“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.)
Number 9 is not entirely accurate. All animals live to 1 billion heartbeats, with the exceptions of humans and some domestic animals. Humans live more than twice that long; if humans only lived for 1 billion heartbeats, we would all be dead by age 30.
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I would add that we build our image of reality based on a finite number of events.
Isn’t #2 fatuous, given that we cannot ever possibly know things like when radionuclides will decay?
The last I recall, there was an effort to reconcile quantum indeterminism with the appearance of reality we see everyday by a method called “sum over histories.” They wanted to use the concept of decoherence I think. Reading as carefully as I could, I believe they discovered that the past was just as indeterminate as the future. Which if you think about it, seems a perfectly natural consequence of the past and future being indistinguishable on a quantum level. If both past and future are indeterminate, in what sense are they real?
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I take a different approach to understanding time but it involves a radical departure from conventional physical cosmology. (I know that makes me sound like a crank, but I haven’t found any counter arguments very convincing so far, so I do think there is a possibility that this hypothesis has real explanatory power).
Time is subjective because energy is subjective and energy is the container of time (as space is the container of matter). Energy subjective??? Yes. Energy can only be experienced. It’s not a glowing cloud that gets coiled up inside of springs and released through action, it’s just the experience of a spring or a nerve or a human being experiencing a change from tension to release or vice versa. Of course I’m simplifying here, but stay with me.
Our experiences of the energy of something else – seeing a spring pop, feeling a log burn, reading a person’s emotional shifts on their face (yes, that’s energy too), is a second hand experience. The first hand experience is our nervous system imitating those changes as the content of our awareness.
That is what we sense, feel, see, etc is the ‘interior’ of our own nervous system recapitulating the changes it is detecting through the body cells, tissues, and organs, of the changes in the environment. Experience = change = energy. No subjectivity, no change, no energy (not talking about any one organism’s subjectivity, I’m talking about the ontological necessity of perception or detection in the cosmos to define energy).
If you can get the gist of that, then maybe it will make sense that time is the container of energy. It doesn’t ex-ist, it in-sists. Since energy is change, and change requires some sort of fundamental capacity to sense that there was an original state to begin with and an extension of that sense into memory to sense that the now state is different in some way from the original state, time can be understood as energy which is not occurring ‘now’.
The sequential nature of time is important, but it is not primitive. On this list, the #3 trumps the #1. In the vernacular sense of ‘time’, I think that it is just an aggregate measure of physical change modeled in a linear fashion. It has no existence of it’s own beyond our sense of sequential causality (which evaporates predictably under altered states of consciousness – dreams, drugs, trance, etc). We are the ones who interpret the digits on the clocks and the calender squares as a shared temporal text. In reality, there are no days, just astrophysical orientations woven together by our memories and monitoring of regular oscillating patterns.
As far as aging and entropy goes, those are existential functions (so having to do with the reflected secondhand side of our experience…matter). If my hypothesis is correct, entropy is actually a function of space more than time. If you think about two sandcastles, one of which is confined within a glass, castle shaped jar, and the other naked, entropy is going to be staved off for much longer under glass.
Space is to matter what time is to ‘energy’ (experience of change). If you think about a universe with nothing at all in it except a single proton (or a ping pong ball, whatever, some kind of ideal sphere of matter) and no exterior phenomena as a frame of reference, there can be no space. There’s no position or speed or spin. For those things you need some other object to relate to to be able to generate the relation of space – so space would not be a rich hyperdimensional topology of superstrings and intangible vacuum energy, it would in fact be a true void. A void created not by a Big Bang, but by a Big Shatter of the singularity (which exists still across the gaps of spaces and insists still through the stories of times).
I know, it sounds completely nutty, but I’ve spent a lot of time debating with people about it and have not heard any new convincing objections. I think all of the irreconcilable strangeness of entanglement and uncertainty, gravity and electromagnetism, mind and brain, perception and relativity, entropy and cosmology, QM and GR can all be tied up neatly by using a sense-based model of the cosmos.
Sorry if I’m hijacking the thread here, it’s hard to comment on time in this model without commenting on everything else.
I am not a physicist, but physics questions and “theories” are all over my mind, mostly from a philosophical perspective. Lately I have this persistent idea that time is “merely” an illusion, just another part of this whole illusion that we call “reality” as Einstein points out. Therefore I am believing that all of the time pieces are actually happening at the same “time” which complies with #2 (The past and future are equally real), it is just our “consciousness” and the illusion it is being effected by that make us believe that past is gone and future is yet to come, but actually everything is just there happening all at once.
I have always held to the idea that time is just man’s feeble attempt at measuring that which is immeasurable. We just chop it into chunks (days, weeks, months, years, etc…) in order to more easily process something we are unable to fully grasp.
How does every living thing only get 1.5 billion heartbeats? Doesn’t our heart beat faster than once per second? If resting heartrate is around 70 beats a minute, the answer is yes. When we’re babies/children out hearts beat even faster. Well 1.5 billion seconds is equal to roughly 48 years. The math doesn’t add up for humans.
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“You and I won’t live forever. But as for our grandkids, I’m not placing any bets.”
Yeah, well: http://youtu.be/JOxlpVm96QQ?t=1m43s
Is it not an oft-used lie that entropy and disorder are the same thing? Entropy is about energy distribution, not what looks neat to human brains. A ‘disordered’ (messy) room with clothes strewn about does not have access to any more or less energy states than an ordered room. Or if you have some ice cubes floating in a bowl of water, ‘order’ increases as the entire system becomes liquid water, but entropy increases as well.
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Stephen said: “Imagine a society where the industry tycoons of the 19th century still own the majority of money and influence….”
Umm, is that NOT the present reality??
Time (na na, nana na)
Time is time (na na, nana na)
Labadab dab dab time (na na, nana na)
Tiiiiiiiime! (na na, nana na)
“Time is an illusion. Lunchtime, doubly so.” -Ford Prefect
“if reality were completeLY different from moment to moment. ”
I’m quoting, with attribution to you, these points in a science fiction short story, “Time Is Out of Joint.” I try to tie in the recent arXiv paper Joel Hamkins’ The set-theoretic multiverse. I posted a lovely quote from that paper on your Facebook wall.
I’ve taught half a dozen times a 1-week college level adult education course on Time Travel that uses Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction (ISBN 9780387985718) by Paul J. Nahin as de facto textbook.
Of course, my understanding of Time is influenced by our mutual friend Kip Thorne, and my co-authors Isaac Asimov, Ray Bradbury, and Robert Heinlein. Ideas that fall short of working, equationally, and would likely not get past a referee, are still grist for the fiction mill. I’ve been paid a number of times for rather goofy Time Travel stories.
I can email you the complete draft of the story when done, to ensure that I have your permission.
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“The past and future are equally real.” @ STan and others – maybe all slices of time (past, present, future) do happen simultaneously. Considering #2 above, maybe we are not traveling up or down a time stream but rather along an axis that crosses all possible variations of past, present, future. What a mess.
Perhaps ‘life’ could be defined as forces that defy entropy. During the course of ‘life’ our existence is in contradiction to the laws of entropy. When we understand those forces, we will be able to better define this concept of ‘life’. The state of ‘life’ is more ordered than the work of attracting forces like magnetism or gravity. And our point of observation of ‘time’ is from the seat of life. Rocks don’t perceive of, or witness, time.
And as for the refrigerator, it doesn’t design and assemble itself, it requires us to do that. Again, it retains its form and function only breifly.
@Stefen re: #9 – I won’t go willingly, there is more than enough space for us and every future human generation in this universe. In fact, we better keep and create every human we can. The problem is that we as humans are too focused on staying at home. We spend our money and energy on competing for resources and tribal domination, when the human race should be trying to expand beyond this planet. Take life with us and continue to fill the universe, there is no room for decay and death, what a waste.
@Tim, but is did take energy to change the state of those messy cubes of water to ice…
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I think the problem with time is that we think in terms of the present as moving from one event to the next, but the only reality we perceive is the present and it’s the changing configuration of this present that turns future potential into past circumstance. . So it isn’t the present which moves from past to future, but the events which go from future to past. The earth doesn’t travel the fourth dimension from yesterday to tomorrow. tomorrow becomes yesterday because the earth rotates. Reality doesn’t break into different worldlines with every quantum indetermination. It’s the collapse of future possibilities which creates past events. Time isn’t a vector, it’s a process.