# Relativity Theory and the A-Theory of Time **URL:** \ **Category:** Factual Questions\ **Created:** [September 11, 2009, 5:13pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705 "2009-09-11T17:13:58Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 11, 2009, 5:13pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/1 "2009-09-11T17:13:58Z")
Okay, I’m not an expert and I’m not even quite positive that I have all the definitions down, so please bear that in mind. I’ve been reading some philosophy on time, in regards to McTaggart’s position on the B-Theory and the A-Theory and it occurred to me that - at least in my primitive understanding - that the theory of relativity pretty much conclusively shows the A-Theory is false. Of course, since there is no real consensus (that I can find) as to which theory of time is correct, I must admit that I’m skeptical of my conclusion… As I understand it, the A-Theory basically posits that the present is what exists. Now, I’m going to garble definitions to match what I understand them to be and if they are wrong _PLEASE_ correct me. So the A-Theory is essentially that the present is all that exists. The past _existed_ and the future is going to exist. The B-Theory, in contrast, says that past, present, and future all exist at the same time. They are essentially different dimensions. Please see [this picture](http://www.fairfaxunderground.com/forum/file.php?2,file=5219,filename=Time_5.png) (I didn’t create it) as I think it helps. Okay, so if my understanding of all the terms/theories are correct, then doesn’t relativity demonstrate that the A-Theory of time is false? The reason for this is, in my mind, is related to time travel. Since it is theoretically possible to travel throughout time in a relative universe, doesn’t that mean that the past and future exist in a B-Theory conception?? ---
**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 11, 2009, 6:58pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/2 "2009-09-11T18:58:05Z")
> [@](#): > > Since it is theoretically possible to travel throughout time in a relative universe… Only in certain specific circumstances which do not appear to be possible in the actual Universe we live in. Still, from what you’re describing of A-theory and B-theory (I’d never heard the terms before), it sounds like B is much more consistent with current scientific understanding of space and time. The A theory is really just a form of solipsism, since if you’re going to say that only the present time is “real”, then relativity forces you to also conclude that only your exact location is “real”. ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 11, 2009, 7:08pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/3 "2009-09-11T19:08:56Z")
> [@Chronos](#): > > Only in certain specific circumstances which do not appear to be possible in the actual Universe we live in. I’m not sure about that - I seem to recall that there is a scientist who is working on a way to send a particle into the future or something like that. > [@Chronos](#): > > Still, from what you’re describing of A-theory and B-theory (I’d never heard the terms before), it sounds like B is much more consistent with current scientific understanding of space and time. I would encourage you (or others) to look up these terms because I’m on very shaky grounds with their definitions. I think I’ve got it right, but after reading some McTaggart - who I believe made the compelling case in 1900’s - I’m not so sure. > [@Chronos](#): > > The A theory is really just a form of solipsism, since if you’re going to say that only the present time is “real”, then relativity forces you to also conclude that only your exact location is “real”. That’s an interesting point and kind of along the same lines that I’m thinking. ---
**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 11, 2009, 8:10pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/4 "2009-09-11T20:10:23Z")
> [@](#): > > I’m not sure about that - I seem to recall that there is a scientist who is working on a way to send a particle into the future or something like that. Oh, any fool can do that. It’s stopping a particle from going into the future that’s the hard part. ---
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [September 11, 2009, 9:05pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/5 "2009-09-11T21:05:34Z")
> [@Chronos](#): > > Oh, any fool can do that. It’s stopping a particle from going into the future that’s the hard part. 😛 A relativity question, though: Would we be able to tell if a particle was traveling backward through time? Wouldn’t it, to us, simply appear to be going in the opposite direction? (Although I guess it would appear to have antigravitational properties, right?) ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [September 11, 2009, 9:06pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/6 "2009-09-11T21:06:13Z")
> [@Meatros](#): > > As I understand it, the A-Theory basically posits that the present is what exists. You don’t need to posit time travel to reject the A-Theory. The A-Theory assumes that there is such a thing as a “present moment”. But relativity tells us that what constitutes the “present moment” varies from observer to observer. Say there are two events X and Y. Depending on his relative motion, one observer may observe that X and Y occur simultaneously (i.e. within the same present moment), another observer may experience X preceding Y, and a third may experience Y preceding X. If different observers can even agree on what is contained within the present moment, its very hard to argue that the present is the only thing that exists. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [September 11, 2009, 9:13pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/7 "2009-09-11T21:13:19Z")
> [@toadspittle](#): > > Would we be able to tell if a particle was traveling backward through time? Wouldn’t it, to us, simply appear to be going in the opposite direction? (Although I guess it would appear to have antigravitational properties, right?) A positron traveling backwards through time is indistinguishable from an electron traveling forwards through time. In fact, there briefly was a theory that there is only ONE electron. It only appears that there are many electrons because it keeps bouncing back and forth between the future and the past. This was quickly rejected because that would imply that the number of observable positrons should roughly match the number of observable electrons – we should see roughly equal numbers of backwards and forwards paths. However positrons are relatively scarce compared to electrons, so even if electrons do travel backwards in time, there must be more than one. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [September 11, 2009, 9:14pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/8 "2009-09-11T21:14:24Z")
> [@The\_Hamster\_King](#): > > If different observers can even agree on what is contained within the present moment, its very hard to argue that the present is the only thing that exists. That should read “… CAN’T even agree …”. ---
**Author:** ![Tyrrell\_McAllister](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tyrrell_mcallister/32/16772_2.png) [@Tyrrell\_McAllister](https://boards.straightdope.com/u/Tyrrell_McAllister)\ **Post date:** [September 11, 2009, 9:40pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/9 "2009-09-11T21:40:29Z")
> [@Meatros](#): > > The B-Theory, in contrast, says that past, present, and future all exist at the same time. I would quibble with that characterization. Even under the B Theory, different times don’t exist _at the same time_, any more than different points of space exist _at the same point of space_. What distinguishes the B Theory from the A Theory is that, under the A Theory, _did exist_, _exists_, and _will exist_ are three very different ontological conditions for something to be in. Under the B Theory, on the other hand, all times exist in the same, full, sense of the word. For example, there is no sense in which the future has some kind of vague indeterminate existence. Every proposition about the future is eternally true or false. The distinction between the A and B Theories is one of those fault lines where everyone who thinks about it almost immediately identifies with one side or the other. For my part, I don’t understand how the A Theory can be coherent at all, even setting aside developments in modern physics. But my arguments are the standard ones, which the A theorists have seen before and find unconvincing. But I really don’t know how they deal with some of the developments of post-relativity physics. For example, how does an A-theorist make sense of the relativity of simultaneity? ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 13, 2009, 12:02am UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/10 "2009-09-13T00:02:19Z")
> [@The\_Hamster\_King](#): > > You don’t need to posit time travel to reject the A-Theory. The A-Theory assumes that there is such a thing as a “present moment”. But relativity tells us that what constitutes the “present moment” varies from observer to observer.. I suppose it was easier in my head to think about it involving time travel, but you are correct. > [@The\_Hamster\_King](#): > > Say there are two events X and Y. Depending on his relative motion, one observer may observe that X and Y occur simultaneously (i.e. within the same present moment), another observer may experience X preceding Y, and a third may experience Y preceding X. If different observers can even agree on what is contained within the present moment, its very hard to argue that the present is the only thing that exists. I’m curious how the A-Theory attempts to defend against this. It seems to me that A-Theory should have been discarded when the notion of absolute time disappeared (ie, relativity theory). I feel that I’m missing something though, since, to my understanding (which is limited), the A-Theory still dominates contemporary thought. ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 13, 2009, 12:09am UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/11 "2009-09-13T00:09:32Z")
> [@Tyrrell\_McAllister](#): > > I would quibble with that characterization. Even under the B Theory, different times don’t exist _at the same time_, any more than different points of space exist _at the same point of space_. Ha, yes, you are correct - it’s actually hard to quickly jot down the specifics, since the B-Theory seems counter intuitive to me. > [@Tyrrell\_McAllister](#): > > What distinguishes the B Theory from the A Theory is that, under the A Theory, _did exist_, _exists_, and _will exist_ are three very different ontological conditions for something to be in. Under the B Theory, on the other hand, all times exist in the same, full, sense of the word. For example, there is no sense in which the future has some kind of vague indeterminate existence. Every proposition about the future is eternally true or false. That is a much better way of putting it, thank you. > [@Tyrrell\_McAllister](#): > > The distinction between the A and B Theories is one of those fault lines where everyone who thinks about it almost immediately identifies with one side or the other. For my part, I don’t understand how the A Theory can be coherent at all, even setting aside developments in modern physics. But my arguments are the standard ones, which the A theorists have seen before and find unconvincing. I think that with the A-Theory, cosmology becomes a matter of something from nothing. The B-Theory, it seems to me, doesn’t have this limitation. I know this is imprecise to say, but I think you follow what I’m saying when I say, with the B-Theory, there was no point where time/existence didn’t exist. Urgh, that’s not properly phrased. I suppose what I’m getting at is that it seems to me that the cosmological argument is only effective if the A-Theory is true. If the B-Theory is true, it seems to me that the cosmological argument is false. (This is putting aside whether or not A-Theories are coherent). You seem to have a good grasp on this issue (you can at least phrase it better then I can), so I hope you can see what I’m aiming at. > [@Tyrrell\_McAllister](#): > > But I really don’t know how they deal with some of the developments of post-relativity physics. For example, how does an A-theorist make sense of the relativity of simultaneity? I’m not sure what you mean here, can you explain? ---
**Author:** ![Tyrrell\_McAllister](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tyrrell_mcallister/32/16772_2.png) [@Tyrrell\_McAllister](https://boards.straightdope.com/u/Tyrrell_McAllister)\ **Post date:** [September 13, 2009, 7:42pm UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/12 "2009-09-13T19:42:09Z")
> [@Meatros](#): > > I think that with the A-Theory, cosmology becomes a matter of something from nothing. The B-Theory, it seems to me, doesn’t have this limitation. I know this is imprecise to say, but I think you follow what I’m saying when I say, with the B-Theory, there was no point where time/existence didn’t exist. Urgh, that’s not properly phrased. > > I suppose what I’m getting at is that it seems to me that the cosmological argument is only effective if the A-Theory is true. If the B-Theory is true, it seems to me that the cosmological argument is false. I don’t think that the B Theory gets around the cosmological argument. Under the B Theory, no less than under the A Theory, the well-suitedness of the Cosmos for certain ends is still left unexplained. Why does the Universe satisfy the necessary conditions for life? Construing the Universe as a four-dimensional given thing doesn’t answer that question. Given our current models of physics, it _could_ have been some other four-dimensional given thing that could not contain life. You might turn to some kind of anthropic explanation, but you could have done that just as well under the A Theory. > [@](#): > > You seem to have a good grasp on this issue (you can at least phrase it better then I can), so I hope you can see what I’m aiming at. This thread was a funny coincidence for me. I’ve been reading an anthology of essays on the philosophy of time lately, and I’d just read McTaggart’s piece last weekend :). > [@](#): > > I’m not sure what you mean here, can you explain? I was just referring to the point that **The Hamster King** raised in [post #6](http://boards.straightdope.com/sdmb/showpost.php?p=11545154&postcount=6). There is no canonical way to say which pairs of events are simultaneous. A pair of events may be simultaneous in one inertial reference frame without being simultaneous in another inertial reference frame. Wikipedia has [a nice article](http://en.wikipedia.org/wiki/Relativity_of_simultaneity) about why this is so. My impression is that the A theory has few defenders these days among philosophers. Maybe my sample is biased, though. ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 14, 2009, 1:39am UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/13 "2009-09-14T01:39:08Z")
> [@Tyrrell\_McAllister](#): > > I don’t think that the B Theory gets around the cosmological argument. Under the B Theory, no less than under the A Theory, the well-suitedness of the Cosmos for certain ends is still left unexplained. Why does the Universe satisfy the necessary conditions for life? Construing the Universe as a four-dimensional given thing doesn’t answer that question. Given our current models of physics, it _could_ have been some other four-dimensional given thing that could not contain life. > > You might turn to some kind of anthropic explanation, but you could have done that just as well under the A Theory. Yes, one would still have to explain the anthropic argument. I guess I wasn’t considering that as one of the cosmological arguments. I was more thinking along the lines of the Kalam cosmological argument. It seems to me that _that_ sort of argument is still born. Anthropic arguments on the other hand still have their punch and would have to be dealt with with other rebuttals or what have you. > [@Tyrrell\_McAllister](#): > > This thread was a funny coincidence for me. I’ve been reading an anthology of essays on the philosophy of time lately, and I’d just read McTaggart’s piece last weekend :). Heh, nice - I just attempted to read McTaggart a week ago or so myself. I just received a book on metaphysics which includes one of his essays. I just couldn’t understand the language - I need to spend A LOT more time with it I suppose. It’s weird, and it could just be a matter of my not reading the right things or what have you, but from my perspective the philosophy of time just isn’t talked about. I emphasize the fact that I haven’t taken a lot of philosophy courses and I’ve only read a smattering of philosophy books, but it just seems to me that it’s hardly talked about. > [@Tyrrell\_McAllister](#): > > I was just referring to the point that **The Hamster King** raised in [post #6](http://boards.straightdope.com/sdmb/showpost.php?p=11545154&postcount=6). There is no canonical way to say which pairs of events are simultaneous. A pair of events may be simultaneous in one inertial reference frame without being simultaneous in another inertial reference frame. Wikipedia has [a nice article](http://en.wikipedia.org/wiki/Relativity_of_simultaneity) about why this is so. Ah, I see - it’s clear now and I’m not sure how I misread you before. > [@Tyrrell\_McAllister](#): > > My impression is that the A theory has few defenders these days among philosophers. Maybe my sample is biased, though. I haven’t read much physics, but the popularist accounts that I have read don’t seem to reference the A or B theory, although now that I think about it, I seem to recall that Stephen Hawkin made some mention about there being no begining and that there is no ‘before’ the big bang. Perhaps he was referring to the B-Theory without expressly saying so. I’m not sure - shoot, I don’t even know if physicist even make a distinction anymore, they just assume the B-Theory in the same way that astronomers assume heliocentricism (in this I specifically mean they don’t go out of their way to justify heliocentricism). ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [September 14, 2009, 1:40am UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/14 "2009-09-14T01:40:49Z")
BTW - William Lane Craig - a philosopher/apologist is a main proponent of the A-Theory. Again, I am not up to date on the philosophy of time, but he’s practically the only living philosopher I’ve read who seems to have an opinion on the matter. Richard Carrier mentions both in his Sense and Goodness book. ---
**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 14, 2009, 3:30am UTC](https://boards.straightdope.com/t/relativity-theory-and-the-a-theory-of-time/509705/15 "2009-09-14T03:30:40Z")
Should have replied to this sooner: > [@](#): > > (Although I guess it [a particle moving backwards in time] would appear to have antigravitational properties, right?) Gravity is symmetrical under time reversal. Consider a movie of a person throwing a ball up in the air and then catching it: If you played that movie backwards, it would look basically just the same.