The question is sometimes referred to as the "psychological arrow of time" (Hawking, 1985). Here the past is understood as a moment or time when the entropy of the universe was lower, and contrarily for the future. So it is generally thought that PAOT is a consequence of the thermodynamic arrow of time of our universe. If so (maybe not?), how do the two relate?

Some explanations in the literature:

  1. Practical memory systems work in a way that the formation of new memories entails an overall increase of total entropy of the system and the environment. For example, to create a memory, i.e. to cause our neurons to orient in a particular fashion, requires energy which results in our body heating up a little bit, increasing the total entropy (Hawking, 1985 and 1994); The initialization of memory to make it reusable is an irreversible process that increases total entropy (Landauer, 1961. Wolpert, 1992).
  2. More recently, People have argued that even reversible and non-dissipative memory systems are subject to PAOT (Mlodinow and Brun, 2014). The conclusion is arrived by imposing some constraints on what a memory system should be like. Specifically, they argue that a memory should be somehow robust to small microscopic changes in states of the system it records (what they call "generality" requirement). But the smallest changes in the future state destroy the thermodynamic arrow of time between now and the future. So any memory of the future of the system "could remember only one possible configuration of that system". This fine-tuning disqualifies it as a bona fide memory.

My problem with explanation (1) is that even if it's correct, it doesn't seem to be a complete answer in itself. Yes, increase of (new) memory happens only as total entropy of the universe increases. So what? It doesn't have anything to say on the nature of that memory. Why couldn't it occasionally be a memory of the future for that matter? Explanation (2) leaves no such ambiguity. But the generality requirement seems artificial: surely a memory that records the only future configuration of the system remembers the future in a deterministic world, there being no "what ifs" regarding that state?

Of course, my understanding of the problem is only preliminary. I'd like to know whether there is not some generally accepted explanation, or any other thoughts you have on it.

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    $\begingroup$ As you may recall, I've already explained this to you next week. $\endgroup$
    – rob
    Jul 6, 2014 at 13:17
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    $\begingroup$ The Laplace demon probably has no problem remembering both past and future. So maybe PAOT isn't a necessary consequence of TAOT, but the result of how our particular memory system adapts to TAOT? $\endgroup$
    – Eric
    Jul 6, 2014 at 13:32
  • $\begingroup$ Down voters please explain why. $\endgroup$
    – Eric
    Jul 6, 2014 at 13:34
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    $\begingroup$ I didn't downvote, but I was thinking of it. In physical terms, your question is not clear. Can you provide a physical definition of memory? (Not only of its storage, but how memory of the past and memory of the future are physically distinguishable) I do not see how your question could ever generate an answer that is firmly grounded in physics instead of the answerer's personal philosophy. $\endgroup$
    – ACuriousMind
    Jul 6, 2014 at 13:39
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    $\begingroup$ I don't see how you could have stored something in your head that hasn't actually happened yet. $\endgroup$
    – Kyle Kanos
    Jul 6, 2014 at 14:23

7 Answers 7


All you have direct access to at any moment is the macrostate of your brain at that moment. A (backward) memory is an inference from that state to what the macrostate of the world was at some time in the past and a (forward) prediction is an inference from that state to what the macrostate of the world will be at some time in the future.

A given macrostate is compatible with a great many microstates, each of which yields inferences about the past and inferences about the future, all by running the same laws of physics both backward and forward. This gives you a range of possibilities (and a probability distribution) for what the past was like and a range of possibilities (and a probability distribution) for what the future will be like.

So I think your question should be rephrased this way: Why are we so much more confident about our backward inferences than about our forward inferences? The answer must be twofold:

  1. The inferred probability distribution over past states is much more concentrated than the probability distribution over future states. (That is, our memories of the past are more accurate than our memories of the future.)
  2. At some level, we are aware of this. (That is, we give far greater credence to our memories of the past than our memories of the future.)

The first follows from increasing entropy, which in turn follows from the fact that the Universe was once in a state of very low entropy. The second follows, perhaps, from the first together with natural selection, which rewards true beliefs and punishes false ones.

So I think the answer to your question is something like this: Our ability to remember the past but not the future (or more precisely, our much greater faith in our backward predictions than in our forward predictions) results from Darwinian evolution in a universe that started with a Big Bang.

  • $\begingroup$ Thanks, you've put this really clearly. (I came here after reading Sean Carroll's excellent The Big Picture and feeling confused.) $\endgroup$
    – Eli Rose
    Oct 31, 2019 at 0:00
  • $\begingroup$ Dear WillO, I think this type of argument is deeply flawed, because local entropy can certainly decrease. But in those scenarios you don't observe the local time arrow (or your inferences therein) contradict the global arrow. Put a man in some sealed chamber and steadily decrease the temperature inside, in which case entropy decreases for the system (chamber-man) and future probability distribution will be more concentrated than past. According to your argument he should be remembering the low entropy future inside the chamber, which we know isn't the case. What happens [to be continued ] $\endgroup$
    – Eric
    Nov 16, 2019 at 4:16
  • $\begingroup$ outside the chamber can't effect his inference because he can't sense them. However, his inferences accords to the outside global arrow which he can't sense, instead of the inside entropy decreasing local environment which he does sense. $\endgroup$
    – Eric
    Nov 16, 2019 at 4:23
  • $\begingroup$ @Eric: Has this experiment been done? What justifies the phrase "which we know isn't the case"? $\endgroup$
    – WillO
    Nov 16, 2019 at 5:16
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    $\begingroup$ Because similar situations occur everyday everywhere. Turn on the air conditioning and the room cools and entropy decreases. Do you remember the future in such a room? $\endgroup$
    – Eric
    Nov 16, 2019 at 6:17

It is a reasonable question at the elementary particle physics level , since the mathematical formulae of all the models we have are reversible as to time. It is in the thermodynamic manifestation of the laws that an arrow of time appears, and in special relativity which separates observations in timelike and spacelike regions.

So it is one of those questions of "why" addressed to physics that really have the only answer "because". Macroscopically , where we live and die, there exists the arrow of time described by the laws of thermodynamics, and that is that.

The theory of special relativity has been validated with innumerable measurements and therefor again the answer is "because this is what we observe. A future event cannot register in the present because of the velocity of light which is the limit in the transfer of information.

Edit for clarification:

There are theories for particle physics extending the standard model, and there tachyons can exist. Tachyons in these theories can travel backwards in time and could interact with the electromagnetic structure of our brain if the theory allows it. Thus it could affect the memory banks and leave a record that could be interpreted as a future memory. There is no experimental sign for such particles.

So future memories belong to science fiction and metaphysics (precognition and such).

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    $\begingroup$ I do not believe that special relativity is relevant here. There is nothing in special relativity that makes it any harder (or easier) to infer things about the future than it is to infer things about the past. All I have access to at the moment is the current (macro) state of my brain. Running the laws of physics backward and forward, I can draw inferences about what sort of events might be present inside my brain's past and future light cones. SR provides no reason for me to trust one set of inferences more than the other. $\endgroup$
    – WillO
    Mar 10, 2016 at 1:10
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    $\begingroup$ To put this another way: All SR says is that, from the information available to me, I can draw no inferences about events outside my (past and future) light cones. But it draws no distinction between the sort of inferences I can draw about my past and the sort I can draw about my future. $\endgroup$
    – WillO
    Mar 10, 2016 at 1:13
  • $\begingroup$ @w en.wikipedia.org/wiki/Light_cone the future light cone goes with the velocity of light. for a signal/information to come backwards from the future one would have to break the limit of c backwards too. $\endgroup$
    – anna v
    Mar 10, 2016 at 5:41
  • $\begingroup$ But I am not talking about anything coming backwards from the future. I am talking about predicting the future based on what we know of the present, which is exactly the same procedure as "predicting" the past based on what we know of the present --- which in turn is exactly what memory is. $\endgroup$
    – WillO
    Mar 10, 2016 at 5:45
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    $\begingroup$ To add to that --- Memories do not involve signals arriving from the past. When a photon from the sun hits my eye, it does not trigger a memory; it triggers a sensation. Instead, memories involve drawing inferences about the past from what we know of the present. ("The fact that my neurons are in such and such a configuration strongly suggests that I had eggs for breakfast this morning"). As far as SR goes, drawing inferences about the future is no more difficult than drawing inferences about the past, so SR cannot be the right story here. $\endgroup$
    – WillO
    Mar 10, 2016 at 5:49

If you think of the future as a probabilistic distribution of events, for the far future there are an infinite number of possible events. As you approach those events in time, past (and present) actions force the future to collapse to a single event (assuming two can't happen simultaneously). You could think (and even predict) one event would happen over another, but you can't have a memory of one because it hasn't happened until it happens.

In this respect, you could almost think of events in time as being a larger version of Schrödinger's cat

  • $\begingroup$ Great answer. Makes me realize an implication of remembering the future is there is no freewill. $\endgroup$
    – User314159
    Nov 3, 2017 at 16:55
  • $\begingroup$ Nothing in this answer explains the asymmetry between past and future. $\endgroup$
    – user4552
    Nov 13, 2019 at 6:00

The answer to your question is that we don't remember the future because we haven't yet stored any memory of it.

Your memories arise from connections wired in your brain as a consequence of experience. You have not yet experienced the future, so the configuration of your brain has not been affected by it.

You can, of course anticipate and imagine the future. Indeed, the sensation of imagining, say, a walk in the woods tomorrow, can be as vivid as recalling a walk in the woods a year ago.

The phenomenon is not in the least confined to the human mind. The history file associated with my browser contains only the sites I have already visited. It does not contain information about the sites I am going to visit, because I haven't yet done so.

The one way direction of time arises naturally if you consider time to be a counter of change (as it is in the case of SI units). If I increment a counter every time there is a transition between the states of a given caesium atom the counter will inexorably rise. If the transitions were to somehow reverse themselves they would still count as changes, and the total on the counter would still increase. The count can never decrease.

To take another example, the number of transactions on your bank account can never decrease, because, unlike the balance of the account, it does not differentiate between positive payments in and negative payments out- it is counts both as an increase regardless of their sign. Past transactions are those that have been counted- future transaction have yet to be counted.

There should be no mystery to the arrow of time. Change happens. Time is simply a count of it.

  • $\begingroup$ If it has nothing to do with entropy, why do our memories increase as the entropy of the universe increases (I remember yesterday but not tommorow, and yesterday's entropy of the universe is lower than that of tommorow)? Why does the count increase in the direction of the 2nd law of thermodynamics? Coincidence? Correlation? Or Causation? $\endgroup$
    – Eric
    Jan 22, 2021 at 3:19
  • $\begingroup$ It seems to me that your argument is a bit circular. Because you essentially DEFINED the past as the sum total of our memories. But the very same definition is equally valid if time runs backwards! You have the same number of changes, microscopically, because physically laws are reversible at that scale. So why don't we remember the other way around? $\endgroup$
    – Eric
    Jan 22, 2021 at 11:43
  • $\begingroup$ You are over-thinking this, Eric. Memory is simply a record of events that have happened. Future events by definition have not happened, ergo they do not form part of any memory. $\endgroup$ Mar 28, 2021 at 15:59
  • $\begingroup$ @MarcoOcram and what if existing memories themselves created events in the future? Voila. $\endgroup$
    – Midovaar
    Nov 21, 2022 at 18:20
  • $\begingroup$ @Midovaar I cannot understand what point you are trying to make, $\endgroup$ Nov 21, 2022 at 20:05

This seems like an extremely deep question. The brain is a physical system. So it must have some mathematical object that describes its current state $S$. And this state evolves in time, so we have $S(t)$ representing the entire life of the brain.

Suppose we're given the state at some time $t_0$, $S(t_0) $. The question is, why are we able to extract more information from $S(t_0) $ about the past $S(t<t_0) $ than the future $S(t>t_0) $?. After all, the laws of the universe are time-symmetric. The future retro-causes the past as much as the past causes the future.

The only real asymmetry that I see is in wavefunction collapse. In the information theory interpretation, the wavefunction represents the knowledge of an agent about the universe. The collapse is knowledge becoming known.

In this sense, the future states $S(t>t_0) $ are fundamentally not extract-able from the present state $S(t_0) $

But then, the past states are not extractable from $S(t_0)$ for the exact same reason. It's well known that you can't know the past wavefunction from the collapsed wavefunction.

I believe we have to model $S$ as a cumulative quantity. $S(t_0) $ represents the cumulative knowledge gained from all the past knowledge gains. This way, the definition of $S$ treats time asymmetrically.

In short, we need two "axioms" :

  1. The laws of the universe are non-deterministic. This makes $S(t>t_0) $ non-extractable from $S(t_0) $

  2. $S(t) $ is a cumulativive quantity, accumulating in the direction past to future. This makes the past states extractable from $S(t_0) $

Cumulative quantities seem like they violate time-symmetry, but they don't. Think of a mountain that grows bigger and bigger with time. The laws behind this are obviously time-symmetric, but that doesn't mean that the phenomena have to be time-symmetric.

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    $\begingroup$ Thoughtful answer. But isn't it just as arbitrary to postulate that S(t) is accumulating in the direction from past to future, as from future to past? This part seems like a circular argument where an arbitrary distinction between past and future surreptitiously sneaks in from back door. $\endgroup$
    – Eric
    Nov 22, 2022 at 2:04
  • $\begingroup$ @Eric indeed it sounds circular. But I am only postulating that $S$ is a cumlulative quantity. So, it had to acumulate in some direction. Whichever direction it happens to accumulate in, will be labeled as "past" by the subjective "experiencer" of the memories. But you could still provide an independent definition of the past : like "past is closer to the big bang than future is". Then your question would become " Why are we accumulatig memories in the direction "closer to the big bang" to "away from it"? " $\endgroup$
    – Ryder Rude
    Nov 22, 2022 at 2:21
  • $\begingroup$ @Eric Do you think that the answer to this comes down to biology, as in, you can definitely imagine an alien that acumulates memories in the opposite direction. But they would experience every universal phenomenon, that we observe, in reverse. They will need very different biology to live in those conditions. $\endgroup$
    – Ryder Rude
    Nov 22, 2022 at 2:25
  • $\begingroup$ @Eric And lastly, does postulating that memories acumulate in some direction also violate time symmetry? No. The laws of physics do not forbid the existence of cumulative quantities. It's no different from how a mountain can grow bigger in one direction but smaller in other. The laws are differential equations that are time symmetric. The initial conditions can still allow for asmmetries like cumulative phenomena and entropy increases. These asymmetries are a feature, not of the laws, but of the initial conditions. $\endgroup$
    – Ryder Rude
    Nov 22, 2022 at 2:36
  • $\begingroup$ The same "accumulating" idea had occurred to me, too. It's a natural idea. I think it still begs the question why and how this thing called S(t) accumulates the way it does, which is the crux of the matter we hope to understand. We can even ask why this accumulation somehow has to align itself with either the increase or the decrease of entropy. Why can't it just be random? $\endgroup$
    – Eric
    Nov 22, 2022 at 2:40

Here's an answer that should be taken with a grain of salt.

I believe there is an explanation of the arrow of time. I think the explanation is not the second law of thermodynamics because I don't actually believe that law myself. I think there is another explanation for it but I won't get into that here. I believe that the short answer is that a certain property of the state of the universe at the beginning of time held and if the state of the universe at any time satisfies that property, it's state at a tiny bit later time must also satisfy that property. I'll give one example. According to general relativity and the big bang theory, a white hole can never exist because none existed at the beginning of time and a black hole can never get destroyed once it has been created.

The brain follows the arrow of time. Environmental factors run in an unpredictable way. Once you see something, a memory of it gets stored in the brain but the memory doesn't form until after you see it.


The reason we don't remember the future has to do with an arrow that gets ignored. That of information flow between the brain and the outer world. One could call it the sensory arrow of time. With our senses, we absorb information only in the future direction i.e. along the thermodynamic arrow of time. We never absorb information along the opposite direction of time. In other words we never emit it along the normal direction. We actually do emit information (speaking, touching) but not in the "future-memory' way. I will explain how emitting information is the key to the definition of future memories.

Imagine there were beings who somehow experienced time in reverse. They call the future past and the past future. Now from their point of view everything happens in reverse. Dissipated heat collects into light and goes back into stars, smell molecules waft back into roses, etc. However one should note that they can't observe these reverse-dissipation events. For them it is us who are the future-remembering beings. How they "see" us can give us a clue to what future memory can look like to us. Now from our point of view, information from objects enters our bodies, gets stored as memories and finally decays / gets corrupted. But as per them, who live reverse lives, our memories arise randomly, reverse themselves and then the information from those memories (lights, fragrances, sounds) is sent out to the external world where it gets absorbed by objects. Thus it follows that when we, the past remembering beings, want to talk of future memories, then it must be about forming memories, reversing and converting them into information and then sending that information out.

So, if we wanted to have a memory of the future, we could, with some physiological modifications, do something like this. We could imagine an object like a blue square and remember this imagination. Now we reverse this memory and emit blue photons from our body in such a way that this light forms a blue square on a white screen. This is a valid memory because it has a causal connection with the square and is also a memory of the future because the square came after the memory. Imagined in reverse the photons collect and fall on the white sheet, get reflected back in a blue stream, enter our body, get converted to nerve signals and are finally imprinted on our brains as a memory. This series of these events is not improbable because entropy decreases in the past here.

So just like past memories are 'decodable' impressions of past information, future memories must be decodable sources of future information.

This definition might seem problematic at first because this way you seemingly can't remember "complex" events like looking at a frog. But then even past memories are also always inferences. This has to do with the 2nd law of thermodynamics. It is very likely that information comes from objects than just randomly flowing into us in the shape of objects. So it is improbable that we will see frogs just because light rays collected in such a way by chance and impinged on our eyes.

Future-remembering beings would have a hard time keeping track of the world around them because generally information spreads out from objects. However if perhaps there were some chance events where local entropy was reversed like yellow painting which absorbed yellow and heat gave off white light, then their memories could make sense.

We also need to remember that in the end memories point to information transfer from the body to the brain and not from the outside world to the body. Touching something cold for example makes information from the body flow outward but it still sends impulses to the brain.

Remembering the future is not about time travel or knowing a future event without having any connection to it. And if there was indeed a way of knowing the future, it would still not mean having a memory of the future unless causality was involved.

This definition also avoids any paradoxes. You can't use this memory to change the future. If you do then the memory was simply false. Also, claiming to change the future by destroying a future memory would be akin to claiming to change the past by implanting a false past memory.

We don't remember the future because we don't / can't convert our memories into outside information in a way that is consistent with how we decode that memory in our heads. It makes zero evolutionary sense to align the sensory arrow against the thermodynamic arrow of time.


old post ----------

My rather simple answer, which almost no one has paid heed to or understood, is that we can remember the future; it's just not worth it nor is it really spectacular. And there is nothing mystical about it. Hear me out; no pseudoscience here but quite a simple answer to an often asked question. Please ask me if you don't understand something about it.

So here it goes. You imagined a blue circle in your head and stored it as a memory. You then reversed (forgot) that imagination and that forgetting was the exact reversal of the process of remembering so that it led to emission of blue light from your body. That light produced an image of a blue circle on a white screen for an instant. Thus, that imagination was a memory of the future because you remembered it before it happened. If you reverse this whole process you can see how the blue photons from the fleeting circle flew into you and became your memory. On the other hand if you imagined a blue circle and simply saw one the next day it would be chance and not a future memory because the blue circle was not formed by your memory.

Remembering the future doesn't mean knowing something you have no connection to like the winning number of the next lottery. lt is simply the reversal of the process of remembering (memorization reversal and sensory perception reversal).

Adding some points for further clarity -

Any valid memory must always have a causal connection with the corresponding information outside the memory housing body. And a mechanism should exist in that body to decode that memory because on their own memories don't represent anything. The definition of future memory here meets both these criteria.

In summary, just like past memories are 'decodable' impressions of past information, future memories must be decodable sources of future information.

So, we don't remember the future because we don't / can't convert our memories into outside information in a way that is consistent with how we decode that memory in our heads.

If you are downvoting please leave comments!

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