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.)

250 Comments

250 thoughts on “Ten Things Everyone Should Know About Time”

  1. Pingback: Ten Things About Time (You May Not Know) « Steps & Leaps

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  3. HOW WE CAN SHOW THAT NO ‘TEMPORAL ORDER’ EXISTS.
    (and thus no ‘Time’ just matter and motion).

    (Following from my entry #24)

    This is a great post, love the serious and not so serious entries, but I think in a way it shows the problem of taking an unchecked assumption as true then running with it.

    This unchecked assumption is that extra to matter and motion (what ever they are, whether they are ‘fundamental’ or ’emergent’ etc) – we seem to have assumed there is ‘also’ another thing called ‘Time’.

    Most serious studiers of the subject will think this assumption is safe because Relativity is clearly a very comprehensive, accurate and useful theory – and it consistently mentions SPACETIME.

    However, if we read Relativity carefully from the start…

    http://www.youtube.com/watch?v=14Xv3sA4jPY&t=7m40s

    https://sites.google.com/site/abriefhistoryoftimelessness/comments-on-time-books/einsteins-relativity

    we can see Einstein rapidly assumes that because we can in a sense remember and agree on a ‘temporal’ order of events, that time obviously in some way exists.

    -But if you consider very carefully where 2 people are recollecting and agreeing on some order of events (say things that happened in a basketball game as in ‘Introducing Time, Craig Callender + Ralph Edny) – what they are ACTUALLY doing is looking at ‘the contents of their own minds’, and agreenin that these contents match.

    from this agreement they assume that there is ‘also’ another record created in a ‘place’ or a ‘way’ called ‘the past’.

    http://www.youtube.com/watch?v=14Xv3sA4jPY&t=9m10s

    https://sites.google.com/site/abriefhistoryoftimelessness/comments-on-time-books/introducing-time-callender–edney

    And this assumption is taken as a proof that ‘there is at least a past and a temporal order’
    But gootd science is based on awkward questions, and careful analysis comes up with the question ‘if things could ‘just’ exist, move, change and interact ‘now’ (ie with no such thing as time implied or assumed) – would this be enough to mislead us into thinking time existed.

    in other words ‘is there actually a ‘temporal order’ created and stored somewhere some how in the universe or ‘not’ ?

    because if there is a temporal order then there is and if there is not, then there is not. and if we mistakenly think that the contents of our heads prove more than just that matter can move and change, then we may go on to create a ‘persistent illusion’ like ‘time’.

    http://www.youtube.com/watch?v=14Xv3sA4jPY&t=11m19s

    S0, as i said #24, i think we have to be very careful, and take a step back from asking ‘does time exist, or what is time’ – and instead ask ‘what do we directly observe’ – which is ‘matter and motion’ – then ask ‘would matter and motion be enough to explain all that we observe and attribute to Time’.

    to which, i think rather surprisingly the answer is yes, the universe is full of matter and motion, and also Time-less.

    If you look at relativity you can see that Einstein never proves that ‘the past’ or ‘the future’ exists, it is just assumed after the temporal order is assumed to exist and mean more than it might. And if you look at relativity as if is just beautifully describes how things move, change, and interact ‘now’ you can see it still makes perfect sense.
    But instead of revealing ‘time dilation’ and thus possible ‘time travel’ – relativity only shows that things might change at unexpected ‘rates’, ‘now’. Which doesn’t prove the existence of, or involve any travel in to the ‘past’ or ‘future’.

    Sorry , no time travel , see > http://www.youtube.com/watch?v=PKsfdGI0X-o&t=5m50s

    (Matt Welcome timelessness.co.uk)

  4. Sean,

    Who died and left you in charge of time?
    BTW 1.5B heartbeats = a life. Guess you plan on checking out in your mid 30s. Recheck your math.

    Have a good time.

  5. Pingback: Time…. » Thankful365.com - Thankful 365 days a year!

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  7. Pingback: Ten Things Everyone Should Know About Time | Cosmic Variance | Discover Magazine | That's All I Have To Say About That

  8. Time doesn’t exist, it’s an abstraction…. well it exist’s an abstraction, as an idea. It is not a thing. Show me the thing called time. Show me the empirical evidence of time. Scientists are evidence based when they have evidence, when not, they are as abstract as Terry Pratchet.

  9. This is very interesting. People learn something new. Time was perceived differently in the old days than it is now. In the past there was no understanding of time travel. In the future this could be a reality. Thanks to modern physics mankind achieved greatness in science.

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  12. thanks everyone for this ‘STONE SOUP’ made of too much philosophical SEMANTICS for seasoning and not enough nutrition from LOGIC. Kuddos to #86 if opinions on this topic were trees in a forest you are barking up the right species. #107 also kuddos, I would guess your I.Q. is around 135 and your mental ability for abstract physics is above the 95th percentile. I invite you both to keep Plato in mind. I believe that his ‘ALLEGORY OF THE CAVE’ can get you to the right tree…. if you can see the enormous clue that he provides. Best Wishes…

  13. Time? A billion heartbeats? Mankind’s short lifespan and the lifespan of a tree……Any questions?

  14. I hate my life. I wish I could control time. I would fast forward the next 60 years and get this over with. It’s like I’m trapped here. This is all meaningless.

  15. Biocentric dreams of a creative universe that revolves around what we think ? We have a very very narrow spectrum of existence. A spectrum of existence we can no more fathom than an ameba contemplating the same.

  16. You talk of the Big Bang. Credibility goes out the window. The Big Bang has been disproven many times over. Start with Eric Lerner, then visit the Plasma Universe. Physics in the 20th century was riddled with theories driven by mathematics that were not correct. We continue to suffer from that today.

  17. Aging is not simply a function of time. Progeria, the premature aging of children, makes them look age to 75-100 years old at age six to eight, clearly demonstrates that physical aging is based on something inherent in our physiology, not simply a function of time. Focusing on that will do more to slow or suspend aging than anything!

  18. This is one of the dumbest things I’ve ever read on the internet. Coming from Discover Mag doesn’t surprise me.

    Time is only a construct of man. It doesn’t not actually exist. It is a tool, like math.

    We will all die one day and concepts of god and time help us ameliorate the uncertainty of what will happen to us when we do.

    Remove us from the universe and time dies with it. All that remains is a constant of change.

  19. The Universe is in constant flux. The only constant is change. Re: #3 Time is merely our perception of this change. Regarding #7 The Big Bang represents contraction (Yin) and entropy is the expansion (Yang) mode.

    Re: # 9 When we shift our awareness/consciousness in meditation (traditional, moving, sound, or laughter meditation) and time appears to “stand still” does our aging process stop? Is there a link between perceived time and longevity? Inquiring minds in the laughter field are inquiring…

  20. Anyone who wants to reject the idea that time is real should first look at the observations by Hulse and Taylor of the binary pulsar system PSR B1913+16. The orbits are decaying at a rate consistent with energy being carried away by gravitational waves, ripples in the curvature of spacetime.

    Although not yet observed (for obvious reasons), modelling of mergers of spinning black holes also shows that they can be propelled through space by a process called “gravitational wave recoil”, ripples in time can act like the reaction mass of a rocket.

    http://www.black-holes.org/extreme-kick.html

    Time still has a few surprises in store for us!

  21. @Tom 168, you say that “Physics in the 20th century was riddled with theories driven by mathematics that were not correct. We continue to suffer from that today.” OK fine. Just please don’t be hypocritical and use a computer, iphone, mobile phone, GPS, CD player, credit card, microwave oven, radar system, plane, MRI machine, PET scan, etc, ever again, as they are all based on principles of modern mathematical physics. thank you for your co-operation in this matter.

  22. Does time exist in each dimension independent of time in the other dimensions? If our perception lags our consciousness then no matter what we do or when we do it, by the time it’s realized by others the error in our communication is exponentially relevant. It’s perplexing as it seems that in the perception we are able to experience energy that makes us predict or influence the future even if we appear to do so by accident. Much to ponder.

  23. For a photon, traveling at the speed of light, time in the rest of the Universe is frozen, non-existent. For a human, the past is a memory, the future is a construct, the instant is too short to measure. Time requires motion to be measured and an observer able to remember the past. Time cannot exist without matter, and a very special kind of matter from which consciousness has emerged. Time seems like it might be a construct we impose on the Universe to make sense of our experience.

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