“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.)
Time does change as one ages. Think of time in terms of an individual’s lifetime. To go from age 1 year to age 2 years, an individual has to live another lifetime. However to go age 20 years to age 21 years, an individual only lives an additional 1/20 of a lifetime. Thus the older an individual gets, the faster time, in terms of lifetime, progresses. Of course, Chicago Transit Authority (later to be known as Chicago) had it right in one of their early songs which asked “Does anybody really know what time it is? Does anybody really care?”
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As described by Sean Carroll in “From Eternity To Here”, time IS a coordinate in space-time. The snapshot of the Universe at 6pm today is different than the snap shot of the Universe at 9pm tonight. Your GPS may say you are in the same spot at both of those times, but the Universe would have changed around you, and that new TimeSegment you are in has a different coordinate in the SpaceTime Universe. As I understand…. So, disappointing, time may not be anything other than the observation of change, including entropy. Nothing magical about that. Shucks. I wonder if we could hold an absolute position relative to an unknown point of reference, would time stop (for us). I guess to do that we would have to stop time. It becomes circular.
Time is changeless Space. -Aiya-Oba (Philosopher)
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After reading all the posts I have concluded that time ends when I die.
@Anthony …and (or?) spaceless Change.
Instant Inspiration Translation
When the benefit of goodness translates to the over-bounding plusness of allgetting for the many we are all applause for the cause of clapping as gazing around to find cheek lifting smile style for long while when electronic audio party favors arrive in many flavors so joy juice flows throng grows in expansive trance dance eye glance round n round as bones jump to sonic shoe shuffle song mix tricks transfix clusters of crazy cats toast the most vibe ripe trip hop candy shop score store street smart instant art at the start beyond description sans prescription where sparks sizzle between synaptic snaps as the whole house hangs high above the sky born to fly from here to beyond the margins of any man made map fades into the ether place is then revealed upon your face we let loose the sticky strings which tie us to many things and drift away from gravity and it’s a gas as we become unglued from the get more moods heavy condition we evoke our own rendition via the collective minds ignition of sweet cognition so it’s there we all begin to share the aspect of our eternal travels and the thin mystery of moment unravels to reveal what’s really real as mystic events peal away the layers of paper lies from our naked eyes where time terminates ticks n tocks we step across the invisible line as easy as slipping into the soft abyss of bliss where that, becomes… this!
© 2010 • D.L. Nelson Ironworks Publishing & Focus Fine Arts®
Quote: Maybe, as with the most complex modeling/problem solving accomplished with thousands of inter-connected processors (computers), we humans might be able to adapt and solve our most pressing problems when united together for a survival purpose.
Unquote: Time will tell… 😉
They should put “like” buttons on these comments. Seen some GOOD ones here…
Thanks!
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First, every event in the past and future is implicit in the current moment therefore they are equally real? That is no physics I have ever heard of. What is implicit in the current moment is that there is a direction of time; the arrow of time. It says that because the current moment is the result of the past changes, so will some future specific moment be the culmination of its past events.
The only way that the future is implicit now is that there is no now without a future. In no way at all do future events specifically affect the present moment!! It only means that time hasn’t stopped now, FFS. Three days from now, or three billion years, our universe might cease to exist in the way it does now, perhaps a collision with some other part of the multiverse, or the sudden, spontaneous decay of gravitons into zigzag and corkscrew (flavors, like gluons’) anti-boson pairs, or whatever, we have no idea of how reality crumbles, yet it might or might not. This changes nothing about our present characteristics of reality, we only know that right now, we are in the future of the past and the past of the future, LOL. Having a future allows our present existence, nothing more, nothing less. Don’t give me specific future events crap. There is no physical theory that predicts, let alone concludes, a determinate future, let alone a future that determines or influences its past. The future is indeterminate, as any wave functin wil show you.
Finally, time can exist without matter or energy(they’re the same, Craig), but matter cannot exist without time, for matter is only defined by interaction which is exchange of energy ie change.
@craig#58: Time, energy, physical existence, is not subjective, okay? Energy and matter are equivalent, not two different and separate states that have different physical laws. Matter is ‘experienced’ so is also subjective by your reasoning. Kinetic energy can be ‘released’, from potential energy, and a coiled spring can uncoil, or an atom can fission, without anyone experiencing it, even without anything experiencing it.
Saying that energy is the container of time is a non sequitor. Arbitrarily assigning a perception as subjective is meaningless, because what you mean, I think, is not ‘subjective’ but ‘experiential.’ We experience time(change), matter(inertia), and energy(work), but so does everything – albeit not consciously. All you have done is say that a tree doesn’t make any noise when it falls if no one hears it.
You also said, “and no exterior phenomena as a frame of reference, there can be no space.” How do you know? A sphere has volume, doesn’t it? That is space. If the universe has so decayed(entropy) and expansion has accelerated past the speed of light, then fundamental particles will have no way of interacting and therefore determining spin or anything else, does this mean that time and space have ceased to exist? That the particle suddenly ceases to exist? That expansion ceases to happen/exist? That zero point energy ceases to exist, suddenly?
No. Space cannot exist without time, nor time without space, for without space, there is nothing to change, move, decay, whatever, and without time, space does not have duration, a period in which to exist.
As far as I can tell, this is unresolvable using pure reason without making assumptions(which is my assumption!). Space and time are co-fundamental(lol wut?), or fundamental properties of each other. Obviously, I am just inelegantly restating that we exist in a four dimensional time-space continuum.
I just hope we don’t spontaneously cease to exist before I hit the submit button so someone can set me straight =]
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Sean:
Clearly that’s not true, about aging. You point this out yourself in #9. That’s because of biology, not physics.
I have often thought of time as an invention for our convenience, a construct used to mark units in terms of years, months, days, hours, minutes and seconds (nano included). We even mark the passing of time with birthdays, anniversaries, celebrations, and history. In the natural world, we are governed by seasons, the movements of the earth and the planets, the sun and the moon, and the indigenous peoples were much more in sync without watches and clocks and calendars. That said, I like these ten descriptions and movements and being part of them, and whenever possible, merging past, present and future into one great experience.
Time seems to be a side effect of having a limited speed of communication. If the speed of light or the speed of communication was infinite we could be anywhere in “no time” and there would literally be no time.
I think this may also explain how time started at the Big Bang. Einstein’s famous equation E=mc2 is not currently “flipped” into c2=E/m. If E is in fact quantum energy E(q), vacuum energy, cosmological constant energy or whatever name we give to the expansionary energy that fits into Relativity’s equations this flip may be valid. If so prior to the formation of mass from E(q) (mass = energy) c would have been infinite (since m=0) and there would have been no time.
Put another way then mass creates time by limiting the speed of light.
somewhere between too little and too much is something worth pursuing, somewhere we are going….never to arrive
Sean, everyone is waiting for your comments on Tegmark’s paper.
Check out “Nothing is Random” by Mark Helprin, an excerpt from his book the Winter’s Tale. It may not be true about time, but it’s stuck in my mind for many years…
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The title sounded intriguing, but then in the first item:
“Time organizes the universe into an ordered series of moments”
Er, no. If two points in spacetime are not in each other’s lightcones, you can’t define an ordering. C’mon, special relativity is more than a century old now…
Re. “…why was entropy low near the Big Bang…?”
I am puzzled by the idea that there is a problem here, because in a strong sense, entropy was also nearly maximal.
As I understand it, most of the important physical degrees of freedom were near equilibrium at an early, pre-nucleosynthesis stage of the Big Bang expansion (disequilibrium was largely confined to weakly coupled degrees of freedom). Thus, the early universe was — given its volume — near its maximal entropy with respect to the physical interactions of interest to biologists and astrophysicists.
Expansion of the universe, by expanding the available configuration space for matter, increases the potential maximal entropy of its contents. Thus, expansion allows entropy to increase relative to what had been a state of lower, and yet maximal, entropy.
Viewing this from a thermodynamic perspective, the expansion provides us an ever-increasing volume of ever-cooler vacuum to use as a heat sink.
So, given the expansion of space, I don’t see how the increasing entropy of the universe per se poses an additional problem. The problem of the homogeneity of the early universe is a puzzle involving a surprisingly degree of equilibrium, that is to say, a problem of surprisingly high, not low, entropy.
Am I missing something here?
Quite a lot of interesting subjects in this article, and the comments as well! Great work. Deja Vu may explain para 5 somewhat. The scenario process (normally only remembered as dreams when waking) is accessed by the conscious and is indistinguishable from memory. This gives merit to the idea that reality is delayed between sensing and consciousness awareness to give time to process and prepare for “current” thoughts and actions.
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