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

What Is Time?

[negRo_slim]

Entropy

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?

[...]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.

[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."

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

[Anonymous Coward]

Unfortunately, that doesn't result in another complete egg. (If it did, it would be an entropy violation.) Instead, you need to feed several omelets to the chicken, and you get back one egg plus a load of crap.

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: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.

Time?

[daffey]

A physicist I'm not, nor mathematician, but 'TIME is CHANGE' in my book. No change- no time. What else can you measure it against?

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: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:Or antimatter

[Hurricane78]

I think that that question is just as pointless, as asking what was before time, or what is outside of everything. Because “operation” is only defined in terms of a progressing time.

But you could ask how the world would look if something like that existed, and then compare it to reality with experiments, to find out if it is at all possible. (Just like the final argument of (I think) Bohr against Einstein in the great debate about quantum physics.)

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.

time has no arrow, spacetime does

[sweetser]

Hello:

Time will never have an arrow. Spacetime will, from the space part. If you take Minkowski's advice, that one should only think about spacetime, not time or space, then Carroll's question is poorly formed. It is good English, bad mathematical physics. Since Minkowski's observation was based on work with special relativity, people presume is observation applies only for relativistic systems. Sorry, Nature is more consistent than that: one needs to think about spacetime always, even if it contributes squat. Newton's 2nd law can be written F = m (d/dt. 0, 0, 0)^2 (0, x, y, z). What makes it classical are all the zeroes that appear in the spacetime operators.The handedness of times arrow comes from the space part whose contributions are stupidly small, but add up enough of them, and they are irreversible.

Re:Timeline

[maxume]

Stuff that you remember, or stuff that you wrote down?

You can't trust your brain.

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.

Re:Timeline

[RobDude]

If your brain glitches allow you to see into the future you have no excuse for not being ungodly rich. The insight you'd gain - just in seeing what line of clothing is particularly popular or what the logo is on the front of cars would give you insight to make you the best investor the world has ever seen.

On top of that, demonstrating super-natural abilities - like seeing into the future - would net you millions in rewards from people who claim it can't be done. That James Randi guy will give you a million; probably others too.

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

[maxume]

Given sufficient detail, I would be astonished by a single hit.

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.

Quaternion spacetime reversal is local, not global

[sweetser]

For fun, let me be more technical.

If you want to reverse the time of a spacetime event, you use this member of the Lorentz group, diag{-1, 1, 1, 1}. Have that act on a 4-vector (t, x, y, z) and you get (-t, x, y, z). Now how are you going to get time back to were it started? Use exactly the same element. The Lorentz group is a global symmetry. It is to all levels of accuracy the same darn thing. Makes much math easier, but it is why physicists say the laws are identical if time goes backwards or forwards.

The important laws in physics are local. Both the standard model and general relativity depend on the values of t, x, y, z. Let's construct a local time reversal operator, call it B, such that B (t, x, y, z) = (-t, x, y, z). This can be done by presuming all three of these are quaternions, a 4D rank 1 tensor upgraded to also be able multiply and divide like real and complex numbers (full disclosure: I own quaternions.com). R can be calculated, it is (x^2 + y^2 + z^2 - t^2, 2 t x, 2 t y, 2 t z)/(t^2 + x^2 + y^2 + z^2). That will work every time, but if you want to reverse something, then reverse it again, the second B will not be identical to the first B. The first term is identical, but the 3-vector part flips signs, not magnitudes. When one makes time reversal local using quaternion operators, the arrow of time is not a problem because there is a mathematical difference between reversing the reverse of time.

Re:Timeline

[timmarhy]

so in other words you can't time travel and live forever, it would just seem like forever to everyone not time traveling?

Re:Islamic view of "time"

[MinistryOfTruthiness]

If you're omniscient, it doesn't mean you've forced anyone's hand. It just means that you know which choices will be made, and the ultimate result of those choices.There's a subtle but important difference.

Imagine you see your kid on the other end of the room reaching for a stove burner. You see what he's doing, you know what will happen, you know he's gonna spend the next hour crying. That doesn't mean you made him do it. That was his choice. You just knew in advance about the burns and the crying.

Now your prediction isn't really omniscience because you're basing it on what you expect to happen in the next second or two. Something might catch his eye at the last moment and he decides not to do it, but that's really the only difference between omniscience and prediction. Well, that, and scale. Either way, it doesn't take away the free will of the actors.

The multiverse, again

[jopet]

Theories that involve the multiverse are, in my opinion, nearly as unscientific and embarrassing as religion or theories that involve "god": you can "explain" nearly everything and you can prove nothing. Give me a break with multiverses.

How is the question why there is a multiverse that spawns off universes randomly so much nicer that the question why there is a universe? It is equally unanswerable but introduces complexity: let occam's razor cut away the multiverse part until there is anything that is falsifyable about the story.