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In the answers I do not see any mention of the Second Time Around Problem, which conclusively answers the question with "there is no way to know". The idea is:

by jblow 13y ago

In the answers I do not see any mention of the Second Time Around Problem, which conclusively answers the question with "there is no way to know". The idea is: suppose there is a universe that has elements that are fundamentally random. It is nondeterministic. Well, let that universe run for its lifetime, and record everything that happens. Then make a deterministic universe that just plays back the recording (this is the "second time around"). From the viewpoint of someone living inside the universe, there is no way to tell whether it is the first time around or the second time around. ... But the other thing to point out is that this question presumes an old idea about the passage of time, which is that things happen in a sequence A, B, C, D, ... and that if you are at C then D "has not happened yet". But if you look at relativity, this appears to be a naïve viewpoint. In relativity, the time at a faraway point in space that you would consider "simultaneous" with your own clock depends on the relative speed between you and that point. As you speed up and slow down, you can make a faraway point "go forward or backward in time" with regard to which moment there you would consider "now". The crazy thing is that for angular movements the relative speed is amplified by distance, so when you are moving around at everyday speeds the "now" on planets across the galaxy is going back and forth by thousands or millions of years. (This is hard to observe because you are viewing tiny amounts of light from very very far away that have been traveling for a very long time, and the light that you are about to see was very close to you when you did the angular movement so it will not be much affected, etc, but hey, the math says what it says, you either believe what physics tells you or you don't.) So when you make a distant "now" go forward, then backward, then forward again, do you expect the two forwards to be the same, or not?

siddboots 13y ago
That argument is, roughly, Humean scepticism. Science is a game we play where we do experiments, build theories, do more experiments, and so on. Any scientific answer to how the universe works has to start with the assumption that the nature of universe can be understood in the first place (otherwise we'll never get anywhere.) Edit: This was in response to your comment prior to edit. Up to "..."
- eli_gottlieb 13y ago
  Well yes, which is why we can't really ever know. It's somewhere between difficult and impossible to prove the axiom of your entire epistemological process.
paraschopra 13y ago
This is weird. If the observer in the universe is able to make accurate predictions consistently and repeatedly, doesn't that give confidence to his beliefs that the universe is deterministic. What you might be referring to is another way of saying that "any theory can be proven wrong if and when we observe a data point negating it". So, of course, there is no way of knowing if the universe is __truly__ deterministic or not, but then again we can say the same thing about sun rising from the east (unlikely as it may be, it might well just not rise tomorrow at all and then we will have to revise our theories).
- jblow 13y ago
  Did you read the OP? The whole question is about whether the apparent randomness in quantum mechanics means the universe is fundamentally nondeterministic. We cannot make accurate predictions of measurements of quantum state apart from saying things like "A will happen with some probability, otherwise B will happen". It is that "with some probability" that is the question. (Yes, at the level of the macroscopic world we can make consistently accurate predictions, but physicists would say this is because we are operating at a scale where the statistical nature of quantum mechanics averages out, etc. Unless you are Carver Mead or one of the other wave guide kinds of guys who believe there actually is no randomness in QM and it is deterministic all the way down.)
- crististm 13y ago
  Double slit experiment shows that you can't predict which slit the particles are going to go. This is pretty much non-determinism to me
no_gravity 13y ago
2 things: 1) If we define "universe" as "everything that exists" then you cannot record everything that happens. Because the record would be part of the universe and would have to recorded and the record of the record would have to be recorded... 2) The "first" and "second" universes would not be universes on their own. The "second" is a continuation of what you call the "first". So the system you describe would be random. Because the "deterministic" things that happen later on are based on the random things that happened earlier.
- jblow 13y ago
  Just pause for a moment on "everything that exists" because that is not necessarily a useful distinction. If you think of a universe as "everything I could see/touch/etc given infinite time", someone outside the universe who was in control of it might record it with no problem because they are not subject to the constraints of the universe. e.g. imagine the universe as being like a computer simulation that you programmed. You can probably pause it in a debugger any time you want, look at whatever state variables you want, etc. So in this case the definition cannot be "everything that exists" because you have to define "exists" in the case where different sets of things may have different levels of "reality".
  - no_gravity 13y ago
    Lets say we are in a "game of life" simulation on a computer. It has been proven turing complete. So you would say its deterministic because "game of life" has deterministic rules. But if the computer that runs the simulation is in a random universe, then it might happen that bits of the computer switch in a random fashion. Making our universe inside the "game of life" simulation random too. And we could touch the randomly flipping bits. So there is no point in saying something is "outside" our universe and something is "inside".
    - jblow 13y ago
      There is no point to saying something is "outside" and something is "inside" in the extreme cases of the example you just gave, which may or may not be the case. What if the outer universe is not a random universe? What if its laws of nature are so unlike ours that we do not know what to say about it? (Maybe it does not have space or time, but it has other things instead.) But I think this is a little bit beside the point, because as I mentioned in my first post, all of this is predicated on us adopting the naïve view of time in our own universe, which is maybe not the best idea, given that we have plenty of evidence otherwise. (And anyway, if you believe that "everything that exists" can be infinite, then there is no sense in saying our universe is being produced by some kind of simulation controlled in one particular other place, because of course it is, but actually this is probably happening in an infinite number of different ways from different places such that there is not really point any more in claiming that this is happening at all; the situation becomes such that you could just draw a relation between situation X and situation Y and could state that one could be causal of the other from a certain point of view.)
nhaehnle 13y ago
> when you are moving around at everyday speeds the "now" on planets across the galaxy is going back and forth by thousands or millions of years This feels like it is either entirely incorrect, or you're taking something that is correct and interpreting it in a very misleading way. Your remark about angular movements makes me think that perhaps you don't consider the fact that, when measured from a very distant planet, any angular movements we make in our everyday life are extremely tiny?
- jblow 13y ago
  I first learned this in a book by a prominent physicist, so I am inclined to think it is an accurate reading of the physics. Unfortunately I do not remember which so I cannot give you a citation (it was possibly Brian Greene or Frank Wilczek), but maybe there will be a physicist reading this thread who can chime in. The angular movement thing is this: Imagine you have a frame of reference on the tip of your nose. The X axis points straight away from your face, the Y axis is to your left, Z is up. Now start turning your head to the left. To a tiny observer living on the tip of your nose, the relative speed along the Y axis of a faraway planet has suddenly become very high. The further the planet, the faster the speed (this part is just grade-school geometry).
  - jblow 13y ago
    But let me also just say that you do not have to believe this at all in order to believe my original point; I just brought it up as an extreme crazy case. So if you don't believe the extreme crazy case, think of the standard example: you get into a space ship or something and zip around really fast. And you are thinking about things closer to you. The math says the same thing: as your light cone changes, the set of spacetime intervals, that have time distance 0 from you, changes as well. So from your relative position the "now" at these faraway points goes back and forth. This is basic, basic relativity.
    - jblow 13y ago
      And actually, yeah, forget about the relative speed, since that is not central to the real issue. The real issue is just that when your light cone changes, the set of points that are simultaneous with you changes (all these points are outside your light cone except for the one you occupy). You cannot observe them directly because they are outside your light cone.
zamalek 13y ago
Agreed. StackExchange doesn't like conjecture, but here is my logical argument in support of your conclusion. Let's assume that an argument is made asserting that quantum mechanics are not probabilistic and are actually deterministic over the entire area of the universe. The reason we see QM as a probabilistic feature of the universe is because our "determinism reference frame" is incomplete. For example, within in the room you are sitting in right now there is a determinism reference frame that indicates that when you press a key on your keyboard a letter will appear on your monitor - that reference frame is actually incomplete. Half way across the galaxy a star could explode, the "cosmic ray" from that star could interact with our atmosphere and ultimately flip a bit in your RAM[1] - causing a kernel panic in the worst case scenario. Computers are only deterministic machines if the determinism reference frame does not include the entire universe. This means to assert that something in our reality is truly deterministic we need to have perfect information about the entirety of our reality (I specifically avoid the word "universe" in that statement). The next piece of information that is important is that physicists believe that our universe may have been formed within a Higg's Field (which is why the hunt for a Higg's boson is such an interesting area of physics). This means that there is plausibly a volume or something (not necessarily a universe itself) within which our universe was formed. Conjecture: is this encompassing "metareality" the reason we have non-determinism in quantum mechanics? Is our universe simply a "room" within another reality? Keep in mind, I am avoiding multiverse, the Higg's Field doesn't necessarily have to be compatible with the multiverse theory and it's hard to make the analogy I am trying to make without it sounding like I am venturing into that territory. The problem is that we can only observe our own universe, if our universe does exist within another reality (which the existence of the Higg's boson points toward) it means that our universe is ultimately non-deterministic. It would be impossible to determine if that metareality is deterministic because we cannot observe it. Therefore, there is a good chance that the universe is not deterministic. It is impossible to tell if reality is deterministic. Edit: and then there is this [2]. Determinism is very likely a human concept that has absolutely no meaning in terms of reality, it's a convenient simplification just like Newtonian physics. [1]: http://www.statemaster.com/encyclopedia/Single_event-upset http://www.statemaster.com/encyclopedia/Single_event-upset [2]: http://www.theguardian.com/science/2000/jul/20/technology2 http://www.theguardian.com/science/2000/jul/20/technology2
guygurari 13y ago
> In the answers I do not see any mention of the Second Time Around Problem, which conclusively answers the question with "there is no way to know". This argument is not interesting, because "there is no way to know" is a correct but useless answer for basically any physical question. The best thing we know how to do is to look at the evidence that nature gives us, and try to construct a description of nature from them. We will have different confidence levels for different descriptions (e.g. deterministic vs. non-deterministic), based on how well they match the evidence, but we can never know for sure. Therefore the interesting question is how confident are we that nature is deterministic / non-deterministic. > But the other thing to point out is that this question presumes an old idea about the passage of time, which is that things happen in a sequence A, B, C, D, ... and that if you are at C then D "has not happened yet". But if you look at relativity, this appears to be a naïve viewpoint. In relativity, the notion you described is replaced by the weaker notion of causality. It says that for each observer things will still happen in a well-defined sequence, even though different observers will not agree on questions such as "did these two events occur simultaneously?". The way these statements can be consistent, is that events for which there is no agreement on ordering can never influence each other (this is a mathematical result). Due to causality, the question of whether nature is deterministic is well-defined even when relativity is taken into account.
- jblow 13y ago
  It sounds like you are arguing with me, but everything you said is agreeing with what I said, so I am not sure what to say here.
  - guygurari 13y ago
    Your first point is an argument that you say "conclusively answers the question". I claim this is correct but not interesting. Your second point is: > But the other thing to point out is that this question presumes an old idea about the passage of time, which is that things happen in a sequence A, B, C, D, ... and that if you are at C then D "has not happened yet". But if you look at relativity, this appears to be a naïve viewpoint. Again, this is true but irrelevant to the question at hand, due to causality. I claim that both points are irrelevant. How is that agreeing with you?
colanderman 13y ago
On the flip side, assume it is deterministic. Do we have any means of reconstructing a previous state of the universe in order to test this hypothesis? Hells no. Even assuming we can cordon off a piece of the universe, can we measure/manipulate all the state of that piece of universe? Hence a better question is, "is the universe deterministic, and free of nonlocal effects and hidden state?"
JackC 13y ago
Thanks for throwing in some neat ideas. Doesn't your Second Time Around Problem conclusively answer _every_ question with "there is no way to know," though? For example: is there really gravity? Suppose there was a universe with gravity. We let that universe run for its lifetime and record everything that happens. Then we play back that recording in a universe where stuff (matter/energy/etc) is not affected by gravity, but instead behaves according to a recording. The two are identical. As another example: did I really write this comment? Perhaps there was another universe where this comment was written, but _this_ universe is just a second-time-around recording where playback was started right after alternate-me hit post. Anyway: the determinism question folks are talking about is, does the current state of the universe dictate the next state? In your playback universe, the answer is no -- you could edit any "frame" arbitrarily without affecting the frames that follow.
hrjet 13y ago
> the "now" on planets across the galaxy is going back and forth by thousands or millions of years." I think you are misinterpreting something. If this were true, you would be able to observe a supernova and play it back in time by simply shifting your nose (or camera) around. Which is evidently not true.
ars 13y ago
> In the answers I do not see any mention of the Second Time Around Problem, which conclusively answers the question with "there is no way to know". No it doesn't. You are assuming the existence of a recording device, and worse excluding the recording device from your definition of universe. > Then make a deterministic universe that just plays back the recording (this is the "second time around"). What does play back even mean in this context? When you play back a recording the events in it do not actually occur again. All you are doing is perusing a memory. Your recoding doesn't even need to simulate the laws of physics - it simply places objects where they were - if you wanted to you could make any changes you like. So you could make the recording do anything. i.e. a recording is meaningless - it's not actually a universe. (By universe I mean something that follows a defined set of rules.) > From the viewpoint of someone living inside the universe, there is no way to tell whether it is the first time around or the second time around. In the the second time around they are not actually living in the universe. > make a faraway point "go forward or backward in time" No you can't. You can make it traverse time faster or slower, but you can not make it go forward and backward. > The crazy thing is that for angular movements the relative speed is amplified by distance, so when you are moving around at everyday speeds the "now" on planets across the galaxy is going back and forth by thousands or millions of years. That is simply not true. Angular movement has exactly zero effect on relativity - it's not true motion which requires a relative difference in energy (AKA speed).
- gtr32x 13y ago
  > (By universe I mean something that follows a defined set of rules.) I just want to make a mention for this point. Ahh but you see, for all we know, the universe could have simply followed 'a set of rules' up till the point we are living at and until a set future point in the 'timeline' (or something we perceive as the timeline). Then all of a sudden all the laws we know will break because the 'universe' entity decides to play another recording. Then all of knowledge of the 'universe' collapses. I think this is what the parent was talking about. You cannot predict nor even conclude that such scenario cannot happen.
  - ars 13y ago
    In that situation the universe is not following a defined set of rules, it is following an arbitrary set of rules. Even during the initial playback period.

by jblow 13y ago

siddboots 13y ago

That argument is, roughly, Humean scepticism. Science is a game we play where we do experiments, build theories, do more experiments, and so on. Any scientific answer to how the universe works has to start with the assumption that the nature of universe can be understood in the first place (otherwise we'll never get anywhere.) Edit: This was in response to your comment prior to edit. Up to "..."

eli_gottlieb 13y ago
Well yes, which is why we can't really ever know. It's somewhere between difficult and impossible to prove the axiom of your entire epistemological process.

paraschopra 13y ago

This is weird. If the observer in the universe is able to make accurate predictions consistently and repeatedly, doesn't that give confidence to his beliefs that the universe is deterministic. What you might be referring to is another way of saying that "any theory can be proven wrong if and when we observe a data point negating it". So, of course, there is no way of knowing if the universe is __truly__ deterministic or not, but then again we can say the same thing about sun rising from the east (unlikely as it may be, it might well just not rise tomorrow at all and then we will have to revise our theories).

jblow 13y ago
Did you read the OP? The whole question is about whether the apparent randomness in quantum mechanics means the universe is fundamentally nondeterministic. We cannot make accurate predictions of measurements of quantum state apart from saying things like "A will happen with some probability, otherwise B will happen". It is that "with some probability" that is the question. (Yes, at the level of the macroscopic world we can make consistently accurate predictions, but physicists would say this is because we are operating at a scale where the statistical nature of quantum mechanics averages out, etc. Unless you are Carver Mead or one of the other wave guide kinds of guys who believe there actually is no randomness in QM and it is deterministic all the way down.)
crististm 13y ago
Double slit experiment shows that you can't predict which slit the particles are going to go. This is pretty much non-determinism to me

no_gravity 13y ago

2 things: 1) If we define "universe" as "everything that exists" then you cannot record everything that happens. Because the record would be part of the universe and would have to recorded and the record of the record would have to be recorded... 2) The "first" and "second" universes would not be universes on their own. The "second" is a continuation of what you call the "first". So the system you describe would be random. Because the "deterministic" things that happen later on are based on the random things that happened earlier.

jblow 13y ago
Just pause for a moment on "everything that exists" because that is not necessarily a useful distinction. If you think of a universe as "everything I could see/touch/etc given infinite time", someone outside the universe who was in control of it might record it with no problem because they are not subject to the constraints of the universe. e.g. imagine the universe as being like a computer simulation that you programmed. You can probably pause it in a debugger any time you want, look at whatever state variables you want, etc. So in this case the definition cannot be "everything that exists" because you have to define "exists" in the case where different sets of things may have different levels of "reality".
- no_gravity 13y ago
  Lets say we are in a "game of life" simulation on a computer. It has been proven turing complete. So you would say its deterministic because "game of life" has deterministic rules. But if the computer that runs the simulation is in a random universe, then it might happen that bits of the computer switch in a random fashion. Making our universe inside the "game of life" simulation random too. And we could touch the randomly flipping bits. So there is no point in saying something is "outside" our universe and something is "inside".
  - jblow 13y ago
    There is no point to saying something is "outside" and something is "inside" in the extreme cases of the example you just gave, which may or may not be the case. What if the outer universe is not a random universe? What if its laws of nature are so unlike ours that we do not know what to say about it? (Maybe it does not have space or time, but it has other things instead.) But I think this is a little bit beside the point, because as I mentioned in my first post, all of this is predicated on us adopting the naïve view of time in our own universe, which is maybe not the best idea, given that we have plenty of evidence otherwise. (And anyway, if you believe that "everything that exists" can be infinite, then there is no sense in saying our universe is being produced by some kind of simulation controlled in one particular other place, because of course it is, but actually this is probably happening in an infinite number of different ways from different places such that there is not really point any more in claiming that this is happening at all; the situation becomes such that you could just draw a relation between situation X and situation Y and could state that one could be causal of the other from a certain point of view.)

nhaehnle 13y ago

> when you are moving around at everyday speeds the "now" on planets across the galaxy is going back and forth by thousands or millions of years This feels like it is either entirely incorrect, or you're taking something that is correct and interpreting it in a very misleading way. Your remark about angular movements makes me think that perhaps you don't consider the fact that, when measured from a very distant planet, any angular movements we make in our everyday life are extremely tiny?

jblow 13y ago
I first learned this in a book by a prominent physicist, so I am inclined to think it is an accurate reading of the physics. Unfortunately I do not remember which so I cannot give you a citation (it was possibly Brian Greene or Frank Wilczek), but maybe there will be a physicist reading this thread who can chime in. The angular movement thing is this: Imagine you have a frame of reference on the tip of your nose. The X axis points straight away from your face, the Y axis is to your left, Z is up. Now start turning your head to the left. To a tiny observer living on the tip of your nose, the relative speed along the Y axis of a faraway planet has suddenly become very high. The further the planet, the faster the speed (this part is just grade-school geometry).
- jblow 13y ago
  But let me also just say that you do not have to believe this at all in order to believe my original point; I just brought it up as an extreme crazy case. So if you don't believe the extreme crazy case, think of the standard example: you get into a space ship or something and zip around really fast. And you are thinking about things closer to you. The math says the same thing: as your light cone changes, the set of spacetime intervals, that have time distance 0 from you, changes as well. So from your relative position the "now" at these faraway points goes back and forth. This is basic, basic relativity.
  - jblow 13y ago
    And actually, yeah, forget about the relative speed, since that is not central to the real issue. The real issue is just that when your light cone changes, the set of points that are simultaneous with you changes (all these points are outside your light cone except for the one you occupy). You cannot observe them directly because they are outside your light cone.

zamalek 13y ago

Agreed. StackExchange doesn't like conjecture, but here is my logical argument in support of your conclusion. Let's assume that an argument is made asserting that quantum mechanics are not probabilistic and are actually deterministic over the entire area of the universe. The reason we see QM as a probabilistic feature of the universe is because our "determinism reference frame" is incomplete. For example, within in the room you are sitting in right now there is a determinism reference frame that indicates that when you press a key on your keyboard a letter will appear on your monitor - that reference frame is actually incomplete. Half way across the galaxy a star could explode, the "cosmic ray" from that star could interact with our atmosphere and ultimately flip a bit in your RAM[1] - causing a kernel panic in the worst case scenario. Computers are only deterministic machines if the determinism reference frame does not include the entire universe. This means to assert that something in our reality is truly deterministic we need to have perfect information about the entirety of our reality (I specifically avoid the word "universe" in that statement). The next piece of information that is important is that physicists believe that our universe may have been formed within a Higg's Field (which is why the hunt for a Higg's boson is such an interesting area of physics). This means that there is plausibly a volume or something (not necessarily a universe itself) within which our universe was formed. Conjecture: is this encompassing "metareality" the reason we have non-determinism in quantum mechanics? Is our universe simply a "room" within another reality? Keep in mind, I am avoiding multiverse, the Higg's Field doesn't necessarily have to be compatible with the multiverse theory and it's hard to make the analogy I am trying to make without it sounding like I am venturing into that territory. The problem is that we can only observe our own universe, if our universe does exist within another reality (which the existence of the Higg's boson points toward) it means that our universe is ultimately non-deterministic. It would be impossible to determine if that metareality is deterministic because we cannot observe it. Therefore, there is a good chance that the universe is not deterministic. It is impossible to tell if reality is deterministic. Edit: and then there is this [2]. Determinism is very likely a human concept that has absolutely no meaning in terms of reality, it's a convenient simplification just like Newtonian physics. [1]: http://www.statemaster.com/encyclopedia/Single_event-upset http://www.statemaster.com/encyclopedia/Single_event-upset [2]: http://www.theguardian.com/science/2000/jul/20/technology2 http://www.theguardian.com/science/2000/jul/20/technology2

guygurari 13y ago

> In the answers I do not see any mention of the Second Time Around Problem, which conclusively answers the question with "there is no way to know". This argument is not interesting, because "there is no way to know" is a correct but useless answer for basically any physical question. The best thing we know how to do is to look at the evidence that nature gives us, and try to construct a description of nature from them. We will have different confidence levels for different descriptions (e.g. deterministic vs. non-deterministic), based on how well they match the evidence, but we can never know for sure. Therefore the interesting question is how confident are we that nature is deterministic / non-deterministic. > But the other thing to point out is that this question presumes an old idea about the passage of time, which is that things happen in a sequence A, B, C, D, ... and that if you are at C then D "has not happened yet". But if you look at relativity, this appears to be a naïve viewpoint. In relativity, the notion you described is replaced by the weaker notion of causality. It says that for each observer things will still happen in a well-defined sequence, even though different observers will not agree on questions such as "did these two events occur simultaneously?". The way these statements can be consistent, is that events for which there is no agreement on ordering can never influence each other (this is a mathematical result). Due to causality, the question of whether nature is deterministic is well-defined even when relativity is taken into account.

jblow 13y ago
It sounds like you are arguing with me, but everything you said is agreeing with what I said, so I am not sure what to say here.
- guygurari 13y ago
  Your first point is an argument that you say "conclusively answers the question". I claim this is correct but not interesting. Your second point is: > But the other thing to point out is that this question presumes an old idea about the passage of time, which is that things happen in a sequence A, B, C, D, ... and that if you are at C then D "has not happened yet". But if you look at relativity, this appears to be a naïve viewpoint. Again, this is true but irrelevant to the question at hand, due to causality. I claim that both points are irrelevant. How is that agreeing with you?

colanderman 13y ago

On the flip side, assume it is deterministic. Do we have any means of reconstructing a previous state of the universe in order to test this hypothesis? Hells no. Even assuming we can cordon off a piece of the universe, can we measure/manipulate all the state of that piece of universe? Hence a better question is, "is the universe deterministic, and free of nonlocal effects and hidden state?"

JackC 13y ago

Thanks for throwing in some neat ideas. Doesn't your Second Time Around Problem conclusively answer _every_ question with "there is no way to know," though? For example: is there really gravity? Suppose there was a universe with gravity. We let that universe run for its lifetime and record everything that happens. Then we play back that recording in a universe where stuff (matter/energy/etc) is not affected by gravity, but instead behaves according to a recording. The two are identical. As another example: did I really write this comment? Perhaps there was another universe where this comment was written, but _this_ universe is just a second-time-around recording where playback was started right after alternate-me hit post. Anyway: the determinism question folks are talking about is, does the current state of the universe dictate the next state? In your playback universe, the answer is no -- you could edit any "frame" arbitrarily without affecting the frames that follow.

hrjet 13y ago

> the "now" on planets across the galaxy is going back and forth by thousands or millions of years." I think you are misinterpreting something. If this were true, you would be able to observe a supernova and play it back in time by simply shifting your nose (or camera) around. Which is evidently not true.

ars 13y ago

> In the answers I do not see any mention of the Second Time Around Problem, which conclusively answers the question with "there is no way to know". No it doesn't. You are assuming the existence of a recording device, and worse excluding the recording device from your definition of universe. > Then make a deterministic universe that just plays back the recording (this is the "second time around"). What does play back even mean in this context? When you play back a recording the events in it do not actually occur again. All you are doing is perusing a memory. Your recoding doesn't even need to simulate the laws of physics - it simply places objects where they were - if you wanted to you could make any changes you like. So you could make the recording do anything. i.e. a recording is meaningless - it's not actually a universe. (By universe I mean something that follows a defined set of rules.) > From the viewpoint of someone living inside the universe, there is no way to tell whether it is the first time around or the second time around. In the the second time around they are not actually living in the universe. > make a faraway point "go forward or backward in time" No you can't. You can make it traverse time faster or slower, but you can not make it go forward and backward. > The crazy thing is that for angular movements the relative speed is amplified by distance, so when you are moving around at everyday speeds the "now" on planets across the galaxy is going back and forth by thousands or millions of years. That is simply not true. Angular movement has exactly zero effect on relativity - it's not true motion which requires a relative difference in energy (AKA speed).

gtr32x 13y ago
> (By universe I mean something that follows a defined set of rules.) I just want to make a mention for this point. Ahh but you see, for all we know, the universe could have simply followed 'a set of rules' up till the point we are living at and until a set future point in the 'timeline' (or something we perceive as the timeline). Then all of a sudden all the laws we know will break because the 'universe' entity decides to play another recording. Then all of knowledge of the 'universe' collapses. I think this is what the parent was talking about. You cannot predict nor even conclude that such scenario cannot happen.
- ars 13y ago
  In that situation the universe is not following a defined set of rules, it is following an arbitrary set of rules. Even during the initial playback period.