[Dimensional (OP)]

Here may be a clearer way to see my issue.  Imagine a very simple universe where there only exists object A and an object B.  They are on a collision course.  Scientist A, (from another dimension) uses object A as a point of reference.  But scientist B (from yet a different dimension than scientist A) uses object B as a reference.  Each scientist is going to end up with a different description of this universe from using special relativity.

So what's your issue?

That each scientist is going to end up with a different description of this universe from using special relativity.

[Eternal Student]

Hi.

That each scientist is going to end up with a different description of this universe from using special relativity.

     Yes, exactly this sort of thing does happen.
    There are some questions you can ask which are going to be frame dependant -  they have answers but the answers will depend on the frame of reference you have chosen.
    For example,  the question  "is object A moving?"  is something that the two scientists will not agree on.   Hopefully they will know the answer is frame dependant so they will answer more cautiously:   "Well, no, not in this frame of reference anyway".

    Fortunately, there are some questions that can be asked and answered more objectively:    "Will the two objects collide"  is something that both scientists will agree on.

Best Wishes.

[Dimensional (OP)]

Hi.

That each scientist is going to end up with a different description of this universe from using special relativity.

     Yes, exactly this sort of thing does happen.
    There are some questions you can ask which are going to be frame dependant -  they have answers but the answers will depend on the frame of reference you have chosen.
    For example,  the question  "is object A moving?"  is something that the two scientists will not agree on.   Hopefully they will know the answer is frame dependant so they will answer more cautiously:   "Well, no, not in this frame of reference anyway".

    Fortunately, there are some questions that can be asked and answered more objectively:    "Will the two objects collide"  is something that both scientists will agree on.

Best Wishes.

But when looking at my scenario, SR seems to give two different answers.  Since either object can be considered the object at rest, then each scientist is going to get a different answer.

[Origin]

But when looking at my scenario, SR seems to give two different answers.  Since either object can be considered the object at rest, then each scientist is going to get a different answer.

I don't really know how many different ways it can be explained to you that there ARE 2 different answers and both of the answers are correct for the frames they are measured in.  I understand that this goes against our intuition, but sometimes our intuition is not correct.  Our intuition tells us that if we are moving towards a light then the speed of that light should be c + our speed, but that is wrong.  We can measure the speed and it is a fact that our intuition is wrong.

[Colin2B]

Here may be a clearer way to see my issue.  Imagine a very simple universe where there only exists object A and an object B.  They are on a collision course.  Scientist A, (from another dimension) uses object A as a point of reference.  But scientist B (from yet a different dimension than scientist A) uses object B as a reference.  Each scientist is going to end up with a different description of this universe from using special relativity.

So what's your issue?

That each scientist is going to end up with a different description of this universe from using special relativity.

This really doesn’t need scientists from a different universe and it isn’t down to special relativity.
It was Galileo who first pointed out that if you were in a cabin on a ship you would not be able to tell if the ship was moving, or in which direction unless you had information from outside the room. He concluded that it was reasonable to consider two moving ships as one stationary and the other moving or vise versa and the laws of physics remain the same. For example, if a ship is moving and firing a cannon at a stationary ship, the trajectory of the cannon ball relative to the first ship is exactly the same if the first ship is stationary and the second moving. When we consider this scenario in space where there are no fixed references, it becomes more obvious. It is called Galilean Relativity. Einstein added the effects of the constancy of the speed of light and extended Galileo’s principle to Special Relativity.

[Dimensional (OP)]

But when looking at my scenario, SR seems to give two different answers.  Since either object can be considered the object at rest, then each scientist is going to get a different answer.

I don't really know how many different ways it can be explained to you that there ARE 2 different answers and both of the answers are correct for the frames they are measured in.  I understand that this goes against our intuition, but sometimes our intuition is not correct.  Our intuition tells us that if we are moving towards a light then the speed of that light should be c + our speed, but that is wrong.  We can measure the speed and it is a fact that our intuition is wrong.

If both answers are correct, then doesn't that mean that 2 different universes can exist depending on which object you choose to be at rest?

More specifically, if A is chosen to follow the direction of the time dimension, or, "be only in time" (I can't remember the proper wording; it's been a long time since I took linear algebra), then B will have a spatial component and temporal component, and vice versa.

[Dimensional (OP)]

Here may be a clearer way to see my issue.  Imagine a very simple universe where there only exists object A and an object B.  They are on a collision course.  Scientist A, (from another dimension) uses object A as a point of reference.  But scientist B (from yet a different dimension than scientist A) uses object B as a reference.  Each scientist is going to end up with a different description of this universe from using special relativity.

So what's your issue?

That each scientist is going to end up with a different description of this universe from using special relativity.

This really doesn’t need scientists from a different universe and it isn’t down to special relativity.
It was Galileo who first pointed out that if you were in a cabin on a ship you would not be able to tell if the ship was moving, or in which direction unless you had information from outside the room. He concluded that it was reasonable to consider two moving ships as one stationary and the other moving or vise versa and the laws of physics remain the same. For example, if a ship is moving and firing a cannon at a stationary ship, the trajectory of the cannon ball relative to the first ship is exactly the same if the first ship is stationary and the second moving. When we consider this scenario in space where there are no fixed references, it becomes more obvious. It is called Galilean Relativity. Einstein added the effects of the constancy of the speed of light and extended Galileo’s principle to Special Relativity.

I would like to know how the components of time vs space work in SR.  It seems, from where I am at in my knowledge, that the temporal component and the spatial component can swap, aka fabric of spacetime.  Sure that's fine if it gives us correct results for what we need to know now, but if it doesn't make sense objectively, then it is only a matter of time before SR/GR gives false answers and paradoxes, which it already has.

[Eternal Student]

Hi.

If both answers are correct, then doesn't that mean that 2 different universes can exist depending on which object you choose to be at rest?

    Does there need to be two universes just because there's two different ways of describing it?

You can go outside and measure your bicycle and find it's 1 metre long.
I can go outside and measure your bicycle and find it's 39 inches long.

Presumably, you wouldn't have thought that there must be two different universes, one where the bicycle is 1 metre and the other where it is 39 inches.   You'd probably be quite happy to accept that I was using inches and you were using metres, we were just using different measurements to describe the same physical thing.

    However, if what you meant was that (for at least some things) there's no objective reality, then I'd agree with you. 
None-the-less, at the moment (as described by Colin2B) you haven't really described a situation that required much more than simple Gallilean relativity and it might be best to just push complications from special relativity to one side for a moment.   It's not important here.

More specifically, if A is chosen to follow the direction of the time dimension, or, "be only in time" (I can't remember the proper wording; it's been a long time since I took linear algebra)

    I think this could be a problem or misunderstanding.   You could choose to make object A stationary and that would make the spatial component of its 4-velocity 0.   However  0  is something, it is a numerical value.   The object still has an ordinary spatial velocity, it's just that the magnitude of that velocity is 0.   I probably need to rephrase this - the object is always found somewhere in space it doesn't vanish and exist "only in time" or anything weird like that.    The object is found at the same place in space at all times, it has 0 velocity which just means it's location doesn't change with time.   
    The spatial 3-velocity is  (0, 0, 0)      it is not   ( n/a,  n/a,  n/a )

Best Wishes.

[Eternal Student]

Hi again.

I would like to know how the components of time vs space work in SR.  It seems, from where I am at in my knowledge, that the temporal component and the spatial component can swap, aka fabric of spacetime.

   It is a fascinating topic and I'm glad you're interested.  I'm sure people can recommend references and texts.    We might need a little more guidance as to what your current level of experience is.   It doesn't matter, by the way, none of us were born knowing anything about SR.  More-over, you're going to need to be the one giving me some good references in a few years, please.

Here's an example:
 
Pre-requisite:   Someone who studied some Maths and Physics at school to about age 17.    For example a United Kingdom AS level.

The resource:   Freely available YT videos of lectures presented by Prof. Leonard Susskind of Stanford University.     

Time required:    There are many (I think 10)  lectures although much more than just SR is covered.   Each lecture is about 1.5 hours.    You could sensibly skip some introductions (you won't care about essay deadlines etc.),   however, realistically you should expect to watch several hours of lectures and give yourself some extra time to think through and try some problems yourself.

Alternatives include some good textbooks.   Much faster alternatives include assorted "Pop Sci" videos which will race over the topics in under 20 minutes - but that's never going to offer the full understanding.

Anyway.... the main point is that I couldn't present SR any better than the existing texts and resources for the topic and I'm also fairly sure a forum is not ideal for the task.   However, if there were some specific issues you wanted to discuss then I'm sure people on this forum will try and help.

Best Wishes.

[Dimensional (OP)]

Hi.

If both answers are correct, then doesn't that mean that 2 different universes can exist depending on which object you choose to be at rest?

    Does there need to be two universes just because there's two different ways of describing it?

You can go outside and measure your bicycle and find it's 1 metre long.
I can go outside and measure your bicycle and find it's 39 inches long.

Presumably, you wouldn't have thought that there must be two different universes, one where the bicycle is 1 metre and the other where it is 39 inches.   You'd probably be quite happy to accept that I was using inches and you were using metres, we were just using different measurements to describe the same physical thing.

    However, if what you meant was that (for at least some things) there's no objective reality, then I'd agree with you. 
None-the-less, at the moment (as described by Colin2B) you haven't really described a situation that required much more than simple Gallilean relativity and it might be best to just push complications from special relativity to one side for a moment.   It's not important here. 

I think this is outside of the scope of Galilean relativity because nothing is actually moving in GR.  GR implies a block universe. 

So the idea of an object to be moving relative to another object doesn't really make any sense in a block universe.

   

I think this could be a problem or misunderstanding.   You could choose to make object A stationary and that would make the spatial component of its 4-velocity 0.   However  0  is something, it is a numerical value.   The object still has an ordinary spatial velocity, it's just that the magnitude of that velocity is 0.   I probably need to rephrase this - the object is always found somewhere in space it doesn't vanish and exist "only in time" or anything weird like that.    The object is found at the same place in space at all times, it has 0 velocity which just means it's location doesn't change with time.     
    The spatial 3-velocity is  (0, 0, 0)      it is not   ( n/a,  n/a,  n/a )

Best Wishes.

Yes, I certainly agree.  I tried to explain that I forgot the proper term of a vector that is only in a specific R.  Sorry for the confusion.

[Eternal Student]

Hi again,

I think this is outside of the scope of Galilean relativity because nothing is actually moving in GR.  GR implies a block universe.

   I'm easily confused.   GR is usually an abbreviation of "General Relativity" but I'm going to assume you are using it as an abbreviation of Galilean Relativity.

   I'm not really sure that Galilean Relativity does fully demand the idea of a block universe.   I think we usually forumlate the notion of a block universe after we accept Special Relativity.   However, the fine details about what a "block universe" means may not be all that important.    Just simple Newtonian mechanics already suggests something we would call a "deterministic universe".   This just means that, if Newtonian mechanics is correct, then you should be able to calculate where everything will be and how it is moving at any time in the future  from knowledge about where it is and how it is moving now.    Similarly, you should also be able to calculate where it was and how it was moving at any point in the past  from knowledge about the present.    Overall then, if you had knowledge about the present than it's just a matter of doing some calculations - there's nothing uncertain about the future or past, you would have full knowledge of that.
    A deterministic universe doesn't necessarily mean that the future has already happened, just that that there's nothing especially uncertain or undetermined about it.

    Special Relativity (SR)  probably pushes the notion of a deterministic universe a bit further.   One of the things a good text or lecture course about SR might discuss is the example of what is called the "Andromeda paradox" or Rietdijk–Putnam–Penrose argument  ( https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument ).    These sorts of topics are one of the ways in which you could build up to a more complete notion of a block universe.  Specifically, where you could start to argue that all instants of time and space seem to need to exist.   It's worth mentioning that although these things often have names like "paradox" it's just because Physicist's and Philosopher's have good publicity and media agents and they select the titles.   They aren't usually impossible or logical paradoxes,  it's just that it sounds more interesting to call it a "paradox" instead of "an intersting thing".   
    Additionally, it's not entirely fair to say that the idea of a "Block Universe" is an idea in Physics.  It's an idea in Philosophy that is based on some ideas in Physics, although they often call it "Eternalism".  Physics doesn't really care if you favour a Block universe or Presentism, it just offers some models and allows some predictions to be made.   It's not offering any fundamental truth about how the universe really is, only a few models that seem to be useful.   More to the point, you can easily reconcile Special Relativity with Presentism if you try.   One option is to accept that when an object experiences an acceleration than the universe around them is changed.

    (Please don't get too worried about ideas like a deterministic universe - there's loads of things in physics that can offset or alleviate any concerns, like Chaos theory and quantum uncertainty but that falls outside the scope of this thread).

So the idea of an object to be moving relative to another object doesn't really make any sense in a block universe.

    Are you sure?   Movement is described just as a rate of change of one thing (position) with respect to something else (time).   So this is just a ratio of a small change in one variable to the corresponding small change in another variable.   Just because we, human beings, tend to experience and think of time a certain way, doesn't necessarily grant it any special nature.   It's just something, some variable, we can use to calculate rates of change with respect to.    We could have some other variable like S   - let's give it a silly name like "entropy of the universe" - and just calculate rates of change of position with respect to S.   
    Sometimes the thing we've chosen to calculate rates of change with respect to won't be all that useful.   Older scientists probably did think that determining rates of change with respect to the visible width of the moon could be useful - I mean it's a perfectly good rate of change to determine.   However, it just doesn't seem to be as useful for building mathematical models as determining rates of change of position with respect to something else.

Best Wishes.

[Colin2B]




I think this is outside of the scope of Galilean relativity because nothing is actually moving in GR.  GR implies a block universe. 

So the idea of an object to be moving relative to another object doesn't really make any sense in a block universe.

I think @Eternal Student  has answered this, but some personal views.
I tend to think of General Relativity (GR) as a chart such as used by a ship navigator. S/he can plot the ship’s current, past and future positions in both space and time, but it doesn’t imply that the ship still exists at all those positions. Alternatively you can think of your sat nav which shows your current position and past and future - compatible with a moving spotlight view of time.
Galilean Relativity is most definitely about movement and different views of that movement, but in no way does it imply different universes. If I’m watching a road race from one side of a road I might describe the competitors as moving left to right, whereas someone on the other side of the road would say right to left. One universe two viewpoints.

[Dimensional (OP)]

Hi again,

I think this is outside of the scope of Galilean relativity because nothing is actually moving in GR.  GR implies a block universe.

   I'm easily confused.   GR is usually an abbreviation of "General Relativity" but I'm going to assume you are using it as an abbreviation of Galilean Relativity.

   I'm not really sure that Galilean Relativity does fully demand the idea of a block universe.   I think we usually forumlate the notion of a block universe after we accept Special Relativity.   However, the fine details about what a "block universe" means may not be all that important.    Just simple Newtonian mechanics already suggests something we would call a "deterministic universe".   This just means that, if Newtonian mechanics is correct, then you should be able to calculate where everything will be and how it is moving at any time in the future  from knowledge about where it is and how it is moving now.    Similarly, you should also be able to calculate where it was and how it was moving at any point in the past  from knowledge about the present.    Overall then, if you had knowledge about the present than it's just a matter of doing some calculations - there's nothing uncertain about the future or past, you would have full knowledge of that.
    A deterministic universe doesn't necessarily mean that the future has already happened, just that that there's nothing especially uncertain or undetermined about it.

    Special Relativity (SR)  probably pushes the notion of a deterministic universe a bit further.   One of the things a good text or lecture course about SR might discuss is the example of what is called the "Andromeda paradox" or Rietdijk–Putnam–Penrose argument  ( https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument ).    These sorts of topics are one of the ways in which you could build up to a more complete notion of a block universe.  Specifically, where you could start to argue that all instants of time and space seem to need to exist.   It's worth mentioning that although these things often have names like "paradox" it's just because Physicist's and Philosopher's have good publicity and media agents and they select the titles.   They aren't usually impossible or logical paradoxes,  it's just that it sounds more interesting to call it a "paradox" instead of "an intersting thing".   
    Additionally, it's not entirely fair to say that the idea of a "Block Universe" is an idea in Physics.  It's an idea in Philosophy that is based on some ideas in Physics, although they often call it "Eternalism".  Physics doesn't really care if you favour a Block universe or Presentism, it just offers some models and allows some predictions to be made.   It's not offering any fundamental truth about how the universe really is, only a few models that seem to be useful.   More to the point, you can easily reconcile Special Relativity with Presentism if you try.   One option is to accept that when an object experiences an acceleration than the universe around them is changed.

    (Please don't get too worried about ideas like a deterministic universe - there's loads of things in physics that can offset or alleviate any concerns, like Chaos theory and quantum uncertainty but that falls outside the scope of this thread).

Sorry, I should not have brought any of this up.  It does not add much to my initial issue.

So the idea of an object to be moving relative to another object doesn't really make any sense in a block universe.

    Are you sure?   Movement is described just as a rate of change of one thing (position) with respect to something else (time).   So this is just a ratio of a small change in one variable to the corresponding small change in another variable.   Just because we, human beings, tend to experience and think of time a certain way, doesn't necessarily grant it any special nature.   It's just something, some variable, we can use to calculate rates of change with respect to.    We could have some other variable like S   - let's give it a silly name like "entropy of the universe" - and just calculate rates of change of position with respect to S.   

I am currently watching the videos that you posted from Leonard Susskind.  Maybe this will help me.