“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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Time is a human construct and nothing more. It’s real in the way we use it, to measure events, to order our lives, etc… but other than that time doesn’t exist in nature, in the universe, and even at the quantum level. We perceive the idea of time as the progression of natural phenomenon, but at the fundamental level there is no time. There is nothing in nature that says after so many pulses of a cesium atom (a crude definition how we define a second) that a cell should split or an atom should decay or even that a light wave should travel (x) amount of distance. Everything in nature, even down at the quantum level works in either cycles or happens because of interaction with something else. Because of this and because of how we have defined time in our human lives it appears as if nature does work by a clock, but once you really understand time as we define it, then you can see that time is truly relative and because of that, fundamental time does not exist.
Think of it like this. The speed of light is considered fundamental and unbreakable but as we define it, the speed of light is 299 792 458 m / s. What if we chose to define the length of a second differently? Then the second would either be longer or shorter and thus the speed of light would be different. You see time is completely relative and has no real fundamental connection.
Time is merely what we use to create order out of our lives and/or chaotic systems we don’t fully understand.
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Space/Time are properties of Dark energy/matter like sweetness is to sugar.
Just heard Sean on the TV program “Through the Wormhole” asserting that the direction of time is related to the ongoing increase in entropy since the beginnings of the universe. I’m a layman, not a physicist, but I challenge the notion that entropy (as it was defined, meaning more and more disorganization) has increased since the beginning. What greater chaos could there be than the soup of particles immediately after the Big Bang. Over the eons, these primary particles have organized themselves into galaxies and stars, complex atoms and molecules, and planets like ours (I presume we’re not unique) rich with weather patterns, geologic structures and even, in our case, life. Humans continue to create increasingly organized societies and ever more sophisticated buildings, cars and electronic devices, among many other things. This is increasing entropy??? Yes, the energy driving all this may eventually dissipate, but I would guess that the chart of the universe’s overall entropy is still trending down and has not yet hit its trough. And, again as a layman, I suspect that this energy may ultimately be conserved, with “spent” matter coalescing into massive forms that re-explode or re-convert matter back into new cycles of energy, leading to new types of physical order. Let’s speculate about time in some way other than linking it to this alleged entropy increase.
The ’80 milliseconds in the past’ doesn’t really work well with known, researched facts about human experience in audio processing. For example, when playing music, a 20 ms increase in latency (time between your hands causing the sound and hearing it) is easily felt and noticed by the musician. 80 millisecond difference is clearly noticed as not simultaneous – 8 milliseconds might be a limit, but not 80.
Mikmik #86 seems in my view to have it about correct. Absolutists will continue to adhere to a linear description of their universal precepts. I am late to this discussion, but believe anyone interested in this topic might be rewarded by a look at fractals and chaos theory.
We may like the concept of past and future, but Augustine of Hippo stated over 1600 years ago that neither exist. The past once was present, but is gone now and cannot be retrieved (only partically and poorly recreated by memory) and the future is not here yet and cannot be known. That leaves the present, which in quantum theory, is a duration of 10^-43rd seconds (for those who like the exactness of counting). Quantification of the successive “nows” by clocks of various types does nothing to answer the more fundamental metaphysical question: What is Time?
I suspect that thinking of Time as a fourth dimension only muddles the consideration. Ditto the stuff about psychological experience of time (age of perciever; pain vs. pleasure, et. al.).
Physics is not sufficiently philosophically inclined to help my understanding of this Meta-concept.
“Of course time exists — otherwise how would we set our alarm clocks?”
Proof that “time” is a man-made institution.
Brain fart in post #55: It’s consistent histories approach, not sum over histories approach. Consistent histories cannot find a determinate past any more than it can find a determinate future. Again, this is to be expected as it seems to be consistent with the reversibility of the equations of fundamental interactions. This also seems to be inconsistent with the block theory of spacetime Prof. Carroll seems to telling us is the prevailing opinion of the conference members. But this too is to be expected since no one, so far as I know, has explained how mundane reality is to be reconciled with universal quantum entanglement originating in the Big Bang.
It seems a little odd that a cosmologist would opt for entropy as cause for emergence of time. I would have thought that expansion of the universe would be their first thought.
Total chaos is low entropy. I think a simple explanation could go like this: Since every part of chaos is indistinguishable from another, there are no parts to arrange in different patterns. Essentially, there is one universal hodgepodge. Entropy is the tendency to increase the number of ways the parts of a system can be arranged. (This approach tries to relate entropy to Kolmogorov complexity. Which may be a gross amateur error, but I think is the standard view.)
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Anyone interested in nostalgia and anticipation should take special note of the fifth thing in the list. The points about memory and imagination only reinforce de Selby’s theory of time and travel included in the Flann O’Brien novel “The Third Policeman”: “Instead of going to the railway station and inquiring about trains, he shut himself up in a room in his lodgings with a supply of picture postcards of the areas which would be traversed on such a journey, together with an elaborate arrangement of clocks and barometric instruments and a device for regulating the gaslight in conformity with the changing light of the outside day. … Like most of de Selby’s theories, the ultimate outcome is inconclusive.” But I feel now it is less so.
i think time is related to our weird ability to have memory. imagine if you have no memory, then there would be no way to say if things move or not, i.e. we could not compare. world would seem totally different then
McTaggart is turning in his grave with affirmation 1.
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Sean,
Can you express these ideas in terms of cubes?
;^)
Reading #2 (past & future) I immediately thought of the 2-slot experiment, which indicates that every quantum particle (past, present and future) “knows” each and every other quantum particle.
Is this relevant?
The Stillness Before Time
http://www.thestillnessbeforetime.com/
There is really only one Way.
It is without division or boundary.
It is without name or theology.
Awareness is its scripture,
Here now its venue,
You its witness,
Your life the journey.
* * * *
There has ever been now,
Will ever be now,
Is ever now.
Never has there been any time
Other than now.
* * * *
The manifest dance is timeless, momentary, eternal.
A reverie without beginning, without end,
Without cause, purpose or meaning,
Neither definable or explicable,
For it is beyond all rational appearances.
It can never be known, comprehended or understood
Except in the most roundabout, circumspect, fluid, abstract ways.
And in that which is intuited there is no gain or reward,
One simply wanders spontaneously free,
Whatever the course.
* * * *
All mythos, all sense of time and history,
Is the make-believe of adults.
These are random “points”, and I am not convinced they are all true. “1. Time exists” (?) S. Hawking said it better that at a very low level, e.g. quantum scale, time is a foam, with particles appearing and disappearing. If “7. Disorder increases as time passes” were true, why are humans so advanced in comparison to our local universe?
Some of the statements which people have made appear wrong or unwise to me. Time is a product–or more accurately perception–of mind. Time does not “exist” independent of mind. Unless there is mind to perceive time, there is no time.
What is relevant? I don’t believe time is one of those relevant discussions.
Does anyone remember that line from the song — “time keeps on slippin’. slippin’, slippin’ into the future.” that’s what came to my mind…. maybe not relevant but funny what you think of when you read an article like this….
Time doesn’t exist! Time is space in movement. Think about this.
Before you buy into the “time” story try reading Julian Barbour’s “The End of Time”. You can also go on his website http://www.platonia.com/ideas.html.
The book really puts a twist in the conventional understanding of reality and time.
Time is Magnificent. It’s there to tell us we’re late for work, a party, the first race of the day. ETC. Time is running down slowly till when it stops the universe will die. “Can Entropy be reversed.” that question is “The Last Question.: it’s my most favorite story by Dr. Issac Asimov. if you can find it read it. The last page will blow your feeble minds away.
I love how people keep asking the author, Sean, questions, yet he dasn’t deign to reply. Maybe, in time, he will reply. Pun most thoroughly intended.