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Is time real?

  1. May 10, 2010 #1
    This is a troubling question for me. Certainly there exists in our universe sequences of events which may or may not be undone easily. For example, I can walk 31 steps forward, then walk 31 steps backward and return to where I was. However, I can not easily unmix milk from my coffee after I have poured it in, though I shouldn't think it is impossible.

    This isn't really what we mean when we think of the concept of "time". A simple sequence of events is something we can do or undo, but no one here thinks they can return to yesterday, or last year. When we think of time, we think of us somehow floating along in a bubble that we call the present, and behind us is the past and before us the future. We can not look forward into the future, but we can look behind us into the past and around us inside the bubble at the present. I personally suspect that this is really an illusion, and that the concepts of "past' and "future" have no physical meaning, but trying to prove it in any concrete way I think is beyond my scope for now. Any thoughts on this topic?
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  3. May 11, 2010 #2


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    I think the problem we run into in trying to understand time is that both in the physical world and in everyday life we're dealing with two very different notions of time. And to me it's nonsense to call either aspect of time an "illusion"... since if something so basic to all our experience of the world is illusory, what basis do we have left for deciding whether anything is "real"?

    The problem is made very difficult by the fact that one of the two aspects of time -- call it clock-time or calendar-time -- is very well conceptualized, again both in physics and in everyday life. The other aspect -- the time we actually experience in this ongoing present moment we call "now" -- is almost unconceptualized, even though it's the most familiar thing in the world to us.

    To me, what's illusory in this situation is that "time" refers to something simple. On the one hand we're very much at ease with clock-time as a conceptual structure. We picture time laid out in front of us, from the beginning of the universe on into the future, and mark off the points when certain things happen, and measure the distance between them. (It turns out that in Relativity, time is connected with space in a very counter-intuitive way, but we're still dealing with a classical continuum, so we tend to overlook that complication.)

    On the other hand, we're so familiar with the way things happen in the present moment that we tend to assume this fits nicely into our picture of clock-time. But the "present" we experience is nothing like a "point in time" -- like the frozen instant captured in a snapshot. Of course "now" has something to do with the time I see on the clock, but to try to conceptualize it as a "moving point" on the time-line doesn't do it justice at all. It's more like a continuously evolving event in which a factual situation we inherit from the past opens up certain possibilities, such that present interaction with things can "choose" certain of these possibilities and reject the rest, and so change the possibility-structure inherent in the situation. This aspect of time as an ongoing interactive selection process that evolves possibilities seems to me to have a strong resemblance to aspects of Quantum theory. But we have yet to work out what's involved in this aspect of time, so it seems mysterious and maybe unapproachable.

    Heidegger made some real progress with this issue in his unfinished Being and Time. And Bergson at least pointed out the radical difference between these two aspects of time. But the issue hasn't even been approached in physics -- even though I think there are strong indications both in the Quantum theory and in Relativity that it's important. I agree with Lee Smolin's guess that the key to fundamental physics will be a better understanding of time, specifically the aspect that's essentially different from space. But many other physicists are willing to accept the traditional time-continuum picture and treat the actual happening of the present moment as an "illusion" produced by human consciousness. And many philosophers have misread Relativity as "proving" that time and space are essentially the same.
  4. May 11, 2010 #3
    It's a dimension, just as real as "up."
  5. May 11, 2010 #4


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    That's definitely one of the things it is.

    Up and down are different, because of gravity. Past and future are different in a different way, because something definite has happened in the past, that's brought us to where we are now, and the future is only possible. To some extent it's predictable, but it hasn't happened yet.

    The easy aspect of time is laying this out like a map, with the past "back there" and the future "in front of us". We can refer to last Wednesday or next Wednesday with equal clarity.

    The hard part about time is understanding what it means "to happen"... how this "present moment" (which is all we've ever experienced) actually works, to make new facts out of possibilities. As always, we're dealing both with a subjective experience and with something physically real. We have a lot of research to work with both in psychology and in Quantum physics. But I don't think we yet have adequate concepts for describing this "process"... largely because it's very foreign to the orientation of the philosophical tradition. And because happening is so familiar to us that we tend just to take it for granted.
  6. May 11, 2010 #5

    I agree with Rovelli that time is a semi-classical concept. What would that mean for space? Well, the same. Both are not fundamental constituents of reality(and not real in the ordinary sense) but emergent manifestation of the way underlying fundamental constituents(strongs, loops, ???) interact.

  7. May 11, 2010 #6


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    Good points Conrad. Space is our way of modelling/measuring stasis or the location of things, time is our way of modelling/measuring flux or change. Space is thus defined by its available physical symmetries (you can go one direction and then return the other way) while time is defined instead by its asymmetry - the changes that make a lasting change.

    It all gets confused in physics models because both Newton's mechanics and Einstein's relativity treat change as a symmetric operation (the equations allow the world to be run in either direction). It is only thermodynamics that has the extra constraints to give time an arrow in physical theory. Thermodynamics introduces the notion of a thermodynamic gradient of entropy down which a system must slip, and cannot return. Although the Boltzmann ensemble interpretation of thermodynamics then re-introduces locally symmetric processes and once again time becomes a dimension that seems reversible (the ergodic hypothesis).
  8. May 11, 2010 #7
    Is it safe to say that only the past exist. The present is never present, always in the past. The future does not exist until it becomes past. But then again wouldn't the past just be what we beleive has happened. WOW, someone has got to figure out this whole time business.
  9. May 11, 2010 #8
    I've thought of time in the following way in the past. To simplify things a bit, imagine a two dimensional universe as simply a flat plane, you can call it the standard (x,y) plane.

    Imagining time as a dimension, then time would constitute the z axis, or we could label it the 't' axis for simplicity. Now, if you think of this (x,y) plane as representing space, and the t axis representing time, then the flow of time, or arrow of time, is represented by this plane moving along the t axis in one direction. In this sense, your ultimate position in "spacetime" is always changing. Your (x,y) position may remain constant, but your position given by (x,y,t) is always changing because your position on the 't' axis is always changing.

    I can instantly recognize flaws in this analogy though, which tie into flaws in the way we define time in the real world. The major flaw being, that the "rate of time" in such a scenario is given by the velocity the (x,y) plane is traveling along the 't' axis, and velocity is given as a function of time... therefore, the definition of time in such a universe is cyclical!

    I find the same problem to be true in the real world. We always measure time by the motion of things. Either the motion of gears in a clock, the oscillation of a cesium atom, or a number of other ways - we can not measure time except by measuring motion. An the measure of motion is really the measure of the rate of change of a position - or a velocity. So even in the real world, the definition of time is cyclical, at least the way we measure it!

    The fact that, I for one, can not think of a non-cyclical way of defining time leads me to the idea that time is not a real thing, but some sort of abstraction we humans have invented. I hope I have made my idea clear, if not I will try to explain it in another way.
  10. May 11, 2010 #9


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    If you take the thermodynamic view, then you can measure the average temperature of the universe - the cosmic background radiation - and use this as your clock. Looking around at different point's of the universe's history, this is one thing that will change for observers "at rest" and so can define the current "now" as different from previous or future "nows".
  11. May 11, 2010 #10
    Either action still respects the arrow of time. Your backward steps that have "undone" your actions don't undue the loss of free energy that came from your forward steps, and in fact created their own loss of free energy.
  12. May 11, 2010 #11
    I really like this idea, but I think it only works in an open universe. In a flat or closed universe, there should be points in "time" where the average temperatures are equal... it's definitely a fascinating way of thinking about time though, or at least a way of defining or measuring it.
  13. May 12, 2010 #12
    Time only exists with change. Without change, time can not exist.
  14. May 12, 2010 #13


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    By the thermodynamic view, if gradients don't exist, then yes, time also ceases effectively.

    An equilibrium state such as an ideal gas or the heat death universe is an example of a realm where change becomes no change. There is no gradient even though there is still dynamism. So no change despite still continual change. And thus no real time in the sense of a still developing history. Development has halted.
  15. May 12, 2010 #14
    Are you implying that time does not physically exist without a thermodynamic gradient? I would prefer the view that time is something mentally constructed, that needs a thermodynamic gradient in order to function the way it does.
  16. May 12, 2010 #15


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    When you speak of time as a mental construct, I think you’re referring to our moment-by-moment experience of things happening. As distinct from the time-continuum pictured in physics, on which one would map a thermodynamic gradient.

    Because the time-continuum is so fundamental in physics, there’s a strong tendency to assume that’s the only aspect of time that’s relevant. In order to push the “happening” aspect of time out of physics, it’s often treated as something our minds do, somehow, independently of the physical world. Looked at in a broader scientific context I think this makes no sense at all – but it serves the purpose of letting physicists treat time as something simple.

    But the “happening” aspect of time is hard to dismiss in Quantum theory. And even in Relativity, there’s an important difference between the time-continuum and the proper time of any observer. To illustrate:

    Here we’re looking at the universe as if we could stand outside of space and time and observe it globally, like an object we can set on our desks and observe from all angles. Time is then pictured as another dimension of space – and as our calendars and historical time-lines attest, the is a very useful way to think about time.

    However, this familiar picture misrepresents the way space and time are combined to make spacetime in Relativity. In this picture – assuming a “flat” spacetime – the “distance” between two events in spacetime would be the square root of x2+y2+t2, whereas in Relativity the invariant interval is the square root of x2+y2 minus t2. That is, we’re in Minkowski’s spacetime, not Newton’s.

    The structure of Relativistic spacetime is very difficult to picture “from the outside”... but it’s not hard to see “from inside” – i.e. from my own point of view as an observer. What the minus-sign means is just that my present moment now contains events that are happening right around me, as well as distant events that happened some time ago. When I look at a star, my “now” includes an event on the surface of that star that's as distant from me in time as it is in space. Because of the minus-sign, the space-distance and time-distance offset each other.

    There’s nothing “subjective” about this situation – we’re talking about the physical structure of spacetime, not a “mental construction”. But Relativity tells us that distant observers do not share the same “now” – the present moment each of us lives in is an essentially local aspect of the structure of time. So even though for some purposes it makes sense to map the universe on a time-line, as if it were all “moving forward in time” as a single entity, that’s not how physical time actually operates. The physical context of my “now” is made up of events that just happened right around me, and the events “on my past light-cone” that are equidistant in space and time. Likewise what happens here and now is relevant only to what can happen close by and to possible events “on my future light-cone.”

    So the physical structure of happening, in our world, is not at all like an x,y plane “moving” along the z-axis. It’s more like a complex web of events influencing other events nearby and far away. There is definitely still a “direction” of time, in a Relativistic universe – no event ever influences an event in the past. But the time of each observer is physically distinct from that of other observers.

    My point is that even in Relativity – where we can still imagine what happens in the world in a classically deterministic way – the temporal structure of happening is complex. The global time-continuum is only one aspect of the physical structure of time. What we observe in our own local present moments reflects a different aspect of physical time, not something mysteriously created by our minds.
  17. May 12, 2010 #16
    What I really meant is that dynamic time rather than the "frozen river" time is a mental construction. And physical theories modelling spacetime as manifolds etc. are also mental constructions in a sense - they are abstract models designed to generalise our direct experience (observations). I do not really believe space or time exist except as a way to categorise and understand objects and events.
  18. May 12, 2010 #17
    I think that there are two notions of time.
    One is the mental one, which is a side effect of consciousness and memories, and the other one is physical time.

    I believe physical time is a side effect of the inner workings of quantum physics, and in general how the universe is built.
    The universe can only have one state, and that is the current state it is in.
    This state just is, it doesn't have a time property except the inner workings that define why and how things can move, and events can happen over a period.

    So I guess if we want to know why the universe is changing, and why things move around, we will have to create the math and physical models that describe this based on observation of quantum particles, and whatever else is out there.

    Also, since the universe is in a constant single state, the past and the future do not exist physically.

    Or at least, that's my take on it.
  19. May 12, 2010 #18


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    Good explanation. The interesting thing to me is that really relativity means we should be taking the relativistic story as the baseline, no time, equilibrium state. So in some important sense, time was in a different state during the early big bang, before the Higgs transition that created located massive particles moving slower than light.

    Before the Higgs mechanism kicked in with sufficient expansion/cooling, all particles existed only relativistically, moving at the speed of light. What it actually means to say electrons or quarks even "existed" in that environment seems debateable. There would be no locality as such. The time-like separation between locations would shrink to zero - which is why a particle going at the speed of light like a photon is said to experience no time.

    In a hot relativistic realm, the entire system would be in communication at the same rate, and so there would be no gradient to define an arrow of time locally.

    However once mass condenses out of the quark-gluon soup, suddenly you have lumps of mass travelling slower than light speed. These lumps can slow all the way down to be "at rest" even. And lumps at rest would thus maximise a local potential for temporality. They would create a new kind of gradient, a gap that exists between their inertial position and their potential to interact at lightspeed via EM and gravity in particular.

    So we can see that once time is broken down into the notion of changes that are changes (rather than merely the milling non-change of a dynamic equilibrium), and then further broken down to a model based on entropic gradients, time becomes an emergent property of systems. And it can come in a variety of grades.

    In a relativistic realm, all actions between locations are happening at the same rate - lightspeed - so it is a rather vanilla notion of "time passing".

    Then when mass condenses out, suddenly there are lumps of matter going slower than light, and so introducing a time lag between their physical actions (mechanically bumping about) and their energetic exchanges (radiating photons, communicating slow shifts in position as lightspeed changes in the gravity fields associated with their mass).

    Time immediately becomes richer because there is a gradient, a game of catch-up to be played. A local component is introduced and there is no longer just a vanilla relativistic equilibrium realm.

    It could be that following this approach, we could argue for a hierarchy of temporal richness. So first the step up in complexity that comes with the condensation of massive particles. Then further steps up with the more complicated dissipative structures we are talking about when we deal with life and mind.

    But anyway, the key thing here is that there is already in the cosmological story a clear phase transition in the nature of "time". At the point mass condensed out, a new kind of locality was invented. Time visibly passes for objects able to move slower than light because of the gap between their motions "in space" and their energetic exchanges.

    Newton's model of time arose from the wrong assumption - that the natural baseline for measuring time was from the point of view of an observer at rest. Relativity fixed that by making the baseline dynamic - the lightspeed realm - and we can instead measure time in terms of deviations from that natural rate (with "at rest" being the greatest possible deviation, the one that creates the largest out-of-equilbrium gradient to drive change, locally).
  20. May 13, 2010 #19


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    It is always a matter of modelling, and the map is not the territory, just the map. So we could have various views of what time or space are, chosing the ones most suited to our current purposes.

    Having said that, the Newtonian notion of time is very simple and unrealistic. The thermodynamic view seems richer because it is based on a self-organising systems model of reality.

    So what I was saying is that time is generally agreed to be our way of measuring change. And change in turn must depend on the existence of a gradient. So with no gradient, a system would be at equilibrium and this indeed seems a pretty timeless place.

    The way this changes the notion of time is that we no longer need to worry about the fact it has an asymmetric arrow (that comes with the idea of the gradient). And it also makes time not a single thing (so time always flows the same even before universes exist). Instead it becomes a systems property and so something that can be found in various grades of being.

    The universe becomes a nested hierarchy of gradients. So you have the basic gradient set up by the big bang - the one that runs from a hot point to a cold void. This is the naked spreading radiation story. Then a second more complex gradient was created on top of this basic entropic story by the appearance of located mass able to go slower than light speed. Time was now a richer, more complex thing, in our universe as more kinds of change became possible.

    Life and minds exist in an even richer variety of time. Organisms have memories and expectations. By harnessing natural entropic gradients like sunshine - by steepening those gradients locally by accelerating the production of entropy - an even more complex kind of time passing is created.

    It could be said that this is now stretching the physical definition of time. And I would agree. The really major transition in the nature of time was the one where mass condensed out. Yet I can still relate organismic time to physical theory. As I said, life pays for its existence by steepening entropic gradients. It literally changes the universe faster, and that does mean more is happening.
  21. May 13, 2010 #20


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    Yes, this begins to sketch out some of the complexity that belongs to the physical structure of time. I think eventually it will be clear that the concept of "time" in physics is a lot like the concept of "life" in biology – a complex notion with many important facets, all of which play a role in an evolutionary process.

    The example you point out – Relativity makes an absolute distinction that had never before been suspected, between light-speed and speed in general, which is equivalent to being at rest. Instead of just “motion” we have a radical difference between the way masses and massless particles relate to space and time. It’s almost as though the physical world has two completely distinct spacetime structures superimposed on each other.

    There’s the more “primitive” structure of light-speed connections – in this regime, the early universe is still happening right now, in my present moment – though it’s 13.7 billion miles away. And there’s the completely distinct structure of dynamics between masses, that approximates Newtonian dynamics except when things are moving very fast, relative to each other, or where there are high concentrations of mass.

    So as to the philosophical question – “Is time real?” – Yes, certainly, but how it works is not so simple as we tend to think. Our notions about time today are like the way some people thought about “life” in the 18th century – as a simple, elemental force that animated things.

    Discussions of time still tend to get caught in the same dichotomy that the ancient Greek philosophers invented – “everything is in flux” / “change is really just an illusion.” But time in modern physics is not just an empty background against which things change or stay the same.

    To me, this is where philosophy can play a significant role, as distinct from science. The task of science is to look at the available hypotheses and figure out which best fit the facts. The task of philosophy is to invent new ways of imagining the world and what’s going on in it. Because if there’s one thing that’s clear in physics today, it’s that our knowledge of the world has gone far beyond what we know how to imagine.

    Nearly all observable physical phenomena are theoretically “well understood” today. But we don’t have any hypotheses that begin to explain why the world should be built the way it is. Why Relativity, why Quantum theory – and why does so much of the world behave just like classical, Newtonian physics? There are several profoundly different kinds of structure involved in fundamental physics – each with its own “temporality”, to borrow Heidegger’s term... its own way of “doing time.” The ancient arguments about flux vs stasis don’t shed any light on what’s going on here.
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