# Is spacetime independent of its universe?

1. Apr 16, 2013

### Seminole Boy

Something is throwing me here.

No matter how fast one is going, relatively speaking, one is in the same universe as everyone (and everything) else. We're all going through the same spacetime, albeit at different velocities. You're in the same universe as I am and we both see the same space, light, and bikinis.

If what I've already said is wrong, it makes me believe the universe itself is an idea that has no absolute form. Because, regardless of different speeds of motion, if we are "trapped" in the same universe and going through the same "spacetime", how can time dilation make any sense? For time dilation to make sense, it's almost like we're altering the universe by virtue of our speeds through it.

WannabeNewton is in a spaceship (working on some huge equation that would take me 6,000 years to understand) and is going fast.

The Great One (Peter Donis) is in a spaceship (flexing his muscles) going very fast.

I'm in a spaceship (and throwing the ball for my golden) going very, very fast.

However, we're all in the same universe and we're all going through the same spacetime.

Unless our relative speeds are creating different universes (I'm sure I'm not saying this exactly right, but work with me), I don't see how the speeds matter.

Is spacetime independent of the universe in which it's understood?

2. Apr 16, 2013

### Mentz114

I doubt if you know what time dilation is, so it is bound not to make sense to you.

Does the universe not encompass everything ? That was the usual definition when I last checked.

3. Apr 16, 2013

### Fredrik

Staff Emeritus
I don't understand the question in the title. What I can tell you is this:

SR defines a mathematical structure that we call spacetime. Each specific theory of matter and interactions in the framework of SR is defined by a specification of the matter content of spacetime.

Velocity matters because the numbers displayed by clocks are not the time coordinates of the events on the clock's world line in some specific coordinate system. If a clock displays t and one event and t' at a later event, then the proper time of the part of the clock's world line from the former event to the latter event is t'-t.

4. Apr 16, 2013

### Seminole Boy

Frederick: thanks for your input. I believe you're making this a bit too technical. What I'm saying is this: you and I are "supposedly" in the exact same universe. We are moving through the exact same spacetime. However, our relative speeds through this spacetime can make my universe different from yours. This is telling me that spacetime itself is independent of the universe in which it's understood. The universe is altered, in other words, by one's relative speed through it.

The General Relativity of the Universe is what I'm talking about here, and I believe that's a good name for it.

5. Apr 16, 2013

### Staff: Mentor

Mentor note -- Seminole Boy is no longer with us (not for his posts in this thread, BTW).

6. Apr 16, 2013

### Fredrik

Staff Emeritus
The universe isn't altered. Only your perception of it is altered. If you're interested in relativity, I suggest that you study a book that uses the approach based on spacetime diagrams, like Taylor & Wheeler. (I've been told that the first edition is better because the second is somewhat confusing).

7. Apr 16, 2013

### TheBC

I'm not sure I fully understand the opening post, but this is how I interpret SR:
I am not sure SR only defines a mathematical 4D spacetime. There is more involved. To measure between events, the events have to be available for measurement, i.e. observable, (i.o.w. they have to 'exist', be 'real': physics is about reality out there, isn't it?). 4D spacetime is made of 4D worllines (made of billions of 'past, present and future' events). All events are located in specific spots in 4D spacetime, relatively separated by absolute spacetime intervals. This makes SR's relativity of simultaneity of events possible. Hence different observers consider different 3D spaces of events, but they are all part of one and the same 4D spacetime.

8. Apr 16, 2013

### Fredrik

Staff Emeritus
I made a comment about that in a post yesterday. This is what I said:
My previous comment in this thread was a bit incomplete, since I didn't mention the correspondence rules.

9. Apr 16, 2013

### bobc2

You are on to something here, TheBC. However, it would be tough sledding to pursue your line of thought on this forum in the context of concepts like "exist" and "real" (that's considered more philosophy than physics). I get the impression that you are alluding to the "block universe" concept of special relativity (I dont' know if you are familiar with the term). If you do a thread search on that subject you will get a sense of the attitudes about that concept.

Of course if the block universe model were the correct model for the universe your questions and notions would find easy explanations (as would Seminole Boy's questions). I think that most physicists here would consider the block universe to be useful as a pedagogical tool for understanding special relativity, but would consider the model as correctly representing reality to be unproven.

Last edited: Apr 16, 2013
10. Apr 16, 2013

### bobc2

TheBC,

I was a little unsure about attempting an answer to your question (I just wanted to comment on the difficulty of your probing). The others have given you reasonable direct answers.

The only thing I would add would be first to repeat what's already been said. No, the space-time of special relativity is not separate from the universe. However, I sensed that you had some picture in mind of what the universe is and perhaps felt that the special relativity description was not quite fitting. It might help to understand that there are at least two views of the universe:

1) The physical universe is 3-dimensional, evolving in time. The special features such as constant speed of light for all observers (regardless of relative velocities among them), relativity of simultaneity, time dilation, length contraction are best understood mathematically (Minkowski metric and Lorentz transformations, etc.).

2) The physical universe is 4-dimensional (the "block universe"--it's all there at once) and different observers moving relative to each other have different instantaneous 3-D cross-section views of the 4-D universe (after you've accounted for light travel time delays).

One of Seminole Boy's comments stood out for me:

"The universe is altered, in other words, by one's relative speed through it."

He wouldn't have any trouble resolving this misunderstanding if he understood the block universe (in a pedagogical sense).

Most physicists, while favoring one picture or the other of the universe, will concede that neither model has been proven to represent the "true" picture of reality (aside from competing boundary conditions, details, etc., of the various 4-D curved universe models).

I couldn't tell whether you had a particular view from which you were launching your probing questions.

Last edited: Apr 16, 2013
11. Apr 19, 2013

### TheBC

I don't think Einstein was talking philosophy when he wrote (my bold):

<< From a "happening" in three-dimensional space, physics becomes, as it were, an "existence" in the four-dimensional "world". >> Albert Einstein. "Relativity: The Special and the General Theory." 1916. Appendix II Minkowski's Four-Dimensional Space ("World") (supplementary to section 17 - last section of part 1 - Minkowski's Four-Dimensional Space).

<< Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence. >> Albert Einstein, "Relativity", 1952.

The more I read this forum, the more I get confused. Am I correct you propose two different theories of Special Relativity? Never heard of this.
Do you mean an evolving 3D space (invariant, one and the same for all observers) can contain non-simultaneous events? That's new to me. I always thought 3D space = simultaneous events. If relative moving travelers consider a different set of events to be simultaneous, then obviously they consider different 3D spaces.

What else is 3D space 'made of' if it is not simultaneous events?

If for the car driver the event "hitchhicker with hands up in the air" is simultaneous with event "car at flag pole", then obviously the car's 3D space environment can not be any of the indicated horizontal 3D spaces.
If simultaneity is not related to 3D space, then what is 3D space?

The more I think about all this I slide into semantics and the more obscure it gets. Is it wrong to state that the simultaneous events part of 3D space 'exist'?
If not, then what are events? If I observe an event (or rather: observe the image of an event), then that event 'existed' spatially separated from me before the lightbeams from that event reach me. Or what else is observation?

I also am of the opinion that a distance between simultaneous events means spatial separation between the events. Mathematics measures something that's out there to be measured. If the car driver 'measures' a spatial distance between his car and the event 'hitchhiker hands up in the air', then the events 'car at flagpole' and 'hitchhiker with hands in the air' have to be there (exist), and they are 'space related', i.o.w events of 3D space.
In your (one for all observers) evolving 3D space scenario where the above two events are not part of (a) 3D space, the simultanity of above two events can only be a fictitious mathematical result of calculation. Is this part of Special Relativity physics?

I guess you refer to an old ether theory before Einstein's relativity in which the primed space and time coordinates of the Lorentz Transformations were indeed only fictitious data (Lorentz called them "auxiliary mathematical quantity"). Lorentz later admitted this shortcoming as a failure.
Only Einstein's 4D spacetime with different 3D spaces for relative moving travelers can give physical transformations to equally valid frames. Nothing fictitious.
In 4D spacetime all events are a given, as one lot, one entity. For Newton and Lorentz the 3D space was invariant, but in Special Relativity the invariant is 4D spacetime, in which 3D space is only relative: relative moving travelers through (in) 4D spacetime consider different set of events to be simultaneous, i.e. different 3D spaces.

I do not see how 4D spacetime can be compatible with a (for all observers invariant) evolving 3D space. And reading Einstein's words (see above) I am not (yet?) the one to contradict him.

12. Apr 19, 2013

### ghwellsjr

There's only one Theory of Special Relativity but many compatible interpretations.
Each Inertial Reference Frame (IRF) assigns Coordinate Time to events differently so that two evens that are simultaneous in one are not simultaneous in another.
I just made some spacetime diagrams for another thread but they also work for your scenario. Here's one for the IRF in which the hitchhiker and the flag are at rest. The hitchhiker is depicted in blue and the flag is in red. The car is in black and is traveling at 0.8c from the hitchiker to the flag. The dots show each observer's/object's Proper Time of one-month intervals. (Ignore the green lines.)

Now we could say that the event of the hitchhiker throwing up his hands occurs at the Coordinate Time of 10 months (which is also the Proper Times for the hitchhiker and the flag because they are not moving) and for the Proper Time for the car of 6 months. It's clock is Time Dilated because it is moving in this IRF.

Now we can show the same information in the rest frame of the car:

Now if you look at the Proper Time of 6 months for the car (which is now also the Coordinate Time since the car is stationary), it is no longer simultaneous with the Proper Times of 10 months for the event of the hitchhiker throwing up his hands and the Proper Time of 10 months for the flag. In fact, the first happens way later and the second happened way earlier.
Now you're talking about something else. If you want to depict how observers actually see events, you need to draw lines at 45-degree from the event to the observer. For example, you can see in the above diagrams how the flag sees the event of the car leaving the hitchhiker as the first green line. The line starts at the event (Coordinate Time and Distance both equal zero) of the car leaving the hitchhiker and arrives at the flag at its Proper Time of 12 months. This is the same in both IRF's. You can pick out any other event and its observation by any observer and it will be the same in either IRF. Unfortunately, there are not too many such examples that you can show on those diagrams but you can print them out and extend them to include more examples.

13. Apr 19, 2013

### TheBC

Ghwellsjr, nice charts you gave there. There is nothing confusing about this. And they are indeed very simple and easy to understand.
But your post does not deal with what Bobc2's post and my reply to his is about.
The issue is whether your IRF's and corresponding simultaneous events do make sense in a (one for all observer's) evolving 3D space scenario, or only in a 4D spacetime...

14. Apr 19, 2013

### Staff: Mentor

Why not? Was Einstein not permitted to philosophize from time to time? The key difference between science and philosophy is experimental evidence. I cannot think of any experiment which could distinguish the two concepts Einstein mentioned, can you? So I would classify it as philosophy, not science.

Furthermore, the words you put in bold are primarily philosophical terms, not scientific terms.

Why not? Which specific expermiental outcome would be different in the two cases?

15. Apr 19, 2013

### ghwellsjr

You're right, my post was not dealing with Bobc2's posts as I find them confusing, complex and impossible to understand. You have a choice. Stick with what is clear, simple and easy to understand or go with something that is not.

But I did answer many of your questions, did you not recognize that?

As I see it, your major concern is regarding what is real. I was pointing out that all observers' measurements and observations are real and how even though we apply different IRF's with different coordinates (which are not real in the sense that you are asking), they all adhere to the constant speed of light and they all support the same real measurements and observations that everyone makes as related to their own Proper Times. This means that simultaneity issues between remote events are not real (in the sense that you mean it) because they are based on Coordinate Times.

16. Apr 19, 2013

### bobc2

Hi TheBC,

I think I follow your logic, and you seem to be making some reasonable points. But, yes, there are essentially two interpretations of special relativity: 1) A time evolving 3-dimensional universe and 2) A four-dimensional universe populated by 4-dimensional objects (it's static--all there at once).

Originally, Fitzgerald, Lorentz, Poincare' and others presented papers that attempted to explain the special features of special relativity has resulting from electrical force transmission delays, etc. (the "Lorentz Aether Theory"--or LET). Rindler, in his text book, does not go into the details of the theory, but dismisses it with the comment that the theory faded away into obscurity. However, some people seem to still affirm the theory, although, in spite of my recent literature searches, it still is not clear to me personally whether or what kind of modifications have been applied to overcome the original defects in the theory (e.g., if one rod, moving at relativistic speed, is shrunk due to absolute physical effects, and another rod at rest in the ether is not shrunk, then everyone would always be able to tell which one is shrunk--unlike Einstein's relativity, where each observer sees the other's rod as shrunk). I've not been successful in getting responses on this forum that would further clarify that point. I'm not claiming to have proved the theory to be invalid--just haven't found a convincing demonstration of it (since it is based on a physical mechanism, you should be able to validate the mechanism).

I think that nowadays, those who reject the block universe (a real 4-dimensional universe populated by 4-dimensional objects), may possibly do it for one or more of the following reasons:

1) The block universe is just one among competing interpretations of special relativity (particularly LET)

2) It is not presently possible to understand reality (it remains mysterious), and it is sufficient to just understand the mathematics (Minkowski metric, time dilation, length contraction and relativity of simultaneity).

3) A 4-dimensional universe has not been proven to exist as physical reality. Further, terms such as "exist" and "reality" should not be a part of doing physics--rather those are philosophical terms.

I once presented a summary of Paul Davies's book "About Time" on this forum. After reading his book, I had a short encounter with Davies during a visit at Arizona State University in Tempe (it deals quite a bit with the block universe). You can see the various attitudes that surface when a subject like this is put on the table (I think the thread may have been closed):

Last edited: Apr 19, 2013
17. Apr 20, 2013

### ghwellsjr

Notice the "E" in LET? That stands for ether which simply means a single absolute state in which light travels in all directions at c. Take all of SR and then ignore the second postulate and instead postulate that light travels at c only in a single reference frame and you have LET. Whether or not Lorentz actually formulated or believed in that particular form of LET, that's what we mean today when we speak of LET. It's identical to SR except that it assigns one Inertial Reference Frame (IRF) to be preferred, in the sense that nature operates physically on it, even though we cannot tell which IRF that is.

But you cannot attribute a physical mechanism exclusively to LET and not to SR. The Lorentz Transformation (LT) in SR requires that all the laws of physics transform according to LT. Maxwell's equations already did this prior to Einstein formulating SR. But other laws did not and they had to be modified to correctly reflect relativistic requirements. And once that's done, the physical mechanism that you want to attribute only to LET is also in SR. It's just that no one bothers to look in detail for a demonstration of it because it cannot be detected, just like the propagation of light cannot be detected.

The reason that SR is preferred over LET is that SR is simpler. It frees us of having to assign a candidate ether rest state, even if we believe the ether exists. The question for you is: do you agree that there is never a requirement to assign more than one IRF in any given scenario?

18. Apr 20, 2013

### TheBC

Would you consider 'event' a scientific term? Do you accept 'observation'?
If I observe an event then that event was out there, happened, existed spatially separated from me before the lightbeams from that event reach me.
Or what else is observation? Mental images popping into your mind? C'mon, Dalespam...
An event is out there to be observed, therefore the event 'happens', 'exists' out there before it is observed. Am I talking philosophy here? If you consider all this philosophy, no wonder you encounter problems with Einstein's quotes.

In the -one for all observers evolving 3D space- you will simply not get any experimental evidence for the reciprocal time dilation, lenght contraction etc. If you get full experimental evidence for the Lorentz Transformation it's because 4D spacetime is the context we live in, not an 'absolute' evolving (ether?) 3D space.
Can you give me an experiment to prove an absolute evolving 3Dspace gives experimental evidence? I need more than an calculation of course.
Maybe you can answer my question I posted in #11:
In your (one for all observers) evolving 3D space scenario where the above two events are not part of (a) 3D space, the simultanity of above two events can only be a fictitious mathematical result of calculation. Is this part of Special Relativity physics?. In other words, do you call that experimental evidence?
To have experimental evidence the outcome of a formula (say Lorentz Transformation) not only have to make sense on a calculator or a sheet of paper, but also on the playground, out there in reality. An evolving 3D (ether?) reality can never give you the experimental evidence for the Lorentz Transfomations. One part of the results (the so called primed coordinates) are only fictional auxiliary mathematical data.
Lorentz: << The chief cause of my failure was my clinging to the idea that the variable t only can be considered as the true time and that my local time t' must be regarded as no more than an auxiliary mathematical quantity. In Einstein's theory, on the contrary, t' plays the same part as t; if we want to describe phenomena in terms of x'; y'; z'; t' we must work with these variables exactly as we could do with x; y; z; t. >> Lorentz, H.A (1916), The theory of electrons, Leipzig & Berlin: B.G. Teubner.
Only Einstein's 4D spacetime with different 3D spaces for relative moving travelers can give physical transformations. This is basic Special Relativity.
I guess that on this forum any physical interpretation of mathematical numbers is considered philosophy, which reduces this forum to just mathematics issues. That's fine, but then please don't call it a physicsforum ;).

19. Apr 20, 2013

### TheBC

I am really baffled you consider an ether theory can be 'part' of Special Relativity! Be happy Einstein doesn't hear you.

20. Apr 20, 2013

### TheBC

LET is an awful attempt to mix dynamical ether effects in one frame and pure spatio-temporal 4D effects in another. I am flabbergasted you can take this serious...