Dec 13 2012

What is Time?

One of the joys of having children is the opportunity to vicariously view the world through child-like eyes. Children are generally curious, and are free from the bias of “knowledge.” I am not trying to make ignorance into a virtue – knowledge helps us to think about things on a deeper level and to see the connections that make up the tapestry of reality. But knowledge can also be a trap that constrains how we think about things.

Children may highlight this fact by innocently asking questions that are free of assumptions we didn’t know we had. Every parent has likely faced these questions. In my opinion these moments are tremendous opportunities to engage a young mind with everything that is awesome about science and intellectualism itself.

Alan Alda seems to get this. He has been parlaying his TV and movie fame to promote science communication. He is a founding member of the Center for Communicating Science at Stony Brook University. He wants scientists to explain basic concepts to the public – to children, in fact – in a way that they can understand. He gets questions from 11 year olds and then challenges scientists to explain the answer in a way that is engaging and accessible to 11 year olds, and then has 11 year old judge the answers (although referred to as “11 year olds” it seems the job of submitting questions and judging answers is open to 4-6th graders).

Last year Alda posed the question – what is flame? This is a perfect child-like question that adult may take for granted or just assume there is no specific answer to (a complacency to mystery perhaps not entirely shared by children). Many scientists submitted answers, the winner was Ben Ames whose answer was in the form of a video. Nicely done.

Alda decide to make the “Flame Challenge” a recurring thing, and this year he solicited questions from children and came up with a new challenge – what is time?

This is a perfect question, the kind of thing that most adults may assume does not have a specific answer, or if it does it’s too “sciencey” to be understandable by non-experts, or perhaps they think they know what what the answer is. Try to put the answer into words, however, and it becomes immediately obvious that this is no small challenge.

There are lots of pithy if snarky answers, such as, “time is what keeps everything from happening at once,” (which I believe was the focus of a Doctor Who episode). Such answers, however, tend to be more philosophical than scientific. A thorough scientific answer can be extremely technical and mathematical, involving relativity and thermodynamics, for example. This has the makings of a good challenge and I will be interested in seeing what real experts come up with.

My off-the cuff crack at an answer is this:

Time is a property of the universe – a measurable quantity that separates two events. It is like a stream that moves in only one direction. You are always at one point on that stream, being carried forward at a steady rate. Part of the stream is forever behind you, while the rest is in front of you. You can move more quickly or more slowly along that stream, but never backwards. Further, you cannot see in front or behind you. You can only see the boat that is your “now.” You can remember where you have been, but you cannot see or remember the future in front of you.

That is, perhaps, a simplistic analogy. I will have to think about this some more to see if I can come up with anything better. Feel free to take a crack at it in the comments as well, or even submit an official answer to the challenge.

33 responses so far

33 thoughts on “What is Time?”

  1. tmac57 says:

    Can two spatially separated events be said to have occurred at the ‘same time’ ?

  2. korin43 says:

    It’s funny that you just posted this. I’m in the middle of reading a book called “The End of Time”, which argues that time doesn’t exist..

  3. Eric Thomson says:

    tmac that’s the problem: try to answer in one paragraph in a way that includes special relativity, in which the view of time expressed above would need some heavy revision. Revision that is *very* hard to do in a single paragraph. E.g., this view that there is this universal “now” is pre-relativistic.

  4. AdrianG says:

    Here is a very good talk about time travel by Sean Caroll. He gives an interesting way of thinking about time : Time Cones.

  5. tyro says:

    The long and the short of it is this: time is what clocks measure.

    It’s not a very satisfying answer, but it’s actually got interesting implications. It leads to the relativistic idea of time moving at different rates (because light-based clocks appear to tick slower) in a fairly straight-forward way.

  6. Jerry in Colorado says:

    Years ago, I overheard a 8 yr old explain “time” to his 6 yr old sister: “Time happens when you get older”.

  7. BobbyG says:

    It’s always “now” anyway.

  8. ccbowers says:

    “In my opinion these moments are tremendous opportunities to engage a young mind with everything that is awesome about science and intellectualism itself.”

    Nearly as important is that these are moments that can allow us to deepen our understand by forcing us to clarify our own thoughts enough to communicate them to a young mind. To be able to distill a concept down to a basic explanation, while maintaining the important components and structure of the concept is a great mental exercise.

  9. Blair T says:

    I always thought time was simply movement. That is, there are 3 spacial dimensions (3D) and the 4th dimension is movement within the 3 spacial dimensions. No movement, no time. No time, no movement.

  10. BillyJoe7 says:

    tmac57: “Can two spatially separated events be said to have occurred at the ‘same time’ ?”

    Yes, if the same gravitational forces act on them and they are not in relative motion.
    Because of the finite speed of light, they will mostly be observed to have occurred at different times, but all observers can make the calculation that tells them that the two events occured simulataneously.

  11. BillyJoe7 says:

    korin43: “I’m in the middle of reading a book called “The End of Time”, which argues that time doesn’t exist.”

    Does the author explain this on the basis that time and space are illusions of the overlying reality that we call spacetime? Everything, including light, travels through spacetime at the same rate. The illusions of space and time are like the shadows on the backdrop of a theatre, whilst spacetime is the actual passion play.
    Am I close?

  12. BillyJoe7 says:

    BobbyG: “It’s always “now” anyway.”

    Well, that’s the solipsists view anyway.

  13. BillyJoe7 says:

    BlairT: “No movement, no time. No time, no movement.”

    In most of universe there is an absence of movement.
    Does time not exist there?

  14. quen_tin says:

    I think time is a terrible subject for this challenge, while flame was a very good one.

    Scientists know very well what flames are and our conception of flames will probably not be updated by any new scientific theory (even if our knowledge of fundamental particles happens to be completely revised, which change the way we think of atoms, the way atoms play in combustion will probably not be revised). Flames are not a matter of scientific or philosophical debate today (they don’t involve existential problems or the like). This makes of it the perfect subject to be explained to children: you can be clear and objective.

    For time, this is all the contrary: time plays a key role in the problem of unification of relativity and quantum physics, so future theories will probably change drastically how we think about time. Its very status (whether it is an “illusion”, whatever that means) is a matter of philosophical debates (presentism, eternalism, …) which encompass science rather than fall within the scope of science. Time is an existential question. I am pretty sure this “flame challenge” on time will turn into either pieces of hardcore scientism, or bad “woo” philosophy — metaphysical positions in either cases.

    An honest and cautious answer to the question could be: “You know just as much as us on time (=nothing). However we are able to measure and compare cyclic phenomenons, and on this I can tell you a bit more”. I truly hope that the winner will choose this approach.

  15. BillyJoe7 says:


    “Its very status (whether it is an “illusion”, whatever that means)”

    It simply means that time is not what it seems to be.
    (Like the colour of those two squares in the checkerboard illusion)
    Time does not seem to be something that changes according to relative motion or the effects of gravity.
    Time does not seem to be something that can interchange with space.
    Yet science has shown that this is the case.
    Therefore, the one thing that can be said about time is that it is an illusion.

    There is just a widespread misunderstanding about what an illusion is.
    Illusions are not delusions. They exist. They just aren’t what they seem to be.

  16. maineiac48 says:

    In most of universe there is an absence of movement.

    Not so. Everything is in motion. The universe itself is expanding. Particles and subparticles are in motion. Motion never stops. Curiously, if motion were to cease, such cessation would be impossible to prove without the use of “time”.

  17. locutusbrg says:

    Time is the ability to understand that there is a past, present and future. For creatures that have only “now” time is meaningless.

  18. quen_tin says:


    I was not intending to have a philosophical debate, I was just pointing out that it *is* a matter of debate.

    When time is said to be an illusion, usually the flow of time is refered (not its relativity). This refers to a philosophical doctrine called eternalism, which some think is favoured by the relativity of time, but other don’t.
    By the way, time is not “interchangeable” with space… Both hold in the same representation but time-like intervals are distinct from space-like intervals.

  19. quen_tin says:


    Actually, saying that the flow of time is an illusion is a stronger statement than mere eternalism, and I am not sure it makes sense at all.

  20. BillyJoe7 says:

    “By the way, time is not “interchangeable” with space… Both hold in the same representation but time-like intervals are distinct from space-like intervals.”

    Yes, “interchangeable” was the wrong word.
    I was trying to be succinct.

  21. ccbowers says:

    “Children may highlight this fact by innocently asking questions that are free of assumptions we didn’t know we had. Every parent has likely faced these questions.”

    I wonder if this is how children lose some of their curiosity as they age (or is this perception a myth- I may be more curious than when I was younger). If adults dismiss their questions, because they don’t know or don’t feel like answering a simple question with a complicated answer- does this discourage them from being curious about the world around them? My children may have the opposite problem of not asking me a question, because I will try too hard to answer them, but I’d rather err on the side of TMI

  22. tmac57 says:

    ccbowers-You pose an interesting question about why or whether children lose their curiosity as they grow older.One possibility,is that as you age,you do learn things about the world that ‘answer’ (rightly or wrongly) the questions that you have,so those are done.Some people see further questions within the answers that they get,but others are satisfied with (and really want) a simple answer (even if it is incomplete or on shaky ground).
    One of the characteristics of skeptics,seems to be that when they discover “answers” that they have previously believed to be correct are in fact wrong,they are energized to go on a quest to not be fooled again,and try to understand how we can come to truly understand our world by the most rational methods.
    Credulous people will instead grasp on to the first explanation that ‘feels right’ in their gut,and cling tenaciously to it.These might be the type of people who lost their desire for questioning early on,and perhaps were discouraged from asking questions.

  23. tyro says:

    @Blair T

    No movement, no time. No time, no movement.

    As I understand it, the reason that we have time dilation and other weird GR effects is that our movement through time and space is always a constant, c (the speed of light). When our movement through space is 0, our movement through time is at a maximum. That’s why when particles are moving through space close to the speed of light, their movement through time is very low. So ironically, as intuitive as your statement sounds it’s actually backwards.

  24. tyro says:

    I’ll be very interested to see what sort of answers people come up with. For flames, the forces and particles involved are accessible to children and there are day-to-day analogues. As Feynman once said about magnets, some forces don’t have any analogies and have to be taken as they are and intuitive understanding may be elusive. I think that despite the simplicity of the question, time doesn’t have any analogues so it simply isn’t something that can intuitively understood. I suspect the best answers for children will have to emphasize the complexity and our lack of (non-mathematical) understanding.

  25. BillyJoe7 says:


    That was a pretty clever response and entirely accurate. Everything in the universe travels through spacetime at the same rate in all reference frames. That includes light. In fact, it is the reason that the speed of light is the same in all reference frames. The time dimension of light’s spacetime is zero, so all of it’s spacetime is taken up in the space dimension. This also shows why the constancy of the speed of light is not really all that special.

  26. Jared Olsen says:

    I’m inclined to agree with quen_tin on this one. The best Einstein could do was “what a clock reads”, as quoted above. If anyone really knows what time is, I think explaining it to a child is next to impossible. Why not ask ‘what is quantum entanglement?’. That being said, I think it is a fruitful exercise, if not for the askers, then at least for the answerers (as the posts above show :-)).

  27. BillyJoe7 says:


    There is a retired high school teacher in Australia, who has gone back to teach physics to primary school children in grades 3 and 4. He uses diagrams and models and the kids seem to be engaged and interested. At another school in Western Australia, children in grades 5 and 6 are being taught relativity.

  28. selfification says:

    “Let us say that you had the ability to view the entire universe at once. As you watched it, you make not of the randomness or the messiness of the universe. We say that “positive time” has passed if your see more randomness overall (like milk mixing in water to become a uniformly watered down mixture). We say that “negative time” has passed if you see less randomness overall (like a broken vase re-arranging itself). We use devices called clocks that give us a measuring stick so that we may talk to one another about amounts of time. Now, we know that how quickly you are running around, where you and how close you are to other big objects and things affects how a clock works. There is no standard clock – everything is based on your own clock. But for people on earth, the changes are tiny and we still manage to mostly get along with crude measurements like seconds, minutes and hours.”

  29. Jared Olsen says:

    Nice link, BillyJoe. Inspirational. The little girl talking about filled electron shells…wow. Not a helluva lot of deep abstraction though, which I think would be required for the ‘time’ question.
    BTW Jaded? Was that a typo??

  30. BillyJoe7 says:

    Maybe it was the iPad autocorrect function. Let’s see:
    Nope, must have been a typo, sorry.

  31. roadfood says:

    “Time. Time. What is time? Swiss manufacture it. French horde it. Italians want it. Americans say it is money. Hindus say it does not exist. Do you know what I say? I say time is a crook.”

–Peter Lorre
 in the movie “Beat the Devil”

  32. rchapra says:

    What we call time is the process by which we organize our perception of movement.

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