What Is Time? One Researcher Shares His Exploration

Physicist Sean Carroll has built up a bit of a name for himself by tackling one of the age old questions that no one has been able to fully explain: What is time? Earlier this month he gave an interview with Wired where he tried to explain his theories in layman's terms. "I’m trying to understand how time works. And that’s a huge question that has lots of different aspects to it. A lot of them go back to Einstein and spacetime and how we measure time using clocks. But the particular aspect of time that I’m interested in is the arrow of time: the fact that the past is different from the future. We remember the past but we don’t remember the future. There are irreversible processes. There are things that happen, like you turn an egg into an omelet, but you can’t turn an omelet into an egg."

Comments

Time

[Anonymous Coward]

About half past ten, give or take a couple of minutes.

Re:Time

[Anonymous Coward]

LOL, posted at 2:33.

Edward Teach meet Time Zones. Times Zones, Edward.

Re:Time

[Anonymous Coward]

Maybe you need to study the concept more.

Re:Time

[twidarkling]

Well get him out, then. We just cleaned it.

What Is Time?

[negRo_slim]

Entropy

Re:What Is Time?

[Jorl17]

Money

Re:What Is Time?

[N7DR]

As Einstein famously said: "Time is what a clock reads". I always thought that was rather a clever evasion: true but not particularly helpful.

For many years that quotation was on a poster that greeted one in the lobby of what was then the National Bureau of Standards in Boulder, CO.

Re:What Is Time?

[Toonol]

Time is an artificial construct of the Human mind that allows us to mark our pitiful existence in an uncaring universe.

Not far from the truth, but I'd say it is an "amazing creation of evolution" that allows us to "experience the unfurling glory of our life in a rich universe."

Re:What Is Time?

[AthanasiusKircher]

It's not an evasion. It's a very solid, concrete answer to the question.

In the normal sense of the question in physics, I completely agree with you.

People get caught up on the question of "what is time?", but in reality, the question is no more deep or metaphysical than the question "what is length?". Both are equally distracting quagmires of philosophy and both are neatly (and appropriately) dealt with in physics by simply defining them to be something that is measured by a device, as a multiple of some defined quantity. [...] and so physics and science can proceed, while philosophy remains in its quagmire.

I also completely agree with your assessment of the role of "time" in standard formulations of physics.

However, I do think that your emphasis on scientific definition and measurement misses something important about the question "What is time?" The question is not simply, "How is time measured?" That is an interesting question, and perhaps it's the most concise and important one to answer when considering the use of time in physics, but it only scratches the surface of "What is time?" And for all your dismissiveness toward "philosophical quagmires," the fact is that the vast majority of the time, the vast majority of the people in the world don't deal in the wondrous abstraction of time as defined as a measurement in physics. To most people, "time" is something we experience.

"Time" existed before people measured it, and therefore it is quite productive to ask the question of what it means without a measuring device. Our understanding of the human experience of "time" may in fact be useful in understanding, for example, the workings of the mind.

To give a simple comparison, think about temperature. No doubt, you'd define it just as you defined length or time. But that's not what most people in most situations mean by "temperature." They instead have in mind something about how cold it feels outside or how hot a pan is. More meteorologically-inclined people might realize that "wind chill" and "heat index" give a better sense of how it feels than the measurement of temperature alone, but many people still think of temperature primarily when they think about how they experience the world.

And yet we don't experience temperature directly. At best, we experience rate of heat transfer. How "cold" it feels depends on temperature, humidity, wind speed, whether the sun is shining on us with radiative heat, etc. A coin on my desk will feel "colder" than a piece of paper, because metal has a greater transfer rate for heat than paper. I could burn myself on a metal pan at a particular temperature and yet quickly move around a hot piece of wood at the same temperature without injury. None of this downplays the usefulness of temperature as an abstract measurement in science, but it isn't really a practical way of dealing with our direct experience of the world. Abstract temperature is simply not relevant to most people under most circumstances.

In a similar way, I think by asking the question "What is time?", we can begin to think about the phenomenological aspects of the experience of time and what that might tell us about the way we interact with the world. You might think of such questions as "soft" or "philosophical meanderings," but if you simply dismiss them in favor of an abstract physical concept (which has little direct impact on our experience of the world), you might be missing out on some significant aspects of what time is -- at least to humans.

And, ultimately, isn't that what really matters for most people? Evolution created an amazing apparatus that experiences something called "time," and your physical measurement definition doesn't model that apparatus well at all. So maybe, rather than just begrudging such "philosophical meanderings," perhaps -- if you care at all about humanity and how humans work within their environment -- a better understanding of such a question could really be useful for advancing science. After all, science is a human endeavor, based on our human experience of the world. Might we not become more efficient or better in working within science if we understand ourselves better?

Time is the goo...

[gestalt_n_pepper]

that connects state one to state two.

Re:Time is the goo...

[eepok]

ding ding ding!

Time is just another dimension, but one we can only experience shallowly.

Two points make describe line.
Two lines make describe plane.
Two planes make describe space.
Two states of space describe time.

Time as we experience time in the same way a single-cell organism on a slide experiences 3-dimensional space.

Re:Time is the goo...

[Digital Vomit]

A point doesn't have to move across a line.
A line doesn't have to move across a plane.
A plane doesn't have to move across space.
So why, then, is space constantly moving across time, always in the same direction? Is "God" pushing our "space" through "time"?

Why do we "experience" anything at all? Why are we not just static sequences of space?

I think time is a little something other than "just another dimension". But who can really say?

Easy

[Yvan256]

FORD: No, No listen. Just imagine that you’ve got this ebony bath, right? And it’s conical.

ARTHUR: Conical? What kind of bath is -

FORD: No, no, shh, shhh, it’s, it’s, it’s conical okay? So what you do, you fill it with fine white sand right? Or sugar, or anything like that. And when it’s full, you pull the plug out and it all just twirls down out of the plug hole but the thing is

ARTHUR: Why?

FORD: No, the clever thing is that you film it happening. You get a movie camera from somewhere and actually film it. But then you thread the film in the projector backwards.

ARTHUR: Backwards?

FORD: Yeah, neat you see. So what happens is you sit and you watch it and then everything appears to swirl upwards, out of the plug hole and fill the bath amazing.

ARTHUR: And that’s how the universe began?

FORD: No. But it’s a marvellous way to relax.

TRILLIAN: Funny man.

FORD: Well it broke the ice didn’t it?

Re:Easy

[Anonymous Coward]

FORD: Time is an illusion. Lunchtime, doubly so. ARTHUR: Deep. Very deep. They have a page at the Readers' Digest for people like you.

Re:Easy

[Trepidity]

It's an old comment from Henri Bergson, though his version didn't include "nature", but was instead something more like, "time is a resistance against everything happening at once".

Re:Timeline

[spiffmastercow]

You're either a Philosophy student, or you just watched Donnie Darko for the first time, right?

[...]you can't turn an omelet into an egg.

[snowraver1]

Maybe not directly, but you can feed that omelet to a chicken, and then take the resulting egg.

Re:[...]you can't turn an omelet into an egg.

[JustOK]

automate it with a shell script

Re:[...]you can't turn an omelet into an egg.

[timeOday]

Maybe not directly, but you can feed that omelet to a chicken, and then take the resulting egg.

But the chicken produces fewer eggs than you feed it. Not just "not more", but "fewer."

Sean Carroll's "Real Rules for Time Travelers"

[Bobtree]

http://discovermagazine.com/2010/mar/02-the-real-rules-for-time-travelers/article_print

Re:Sean Carroll's "Real Rules for Time Travelers"

[painandgreed]

I read it on a plane trip earlier this month and was fairly disappointed with it. In fact, the entire issue made me decide to never bother with Discover magazine again. I have a physics degree and used to not getting any actual math with my physics in mainstream culture. However, everything it in was pretty much uninformative if you've ever even heard of the subject before. Seriously, wikipedia does a better job and is probably more up to date than Discover magazine.

This aritcle in question, there was no actual discussion of physics. No talk of the lack of time direction in Feynman diagrams. None of the solutions for time travel that can be come up with using Einstein's equations. Nothing really, just a bit like "you can't go back in time and kill your father because then you wouldn't exist to go back and kill your father" logic. Never mind that this isn't actually supported by physics and Tippler showed that acasual time like paths can occur, it completely ignores the many-world interpretation and it's possible relevance to time travel. never mind that you don't have to actually go kill your dad but just showing up is going to cause the same effect simply from your changes in weather do to chaos theory/butterfly effect. I was hoping for a simple article talking about things I already know with the possiblity of a mention of some new development that I could research later, but ended up with no actual physics (and not even a good philosophical discussion) of the subject.

Real rules for time travellers? Einstein's theories currently say that a time machine is possible but you can't go back in time to a point before the time machine was .turned on'. Entropy is in there so if you are going back in time it's going to take energy to reverse it. What happens when you go back in time to kill your father is an interesting question, but not one that the article actually addresses in any way that actual addresses physics of the subject. My personal hypothesis is that either you can't change history, only fulfill it because it has already happened, or you end up in a different time line. Yay! now we have a testable hypothesis and science. We just need a way to test it.

But...

[Black Parrot]

We remember the past but we don't remember the future. There are irreversible processes. There are things that happen, like you turn an egg into an omelet, but you can't turn an omelet into an egg.

But if time is non-monotonic, wouldn't we un-remember, un-break things, during the backturns?

How would anyone know if time isn't always forward?

Re:But...

[timeOday]

In Slaughterhouse Five, Vonnegut advances the theory that the perception of time is simply a limitation built into us - that everything from all times simply exists, but we can only sample it monotonically (like a flat-bed scanner head moving along).

If the universe were deterministic, then time is essentially meaningless even if it exists, since the start state and dynamics are all you need to know. And if the dynamics are information-preserving, any state (not just the start state) suffices. Apparently there are even deterministic interpretations of quantum mechanics, although I really don't know what that means.

Re:But...

[SoftwareArtist]

He actually discusses this fairly well in the interview. Here is where it's put most succinctly:

Wired.com: In this multiverse theory, you have a static universe in the middle. From that, smaller universes pop off and travel in different directions, or arrows of time. So does that mean that the universe at the center has no time?

Carroll: So that’s a distinction that is worth drawing. There’s different moments in the history of the universe and time tells you which moment you’re talking about. And then there’s the arrow of time, which give us the feeling of progress, the feeling of flowing or moving through time. So that static universe in the middle has time as a coordinate but there’s no arrow of time. There’s no future versus past, everything is equal to each other.

The essential point is that the 2nd law of thermodynamics is really a backward way of looking at the question. It isn't that entropy increases with time. It's that we define "forward in time" to mean, "the direction of increasing entropy". Our local region of the multiverse happens to have an entropy gradient in one direction, so that's the direction we perceive time to increase in. But other regions of the multiverse might have different directions of increasing entropy, and hence different "arrows of time". And still other regions of the multiverse are completely flat with regard to entropy. In those regions, it isn't meaningful to define any arrow of time at all.

We feel like we're "moving through time" because we can only remember the past, not the future. If we could remember past and future equally well, we wouldn't have that sensation. Every moment would feel static, equally connected to past and feature. We wouldn't have a sense of moving in one particular direction.

So the question is, why can't we remember the future? And the answer is, because increasing entropy is needed to form memories. Just after the big bang, the universe was in a state of very low entropy. All the energy in the universe was concentrated in a tiny region of space. Since then, that energy has steadily spread out and become more diffuse (that is, entropy has increased), but the process still has a long way to go. We still have enormous amounts of energy concentrated into small areas known as stars. But energy is continuously flowing out from our sun and getting transferred from one form to another. Nuclear reactions produce high energy photons, which are used by plants to produce sugars, which our bodies use to produce ATP, which we use to manufacture proteins and form synapses and do all the other things needed to form a memory. At each stage, energy is converted from one form to another, and the entropy of the universe increases a little bit. Forming a memory is one of those transitions. On one side of the transition, the energy is still stored in ATP and the memory doesn't exist yet. On the other side of the transition, ATP has been used to form a memory, so the memory exists and the ATP has been split. Forming a memory requires using energy (and hence producing entropy), so the memory can only exist on the high entropy side of the transition.

What Is Time?

[Monkey_Genius]

Time is an artificial construct of the Human mind that allows us to mark our pitiful existence in an uncaring universe.

Re:Timeline

[BitZtream]

Except ... medical studies show that 'Deja Vu' is really just brain glitches that are nothing more than thinking after the fact that you knew it was going to work that way. You're having a minor seizure, not predicting the future.

Re:Timeline

[TheCouchPotatoFamine]

nope, your referring to prediction - not prescience - something that neural networks, such as the ones in your head - are very good at.

if you'd like to explore a real philosophical issue, consider whether or not you, as a neural network world-predictor, could ever experience anything truly random? Pretty much, no, your mind cannot refuse to map patterns, even if your senses pick up something that has no pattern at all, since your brain is just so wired to the gills to put a pattern on EVERYTHING.

Time Travel

[Citizen of Earth]

Hello. I am a time traveler. Be not afraid. I come from the past and I travel into the future at a rate of one second per second.

St Augustine already figured it out:

[joebok]

From St. Augustine's Confessions, Book XI:

CHAP. XIV. -- NEITHER TIME PAST NOR FUTURE, BUT THE PRESENT ONLY, REALLY IS.

17. At no time, therefore, hadst Thou not made anything, because Thou hadst made time itself. And no times are co-eternal with Thee, because Thou remainest for ever; but should these continue, they would not be times. For what is time? Who can easily and briefly explain it? Who even in thought can comprehend it, even to the pronouncing of a word concerning it? But what in speaking do we refer to more familiarly and knowingly than time? And certainly we understand when we speak of it; we understand also when we hear it spoken of by another. What, then, is time? If no one ask of me, I know; if I wish to explain to him who asks, I know not. Yet I say with confidence, that I know that if nothing passed away, there would not be past time; and if nothing were coming, there would not be future time; and if nothing were, there would not be present time. Those two times, therefore, past and future, how are they, when even the past now is not; and the future is not as yet? But should the present be always present, and should it not pass into time past, time truly it could not be, but eternity. If, then, time present -- if it be time -- only comes into existence because it passes into time past, how do we say that even this is, whose cause of being is that it shall not be -- namely, so that we cannot truly say that time is, unless because it tends not to be?

Re:St Augustine already figured it out:

[maxume]

If you find yourself slightly convinced by an existential argument, eat breakfast.

Re:Time?

[Monkeedude1212]

What else can you measure time against, or what else can you measure change against? Because you can measure a change in distance, a change in volume, a change in temperature, the list goes on.

As for measuring time - you can have instances where nothing changes BUT the time - so thus begs the question, what is time if nothing changes?

Imagine a single Molecule, Well if you can imagine it moving you know it has speed and then you just take the change in distance to find the amount of time that had passed.

Well, imagine if it didn't have a speed - it wasn't moving. How would you calculate the change in time?

Re:Timeline

[bill_mcgonigle]

It seems like it is just on the tip of the tounge, but just out of reach. Has anyone been able to announce a reasonaby random event before it happened while experiencing a deja vu? Something like "Bob will walk in though that door now" or "Bob is going to spill his drink".

No, they can't because it's an illusion. Your brain gets into a tight sensing/remembering loop for a short time, so it seems like you're recalling stuff that just happened, but it's the other way around. You're not used to that, so it's confusing and easily misinterpreted.

There's no more reason to be embarrassed by this than being fooled by optical illusions (happening in your visual cortex, not your eye in many instances) - our brains aren't perfect arbiters of the physical world, they interpolate quite a bit, so occasionally they get tripped up. This imperfection lets us laugh at Penn & Teller - it's all good.

Besides, we already know that memories are chemically encoded, so the only way to have memories of the future is magically putting chemical patterns in your brain. And between 'magic' and 'brain fart' - well, apply Occam's Razor.

Or antimatter

[mangu]

Thermodynamics is one of two sets of phenomena that are irreversible. The other is rather obscure, but is related to the fact that "ordinary" matter seems to be so much more abundant in our universe than anti-matter.

All other phenomena in our universe are reversible in time, which raises an interesting question: are we unable to see the future because our brains work on thermodynamic operations?

Not only biologic brains, but digital computers also depend on non-reversible operations. A two-input AND gate has a "0" output in three different input conditions: "00", "01", and "10". Now imagine a computer that uses a reversible logic system that is reversible, would that computer have a time-symmetric operation?

Re:Timeline

[snowraver1]

Has anyone else noticed a decrease in the frequency of deja vu as they get older? I think that the peak was when I was about 13-14. Just curious....

Time does not exist

[afabbro]

Prove me wrong.

The future obviously does not exist. The past? Doesn't exist either. Hence, only this present moment exists.

You can't even prove that the past existed. The only thing we have is present-moment memories, etc. I remember typing "Prove me wrong" but my memory is hardly reliable. If thirty seconds ago you spilled milk on your pants, all you have now is wet, soggy pants, not any "chain of events". Even if you filmed it, all you have is the present-moment series of images, not some actual piece of the past.

Only this present moment exists. All else is wild speculation and fantasy. Time does not exist.

Re:Timeline

[zach_the_lizard]

could ever experience anything truly random?

Halt! As Captain of the Internets, I cannot stand to hear you lies anymore! No randomness? Blasphemy! Lunacy! Have you ever been to Wikipedia? The dark corners of the Internet, such as 4chan? Fark? IRC? Had you been there, you would have seen the reality of randomness! Now repent your crimes before I am forced to put you into the Total Perspective Vortex with a half naked anime character, a motivational poster, and a Wiki article on the nature of the Etruscan language, which you got to by starting on the page on pastrie!

Re:Timeline

[Gerzel]

It is explained by the fairly well substantiated fact that humans are poor judges of exact time and memory is often faulty. You remember knowing before hand, but did you actually know it or do you just think you knew? It is easy to misjudge a few seconds.

If you don't believe me that humans are poor at keeping time then I ask you, why do we have so many clocks around? Far more clocks than say thermometers or even distance. We don't need an alarm thermometer to tell us it is getting hot outside, but we often do need alarm clocks to tell us it is time for an appointment or if enough time has elapsed for an egg to boil or how long the microwave has run.

Re:Timeline

[maxume]

If you write it down beforehand and document it when it happens, James Randi will give you 1 million dollars.

Re:Timeline

[mforbes]

Sure, counting the hits and ignoring the misses can work for anyone.

That's how road-side crystal-ball gazers make their money.

Re:Timeline

[Cassius Corodes]

Anyone who says otherwise is either narrow-minded or hasn't ventured out very far into the real world.

Or maybe they have been in the real world long enough to know the truth from wishful thinking. For a while I too thought I had such powers but then when I started writing down actual details it turns out they weren't very accurate at all. I am willing to bet a good percentage of people has these fantasies but at some point you just have to face facts.

However imagine for a moment you are right:

If you think about it the ability to see into the future would be such a massive evolutionary advantage that there is absolutely no way it would remain hidden in dreams or only vaguely available. The first species to do this would dominate all others and would evolve and eventually take over all the other niches until all living species can see the future.

Secondly if some could see the future (even partially) then we would on average see more people who made such claims at the top of industries / careers as they have an advantage. Yet the people who make such claims are usually at the bottom. I'm not including people who work in the psychic industry as these are obviously frauds.

Thirdly - if some people could see the future they would be famous, and we would have positions designated within all power structures for such advisors. As things stand now we have a guy offering a million dollars for anyone who can prove it and still nothing.

Finally - in the 60's scientists were very interested in these questions and had look at every idiot of the street who made claims of supernatural powers. Nothing. Remember that scientists routinely deal with discovering things that function barely above 50/50.

Re:Timeline

[dissy]

How do you explain when it doesn't happen after the fact? For example there are times when I have a second or two advance warning. I know exactly what someone is going to say, and then they say it. I never know more than a few words, but I know exactly what those few words will be.

That is the interesting thing when the brain and mind come into play.

How would one be able to actually tell the difference between:

A) You have a 'prediction' first, then that happens in reality next, and finally you think 'i predicted that!'

and

B) First you hear what the other person said. Next your brain/mind do some form of trickery so you THINK that you predicted what they said prior.

Note the time line of events between A and B are almost perfectly reversed, yet both will have the same identical effect on the observer in the end.

Taking things to a totally nonsensical example, if I read a book to you and you enjoyed it first, then second I modified your memories so you now have the memory of reading that book long ago.
How could you tell?

Until we learn more about the physical structure of the brain, and possibly (probably) the functions of the mind, we really can't tell.

Now, I'm not at all saying this is actually what happened to you with Deja Vu!
Just posing the question of how one can know either way when the device (brain) we are using to measure, is the very device being modified constantly in real time during the measurement.

Re:Timeline

[Anonymous Coward]

Clearly you haven't done enough. Nothing is random man, everything is connected!

Re:My head hurts....

[sjames]

The semantics is more an artifact of trying to express something that we have no proper words for because it never happens and we can't exactly imagine what it would be like if it did happen.

At the subatomic level, everything is reversible with equal probability. If a particle can decay into two others, the two others can join to form the particle just as easily. However, at our scale, making all the bits of egg on the floor come back together and the egg then fly up into your hand only happens if you run a movie backwards. Beyond being nearly infinitely funny to first graders, physicists are lead to wonder why that is. What is different between the scales such that equally likely at the small scale becomes "never happens" at ours.

Pidgeon dance

[TapeCutter]

"it's just something that comes to your mind and because of the noise ratio you only notice it later"

The human brain is very good at creating non-existant patterns in random noise. There is a classic phycological experiment (IIRC by Skinner), showing that pigeons do exactly same thing (ie: engage in superstisious behaviour).

In the experiment a feeder was set up so that it would drop a pellet of food randomly with a mean time between pellet drops of a few minutes. The feeder was placed in the pigeon cage for an hour or two at normal feeding times.

The hungry pigeon would just happen to make some random movement just before the pellet happened to drop. It then mentally connected that movement with food and would repeat it a few times in the hope another pellet would appear.

Occasionally it would make a different movement just before the pellet appaeared. It would then mentally connect this new movement with food and join the two movements together in the hope of getting more food. After a while the pigeon(s) had all created their own unique an complex dance that they would start endlessly performing whenever the dispenser was introduced to their cage.

The really interesting part is that the time it took to perform a fully developed pigeon dance was always equal to the mean time between random pellet drops, meaning the pidgeon was virtually garenteed to recieve the reward after one or two performances of it's dance. Connecting random dreams to future events after the fact is just one of the many human forms of the pigeon dance.