is me as a baby. There's another object, which is me as an old man. Time connects the two and allows us to think of them as two different states of the same human being.

 

[PeroK]

But wouldn’t someone in motion with respect to Earth measure those same coordinates as mixed between space and time as described by the Lorentz transformation?

I don't really see the relevance of that. In fact, information technology in general is a human endeavour which has a time parameter, but no recognisable map to spacetime.

For example, a computer program, is not a physical object, has no spacetime coordinates - the physical location of the program is largely irrelevant - but it does have a history. Both as a logical object, it has a version history, and as a run time object it has a usage history, say.

Time, but not space, is very much a factor in IT systems. Especially in any sort of logical rather than physical view of a system.

 

[kent davidge]

Time connects the two and allows us to think of them as two different states of the same human being

And with that being noticed, a better explanation is that time happens when there's change of state of a system, not necessarly of motion. (This always has been my thought about time, good to see that someone else also thinks so.)

 

[kent davidge]

So I think @Arman777's point is valid, the only thing is that change of state is not always change of position --it's only one possible case.

 

[Arman777]

So I think @Arman777's point is valid, the only thing is that change of state is not always change of position --it's only one possible case.

Thanks. But for example what kind of state of changes ?

In the baby and old man case. I can say that there's change in the molecular level in the body of the person. That molecular changes causes the to be person to be old. It doesn't happen by suddenly. And those molecular change can be expressed as motion. By motion, I don't consider only the change in the time dimension but also in the space. For example, when the body gets old there are molecular motions in our body, Like chemical things. We can understand that the two different states. But for change in that state again, we need a molecular motion.

Or without any motion (in atomic level or in general ) how can we understand the change in the state?

Or again in other words, change of state doesn't happen by motion?

 

[stevendaryl]

Thanks. But for example what kind of state of changes ?

In the baby and old man case. I can say that there's change in the molecular level in the body of the person. That molecular changes causes the to be person to be old. It doesn't happen by suddenly. And those molecular change can be expressed as motion. By motion, I don't consider only the change in the time dimension but also in the space. For example, when the body gets old there are molecular motions in our body, Like chemical things. We can understand that the two different states. But for change in that state again, we need a molecular motion.

Or without any motion (in atomic level or in general ) how can we understand the change in the state?

Or again in other words, change of state doesn't happen by motion?

There is certainly no necessary reason for changes of state to be associated with motion. For example, in particle physics, the neutral kaon, K^0 oscillates with time to change into its own anti-particle, \bar{K^0}.

 

[kent davidge]

for example what kind of state of changes ?

We have to be carefull about the use of the term "state". For your question I would use the situation given by @rede96. In that case there is change of state, because the very act of thinking about something involves brain activity, which changes the state of the brain from "before thinking about something" to "after thinking about something" --and this something may be the simplest of all the thoughts.

But, yet, the inteligent being may not be moving at all in space (we could not say that he/she is or is not moving in time, because we are actually trying to define time.)

 

[PeroK]

Thanks. But for example what kind of state of changes ?

In the baby and old man case. I can say that there's change in the molecular level in the body of the person. That molecular changes causes the to be person to be old. It doesn't happen by suddenly. And those molecular change can be expressed as motion. By motion, I don't consider only the change in the time dimension but also in the space. For example, when the body gets old there are molecular motions in our body, Like chemical things. We can understand that the two different states. But for change in that state again, we need a molecular motion.

Or without any motion (in atomic level or in general ) how can we understand the change in the state?

Or again in other words, change of state doesn't happen by motion?

I don't find this argument satisfactory, because age is not defined by motion. If you count my grey hairs, for example. That must be due to a difference in my body's processes from when I was younger. Okay, things must have "moved" somewhere along the line as part of that change, but it's stretching a point to say that the biological processes that produce grey rather than dark hairs are defined by that motion.

 

[Sean Nelson]

A problematic description. First thing is how can you define only yourself in the entire space and nothing else?

I don't see any math in your description. I don't think this argument will be valid.
How you defined time? by just counting seconds in your mind? Whats seconds for you ? just counting numbers? Even you count numbers there's "speed" (motion). I can count 10 number faster than you.
In this sense there's also speed of how fast you count.

In any case I don't think this is valid

I agree. To say that using this description, also leaves question to an inability to know if you are static or in motion. Without a point of reference with your description of your solidarity in space. How do you know if you are not traveling. What is your frame of reference to show you are not traveling 25,000 km per hour. This said you have removed your question of time.

 

[jbriggs444]

also leaves question to an inability to know if you are static or in motion.

That is precisely the point. That the notion of "motion", a variation of "position" in some (three dimensional?) space is not essential to the notion of time.

 

[PeroK]

Additionally, if we take a quantum system, then the conventional concept of motion.of a particle breaks down and it's actually the state (vector) that is changing.

In a harmonic oscillator, for example, it's not that the particle has moved from A to B, but that the state has changed, leading to the change in the expected value of a position measurement from A to B.

And, if we consider the spin state of a particle, then that and the expected value of spin measurements can change, with time, independent of the motion of the particle. Or any motion.

 

[Arman777]

That is precisely the point. That the notion of "motion", a variation of "position" in some (three dimensional?) space is not essential to the notion of time.

Then I am asking again how can you define time without using the notion of motion ?

 

[PeroK]

Then I am asking again how can you define time without using the notion of motion ?

As in post #97, you could, theoretically, use spin precession of an electron in a uniform magnetic field to measure time.

In general, the state of a quantum system changes with time.

 

[Dale]

Even you count numbers there's "speed" (motion).

. "Decay" itself is the motion.

I disagree with those descriptions. That is just redefining “motion” so that it loses all meaning and therefore can be used to justify your (now meaningless) claim.

The process of decay is motion. In decay, it emits an electron and an electron antineutrino. So there's "motion".

None of that is motion. After the decay then the decay products move, but the decay itself is not motion.

The usual hyperfine transition used in standard atomic clocks is not motion, and in fact must be corrected for any motion that does occur.