Ten Things Everyone Should Know About Time

“Time” is the most used noun in the English language, yet it remains a mystery. We’ve just completed an amazingly intense and rewarding multidisciplinary conference on the nature of time, and my brain is swimming with ideas and new questions. Rather than trying a summary (the talks will be online soon), here’s my stab at a top ten list partly inspired by our discussions: the things everyone should know about time. [Update: all of these are things I think are true, after quite a bit of deliberation. Not everyone agrees, although of course they should.]

1. Time exists. Might as well get this common question out of the way. Of course time exists — otherwise how would we set our alarm clocks? Time organizes the universe into an ordered series of moments, and thank goodness; what a mess it would be if reality were complete different from moment to moment. The real question is whether or not time is fundamental, or perhaps emergent. We used to think that “temperature” was a basic category of nature, but now we know it emerges from the motion of atoms. When it comes to whether time is fundamental, the answer is: nobody knows. My bet is “yes,” but we’ll need to understand quantum gravity much better before we can say for sure.

2. The past and future are equally real. This isn’t completely accepted, but it should be. Intuitively we think that the “now” is real, while the past is fixed and in the books, and the future hasn’t yet occurred. But physics teaches us something remarkable: every event in the past and future is implicit in the current moment. This is hard to see in our everyday lives, since we’re nowhere close to knowing everything about the universe at any moment, nor will we ever be — but the equations don’t lie. As Einstein put it, “It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.”

3. Everyone experiences time differently. This is true at the level of both physics and biology. Within physics, we used to have Sir Isaac Newton’s view of time, which was universal and shared by everyone. But then Einstein came along and explained that how much time elapses for a person depends on how they travel through space (especially near the speed of light) as well as the gravitational field (especially if its near a black hole). From a biological or psychological perspective, the time measured by atomic clocks isn’t as important as the time measured by our internal rhythms and the accumulation of memories. That happens differently depending on who we are and what we are experiencing; there’s a real sense in which time moves more quickly when we’re older.

4. You live in the past. About 80 milliseconds in the past, to be precise. Use one hand to touch your nose, and the other to touch one of your feet, at exactly the same time. You will experience them as simultaneous acts. But that’s mysterious — clearly it takes more time for the signal to travel up your nerves from your feet to your brain than from your nose. The reconciliation is simple: our conscious experience takes time to assemble, and your brain waits for all the relevant input before it experiences the “now.” Experiments have shown that the lag between things happening and us experiencing them is about 80 milliseconds. (Via conference participant David Eagleman.)

5. Your memory isn’t as good as you think. When you remember an event in the past, your brain uses a very similar technique to imagining the future. The process is less like “replaying a video” than “putting on a play from a script.” If the script is wrong for whatever reason, you can have a false memory that is just as vivid as a true one. Eyewitness testimony, it turns out, is one of the least reliable forms of evidence allowed into courtrooms. (Via conference participants Kathleen McDermott and Henry Roediger.)

6. Consciousness depends on manipulating time. Many cognitive abilities are important for consciousness, and we don’t yet have a complete picture. But it’s clear that the ability to manipulate time and possibility is a crucial feature. In contrast to aquatic life, land-based animals, whose vision-based sensory field extends for hundreds of meters, have time to contemplate a variety of actions and pick the best one. The origin of grammar allowed us to talk about such hypothetical futures with each other. Consciousness wouldn’t be possible without the ability to imagine other times. (Via conference participant Malcolm MacIver.)

7. Disorder increases as time passes. At the heart of every difference between the past and future — memory, aging, causality, free will — is the fact that the universe is evolving from order to disorder. Entropy is increasing, as we physicists say. There are more ways to be disorderly (high entropy) than orderly (low entropy), so the increase of entropy seems natural. But to explain the lower entropy of past times we need to go all the way back to the Big Bang. We still haven’t answered the hard questions: why was entropy low near the Big Bang, and how does increasing entropy account for memory and causality and all the rest? (We heard great talks by David Albert and David Wallace, among others.)

8. Complexity comes and goes. Other than creationists, most people have no trouble appreciating the difference between “orderly” (low entropy) and “complex.” Entropy increases, but complexity is ephemeral; it increases and decreases in complex ways, unsurprisingly enough. Part of the “job” of complex structures is to increase entropy, e.g. in the origin of life. But we’re far from having a complete understanding of this crucial phenomenon. (Talks by Mike Russell, Richard Lenski, Raissa D’Souza.)

9. Aging can be reversed. We all grow old, part of the general trend toward growing disorder. But it’s only the universe as a whole that must increase in entropy, not every individual piece of it. (Otherwise it would be impossible to build a refrigerator.) Reversing the arrow of time for living organisms is a technological challenge, not a physical impossibility. And we’re making progress on a few fronts: stem cells, yeast, and even (with caveats) mice and human muscle tissue. As one biologist told me: “You and I won’t live forever. But as for our grandkids, I’m not placing any bets.”

10. A lifespan is a billion heartbeats. Complex organisms die. Sad though it is in individual cases, it’s a necessary part of the bigger picture; life pushes out the old to make way for the new. Remarkably, there exist simple scaling laws relating animal metabolism to body mass. Larger animals live longer; but they also metabolize slower, as manifested in slower heart rates. These effects cancel out, so that animals from shrews to blue whales have lifespans with just about equal number of heartbeats — about one and a half billion, if you simply must be precise. In that very real sense, all animal species experience “the same amount of time.” At least, until we master #9 and become immortal. (Amazing talk by Geoffrey West.)

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250 Responses to Ten Things Everyone Should Know About Time

  1. Mike says:

    I’ve always liked this quote about time:

    “The universe may be timeless, but if you imagine breaking it into pieces, some of the pieces can serve as clocks for the others. Time emerges from timelessness. We perceive time because we are, by our very nature, one of those pieces.” — Craig Callender


    Are the talks going to go on-line any time soon?

  2. Stan says:

    Time organizes the universe into an ordered series of moments

    Or as Ray Cummings pithily put it, in The Girl in the Golden Atom, time is “what keeps everything from happening at once.”

  3. Ajay says:

    Sean Carroll, You are out there.

  4. RM Zulkipli says:

    Then its a day, and the holy sun emerged to roll back the mists..

  5. Steffen says:

    On #9: Also our grandkids won’t live forever. Perhaps they will enjoy a prolonged lifespan and much higher quality of life in old age, but they will not live forever.

    There is a very good reason evolution invented death. The cycle of life and death ensures that a species adapts much better to changing circumstances. Imagine a society where everybody is immortal. It quite inevitably culminates in an ultra-conservative nightmare, where “everything is like it was forever”. Imagine a society where the industry tycoons of the 19th century still own the majority of money and influence.

    A static society must break down sooner or later, one of the best examples for this is the late Soviet Union, where nothing new happened and which was completely dominated by old men who still evaluated everything in terms of an economy based on heavy industry. And when society breaks down (what it does regularly in human history, this is inevitable), most people will then lose the access to this life-prolonging technology.

    I am glad that death exists, and when my time arrives, I will go, to make place for the young generation. They deserve their chance.

  6. Drew says:

    Couldn’t time be emergent from motion? If all motion stopped, what would “time” be? Then again, it seems a chicken/egg scenario. For motion you need space. But time is just another dimension…

  7. Rick says:

    “Then its a day, and the holy sun emerged to roll back the mists..”

    Very pretty, but I would hope that folks with a religious bent don’t take a post focusing on science and time on another long and ultimately unresolvable discussion about God, etc.

    I know it’s a free country and all, but please at least wait until Sean puts up another post more directly addressing the issues you care about — he has a history of doing that.

    Let folks who care about this explore it without having to search through dozens of relatively unrelated comments.

    Not my blog of course, just a request.

  8. Alphacarey says:

    “A lifespan is a billion heartbeats” is true for mammals other than humans. We typically reach a billion heartbeats sometime around our mid-twenties; roughly 3 billion heartbeats for a life span of 75 to 80 years.

  9. Howard says:

    Agree with Drew. If there is no motion there is no temperature. Heat death means no entropy and no way to measure time. Doesn’t time cease to exist at that point?

    Additionally could maximum entropy and minimum entropy be identical? Would that be an explanation to why entropy was low at the big bang?

  10. Kevin C says:

    I’ve always (since about the age of 15) defined Time as “the length of space in which it takes to do something.”

    In a nutshell, if you take a “thing” that has absolutely no motion/activity/action/etc. Even the electrons in the atoms that make it up are not moving, does time exist for it? The (theoretical answer) is “No”, because if nothing happens, then it has no way of measuring that anything has changed, and if nothing changes, there is no (subjective) time.

    To an outside observer, time would have passed….but if there are no outside observers then all is encompassed within a non-active state, and therefore there is nothing to compare the difference in location to…thus, nothing happened, including the passage of time.

    Just my thoughts…which to this day (25+ years) still makes sense to me. 🙂


  11. ” The past and future are equally real” – in a classical deterministic universe, and just maybe if the many worlds idea is true, but no. An “emitter” produces a wavefunction that expands out and then either a collapse occurs or else you believe all of the absorptions really happen (which, despite beggiing off, really does violate conservation laws.) Can you really imagine that running backwards, toward that single emitting point?

  12. “Every event in the past and future is implicit in the current moment.”

    How about an atomic bomb going off or not according as a radioactive atom decays or not.
    (Many worlds is only speculative.)

  13. alyxandr says:

    “every event in the past and future is implicit in the current moment” — My impression was that the Determinists hadn’t won the argument *quite* yet… surely, while people are still arguing about how to interpret wavefunction collapse, there’s still room for some randomness?

  14. boreds says:

    (2) “every event in the past and future is implicit in the current moment”

    What about the quantum fluctuations which ‘freeze in’ to become the cosmological large-scale structure we see today? Isn’t this structure thought to be truly random, with its origin in these quantum mechanical fluctuations?

    I hadn’t thought about this for a while (I more work closer to #10 these days), so I’ve looked in Kolb and Turner
    to see what they think. Basically they seem to say that as a wavenumber k
    crosses the horizon, the quantum field mode associated with momentum k
    “freezes in” as a classical perturbation. It seems the process of this
    classical freezing is irreversible, or is that not right?

  15. Sid says:

    Would love to know if the talks of this conference will be posted online? Also, the other conference attendees might have posted blogs of their own about their take-away from the conference? If you know of any such blogs, it’d be amazing if you could post their links here. Thanks!

  16. Fred says:

    Sean, in your view, does the recent paper 1108.3080 solve the entropy problem in 7, as the paper claims?

  17. Chris says:

    Humans must not fit into that 1.5 billion heartbeats. Our lifespans have going from 30 to 80 years so unless our hearts were beating a lot faster back then (maybe when we were outrunning tigers) it clearly isn’t as simple as you made it seem.

  18. Tony says:

    Chris – That’s just a factor of two difference. These are just rough figures.

  19. bboyneko says:


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  21. Ruda says:

    Maybe time doesn’t exist, and only the far future at the end of time is real. The timeless eternal reality of eternal memories by one consciousness remembering. The future is far more real than the past. And yes, that one consciousness has a fallible confabulating memory.

  22. Tom Clark says:

    “The past and future are equally real. This isn’t completely accepted, but it should be. Intuitively we think that the “now” is real, while the past is fixed and in the books, and the future hasn’t yet occurred. But physics teaches us something remarkable: every event in the past and future is implicit in the current moment. ”

    So you’re saying the future too is fixed and in the books, and that this should be completely accepted. Given the implications for commonsense notions of human freedom and dignity, it’s unlikely this will ever come to pass, but I appreciate your advocacy.

  23. Matt Welcome says:

    Ok =)

    how do i put this ?

    Einstein suggested – ‘For we convinced physicists, the distinction between the past,present, and future is just an illusion, no matter, how persistent’.

    I’ve looked at the notion of time in great detail. I seem to have a rather analytical mind. And, it seems to me that what we are trying to work out essentially is ‘how does the world work?’

    If we ask this question, we may find some meaningful answers. But if we ask loaded questions, such as “does ‘TIME’ exist”, or “What IS TIME”, then perhaps these are biased and misleading questions that may send us down a very long, and very confusing, wrong path (if it transpires ‘our ‘assumption’ that Time’ exists is actually unfounded, and false).

    Perhaps instead, we can ask the unloaded question, ‘what do we actually, directly,observe?’

    To which we might answer, at the most basic level, ‘I directly observe matter existing, moving, changing, and interacting’.

    Then we could ask “Is matter, existing, moving, changing and interacting, enough to explain all we observe and attribute to the existence of (mysterious, and intangible) ‘Time’ ?”

    To which, I would, after very careful consideration say,

    “Yes, the suggestion that perhaps in the universe, matter just exists, moves, changes and interacts ‘now’ (to use a redundant word), can explain, without exception, all, of the observations that lead us to the (incorrect) idea that, extra to what we directly observe, a thing called ‘TIME’ also exists”.

    In other words, it can be logically deduced that there is no ‘extra’ thing called Time.

    Things may ‘just’ exist, move, change, and interact. And this alone can lead us to wrongly assume that a ‘past’, and a ‘future’, and a flow of a thing called ‘time’ between them, exist.

    ‘TIMELESSNESS.CO.UK’ links to more info on this, my book draft ‘A BRIEF HISTORY OF TIMELESSNESS’, and some videos of a couple of talks I have given on the book.

    Matt Welcome – London.

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