# B How tightly coupled are matter and spacetime?

#### kfielder

Hello, I’m not a physicist or studying physics in school; I’ve just read some books and have some questions that I was hoping someone could help with. Sorry if they’re a little basic.

I’m trying to understand how tightly coupled matter is to spacetime. In other words, if you could look at a single point in space across time, would you see all of the places where matter passes through that intersection of space and time as a static event?

Some of what I’ve read seems to indicate that, but I’m not sure how that works with the relativistic nature of space and time individually since events don’t happen at the same intersection for all observers. Basically, if I had a bird's eye view of all four dimensions of our universe, what would I see?

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#### Ibix

You seem to me to be attempting to describe the block universe, which says that "the whole of space, now" is a slice through a 4d solid - rather like a page in one of those books where you flip the pages to see an animation.

This is far and away the most popular interpretation of the maths of relativity. To my mind, it's the easiest way to make sense of it all and a lot of physicists will talk as if That's The Way It Is. However, it's just an interpretation. Whether the world is "really" like that, or even asking the question, is strictly a matter of philosophy, not physics.

In other words, your guess is as good as mine. We usually assume that the block universe is true for descriptive purposes, but there's nothing in the maths that requires it. Future theories may modify our opinion.

#### cosmik debris

Basically, if I had a bird's eye view of all four dimensions of our universe, what would I see?
Look out of the window.

Cheers

#### MikeeMiracle

Likewise i am not a scientist, but my explanation for a non scientist like myself would be:

The rate of the "flow of time" of an object is dependant it's speed and it's proximity to other objects. The faster an object travel's, the slower it's "flow of time." The closer the object is to another object, the slower it's "flow of time."

For special relativity a simple way to view it is as you currently travelling through "spacetime" (not space) at the "speed of light." If you draw a graph with speed one 1 axis and time on the other, you can see that if you travel through space at the "speed of light" you are not travelling through time, if you travel through time at the "speed of light," your are not travelling through space. It should be noted that there is no actually such thing as not travelling through space, you may be standing still but the earth is still spinning, the earth is also circling the sun, the sun is circling the galaxy etc.

For general relativity, objects warp the space around them. The more mass an object has, the more it warps space. The more that space is warped, the slower the "flow of time" is in that warped space.

So to anwer your question, they are very tightly coupled, though that answer implies they are two separate entity's which is not correct. They are both properties of the same entity called "spacetime."

#### kent davidge

If you draw a graph with speed one 1 axis and time on the other, you can see that if you travel through space at the "speed of light" you are not travelling through time
Im afraid this is not correct. For such situation we would have a graph like this

as you can see, we would be travelling through time.

#### kent davidge

if you travel through time at the "speed of light," your are not travelling through space
I believe this is incorrect too. Speed is rate of change of space with respect to time, you would need to define what "speed relative to time" is. In any case, it would not be the conventional speed, so it would not have any significance at all for the case in discussion.

#### MikeeMiracle

Ok, my source for this is example a CassioPeia video here:

From 2.45 - 4 minutes.

I could have explained it better, I am not sure how you embed pictures / graph's into a post to demonstrate what I wanted.

#### jbriggs444

Homework Helper
Ok, my source for this is example a CassioPeia video here:
Pop science videos are not acceptable references here on PF. They tend to simplify things to the point of incorrectness.

Relevant notions are those of coordinate time and proper time. In special relativity, if one looks at the rate of passage of proper time compared to coordinate time [in a specified coordinate system] then the result might be called "speed of time".

But that "speed of time" is just as much a reflection of your coordinate system choice as it is of anything physical.

#### MikeeMiracle

The poster is asking a basic question and by his own admission is a physics laymen which in terms of the knowledge of members on his forum I also put myself in that catagory. People with our basic level of knowledge require over simplifications in order to try and grasp basic concepts. Pop science videos are a good way for people with our level of knowledge to learn these basic concepts.

It was with this in mind that I made the post and clearly stated that I am not a scientist either. Going into a discussion of coordinate time and proper time would not in my opinion help the original poster get a grasp on the issue.

I have been pulled up on these oversimplification before by other members and was told I had to state that this was a basic approximation, hence why I specified that I am not a scientist either. I am just a laymen trying to help anoter laymen with their understanding of basic concepts in a manor I felt they could relate to. So my question is, am I doing something wrong and should I not attempt to try and help fellow members with a similar basic level of knowledge in future?

Also with regards to pop science videos being used, I have seen experienced members reference these videos in other posts when communicating concepts to other members with only a very basic level of knowledge such as myself. I would never dream of trying to answer a properly formulated scientific question clearly expecting a proper scientific explanation with a pop science video. I am aware of the rules of using acceptable sources of information when engaging in a proper scientific debate between knowlegable members of the forum.

#### kfielder

You seem to me to be attempting to describe the block universe, which says that "the whole of space, now" is a slice through a 4d solid - rather like a page in one of those books where you flip the pages to see an animation.
Thanks to everyone for their replies!

And thanks to the above poster for stating my premise better than I did. That's exactly what I was referring to. I'm pretty clear on the basics of both special and general relativity such as time dilation (not the formulas, but the concepts), so the question wasn't really from the perspective of an object as it moves through space and/or time.

Maybe it is more philosophical, but I do hear things described in that block universe visualization and just can't wrap my head around what that really looks like when there is no synchronicity of events across observers. Part of this too is hearing physicist talk about the universe being approximately 14 billion years old. I think of time as a property of the universe, so I think of it more as there's been stuff on the time axis going back 14 billion years (to the best of our knowledge, based on what we're able to observe) from this point and an unknown distance forward. When I do hear physicists tightly couple the two and imply that there was no universe before the bang, I question whether my thinking is correct and if the fundamental structure is actually space/time/matter.

Just for a little extra background, part of the root of the question comes from trying to mentally reconcile that picture of a very static, block image with the statistical nature of quantum mechanics.

Sorry if my physics vocabulary is lacking. I really do appreciate everyone's time and patience.

#### Ibix

the question wasn't really from the perspective of an object
The problem is that there is no other basis for a scientific answer. We can come up with visualisations of the universe from some sort of "outside perspective", but since we don't even know if "outside the universe" is even an honest concept such things can only be interpretations of the maths. If there isn't actually an outside, any such concept is in not-even-wrong territory. That said, the block universe is an incredibly useful way of conceptualizing the maths. Just don't assume it's Actually What's Happening.

just can't wrap my head around what that really looks like when there is no synchronicity of events across observers.
Subject to my comments about it being just an interpretation, the lack of a global synchronisation process is what makes the block universe so popular. Take the flip-book model I proposed and disolve the paper, leaving just the ink. A dot on each page becomes a little column of ink. A moving dot becomes a slanted column. But now there's no preferred direction imposed by the pages. I can tip my head so that the slanted column is vertical - now my moving dot is stationary and my sense of "the same time" has changed.

That's the block universe in a nutshell. It's not quite that simple, because Minkowski geometry is different from Euclidean geometry. You can turn such a model over so that "time runs backwards" but in reality (or a slightly more sophisticated representation like a Minkowski diagram) the geometry prevents this.
physicist talk about the universe being approximately 14 billion years old
It's 14bn years old to people who see the cosmic microwave background as isotropic. Other observers disagree. The "isotropic CMB frame" just turns out to be convenient to use.
part of the root of the question comes from trying to mentally reconcile that picture of a very static, block image with the statisti
Quantum field theory is a completely relativistic theory (we just don't know how to link it up to gravity at the moment). I'm no expert on quantum, but I don't think reconciling it with the block universe is qualitatively any different to the fact that you don't yet know what's going to happen tomorrow. You either know the outcome of a measurement of a quantum system or you don't yet.

#### Ibix

you can see that if you travel through space at the "speed of light" you are not travelling through time,
Please don't say this, even though Brian Greene does. It's wrong. Time is not a definable quantity for objects moving at $c$, and you cannot travel at $c$ anyway.

#### weirdoguy

Pop science videos are a good way for people with our level of knowledge to learn these basic concepts.
The problem with quite a lot of pop-sci videos or books is that they oversimplify things to the point where what they say is simply not true. It doesn't even matter how famous and good the author is. That is why this kind of videos are generally not allowed here as references. Of course there are exceptions, but to know that a video is good and can serve as a reference you have to know physics on an appropriate level first.

If Hawking or Greene registered here incognito and posted only those things that they wrote in their pop-sci books, they would probably be banned quite quickly for spreading misinformation

#### Ibix

If Hawking or Greene registered here incognito and posted only those things that they wrote in their pop-sci books, they would probably be banned quite quickly for spreading misinformation
Hawking particularly, because the afterlife is the kind of woo topic we don't allow.

This probably deserves a little expansion. The problem, from our perspective, with a lot of popsci is that authors (who do understand) go further in simplifying complicated concepts than we would. Sometimes to the point that what they say is wrong - I think because they forget how much they know and forget that it isn't common knowledge. So what they say is misinterpreted, and people like the OP end up confused. Then they post here. And that's why we don't allow popsci as a reference, even if Einstein himself wrote it...

#### phinds

Gold Member
The rate of the "flow of time" of an object is dependant it's speed and it's proximity to other objects. The faster an object travel's, the slower it's "flow of time." The closer the object is to another object, the slower it's "flow of time."
None of that is even remotely true. The rate of flow of time is ALWAYS one second per second. It can APPEAR different to someone in motion relative the the thing being timed, but in a local frame the rate is always the same.

#### PeterDonis

Mentor
The rate of the "flow of time" of an object is dependant it's speed and it's proximity to other objects. The faster an object travel's, the slower it's "flow of time." The closer the object is to another object, the slower it's "flow of time."
There is a way to define "the flow of time" so that this is true, but (a) it's not a very useful way, and (b) it only applies to a certain special class of spacetimes.

A much more useful way to define "the flow of time" of an object is proper time along its worldline. By that definition, as @phinds has already pointed out, the "flow of time" is the same for all objects: 1 second per second.

(Note that here we are only talking about objects like us that move on timelike worldlines. Trying to discuss light and things that move on null worldlines raises a whole other set of issues.)

For special relativity a simple way to view it is as you currently travelling through "spacetime" (not space) at the "speed of light."
This view has many limitations, which is why you will only find it in pop science books and articles and not in actual textbooks or peer-reviewed papers. Brian Greene's popularizations of this view probably come close to holding a PF record for the number of threads started based on a misunderstanding.

If you draw a graph with speed one 1 axis and time on the other, you can see that if you travel through space at the "speed of light" you are not travelling through time, if you travel through time at the "speed of light," your are not travelling through space.
Unfortunately, such a graph misrepresents the physics because it is not a spacetime diagram and it does not properly map events in spacetime to points in the graph.

First, since your proper time always advances along your worldline at one second per second, it's not possible to not travel through time. (Note that light itself, which travels at the speed of light, moves on null worldlines and its behavior in this respect is discontinuous from that of timelike objects--another way in which the graph you describe misrepresents the physics, since it makes the transition from "almost at the speed of light" to "at the speed of light" seem continuous when in fact it is not. The concept of "proper time" and the concept of "speed through spacetime" derived from it do not even make sense for light.)

Second, there is no invariant notion of "not traveling through space", because, as you yourself have recognized, space and time are interconnected and can't be separated. The only invariants are relative motions--you can say that you and I are at rest relative to each other (if we are), but neither of us can say that we are "at rest" (not moving through space) in any absolute sense.

For general relativity, objects warp the space around them.
No, they warp spacetime. Big difference.

The more mass an object has, the more it warps space. The more that space is warped, the slower the "flow of time" is in that warped space.
No, this is wrong. For the spacetimes in which the "warped space" description makes sense at all (which is the special class of spacetimes I mentioned above for which the "flow of time" concept you were trying to use makes sense), the effect on "flow of time" of objects at rest relative to the gravitating mass (or moving slowly relative to it) actually comes from warpage of time, not space; space warpage only becomes significant for objects moving at or near the speed of light relative to the gravitating mass, and it shows up in effects like light bending, not "flow of time".

my source for this is example a CassioPeia video here
The "rubber sheet" analogy shown in this video is misleading because it only depicts warpage of space, not time, and as I noted above, the effect on "flow of time" in spacetimes like this comes from time warpage and can't even be shown in the "rubber sheet" diagram.

#### PeterDonis

Mentor
am I doing something wrong and should I not attempt to try and help fellow members with a similar basic level of knowledge in future?
A good general rule is, if you find yourself saying things like "I am not a scientist" and "I am a physics layman" in a thread on a given topic, it probably means you should not be trying to help other people with that topic, even if those other people are also laymen. The fact that someone who asks a question is a physics layman and is looking for a simple explanation at a layman's level does not mean that another layman is going to be able to give them such a simple explanation that is also reasonably correct. It's much better to let experts who are able to simplify their expert knowledge down to a layman's level give answers. As a layman, you can help best by checking to see whether the expert's answer actually answers what you understood the layman's question to be, and if it doesn't, asking further questions to clarify.

"How tightly coupled are matter and spacetime?"

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