Why do we remember the past and not the future? Why do we perceive time as moving forward?
These are more serious questions than you might think. Most laws of physics don’t really care if time moves forward or backwards, yet we see that time keeps slipping, slipping, slipping into the future. Somewhere, deep in our understanding of physics, we’ve missed something.
This stuff ain’t easy to understand, either. Sean Carroll at Cosmic Variance has posted an Arrow Of Time FAQ, and it has some good, solid writing about this. Some of it will make your head hurt, but it’s a good thing to be aware of. There are still some baffling questions we don’t quite understand, and that’s where a lot of the fun is!
December 6th, 2007 at 3:57 pm
Time is a tough nut to crack, surely beyond my mental abilities. It seems to me that time going in one direction is required for a cause and effect order to any universe. It would seem that we judge time to be going forward for the sole reason of cause and effect. How could we assume it was going forward without cause and effect? Perhaps it does reverse at times, but because of perception or the short time we have been a conscious species we are unaware of it. We are certainly evolved to react to a forward moving time. Like I said, a tough nut to crack. I hope that we do somehow discover the underlying physics of why it is so, but I cannot say I am optimistic.
December 6th, 2007 at 4:16 pm
You may be interested in reading a series of books by Piers Anthony, the Mode series. It deals with the concept of travelling between multiple universes, and one of the characters from another dimension only has memories of the future. It really is quite interesting, even though most of his books were written for people around the 11-15 age group.
December 6th, 2007 at 4:18 pm
Time is nature’s way of keeping everything from happening at once.
Space is what prevents everything from happening to me.
* Attributed to John Archibald Wheeler
December 6th, 2007 at 4:25 pm
It is a fairly simple problem to me. Entropy has a definitive direction, and diffusion of ions (which is entropy-driven) is one of the two main factors driving neuronal activity. To put it simply, the “forward” direction of signals in the central nervous system is determined by entropy. So changes in the nervous system must happen in the direction of time in which entropy increases.
To paraphrase Hawking, we measure time in the direction that neuronal signals propagate, and the direction that neuronal signals propagate is determined by entropy. Most other biological processes are in some way driven by diffusion, and diffusion is given direction by entropy. So the direction of time is arbitrarily set as the direction in which our cellular functions proceed.
We cannot remember the future, for instance, because memories are formed to a large extent by the difference in the relative arrival time of neuronal signals. The creation of memories has an inherent directionality to it that is determined by entropy.
December 6th, 2007 at 4:35 pm
Too bad Star Trek never did an episode dealing with time travel.
December 6th, 2007 at 4:47 pm
I’ve always considered Time to be nothing more than the perception of Change. We perceive the passage of Time because we are able to perceive that our environment has changed; that is, we are able to observe how things are now and contrast that to how things were. If we cannot perceive Change, then we cannot perceive Time.
December 6th, 2007 at 4:49 pm
There are several intertwined questions here. Perhaps the easiest one to answer is why our memories obey the universe’s arrow of time, i.e. why we only remember the past and not the future.
The really hard question is why the universe should have an arrow of time at all since the laws of physics are all time-reversible. As the article says- if it’s logically true that entropy should increase in the future then it’s also logically true that entropy should increase in the past. The fact that it only increases in the future (relative to the present) is the real puzzle and points to an asymmetry in the boundary conditions.
I like to imagine a simulation of a pen teetering on its end. If you run the simulation forward then the pen will tip over and dissipate its energy as heat. The problem is that running the simulation backward will give exactly the same result- the pen will still fall over. Taken as a whole the pen starts off on its side- it then rises to balance on its end and then falls back to the table again. But- the universe doesn’t appear to work that way- so there must be something wrong with the simulation even though it obeys the laws of physics at each instant.
December 6th, 2007 at 5:18 pm
…You know, that first line is not something I think of as an average joe. My first reply is “Cuz future’s not here yet”.
December 6th, 2007 at 5:27 pm
The question is nonsense. Therefore, any answer would be meaningless, and the quest for an answer is a waste of time.
December 6th, 2007 at 5:41 pm
I may be out of date here but I recall from the 1960s that weak interactions violate time reversal invarience.
December 6th, 2007 at 5:48 pm
After reading the Arrow of Time FAQ I feel like one of the Gumbies: My brain hurts!
SLC, you should read the FAQ that Phil points to. It specifically addresses your comment. Check out the section that asks, “Don’t the weak interactions violate time-reversal invariance?”
December 6th, 2007 at 5:49 pm
@The Centipede:
I really enjoyed your response to this comment; thanks! ^_^ I hope that BA’s post on the Arrow of time is as illuminating as his post linking to the images from New Horizons. The Pluto system is beautiful. I think that was one of my favorite posts of all time. He was soooo excited!
Right now, I’m getting prepared to watch Comet Holmes as its inbursting comes to its climax. Only about five weeks left.
December 6th, 2007 at 5:59 pm
This is supposed to be Sean Carroll’s specialty, so it is with hesitation I say that he seems to me to present a slightly unorthodox view.
Astrophysicist Jayant Narlikar review the arrow of time as regards the universe in his book The Structure of the Universe, and he notes the missing elephant(s), the other arrows of time. We have the cosmological (expansion) and the electromagnetic (past and future lightcones).
I have also heard that small systems like simple composite particles exhibits the electromagnetic arrow, somewhat deflating Carroll’s statistical physics concept of “microscopic” reversibility.
And the arrow of time doesn’t fully answer why we perceive us as “moved by time” but moving in space. Special relativity, where Lorentz invariance seems fairly fundamental, combines symmetry broken time with symmetrical space into symmetry broken spacetime, better but not yet equivalent with just the asymmetry (the arrow).
On the symmetry breaking, I feel like Carroll that the reason is that the universe can expand. But AFAIU inflation specialists like Linde are satisfied with such solutions like a non-expanding prespace that in some large enough region happened to fluctuate into low enough entropy to enable the process of eternal inflation to start. And this should finally be enough to describe time as in the SR picture, not only broken asymmetry but a process that “moves” us through spacetime.
Btw, Carroll seems to think that cosmology requires large scale symmetry as in his paper. But on that point I liked Greg Egan’s comment #77, what if the universe (multiverse) has regions with different arrows, say from different instantiations of eternal inflation onset? Then we happen to observe large scale symmetry as a consequence, not as a requirement, shining higher after shaving with Occam’s razor.
Heh. Not so good as the first line, which I have heard (unattributed). But restoring some of the symmetry.
December 6th, 2007 at 6:13 pm
Yeah, well, I momentarily forgot I had to expand on the argument to assume Linde’s idea of pushing the upper bound of the inevitable past infinite energy catastrophe of each world line infinitely back, making eternal inflation eternal backwards as well. Prespace time would probably be hard to reconcile with SR causal (space)time along the line I suggested. (So my ‘idea’ is precariously forced too. What else is new? :-P)
December 6th, 2007 at 6:17 pm
The popatus of time?
December 6th, 2007 at 6:18 pm
Err, pompatus. You’d think I could proofread a 4 word post.
December 6th, 2007 at 6:23 pm
Reverse time is so obviously actually Dark Time that doesn’t interact with normal matter. So of course we don’t observe backward time!
Anything else you want to know - I can answer anythihng as long as I’m allowed to make it up as I go along!
jbs
December 6th, 2007 at 6:26 pm
Ouch. This hurts my head.
December 6th, 2007 at 6:40 pm
Justin
You may be interested in reading a series of books by Piers Anthony
Another PA series, Incarnations of Immortality has Bearing An Hourglass, where the protagonist is the incarnation of Time, and lives his life backwards.
[However, the only one of the series I really liked was the first, On A Pale Horse]
J/P=?
December 6th, 2007 at 7:47 pm
John P., I’m currently listening to Bearing an Hourglass as a book-on-tape during my commute. Yay public libraries.
I’m only a little into it and the main character hasn’t even become Time yet. It doesn’t seem quite as good as it did when I first read it, what, 20-some years ago. But I am enjoying it. 
The rest of this time stuff makes my head hurt a little….
December 6th, 2007 at 8:21 pm
> @The Centipede:
But… but… but… this is my first response to this…
Okay, I’ve got a conundrum. One, this could just be a typo. Two, it could be someone being clever about the Arrow of Time and responding positively to any potential post I could make before I make it; which brings up the subcases of 1) MandyDax really wants to hear my opinion or 2) MandyDax is being flippant. In any case, here goes.
The linked blog post considers that most quantum reactions are completely reversible, which I’ll take as true for the sake of argument (I’m not a quantum anything; I haven’t seen these equations, and will assume for purposes of time [and the cost-benefit analysis of the usage thereof] that Carroll knows what he’s talking about). However, many macroscopic reactions and events are irreversible: you can’t unstir the jam from the porridge from its low-entropy state (mixed) to it’s high-entropy state (porridge with jam on top, the pattern of the jam on top being a star, or a smiley face, or anything else in paticular). If macroscopic events were time-reversible, then one really could, in some way, uncook an omelet back into an egg. Clearly there’s a disconnect, and as I see it, there are several possible hypotheses (out of many):
1) Quantum phenomena really are completely reversible and, everything else deriving from it, so is everything else. The Second Law of Thermodynamics is really just fitting science to the apparent view of the data (i.e. us living in the Arrow of Time and assuming it to be an actual physical phenomenon rather than a psychological observer effect) and, if we knew how, we could reverse anything we wanted. This would of course beg the question why, if all reactions are reversible, we don’t see them reverse despite our Arrow of Time pointing in one direction when really there is no Arrow of Time, merely a line. This would also beg the question of why our Arrow of Time points in one direction if it’s natural to have no Arrow. In terms of scientific analysis, this doesn’t really serve any purpose and thus as an option can probably be safely discarded due to a lack of evidence of entropy decreasing without work being done on the system from outside.
2) Quantum phenomena are completely reversible, but the Arrow is real and so is the Second Law of Thermodynamics. This suggests that entropy is an emergent quality of quantum phenomena on larger scales (even chemical ones), which does not, I think, break beyond the realm of reason. Our genetic codes do not predict us, the individual human being, down to the very last thought in our head and the very last detail of our bodies; there’s simply not enough information to do so. We show fractal similarity to our coding, yes, but outside influences as well as extreme sensitivity to initial conditions ensures that clones are not true carbon copies of one another. Let’s say that time and space are quantized into Planck units. Information transfer in the universe is limited by the speed of light because photons are the most basic unit of information and energy transfer. Photons moving like chessmen on the Planck spacetime grid go from particle to particle, but Planck time prevents perfect information paradoxes on the quantum scale and thus makes the entire system extremely sensitive to initial conditions. Mathematical chaos does not demand irreversible reactions but does tend to suggest them, especially when coupled with quantum uncertainty.
3) Quantum phenomena are not actually completely reversible and the irreversible aspects of quantum mechanics are mathematical products of the quantum mechanical model, which don’t actually reflect proper reality. I tend to think this possibility is most likely, because it’s usually a poor idea to assume that the methodology of science is more real than what is observed in reality: evidence trumps theory. For the longest time Newtonian mechanics were reversible and Laplace was waiting for the day when everything could be certainly defined and thus all of time’s clockwork would open up. Entropy and uncertainty put the kibosh on that. Mercury going too fast in its orbit made extremely accurate prediction of its motion using Newtonian methods out of reach; this did not make Mercury unreal, but instead made Newton not as right as he possibly could be. Relativity and quantum mechanics were more right, but are still not as right as they possibly could be. Given that evidence suggests spacetime really is the same animal and tightly connected with lightspeed as the limit of information transfer, it is reasonable to consider this connection to go all the way to the quantum level, which the current theory doesn’t show.
Quantum gravity has always been a sort of bogeyman, and conceptualizing Planck space-time seems to be pretty tricky as well. The counterintuitive lack of an Arrow of Time in quantum dynamics is, I think, an indication that our understanding is not complete rather than indicative of a scientific question of why is there an Arrow at all. Consider it this way: if an Arrow of Time were unnatural, then why do we perceive one? We perceive ourselves as moving through space well enough–events are usually space independent–and so if the direction of the motion of time were also irrelevant, we could probably perceive that as well. I don’t mean to use an anthropic argument but we are natural creatures in our natural environment built with natural building blocks. If the Arrow of Time really were a fiction, then I don’t think we’d perceive it. Assuming that we would have to perceive one even if it wasn’t there is natural, given as we don’t know how time-independent perception would be like, but it comes down to a sort of reversal of the Privileged Planet nonsense: what we perceive is natural to us and assuming it to be ‘required’ outside of the construction of the universe is fallacious; if the Earth were different, Bizarro Earth IDers would still consider it special; if the universe were different in regards to the Arrow of Time, Bizarro Us would be arguing inside of that timespace as naturally as we argue in ours, even though our modes of causal thoughts would be extremely hard to grasp to each other.
So yes, in the end, I expect the Arrow of Time is either an emergent property of quantum spacetime, or else our understanding of quantum time is incomplete and the Arrow is a fundamental part of our universe from tips to toes.
December 6th, 2007 at 10:02 pm
[…] Tip of the blogger’s hat to Phil at Bad Astronomy Blog. […]
December 6th, 2007 at 11:08 pm
You are joking, right? There’s lots of Star Trek episodes dealing with time travel, in fact enough to fill a four DVD set:
http://tinyurl.com/22frsb
December 6th, 2007 at 11:54 pm
[…] The Arrow of TimeBy The Bad AstronomerMost laws of physics don’t really care if time moves forward or backwards, yet we see that time keeps slipping, slipping, slipping into the future. Somewhere, deep in our understanding of physics, we’ve missed something. …Bad Astronomy Blog - http://www.badastronomy.com/bablog […]
December 7th, 2007 at 12:34 am
Thomas:
You may want to set your tricorder to detect sarcasitons.
December 7th, 2007 at 12:43 am
“…time keeps slipping, slipping, slipping into the future.”
That song always bugged me because, as I see it, time is slipping into the past.
December 7th, 2007 at 12:45 am
Does anyone happen to remember that article in the NY Times science section that appeared a few years back, about comparing time to a loaf of bread? Where each instance was a slice of the bread that was, as I remember it, persistent for all eternity? (imagine that statement as mathematically as possible)
Anyway, this post fits in nicely with the anti-creationist posts on here, since our perception of time seems to stem from our evolution. We only evolved to perceive a certain aspect of time (which may not be an aspect of “slice time” at all).
My father made me realize it when we were having dinner at a diner. He said something to the effect that the days were just zipping on by. I was about 19 at the time, and it made me realize that time as we think it is actually a process so far removed from physics it could be called uniquely human.
What my father was referring to was a memory comparison process. When an infant is born, they spend 100% of their total life waiting for the next day. The next day, they spend 50%… and so on, until we are spending only .001% of our lives waiting for the next day (barely a drop in the mnemonic bucket at that point), each second being compared to the last, being compared to a total acquisition of seconds all up to that point. This showed me how time seems to speed up as we get older: because each second becomes less important than the last.
In sleep, we don’t have this mnemonic comparison process active, so time doesn’t seem to exist at all. Time as a mathematical entity or dimension is a completely alien thing to the human psychology. I can’t see a reason for any lifeform (as we know them) to ever evolve a way to directly perceive “slice time”. Such a perception device makes no sense to Darwinian survival… for the evolutionary lines on this planet, at least.
December 7th, 2007 at 12:49 am
Actually, this whole topic is easy to sum up in two words: Anthropic principle.
December 7th, 2007 at 3:06 am
Maybe because you never lived it. And you do not know if will ever live !
December 7th, 2007 at 6:03 am
Re Rowsdower
The argument presented by Prof. Carroll basically states that the laws of physics are invariant under the CPT transformation and therefore non-invariance under T is not relevant. This may be technically true; however, I would argue that a universe based on a left-handed coordinate system and consisting of atoms with positrons in a cloud around nuclei consisting of anti-neutrons and anti-protons with time evolving in the reverse direction is not the same as the universe we know, even though the current laws of physics in such a universe would be the same. This is aside from the question as to whether the CPT theorem holds on a global scale under General Relativity (it is my recollection that an assumption of Lorentz invariance is required to prove the theorem). However, it is possible that, if the multiverse hypothesis is correct, such an anti-particle universe could exist.
December 7th, 2007 at 7:10 am
“Most laws of physics don’t really care if time moves forward or backwards,”
Wouldn’t it be more correct to say that we’re moving through time, instead of implying that time is moving around us?
December 7th, 2007 at 8:08 am
It’s really quite simple: Time is an illusion caused by the passage of History. History is an illusion caused by the passage of time.
If you think this definition is circular, go and look up “time” in a dictionary.
I recall a series of Royal Institution Christmas lectures about time, about 10 years ago (ish). The first thing the lecturer asked the audience was “What is the time?”. Everyone put their hand up. He then asked “What is time?”. Obviously, no-one volunteered an answer. This little paradox (we don’t know what time is, but we measure it all the … um … time) gave quite a nice little entry into thinking about time.
December 7th, 2007 at 8:32 am
“Time has a way of always being on time.”
December 7th, 2007 at 8:54 am
Marx summed it up best: “Time flies like an arrow,
Fruit flies like a banana”
(Err…. Groucho, not Karl)
December 7th, 2007 at 9:29 am
Juergen, that’s not much of an answer. It doesn’t explain why the Anthropic Principle exists or works. It doesn’t explain the mechanism of how the Anthropic Principle constrains the Arrow of Time.
December 7th, 2007 at 10:41 am
It already is.. eh.. no wait.. that’s my phaser, oh dear, what have I done?
December 7th, 2007 at 10:42 am
Ah, time, the eternal question,,,
So, this universe is what happens inside a black hole?
I wonder if we’re recapitulating the information generation of an external(read higher level) dimensionality. Hawking seemed to think that information was conserved inside a black hole, ie, all the mass/energy that went into it must eventually return as Hawking radiation. In between the establishment of the event horizon and it’s discontinuance, exists the “black” hole. Within the event horizon,,,S&%t happens,,,but we assume we can never know what that was.
Perhaps the initial mass/energy that goes into creating the black hole is squashed into the false vacuum, then recondenses into new nuclear particles, etc, consistent with a new universe encapsulated within the hole,,,???
Condensation from a supercooled/phase change would eliminate the need for a singularity of the infinite temperature/density kind and replace it with a phase change starting point of the same density as the initial false vacuum, triggered by a small quantum fluctuation, kinda like dropping a rock in a super cooled lake in the arctic circle,,,
I LOVE speculative,,,fiction,,,
Gary 7
December 7th, 2007 at 10:59 am
Zacharyon wrote:
“Does anyone happen to remember that article in the NY Times science section that appeared a few years back, about comparing time to a loaf of bread? Where each instance was a slice of the bread that was, as I remember it, persistent for all eternity? (imagine that statement as mathematically as possible)…”
I remember that article. There were several analogies in it - let’s see, which stack of NYTs is that in? (Kidding) The image could be mathematically represented by rows of receding cones for each slice but I don’t remember the gist of that analogy in context. There is a distinction as you imply between our psychological perception of time as living beings and the physics that surround or permeate the effect of time.
December 7th, 2007 at 11:08 am
Too much work to look it up, but either Bradbury or Clarke (I think Bradbury) wrote a story “The Arrow of Time” or something like that.
December 7th, 2007 at 11:43 am
YinYang0564: Martin Amis wrote “Time’s arrow”.
In any quantum theory, for any possible process there exists the reverse process (technically due to the unitarity of the evolution). More importantly, a state can evolve into as many higher entropy states as lower entropy states. No emergent phenomena that can in principle be based on quantum theory will satisfactorily explain the perceived arrow of time.
December 7th, 2007 at 11:59 am
@Jason B: Reversals are unlikely, but they do happen.
December 7th, 2007 at 12:03 pm
So, does the arrow of time never reverse, or is a reversal simply very unlikely?
December 7th, 2007 at 12:24 pm
The simple answer (to why don’t we remember the future) is, of course, because it hasn’t happened yet. At any instant of time, things can go any number of different ways, but once past that instant, things solidify into the Past. (Except perhaps in the quantum arena.)
You could say that time is just one of those four dimensions in which we are trapped. We can move around in the other three (though not separately), and when we move, we’re using one of the properties of the 4th - time - to do our moving with. The difference is that we have a choice about how we move in the three - but not in the 4th.
(It was Arthur Eddington, in 1927, who came up with the expression. He concluded that it’s a property only of entropy.)
I couldn’t find anything by either Bradbury or Clarke called “Arrow of Time”, but Bradbury (and a host of others) have written stories about time-travel.
December 7th, 2007 at 12:57 pm
YinYang and PK:
Yes, Martin Amis’ “Time’s Arrow” is a spectacular book.
December 7th, 2007 at 1:33 pm
In “Geometry, Relativity and the Fourth Dimension” published by Dover in 1977, the author Rudy Rucker has a somewhat humorous essay about what life would be like now if, in the future, time flowed backwards. His implication however is that even the most ridiculous scenarios would, relative to all other backward frameworks, have observable natural processes and explanations.
December 7th, 2007 at 1:54 pm
Like most answers to this, it’s pretty much begging the question, but how about some kind of temporal momentum. There’s a big bang and space expands in three space, so why not in four space? The present is the leading edge of said expansion and that’s why we remember the past but not the future. The past is where it’s expanded from and the future is where it’s expanding to. That would mean there’s probably an anti-time universe which is contracting in four space, just like the Backwards episode of Red Dwarf.
December 8th, 2007 at 2:52 pm
ZZMike said:
> The simple answer (to why don’t we remember the future) is, of course, because it hasn’t happened yet. At any instant of time, things can go any number of different ways, but once past that instant, things solidify into the Past. (Except perhaps in the quantum arena.)
Sorry, but all you’ve done is restate the problem of the Arrow of Time. Why is it that things solidify? That’s a process of entropy.
December 8th, 2007 at 5:25 pm
[i]The Elegant Universe[/i] has a brief mention of a possible additional time dimension in Superstring Theory, beyond the additional rolled-up spatial dimensions. The second time dimension could conceivable travel in a direction other than forward, forward at a different rate, or reverse itself once bilions of years or millions of times per second. The very concept of time other than the time we normally experience is incredibly difficult to comprehend with our experience being what it is.