- #1
Lynch101
Gold Member
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- TL;DR Summary
- I've learned about relativity through pop-science and so I had come to associate it with the Block Universe. Now, it seems I need to try and unlearn it, so I have lots of questions.
Just a heads up, this post is quite long. I've tried to be as detailed as possible from the outset because I find it can help avoid the need to clarify things later, or helps when clarifying things later. There is only one question posed at the end, but I think it might be useful to read the body of the post to give the full context.
Where I'm coming from
Firstly, I just want to outline where I'm coming from. The route, so to speak, of how I've come to learn (what little I know) about relativity. It's somewhat of a disclaimer, but I think it might help to give an idea of where I am at now.
My first encounter with relativity was through pop-science videos and books. That prompted me to do further reading and discussing of the subject. To cut a long story short, most of my learning was in the context of the Block Universe, so I have come to associate relativity with the Block Universe.
I believe that, although having struggled with the main concepts [of relativity of simultaneity, time dilation, and length contraction] I now have a relatively (no pun in 10 did) decent grasp on them, for a person without a background in mathematics or physics. That is, I understand them conceptually but cannot necessarily calculate solutions to mathematical problems. Where I kept tripping up and thinking there must be some contradiction, I am now able to reason through issues and understand how the different aspects intertwine to "resolve" apparent paradoxes.
Just as I thought I had a decent grasp of it, I come across this article by @PeterDonis telling me that everything I thought I knew was wrong! Well, not quite everything, thankfully, rather that the Block Universe is not a necessity of relativity. It had been presented as such in the materials I had encountered and the discussions I have had, to the extent that I had come to assimiltate this into my own understanding. Now, I must try and unlearn it, or at least see why it isn't a necessity.
I will probably be referencing statements from this thread, related to Peter's article as well as Peter's article itself. I will try to outline the understanding that I have arrived at, through my engagement with different material and discussions. This will probably involve putting forward the arguments that have been put to me, which helped me develop my understanding. This will probably appear as though I am trying to defend the Block Universe but that is not my intention. By putting forward the arguments as I understand them and having the issues pointed out, I find I can develop a deeper understanding than when I simply read something and try to assimilate it myself.
Right! That was long-winded enough. So, if you're still here...thank you...and I'll get to it.Start From Now
The best place to start is probably where I started myself, with an idea that is captured by this comment from Peter in this thread.
When I approached relativity I had a more Newtonian view, so to speak. The notion I had was that of a deterministic Universe where everything existed in a single, universal present moment, a global "now". A more accurate picture would perhaps be that the configuration of all the particles in the Universe constituted this "now", even if they couldn't all be observed. This now was the only moment in which things happened, or could happen. Again, even if we couldn't say what all those events were, the Universal present ensured that there was only one moment in which they could happen, and that was the moment we experience as "now". A feature, or consequence, of this universal present moment was that past or future events, or configurations of the Universe which we consider to be the past or future, were imaginary, while only the present was real - in any sense of the word.
When I encountered relativity, I found that Einstein's theory overturned this limited view of the world. As Peter succinctly put it, there is no global concept of "now" in relativity. From what I learned, relativity necessitated that past and future events be as "real" as those of our present. I know "real" can prove to be a difficult term to pin down, but I was told that it simply meant that the present, which we experience, has the same ontological status as the past and the future.
This was explained in conjunction with the concept of world lines in spacetime, where the locus of all events that make up our history (and that of every object) extend through spacetime as world lines. This meant that, within the structure of the Universe, all the events in our history or the history of any object co-existed in a 4D Minkowski spacetime structure. In this picture our past, present, and future events are all equally "real". Contrast this with the Newtonian picture which says that only our present event is real i.e. us in the present moment.
An analogy that I often encountered was that of a reel of film, the likes that would be used in old-school projectors. If we imagine the film of our lives stretching out from our birth to our death, then this represents our world line, which would exist in spacetime. While our experience is more akin to watching the movie play out on screen - or perhaps more like the original filming of the movie - this is just an illusion. In the block structure, our world line stretches out like this, where each frame enjoys the same ontological status of every other frame. That is they are all equally "real" (no pun intended). That term "real" might be somewhat nebulous, but whatever it is, it applies equally to all the frames on the reel of film, or to all the events that make up our world line.
I struggled for a long time to get my head around many of the consequences of relativity, but over time it began to make more and more sense, including this Block Universe picture, to the point where I now struggle to see how it isn't necessitated by relativity.First Question
So, finally! To the first question.
If the relativity of simultaneity overturned the Newtonian idea of a universal, or global "now", how is it possible to have such a relativistic Universe where past and future states or configurations aren't as "real" as the present?
I would be inclined to think that, any departure from this Newtonian idea of a universal present moment, must necessitate the existence of past and future configurations of the Universe, in such a way that those configurations are equally as "real" as the configuration of the present. In terms of the analogy of the reel of film, while the content of each frame might be different, each frame is made from the same kind of "stuff".
If such past and future states don't exist, then wouldn't we be left with a global "now" or universal present, by way of necessity? A global "now" similar to (but not necessarily the same as) the universal present of Newtonian mechanics?
Where I'm coming from
Firstly, I just want to outline where I'm coming from. The route, so to speak, of how I've come to learn (what little I know) about relativity. It's somewhat of a disclaimer, but I think it might help to give an idea of where I am at now.
My first encounter with relativity was through pop-science videos and books. That prompted me to do further reading and discussing of the subject. To cut a long story short, most of my learning was in the context of the Block Universe, so I have come to associate relativity with the Block Universe.
I believe that, although having struggled with the main concepts [of relativity of simultaneity, time dilation, and length contraction] I now have a relatively (no pun in 10 did) decent grasp on them, for a person without a background in mathematics or physics. That is, I understand them conceptually but cannot necessarily calculate solutions to mathematical problems. Where I kept tripping up and thinking there must be some contradiction, I am now able to reason through issues and understand how the different aspects intertwine to "resolve" apparent paradoxes.
Just as I thought I had a decent grasp of it, I come across this article by @PeterDonis telling me that everything I thought I knew was wrong! Well, not quite everything, thankfully, rather that the Block Universe is not a necessity of relativity. It had been presented as such in the materials I had encountered and the discussions I have had, to the extent that I had come to assimiltate this into my own understanding. Now, I must try and unlearn it, or at least see why it isn't a necessity.
I will probably be referencing statements from this thread, related to Peter's article as well as Peter's article itself. I will try to outline the understanding that I have arrived at, through my engagement with different material and discussions. This will probably involve putting forward the arguments that have been put to me, which helped me develop my understanding. This will probably appear as though I am trying to defend the Block Universe but that is not my intention. By putting forward the arguments as I understand them and having the issues pointed out, I find I can develop a deeper understanding than when I simply read something and try to assimilate it myself.
Right! That was long-winded enough. So, if you're still here...thank you...and I'll get to it.Start From Now
The best place to start is probably where I started myself, with an idea that is captured by this comment from Peter in this thread.
PeterDonis said:This also means there is no global concept of "now" in relativity.
When I approached relativity I had a more Newtonian view, so to speak. The notion I had was that of a deterministic Universe where everything existed in a single, universal present moment, a global "now". A more accurate picture would perhaps be that the configuration of all the particles in the Universe constituted this "now", even if they couldn't all be observed. This now was the only moment in which things happened, or could happen. Again, even if we couldn't say what all those events were, the Universal present ensured that there was only one moment in which they could happen, and that was the moment we experience as "now". A feature, or consequence, of this universal present moment was that past or future events, or configurations of the Universe which we consider to be the past or future, were imaginary, while only the present was real - in any sense of the word.
When I encountered relativity, I found that Einstein's theory overturned this limited view of the world. As Peter succinctly put it, there is no global concept of "now" in relativity. From what I learned, relativity necessitated that past and future events be as "real" as those of our present. I know "real" can prove to be a difficult term to pin down, but I was told that it simply meant that the present, which we experience, has the same ontological status as the past and the future.
This was explained in conjunction with the concept of world lines in spacetime, where the locus of all events that make up our history (and that of every object) extend through spacetime as world lines. This meant that, within the structure of the Universe, all the events in our history or the history of any object co-existed in a 4D Minkowski spacetime structure. In this picture our past, present, and future events are all equally "real". Contrast this with the Newtonian picture which says that only our present event is real i.e. us in the present moment.
An analogy that I often encountered was that of a reel of film, the likes that would be used in old-school projectors. If we imagine the film of our lives stretching out from our birth to our death, then this represents our world line, which would exist in spacetime. While our experience is more akin to watching the movie play out on screen - or perhaps more like the original filming of the movie - this is just an illusion. In the block structure, our world line stretches out like this, where each frame enjoys the same ontological status of every other frame. That is they are all equally "real" (no pun intended). That term "real" might be somewhat nebulous, but whatever it is, it applies equally to all the frames on the reel of film, or to all the events that make up our world line.
I struggled for a long time to get my head around many of the consequences of relativity, but over time it began to make more and more sense, including this Block Universe picture, to the point where I now struggle to see how it isn't necessitated by relativity.First Question
So, finally! To the first question.
If the relativity of simultaneity overturned the Newtonian idea of a universal, or global "now", how is it possible to have such a relativistic Universe where past and future states or configurations aren't as "real" as the present?
I would be inclined to think that, any departure from this Newtonian idea of a universal present moment, must necessitate the existence of past and future configurations of the Universe, in such a way that those configurations are equally as "real" as the configuration of the present. In terms of the analogy of the reel of film, while the content of each frame might be different, each frame is made from the same kind of "stuff".
If such past and future states don't exist, then wouldn't we be left with a global "now" or universal present, by way of necessity? A global "now" similar to (but not necessarily the same as) the universal present of Newtonian mechanics?
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