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."

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.

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?

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: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

[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

[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.