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. My 12 year old daughter is so inquisitive at times, I begin to wonder about things I had up until that point accepted as fact.

    In the case of the big bang theory, and my limited knowledge of it, my daughter had asked, “What is the big bang theory?” Well, without grabbing numerous texts and scientific journals on the subject, I attempted to explain it in the simplest terms, which coincidentally fit my simple mind.

    As I explained it, I began to wonder about the origin of it all.

    Let me put it simply. I explained there was some huge ball of matter, which somehow exploded, the pieces were sent outward throughout space, and formed the various galaxies. Supposedly the universe is constantly expanding. If we were to build some kind of super-fast spaceship, we would be able to travel far enough past the debris to a point where the universe ends, an eternal blackness of nothingness.

    My question which comes from this rudimentary answer is, “How the h*ck did we come up with this one?” My guess is someone took a telescope and a computer, then progressively more powerful telescopes and computers, and pointed it into outer space, and determined there appears to be a limit to what we can see out there in any infinite direction. The fact that “what we percieve determines what is” is a horrible basis for what is, in my opinion. I get the impression that whomever devised the theory of the big bang have determined that the Earth is the center of the universe. If they have not, I’m guessing SOMEONE is probably wondering where that supposed center is. It must be calculatable taking into account the direction of the various galaxies and particles if such an explosion could occur in the vast matterless vacuum of space billions of years ago and still be in motion from a perfect zero friction environment.

    Funny thing, she also asked about dark matter. Again, armed with my vast knowledge and intellect on the topic, I explained that there are things that exist in the world that cannot be verified light or the reflection of light off it, thus the term dark matter. Not to be confused with the experiments on objects which light is able to bend around, the “cloak of invisibility” experiments of recent history.

    Disclaimer: Lots of sarcasm when discussing the topic of my “intellect”…

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  4. @BlueCollarCritic (187.) I like that explanation a lot. Time is simply a unit of measurement, and we love to create measurements for everything, so we made a name for this measurement (of relativity) which is simply a counter. Not really the same, but think of a car’s odometer. We’d go twice the distance to reverse it back to zero because it’s simply a counter measuring units of something. I know, bad analogy but there aren’t too many. How about a Watt or Amp hour?

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  6. How extraordinary you all are with your opinions and conjectures of time!
    In fact no need for Eisteins/Newtons’Hawkins/Dawkins/Hitchins/Hubble’s Uncle tom cobleys billy connonleys …whatever.. what a band of nonsense-makers you all are.
    Time is sinply a man-made invention a clock -face with two hands to keep you all enslaved
    to money-making for survival to keep the elite and mockery democracies in business.
    Time of itself does not exist, simply a process like your age you live/decay/die like all things
    as portrayed in the four seasons. Time is a man-made invisible enslaving master used by
    pleasure seeking deluded aristocrats,politicians, and all the other human ingredients
    required to keep them in business…armies/navies/celebrities/hollywood arsetists.
    Go on tell me my email is not valid!!! hey hey.

  7. Time does not exist, it’s imaginary. We just think time exists so we can enjoy the present, reminisce about the past or dream about the future.

  8. Alphacarey points out that the Billion heart beat rule doesn’t apply to Humans – They claim that Humans reach the billion heart beats point around age 30. What they fail to point out is that human lifespan is no longer bound by the laws and time spans of nature – Keeping ourselves alive through technology and and drugs.

  9. “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.”
    This is exactly the sort of extrapolation from a theory that tends to be thrown away as science moves on – the kind of extrapolation that can’t be verified directly (unlike, say, mass-energy equivalence) and which comes from taking the theory literally when scientific theories are just working models, and may later prove to only approximations to the truth (as, for example, K.E.=0.5mv^2 is an approximation, and becomes inadequate near lightspeed).
    To say of things based on this, “This isn’t completely accepted, but it should be.” is nonsense. It should never be completely accepted by anybody with the slightest experience of science – if you only know one thing about science, let it be the fact that our theories are always liable to change, unless we have observed them directly. We have observed directly (from space) that the earth is round. We definitely haven’t observed directly that the past and the future exist, and we never will unless a time machine is invented.

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  11. ERROR: as per comment 47, “Part of the “job” of complex structures is to increase entropy,” should read “Part of the “job” of complex structures is to DECREASE entropy.” Living systems produce a localised decrease in entropy at the expense of an overall increase in entropy of the system of which they form a part. Rather like an up slope back eddy in a river flowing downhill.

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  15. It is my opinion that time seems to go faster with age due to an unconcious realization of impending doom, or death.

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  17. I first came across the “billion heartbeats” data in an article by Isaac Asimov, years and years ago. The scaling seems to hold for all mammals except ourselves. Perhaps it was also true for us when our life expectancy was shorter, and it’s only our tendency toward civilization that has allowed our lifespans to more than double the heartbeats allotted to species without MD’s and refrigerated foods.

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  19. Bing Bang why do they keep pushing that
    if there is no beginning and no end
    Infinite
    its to bad we can’t measure it or wrap are small brains
    around it
    I’m sure I have already posted this and will again in the future

  20. I may have missed something, but I’ve read 223 postings, in addition to the article, and I haven’t seen an agreed upon definition of time. So many thoughtful and articulated arguments becoming irrelevant to each other because the fundamental definition of what you’re trying to prove has too many interpretations. I’ve enjoyed reading the thread, but the ‘debate’ is baseless without an agreed upon definition. All this blather about mass, energy, entropy, blah, blah, blah. And 40 years ago when I was a kid, Pluto was a planet. The only thing science proves is what the human race thinks it knows at any given moment in time; pun intended. And no, I don’t want to live without my telephone or cable tv or any other 20th century scientific discovery. But I could. And there’s the rub. The hubris of the scientific community is only surpassed by the volume of s_hit that they don’t know and can’t explain.

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