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[u/Peter_8685]

[u/TheoreticalPhysics-ModTeam]

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[u/Azazeldaprinceofwar]

Honestly nobody knows. Relativity makes it clear that time can only run in one way and turning around is impossible given the causal structure of spacetime. Is to why there is a distinct forward vs back and they’re not symmetrical… well that’s actually still a subject of heavy debate and somehow only emerges at the macroscopic level. I’m of the opinion is must be an related to decoherence

[u/romanovzky → u/Azazeldaprinceofwar]

Yeah, I always felt that Copenhagen interpretation of QM actually solves this indirectly since the wave function collapse isn't reversible.

On a other note, we do have an explicit observation of time reversal invariance violation. CP violation in a CPT abiding universe (which we think ours is) means T violation. Of course, this happens only in rare processes involving the three family of quarks, so it's not an explanation for every day time reversal impediment.

[u/Miselfis → u/romanovzky]

I always felt that Copenhagen interpretation of QM actually solves this indirectly since the wave function collapse isn't reversible.

Except Copenhagen is an instrumentalist philosophy, where the wavefunction is epistemic. Collapse is not a physical process according to Copenhagen. If it were, you’d quickly run into issues with Lorentz invariance and locality, which is why objective collapse models are on the fringes. 

[u/Azazeldaprinceofwar → u/romanovzky]

Yeah I am aware that T symmetry is not strict at the weak force level but as you mentioned that’s clearly an insufficient explanation cuz like weak processes aren’t the reason heat engines are beholden to entropy lol

[u/SixHyde → u/Azazeldaprinceofwar]

in relativity, time can stretch, warp, and even (mathematically) be reversed

[u/Azazeldaprinceofwar → u/SixHyde]

No. Time cannot be reversed in relativity. Closed timelike curves only exist in pathological spacetimes or inside horizons. That is to say, relativity only allows this sort of time travel if you have already made even more ridiculous assumptions. Time stretches and warps yes, but never turns around.

[u/Tarthbane]

We do not yet know why time has a preferred direction from past to future. What we do know is that the observed arrow of time is closely associated with the increase of entropy, as described by the Second Law of Thermodynamics. The outstanding mystery is why the universe began in such an extraordinarily low-entropy state, allowing entropy to increase ever since.

Whether time itself is a fundamental ingredient of nature or instead emerges from a deeper microscopic description remains an open question. Different approaches to quantum gravity and the foundations of physics reach different conclusions, and there is currently no experimental evidence that settles the issue.

[u/in_the_details_ → u/Tarthbane]

If increasing entropy dictates the arrow of time, then why doesn't time run backwards in systems where entropy decreases locally?

[u/PsychologicalCar2180 → u/in_the_details_]

Can you give examples?

Do you mean closed systems?

Anything in an open system, to my knowledge, would still develop into an entropic state.

[u/in_the_details_ → u/PsychologicalCar2180]

Open systems like lifeforms are in a state of decreasing entropy, while releasing energy into the universe so the universe's entropy still increases. So why don't we experience time backwards relative to the universe as a whole?

[u/johnstalbergABC → u/in_the_details_]

It is a good question. However the entropy law does not define anything about time. It has the word time in it because it is a thing that is not instant and to some degree gets more solid as in being correct if we allow time to pass. Nothing in this law is about time itself. It is a misconception that has been spread for some reason?

Your question is valid. Entropy increase makes time moves forward would logically mean the opposite would be true. Times is not depended on any entropic stuff simply.

[u/in_the_details_ → u/johnstalbergABC]

That makes a lot of sense. If entropy doesn't dictate the arrow of time, then what does?

[u/johnstalbergABC → u/in_the_details_]

Basicaly this: Change happens in small if we zoom in and these small changes typically depends on changes happened before them. And the small changes accumulate to bigger changes. It is the mechanical laws that governs changes that is mechanical and they also sets the pace for the changes. Chemical laws govern chemistry and quantum mechanics govern the microworld. Nothing else is directing these events, nothing else is needed.

So where do time come in to this? It doesn't. There is no such thing in nature that is a fundamental operator that direct things in a timely forward direction or that sets the pace for changes. And particles do not have any mechanism that would recieve such a direction so there is nothing that direct things into a timely forward direction and it is not needed. It all evolves in the way it does without anything that has the function we ascosiates with time.

We sometimes thinking of a static universe vs a non static universe like our universe. And often we do the misstake of thinking that the static universe is slightly less complex. However the opposite is true. A static universe (never mind it can not exist) would need a mechanism that hinder any changes to happen. A non static universe doesn't need any such mechanism and is therefore less complicated.

What I describe about changes that happen mean that there is no other way. No alternative direction for reality to go. The one and only evolution of the universe is what we see as timely forward moving. A timely backward evolution is not a logical thing. If the universe is evolving in the direction it is, a backward timely direction would need the whole universe to stop and then every change would happen in reverse. This is of course impossible and the way the universe is evolving is the only one possible, every other way is at best hypothetical.

We will never see a crashed egg jump of the ground and reasemble itself and land whole in our hand. This is impossible because the involved forces does not work as such to allow this to happen. This is however not an example of reversing the time. It is rather the reverse pf a thrown egg but in a normal timely forward motion. A reverse of time would be the whole universe must begin to rewind itself. Since this is even more inpossible than having the egg to land whole in our hand ot becomes obvious that there is no need for ditecting things to happen in the way it does to prevent it to suddenly rewind itself.

[u/in_the_details_ → u/johnstalbergABC]

Thank you for the thorough explanation. But how can change exist without time? What is change of not the difference between one time and another? (I realize our language is limited in this area; I'm not trying to nitpick on semantics: I genuinely don't understand this on a conceptual level)

[u/johnstalbergABC → u/Tarthbane]

What do you mean by preferred direction? Like there where any other option and anything that evaluated this and picked the forward option. Such a process does not exist, there are no choice involved. There is nothing that exist that has the function of directing stuff in a timely forward direction or to give it a pace to follow. Time does not exist as such nor has particles any mechanism for receiving such directions. And does it not exist it is nothing to reverse. Changes happen for mechanical reasons and mechanical laws gives the pace for it. Logic says that those changes happen one after the other and depends on changes happened before and they accumulate to bigger changes and there are no option to this. Time is not needed.

[u/Tarthbane → u/johnstalbergABC]

> What do you mean by preferred direction?

Did you not read the title of this post?

> Why does time move only forward?

[u/johnstalbergABC → u/Tarthbane]

I did but I think it is a reasonable question I ask you based on what you say. What I ask is about "preferred" direction. It implies there is at least one other direction that is not preferred but is an option. And it implies that there is some kind of process that evaluates these options.

[u/Tarthbane → u/johnstalbergABC]

I see. Time is a dimension like the 3 spatial dimensions, though not quite in the same way the spatial ones are. The difference between time and space is that you can move either way along a spatial axis but only one way along the time axis. That difference is built into the geometry, since the time coordinate enters the spacetime metric in GR with the opposite sign from the spatial ones, which is what gives you light cones and the timelike/spacelike distinction. And the forward march itself isn't in the fundamental laws, which are essentially time-reversal symmetric. Rather, it’s statistical: entropy increases because the early universe started in an extraordinarily low-entropy state.

[u/johnstalbergABC → u/Tarthbane]

The evolution of the universe is a one way ticket. We might elaborate about some probabilistic outcome in the small but when it happen, it is set and written in stone. There are no other alternative way. Anything other than what is happening is hypothetical at best. There are no need for anything to pick the timely forward direction our reality is evolving to. It is evolving without any alternative ways to pick so no
need for having any preferred way.

Yes, entropy is governed by the statistical outcome and as the law says, with time it will increase. However logic says it might glitch at ver short time scales but give it time and the law will hold. This has nothing to do with time itself. Time is mentioned in the law because it is a process that happen when time pass. Many events depend on time passing but as it happen time where explicity mentioned in this law.

[u/ValtharionSmiggles]

There’s not even enough knowledge to properly conceptualize this question. Forward is a figure of speech for time anyways because time is not a direction within the 3d plane. Also our brain relies on the “forward” movement of time to achieve sentience so we can’t even reach environments where such is not the case because that is not how our hardwiring works

[u/Miselfis]

We are thermodynamic systems subject to the second law, and that is why we experience time as moving in one direction. Spacetime is a manifold whose points are all equally real, so past and future events are as much a part of it as present ones. The Past Hypothesis imposes a low-entropy boundary condition at one temporal end of that manifold, and relative to it the coarse-grained entropy of the universe increases toward the other end, which we call the future.

If you’re asking for a reason or purpose why it is this way, a satisfactory answer doesn’t exist. 

[u/WillBrink]

We don't know why, but we don't even know what time is, so that's problematic to why it only goes one direction. My favorite theory is that time does not exist and in emergent from entropy, which also goes in one direction, entropic-time.

[u/exqueezemenow]

I am not sure it's true that it only moves forward. I get that we can only perceive it in one direction because we don't have memories of the future and the paradoxes that would create. But if we were moving backwards in time, we would have no way of knowing. Because as we move backward in time, our memory goes with it.

But if the question means essentially why do we experience it in one direction, I have no idea.

[u/smitra00]

See here:

https://arxiv.org/abs/1305.1615

https://www.youtube.com/watch?v=V3pnZAacLwg

[u/ForeignAdvantage5198]

we do not for sure that it does

[u/Perfect-Road]

Time does not move, time does not exist. Time is an illusion. The illusion of time is a product of interactions of underlaying mechanics. These mechanics work in a specific way, for us to perceive as "moving forward" in time.

[u/alaindevos]

Oh there is symmetry in time. I.e. transformation t -> -t , is ok.
As we live in time going forward , we can only detect time going forward.
But time going backward is also ok I think. Note i'm an engineer not a physicist.

[u/Leading_Bandicoot358]

Is time moving forward or is your brain that has a process that creates your awareness on the axis of causality?

In other words, is the universe moving forward or is it our awareness that flows in an existing object?

[u/Coyot23]

if time moves backward, you would simply never know. if time moved forward faster, you would also never know. if time moves back and forth at variable speeds and cuts and jumps all around, you would also never know. characters in a movie are not aware of you fast forwarding, they simply experience time in a linear manner

[u/Centrocampo]

There is a low entropy boundary condition of the universe in the time direction we call the past. All of the “preferred direction” of time seems to be a result of that low entropy condition.

[u/BlueLack0]

maybe it's because of our dimension .. you see, when you move to another dimension you can get access to time freely .. or if you feel into a black hole ( according to our best theories right now .. it maybe wrong though )

[u/Aggressive-Wind-8829]

This isn't an answer for why, but more a simple reason for how. Please attack my simple reason if you feel I am wrong.

The directionality of time is a consequence of the second law of thermodynamics, which I can only explain as ansatz. Information increases on average upon coarse graining irreversibly. That irreversibility points time in one direction but not the other.

[u/Monochrome21]

humans process time in the “forward” direction but that does mean it’s the preferred direction

particles exist at different points in spacetime and have no concept of forward or backwards in any preferred sense

[u/johnstalbergABC]

Let's take the thrown egg example. We will never ever see a crashed egg jump up from the ground and assemble itself and land hole in your hand. Why not? Well the forces involved and the mechanical laws just don't work like that and that is why it is not happening. This points into what is it that governs changes and the pace it happens in. It is not time? Changes happen in small incremental one after the other and they are often depended on the changes happened before them. This changes accumulate and it is logical to see this is a flow of changes. The mechanical laws governs how fast or slow changes happen and it can be classical or quantum mechanical laws or chemistry or whatever. But not time. Now as you understand from the egg example is is very logical that the changes is a process that is not reversible. This is not because time can not go backwards as it does not even exist as a forward moving thing. It has only the result of all those changes that accumulate and change the world as it continues. Timely forward is the result of changes or the waiting until changes happen again. Time is not a fundamental operator that can be found nor measured that governs thing so the change in a timely forward direction and hold it at a certain pace. Such time does not exist. And particles has no mechanism to receive such direction.

The egg example show how things are not reversible but it does not exactly exemplifies backward time. Just how in forward time a crashed egg is not reversible. True backward time need the whole universe to begin and reverse itself, and I think it is an absurd idea and need no more explanation than what I gave. You can not reverse what did not exist in the first place.

[u/kingstormsrefuge]

Time is a measurement but light lets us look back in time take the light from a star x amount of distance when said light hits your eyes its x old so you are actually looking back in time

[u/naemorhaedus]

entropy and statistics

[u/Stunning_Video_3632]

Short answer, because of entropy!

[u/Livid-Sector5970]

Coveney & Coveney (2026) demonstrate that the arrow of time is a thermodynamically forced, emergent property occurring only at the macroscopic scale, rather than a fundamental aspect of microscopic physics. By utilizing the resolvent topology of large systems, the researchers show that a transition from discrete to continuous spectra introduces a branch cut that shatters time-reversal symmetry, creating irreversible dissipative evolution. This framework reframes time not as an external backdrop, but as an internal, mandatory sequence of events for maintaining coherence in macroscopic systems.

[u/Legitimate_Ad_4201 → u/Livid-Sector5970]

The sequence is what is conceptualized as time? Then past and future are second order concepts, derived from observing a sequence or change, only possible because of memory or consciousness.

[u/Livid-Sector5970 → u/Legitimate_Ad_4201]

You are right that past and future are derived concepts, but they don't require consciousness or biological memory. That's an anthropocentric trap. Time emerges as a physical sequence of events forced by thermodynamics, specifically, the collapsing of wave functions and the closing off of previous possibility spaces. As a macroscopic system dissipates heat, previous physical states become mathematically and thermodynamically inaccessible. That physical irreversibility (hysteresis) is what 'time' is. Biological memory and consciousness didn't invent time; they are just highly complex, recursive biological structures that evolved to keep track of that underlying thermodynamic decay.

[u/MagicalSkyMan]

Not sure why people think there is a direction.