# Is Time an illusion? **URL:** \ **Category:** Factual Questions\ **Created:** [September 2, 2010, 2:21pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385 "2010-09-02T14:21:18Z")\ **Posts on this page:** 20\ **Page:** 4
**Author:** ![Bad\_Astronaut](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bad\_Astronaut](https://boards.straightdope.com/u/Bad_Astronaut)\ **Post date:** [September 2, 2010, 10:31pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/61 "2010-09-02T22:31:44Z")
> [@Half\_Man\_Half\_Wit](#): > > Spacetime as a whole may exist, but that doesn’t rule out it being a dynamical quantity – which in general relativity, it is. Can you explain what a “dynamical quantity” is? Spacetime contains all of time as well as all of space, so saying that spacetime “changes” is nonsensical (change implies a difference over time). ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 2, 2010, 10:32pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/62 "2010-09-02T22:32:57Z")
I think the main discussion over time can, perhaps, be boiled down to the following question: Does the past exist? If you say, no, the past no longer exists, then there are several ‘names’ for you - one being an a theorist. If you say, yes, the past does exist, then (again, several names), you are a b theorist. Now, by exist I mean in a similar way that you and I exist: A theorist: The past is gone, no longer accessible, ‘poof’. B Theorist: The past is still there and if you could somehow sit outside of the cosmos (bear with me for a minute) you could _locate_ it. You could theoretically travel to the past via wormholes, faster then light travel, etc. The passage of time: A theorist: We perceive ourselves going through time, because _we_ are going through time. Somehow the present becomes the past (and no longer exists). We are moving towards the future. B theorist: We perceive ourselves going through time, but this is simply a deception. Perhaps similar to how we see faces in the clouds. Chronos, we’ve had this discussion before and it might have been clearer in the previous thread, [here](http://boards.straightdope.com/sdmb/showthread.php?t=531587&highlight=mctaggart). I must admit, this topic is very confusing and I’m probably the main peddler at this point of that confusion (not my intention obviously). ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 2, 2010, 10:34pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/63 "2010-09-02T22:34:18Z")
The timing of that prior thread is almost eerie… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 2, 2010, 10:45pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/64 "2010-09-02T22:45:35Z")
OK, that prior thread has much better explanations of the two models than here. Or at least, more sensical explanations; I don’t know what the original forms of the models proposed by the relevant philosophers were. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 2, 2010, 10:51pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/65 "2010-09-02T22:51:55Z")
> [@Bad\_Astronaut](#): > > > [@TriPolar](#): > > > > Not arguing with what you say in general, but the past is determinate, and the future isn’t. And we remember the past because it has changed our brains through our experiences, while we haven’t experienced the future yet. That’s how we percieve the flow of time. > > You may not think you’re arguing, but your statement directly contradicts the idea that spacetime as a whole exists. Either it’s true or it isn’t, but as I said, the math behind the currently accepted theories of physics suggest that it all exists. If the future exists then it is determined just as much as the past. Predicting the future and remembering the past are equivalent operations in the two different directions of time. Why can we do one and not the other? The same reason that eggs regularly get smashed to bits, but a bunch of egg fragments never seem to converge into a solid egg. Entropy. How do you determine exacly when a particle will decay in the future? ---
**Author:** ![Bad\_Astronaut](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bad\_Astronaut](https://boards.straightdope.com/u/Bad_Astronaut)\ **Post date:** [September 2, 2010, 11:03pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/66 "2010-09-02T23:03:10Z")
> [@TriPolar](#): > > > [@Bad\_Astronaut](#): > > > > You may not think you’re arguing, but your statement directly contradicts the idea that spacetime as a whole exists. Either it’s true or it isn’t, but as I said, the math behind the currently accepted theories of physics suggest that it all exists. If the future exists then it is determined just as much as the past. Predicting the future and remembering the past are equivalent operations in the two different directions of time. Why can we do one and not the other? The same reason that eggs regularly get smashed to bits, but a bunch of egg fragments never seem to converge into a solid egg. Entropy. > > How do you determine exacly when a particle will decay in the future? Maybe we are thinking of ‘determinate’ in different ways. I mean that the future is set, although it may be literally impossible for us to predict. If you are saying that the future is indeterminate in the sense that we are unable to ‘determine it’ (haha) then I agree, but that doesn’t address the deeper questions. As to your question, if no one observed a particle decaying in the past, and the evidence of the decay is no longer available, then we can’t ‘predict’ when that happened either - but I think we both agree that this event is not changeable in any sense. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [September 2, 2010, 11:05pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/67 "2010-09-02T23:05:17Z")
> [@Bad\_Astronaut](#): > > > [@Half\_Man\_Half\_Wit](#): > > > > Spacetime as a whole may exist, but that doesn’t rule out it being a dynamical quantity – which in general relativity, it is. > > Can you explain what a “dynamical quantity” is? Spacetime contains all of time as well as all of space, so saying that spacetime “changes” is nonsensical (change implies a difference over time). Nah, that’s just one way to parametrise change. The height of the terrain changes with location: it’s higher in the hills than on the beach. The colour of light after going through a prism changes with the angle. (If you think that’s somehow cheating, consider that the block universe view provides exactly the same interpretation for ‘change with time’.) In general relativity, spacetime is essentially the gravitational field. Different mass (or stress-energy) configurations yield different gravitational fields, which in turn influence the mass configuration (‘mass tells spacetime how to curve, spacetime tells mass how to move’). To think of spacetime as somehow being fixed misses the point of GR; its background independence, i.e. the explicit absence of a fixed stage upon which the dynamics of the universe unfold, is its greatest conceptual strength: at least in this direction, there’s something like a final answer, rather than an infinite regress of explaining each level in terms of a more fundamental one. The problem is, people tend to use the term ‘spacetime’ without taking to heart the conceptual revolution it brings; they think of it as simply some ordinary three-dimensional spatial coordinate system (x, y, z) indexed by a real variable t that somehow also plays the role of a ‘fourth dimension’. But that’s conceptually only halfway there, since such a decomposition is not in every case possible, and in general, observer dependent – one player’s x may be another’s t, so to speak. ---
**Author:** ![Elendil\_s\_Heir](https://avatars.discourse-cdn.com/v4/letter/e/7cd45c/32.png) [@Elendil\_s\_Heir](https://boards.straightdope.com/u/Elendil_s_Heir)\ **Post date:** [September 2, 2010, 11:09pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/68 "2010-09-02T23:09:34Z")
> [@palindromemordnilap](#): > > Jeez, this thread is a waste of … Agreed. I’m gonna take my hundred bucks and hit the road. ---
**Author:** ![Bad\_Astronaut](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bad\_Astronaut](https://boards.straightdope.com/u/Bad_Astronaut)\ **Post date:** [September 2, 2010, 11:15pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/69 "2010-09-02T23:15:24Z")
> [@Half\_Man\_Half\_Wit](#): > > Nah, that’s just one way to parametrise change. The height of the terrain changes with location: it’s higher in the hills than on the beach. The colour of light after going through a prism changes with the angle. (If you think that’s somehow cheating, consider that the block universe view provides exactly the same interpretation for ‘change with time’.) All of the examples in your first paragraph boil down to changes over either space or time, IMO. And I don’t understand the sentence in parentheses. > [@Half\_Man\_Half\_Wit](#): > > In general relativity, spacetime is essentially the gravitational field. Different mass (or stress-energy) configurations yield different gravitational fields, which in turn influence the mass configuration (‘mass tells spacetime how to curve, spacetime tells mass how to move’). To think of spacetime as somehow being fixed misses the point of GR; its background independence, i.e. the explicit absence of a fixed stage upon which the dynamics of the universe unfold, is its greatest conceptual strength: at least in this direction, there’s something like a final answer, rather than an infinite regress of explaining each level in terms of a more fundamental one. > > The problem is, people tend to use the term ‘spacetime’ without taking to heart the conceptual revolution it brings; they think of it as simply some ordinary three-dimensional spatial coordinate system (x, y, z) indexed by a real variable t that somehow also plays the role of a ‘fourth dimension’. But that’s conceptually only halfway there, since such a decomposition is not in every case possible, and in general, observer dependent – one player’s x may be another’s t, so to speak. I have to say I still don’t understand what you mean. However conceptually bizarre the GR universe is, how could it not be fixed at least from the perspective of a single point in spacetime? ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 2, 2010, 11:24pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/70 "2010-09-02T23:24:27Z")
> [@Bad\_Astronaut](#): > > > [@TriPolar](#): > > > > How do you determine exacly when a particle will decay in the future? > > Maybe we are thinking of ‘determinate’ in different ways. I mean that the future is set, although it may be literally impossible for us to predict. If you are saying that the future is indeterminate in the sense that we are unable to ‘determine it’ (haha) then I agree, but that doesn’t address the deeper questions. > > As to your question, if no one observed a particle decaying in the past, and the evidence of the decay is no longer available, then we can’t ‘predict’ when that happened either - but I think we both agree that this event is not changeable in any sense. Ok, were just using different definitions. I’m not sure that the future is determinate in the sense you are speaking of, but I don’t know, or care. But I doubt you could prove it (haha). We cannot predict the future. We can remember our perception of the past. Some of the past can be determined by measurement to a limited precision. I agree that the past cannot be changed, as far as I know, or anyone else knows so far. But I don’t know what everyone else knows either. As a note, your use of determinate is an arbitrary selection of a term to describe a physical theory, which postdates my use, which is the literal form, something that can be determined. But I assume you were laughing at yourself. My (haha) was laughing at you trying to prove that you can determine the future. ---
**Author:** ![Bad\_Astronaut](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bad\_Astronaut](https://boards.straightdope.com/u/Bad_Astronaut)\ **Post date:** [September 2, 2010, 11:40pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/71 "2010-09-02T23:40:49Z")
> [@TriPolar](#): > > Ok, were just using different definitions. I’m not sure that the future is determinate in the sense you are speaking of, but I don’t know, or care. But I doubt you could prove it (haha). We cannot predict the future. We can remember our perception of the past. Some of the past can be determined by measurement to a limited precision. I agree that the past cannot be changed, as far as I know, or anyone else knows so far. But I don’t know what everyone else knows either. > > As a note, your use of determinate is an arbitrary selection of a term to describe a physical theory, which postdates my use, which is the literal form, something that can be determined. But I assume you were laughing at yourself. My (haha) was laughing at you trying to prove that you can determine the future. But then I think you are just stating what everyone agrees is our perception of time, eg: > [@TriPolar](#): > > Not arguing with what you say in general, but the past is determinate, and the future isn’t. And we remember the past because it has changed our brains through our experiences, while we haven’t experienced the future yet. That’s how we percieve the flow of time. Yes this is how we perceive it, but the question is whether this might be an illusion in some way. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [September 2, 2010, 11:58pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/72 "2010-09-02T23:58:15Z")
> [@Bad\_Astronaut](#): > > All of the examples in your first paragraph boil down to changes over either space or time, IMO. And I don’t understand the sentence in parentheses. OK, electrical field strength changes with charge. The point is that when we speak about ‘change’, we think about quantities like x(t), position wrt time. But that definition of change is valid for all quantities of the form a(b), and those are conversely all the quantities we can ever observe. Everything we observe is a relation between partial observables (such relations may be called complete observables), i.e. a and b, or x and t; x and t can’t be observed on their own (think about how we measure all lengths with respect to a standard length, or all times with respect to a standard time interval). Only in the nonrelativistic limit does the partial observable t get singled out and acquire some ‘special’ meaning; in GR, it stands on equal footing with the other partial observables. > [@](#): > > I have to say I still don’t understand what you mean. However conceptually bizarre the GR universe is, how could it not be fixed at least from the perspective of a single point in spacetime? That’s just the thing – the concept of ‘a point in spacetime’ on its own is in fact physically meaningless in GR. The best we can do is specify spacetime coincidences – intuitively, particles crossing paths. There’s a famous thought experiment known as the hole argument that illustrates this, but it would take us too far to go through it in detail, and I don’t know of any good exposition on the net ([wiki](http://en.wikipedia.org/wiki/Hole_Argument) isn’t terribly helpful). However, with such path crossings, whose viewpoint should we accept as the ‘real’ one? Even if two particles cross paths a second time, in general, to both, a different amount of time will have elapsed since their last meeting (that’s essentially the famous twin paradox), and hence, at this point, we’d have (at least) two notions of time, standing on equal footing. ---
**Author:** ![Kozmik](https://avatars.discourse-cdn.com/v4/letter/k/8dc957/32.png) [@Kozmik](https://boards.straightdope.com/u/Kozmik)\ **Post date:** [September 3, 2010, 1:15am UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/73 "2010-09-03T01:15:27Z")
> [@Leaffan](#): > > Time is an abstract idea that we humans use to plan and schedule our lives. The past is gone and can never be changed; the future hasn’t happened yet. All we have is now. Sure, we can predict what _should_ happen, and are able to do so with great accuracy. For example I’m going for lunch now, unless a large meteorite strikes me dead or something. I don’t know what the soup is though. I’ll let you know in the future if it was good or not. It’s the soup _du jour_. The past is gone and can never be known; the future (probably) won’t happen. Time is a concrete concept that other humans use to plan and schedule our lives. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 3, 2010, 1:21am UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/74 "2010-09-03T01:21:25Z")
> [@Bad\_Astronaut](#): > > Yes this is how we perceive it, but the question is whether this might be an illusion in some way. Oh, that’s easy. Yes, time might be an illusion in some way. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [September 3, 2010, 10:55am UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/75 "2010-09-03T10:55:29Z")
I’m still not sure I understand the point of whether time is an illusion. What new knowledge would come from proving it was an illusion? For the statement that time is an illusion to have scientific value, it needs to have some form of utility. And if it does not have scientific value, why are we trying to use science to make sense out of it? ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 3, 2010, 12:27pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/76 "2010-09-03T12:27:43Z")
> [@Chronos](#): > > OK, that prior thread has much better explanations of the two models than here. Or at least, more sensical explanations; I don’t know what the original forms of the models proposed by the relevant philosophers were. Yes, I feel as though I’ve been muddling things all throughout this thread. I’ve been trying to make sense of both models and it’s very confusing. There’s not just ‘one’ a or b theory, there are multiple ones. Some involve what seems to me to be linguistic arguments (as in, purely). It’s over my head, but I’m trying to figure it out - which is why I believe my explanations have been muddled throughout this thread. ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 3, 2010, 12:31pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/77 "2010-09-03T12:31:10Z")
> [@BigT](#): > > I’m still not sure I understand the point of whether time is an illusion. What new knowledge would come from proving it was an illusion? For the statement that time is an illusion to have scientific value, it needs to have some form of utility. > > And if it does not have scientific value, why are we trying to use science to make sense out of it? The utility of it seems questionable. I think that it can be applied to the origins of the universe and that’s where I primarily see it argued. One camp says that only the present exists and therefore we have to explain how the universe came from nothing. The other camp says that all time exists and therefore the universe has always existed (finitely though). That is not to say that time goes back infinitely. ---
**Author:** ![Diceman](https://avatars.discourse-cdn.com/v4/letter/d/22d042/32.png) [@Diceman](https://boards.straightdope.com/u/Diceman)\ **Post date:** [September 3, 2010, 4:54pm UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/78 "2010-09-03T16:54:06Z")
> [@SmartAlecCat](#): > > > [@Diceman](#): > > > > …now Stephen Hawking thinks that the fact that gravity exists somehow proves that there’s no God. > > That’s not what he said. It was the other way around. > > He said gravity made it possible for the universe to exist without a God to create it. > > There could still be a God. Ah, I see the difference. The theory still doesn’t make any sense, however. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [September 4, 2010, 6:46am UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/79 "2010-09-04T06:46:05Z")
> [@BigT](#): > > I’m still not sure I understand the point of whether time is an illusion. What new knowledge would come from proving it was an illusion? For the statement that time is an illusion to have scientific value, it needs to have some form of utility. > > And if it does not have scientific value, why are we trying to use science to make sense out of it? Well, one point is that the search for a theory of quantum gravity proves difficult in part because of the different way time is treated in general relativity and quantum field theory. If now time were an illusion, in the sense that it isn’t fundamental, quantum gravity could be formulated in a timeless way, solving the difficulty of unifying the two contradictory viewpoints. Not that this means that it becomes easy to find a quantum theory of gravity, but it gives you at least one principle upon which to formulate it. Besides, it’s simply an interesting question, and knowledge can be valuable as its own end, even if there’s nothing as such you can ever ‘do’ with it. Is there a tangible benefit to knowing the exact depth of the Marianas trench (or even to having been there), or the different species of butterflies in the Amazon rainforest, or why the dinosaurs died out? I doubt it, and if there is, it’s most likely incidental. Generally, we don’t do things solely because they have utility – we do them because they’re there to be done, because we’re curious, and I think that’s a good thing. > [@Diceman](#): > > Ah, I see the difference. The theory still doesn’t make any sense, however. Boy, will Hawking ever be embarrassed when you tell him! ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 4, 2010, 7:02am UTC](https://boards.straightdope.com/t/is-time-an-illusion/552385/80 "2010-09-04T07:02:19Z")
> [@Half\_Man\_Half\_Wit](#): > > Well, one point is that the search for a theory of quantum gravity proves difficult in part because of the different way time is treated in general relativity and quantum field theory. If now time were an illusion, in the sense that it isn’t fundamental, quantum gravity could be formulated in a timeless way, solving the difficulty of unifying the two contradictory viewpoints. Not that this means that it becomes easy to find a quantum theory of gravity, but it gives you at least one principle upon which to formulate it. Ok, so time could be an illusion, in the sense that we percieve it (for whatever reason), but actually it doesn’t exist in terms of the physical nature of the universe. Does that mean there would be some way to percieve the future? If our brains are limited somehow, wouldn’t there be a machine which could overcome that? By machine I don’t necessarily mean a physical thing, but maybe just the theory. Or could that just mean time is not a factor in quantum gravity, not that it doesn’t exist? [Previous page](https://boards.straightdope.com/t/is-time-an-illusion/552385.md?page=3) [Next page](https://boards.straightdope.com/t/is-time-an-illusion/552385.md?page=5)