Is there a physical explanation for the relationship between light and space?

[Dale]

But you will never see that plane or car moving faster than light

I will never see them moving faster than light, but I certainly can choose coordinates where they are moving faster than c. Same with distant galaxies.

and they are also being propelled by their own power.

While this is true, I could make similar statements about the planets as I did about the plane and the car.

If science claims that a distant galaxy is accelerating due to expanding space between us and it, then how can I take that to mean that anything other than that the expanding space is responsible

This is a math-to-English translation problem. GR is a geometrical theory, meaning that it describes gravity as the geometry of spacetime. The math is capable of describing arbitrary shapes with precision, but there aren't any normal English words for describing 4D manifolds like the shape of the universe.

The closest I can think of would be "trumpet shaped", but that describes a 2D manifold with the wrong signature. When cosmologists talk about inflation, expansion, and acceleration they are talking about differences in the "flaring" of the trumpet. But that doesn't imply any motion of the trumpet or its material. It is just a lack of accurate words, and an accepted jargon that is used instead.

 

[PeterDonis]

maxwells equations show light to locally always go at c

Only in inertial coordinates in flat spacetime. Your example is using non-inertial coordinates in curved spacetime.

 

[Mister T]

If science claims that a distant galaxy is accelerating due to expanding space between us and it, then how can I take that to mean that anything other than that the expanding space is responsible?

By looking at the experimental evidence that leads people to make that claim.

 

[PeterDonis]

If science claims that a distant galaxy is accelerating due to expanding space

Where does "science" claim this? Pop science sources claim it, but pop science sources are not "science". Can you give a reference to a textbook or peer-reviewed paper that makes this claim?

 

[John Morrell]

There are many models in physics which are intimately familiar to us, and there are many other models in physics which can be fairly well compared to things which are familiar to us. There are also models in physics which do not have a good, intuitive or familiar analog to compare to. GR can be loosely described by saying that space stretches and curves, but that's just an analogy. It isn't an accurate description of what's happening, it's just a relate-able way understand what the mathematical model is describing. It's risky to make conjectures based on the analogy and not the actual model.

 

[Buckethead]

There are many models in physics which are intimately familiar to us, and there are many other models in physics which can be fairly well compared to things which are familiar to us. There are also models in physics which do not have a good, intuitive or familiar analog to compare to. GR can be loosely described by saying that space stretches and curves, but that's just an analogy. It isn't an accurate description of what's happening, it's just a relate-able way understand what the mathematical model is describing. It's risky to make conjectures based on the analogy and not the actual model.

Well said! But still, I'm somewhat skeptical with regard to the nature of space. I fully accept that some things don't have an analogy, such as an electron and for that matter, space itself, but that doesn't mean that these things don't exist. We know an electron exists because we know its properties. But I'm getting the sense in this thread that here we can have something that describes space (GR) without the thing it's describing actually existing. We can't describe an electron if it doesn't exist, so how can we describe space if space doesn't exist? If the trajectory of light through space can be characterized using GR, then the space that GR describes must be controlling that trajectory and if so, there must be a mechanism behind it, unless we decide that the characteristics of space are truly fundamental and GR simply describes something that is fundamental. But have we really reached that point?

On the other hand I suppose it could be argued that an electron actually doesn't exist either. That its mathematical description and that description's effect on other particle models is all that there is, but we have to draw the line somewhere or we would end up saying that nothing exists!

 

[PeterDonis]

I'm getting the sense in this thread that here we can have something that describes space (GR) without the thing it's describing actually existing

Spacetime exists. "Space" is what you get when you slice up spacetime into "space" and "time"--i.e., when you choose coordinates. So it's not that "space" doesn't exist, it's that "space" is the wrong word to use--"spacetime" is the fundamental thing. "Space" is an artifact of choosing a particular way to describe the fundamental thing.

If the trajectory of light through space can be characterized using GR, then the space that GR describes must be controlling that trajectory

Substitute "spacetime" for "space" in the above and it is fine. But it is not correct with "space" in it.

 

[Buckethead]

Spacetime exists. "Space" is what you get when you slice up spacetime into "space" and "time"--i.e., when you choose coordinates. So it's not that "space" doesn't exist, it's that "space" is the wrong word to use--"spacetime" is the fundamental thing. "Space" is an artifact of choosing a particular way to describe the fundamental thing.
Substitute "spacetime" for "space" in the above and it is fine. But it is not correct with "space" in it.

Ahhh...thank you for that clarification. I'm starting to feel better now. So when popular scientists say that "spacetime is curved" and that light will follow this curve, then this is 100% accurate and not just an analogy?

 

[Buckethead]

But the first statement is using a coordinate system in which your house is at rest, and in those coordinates Alpha Centauri has a coordinate velocity of about 10000 times the speed of light;

You lost me here. Under what condition would my house be at rest and AC be moving at 10000 times c?

 

[PeterDonis]

when popular scientists say that "spacetime is curved" and that light will follow this curve, then this is 100% accurate and not just an analogy?

Yes, provided you understand what "spacetime is curved" means, physically. It means there is tidal gravity.

 

[Buckethead]

Where does "science" claim this? Pop science sources claim it, but pop science sources are not "science". Can you give a reference to a textbook or peer-reviewed paper that makes this claim?

Unfortunately, I guess I give too much credit to pop science sources. I was fully under the impression that galaxies are accelerating away from us, some faster than light, because of the expansion of space as mentioned by dozens of pop scientists who all seem to be in agreement with one another.

 

[Buckethead]

Yes, provided you understand what "spacetime is curved" means, physically. It means there is tidal gravity.

You mean caused by a tidal gravity, or actually a tidal gravity? Can you expand a little?

 

[PeterDonis]

You mean caused by a tidal gravity, or actually a tidal gravity?

"Spacetime curvature" is another term for "tidal gravity".

 

[PeterDonis]

I was fully under the impression that galaxies are accelerating away from us, some faster than light, because of the expansion of space as mentioned by dozens of pop scientists who all seem to be in agreement with one another.

Yes, and they're all saying something that is easily misinterpreted, as you were led into doing. I suspect they either don't know the actual technical details, or (in the case of actual cosmologists who write pop science articles) don't know a non-misleading way to describe those technical details in ordinary language.

 

[Buckethead]

This is a math-to-English translation problem. GR is a geometrical theory, meaning that it describes gravity as the geometry of spacetime. The math is capable of describing arbitrary shapes with precision, but there aren't any normal English words for describing 4D manifolds like the shape of the universe.

The closest I can think of would be "trumpet shaped", but that describes a 2D manifold with the wrong signature. When cosmologists talk about inflation, expansion, and acceleration they are talking about differences in the "flaring" of the trumpet. But that doesn't imply any motion of the trumpet or its material. It is just a lack of accurate words, and an accepted jargon that is used instead.

I think I get this. And when they talk about, for example, expansion, they are strictly talking about an expansion of spacetime and not of space. Does this mean that this expanding spacetime is causing both a change in time at any given point in space between us and the galaxy as well as a change in space per unit time and this is why we can't really say whether or not a distant galaxy is actually moving away from us at a given speed?

 

[Buckethead]

"Spacetime curvature" is another term for "tidal gravity".

Why does it have to be tidal gravity and not just gravity? Isn't all gravity in a sense tidal in nature since it comes from mass which is not infinite in size?

 

[Buckethead]

"Spacetime curvature" is another term for "tidal gravity".

Also, can you tell me what the importance is of this distinction? Are you implying here that curved spacetime is not possible without tidal gravity because they are one in the same thing?

 

[Dale]

We know an electron exists because we know its properties.

Yes, as I said above, this is a good approach. So from what you understand, what are the properties of spacetime (not space)? We have mentioned that motion is not one (unless you assume a conspiracy theory of physics). So what other properties have we mentioned here?

And when they talk about, for example, expansion, they are strictly talking about an expansion of spacetime and not of space

Yes. The math of GR is all about spacetime, not space. Occasionally they do talk about space, but with the understanding that it is just a convenient coordinate system that they have artificially introduced on top of the underlying spacetime geometry.

this is why we can't really say whether or not a distant galaxy is actually moving away from us at a given speed?

The reason we cannot really say how fast (relative to us) a distant galaxy is traveling is due to curvature. I will try to explain later.

 

[Nugatory]

You lost me here. Under what condition would my house be at rest and AC be moving at 10000 times c?

That's roughly how fast Alpha Centauri is moving when it makes a full circle around your house in 24 hours. And before you reject that way of calculating Alpha Centauri's speed as ridiculous, consider that it is exactly how you calculate the speed of an aircraft that you see in the sky: distance to moving object times rate of change of angular position.

The point here is that we define speed as the rate of change of the position coordinates with respect to the time coordinate. If we use different coordinates we'll get different speeds, but none of that has any real physical significance.

 

[puzzled fish]

3) inertia felt by acceleration and acceleration felt by rotation are absolute in nature which (it seems to me) must imply they are relative to some kind of physical spacetime structure.

Excuse me, but why this is true? Solutions of EFE inside a spherically symmetric dense mass object produce spacetime with rotating geodesics where no acceleration is present.
Even in Newtonian mechanics inside a massive object where the acceleration is directly proportional to the distance R from its center and not the inverse of its squared distance. there is rotation without any acceleration felt!

 

[Mister T]

But still, I'm somewhat skeptical with regard to the nature of space. I fully accept that some things don't have an analogy, such as an electron and for that matter, space itself, but that doesn't mean that these things don't exist.

In my opinion the source of your confusion lies in a very common misconception about the nature of physics itself. Physics is a process of humans creating models that can be used to describe and explain the behavior of Nature. These explanations are called theories. It's been said that we'd do well to replace "theory" with "explanation" when thinking and speaking about all of science. Thus Einstein's theory of gravity becomes Einstein's explanation of gravity. Darwin's theory of evolution become's Darwin's explanation of evolution. These explanations exist, but not in the same way that the things they're explaining exist. The explanations are creations of the human intellect, the things they're explaining are not.

We know an electron exists because we know its properties. But I'm getting the sense in this thread that here we can have something that describes space (GR) without the thing it's describing actually existing. We can't describe an electron if it doesn't exist, so how can we describe space if space doesn't exist? If the trajectory of light through space can be characterized using GR, then the space that GR describes must be controlling that trajectory [...]

Gravity is "controlling" the trajectory, in the sense that you are using that word. Spacetime curvature is used to explain the trajectory.

On the other hand I suppose it could be argued that an electron actually doesn't exist either. That its mathematical description and that description's effect on other particle models is all that there is, but we have to draw the line somewhere or we would end up saying that nothing exists!

Electrons exist in the sense that they are not created by humans. The models we use to describe and explain the behavior of electrons are created by humans.

Unfortunately, I guess I give too much credit to pop science sources. I was fully under the impression that galaxies are accelerating away from us, some faster than light, because of the expansion of space as mentioned by dozens of pop scientists who all seem to be in agreement with one another.

They indeed are accelerating away from us. In other words they are receding from us at ever-increasing speeds. The reason remains an unsolved mystery in physics. Perhaps one of the greatest unsolved mysteries of all time.

Also, can you tell me what the importance is of this distinction? Are you implying here that curved spacetime is not possible without tidal gravity because they are one in the same thing?

Tidal gravity refers to the observation that the magnitude and direction of gravity varies with position. Spacetime curvature is the explanation of that observation.

Let's go back to your use of the word "controlling", mentioned above. As humans we like to pretend that Nature obeys the laws we've created. Those laws are simply part of the explanations we call theories. Nature doesn't obey our laws. Our laws describe Nature's behavior. Consider the fact that these laws have limits of validity, they are not perfect descriptions.

 

[PeterDonis]

when they talk about, for example, expansion, they are strictly talking about an expansion of spacetime

No, they aren't. Spacetime is not expanding. If you want to pick out something that can be said to be "expanding" in an invariant sense, it is the set of worldlines describing "comoving" observers--observers who always see the universe as homogeneous and isotropic. Those worldlines form a timelike congruence whose expansion scalar is positive; that is the only invariant in the actual math that corresponds to the ordinary language term "expanding".

Why does it have to be tidal gravity and not just gravity?

Because "gravity" is too vague. For example, it can refer to "acceleration due to gravity", the fact that a rock falls when you release it while standing on the Earth's surface. That is not tidal gravity.

Are you implying here that curved spacetime is not possible without tidal gravity because they are one in the same thing?

Yes. "Spacetime curvature" and "tidal gravity" are just two different names for the same thing.

 

[PeterDonis]

Solutions of EFE inside a spherically symmetric dense mass object produce spacetime with rotating geodesics where no acceleration is present.

What solution are you referring to?

 

[nitsuj]

Unfortunately, I guess I give too much credit to pop science sources. I was fully under the impression that galaxies are accelerating away from us, some faster than light, because of the expansion of space as mentioned by dozens of pop scientists who all seem to be in agreement with one another.

Where does "science" claim this? Pop science sources claim it, but pop science sources are not "science". Can you give a reference to a textbook or peer-reviewed paper that makes this claim?

They indeed are accelerating away from us. In other words they are receding from us at ever-increasing speeds. The reason remains an unsolved mystery in physics. Perhaps one of the greatest unsolved mysteries of all time.

In this thread the difference between choice of coordinates and the measured/calculated results was clearly described as an arbitrary choice, a preference.

Acceleration too can just be a result of choice of coordinates, surely those distant galaxies, coordinated as accelerating >c by YOU would disagree and support it with an accelerometer measuring null. Mister T was specific here qualifying their statement with "In other words receding from us at ever increasing speeds." This is distinctly different from what we rough necks know and feel as "G force".

Just wanted to highlight the distinction between the words acceleration and acceleration as used in this thread, aka Proper acceleration and Coordinated acceleration...

Seems more of a human behavior concern than one specific to "pop science"

 

[nitsuj]

Because "gravity" is too vague. For example, it can refer to "acceleration due to gravity", the fact that a rock falls when you release it while standing on the Earth's surface. That is not tidal gravity.

Err when the rock is still in hand it is accelerating "because of gravity" (without taking multiple measure over the body of the rock). When it falls, it's changing position relative to the centre of mass of the Earth, while not remarkable OP was asking about tidal gravity. There would always be some none zero tidal effect with such changes.

 

[PeterDonis]

when the rock is still in hand it is accelerating "because of gravity"

I meant coordinate acceleration, not proper acceleration, but the point from your previous post that we should be more careful about specifying such things is valid.

There would always be some none zero tidal effect with such changes

With accurate enough measurements, yes, you could, for example, drop two rocks, one slightly above the other, and measure the change in their separation due to tidal gravity. But the point I was making is that this phenomenon is still different from "gravity" as "that which makes the rocks fall at all".

 

[Buckethead]

Yes, as I said above, this is a good approach. So from what you understand, what are the properties of spacetime (not space)? We have mentioned that motion is not one (unless you assume a conspiracy theory of physics). So what other properties have we mentioned here?

Uh-oh, a test! Well it seems spacetime is physical in nature and therefore I suppose you can contain it in a boundary using 4 dimensional points. It also seems you can measure the distance of that boundary using a laser, mirror, and proper time clock. It can expand and carry any object or photon with it as it does so. It can be curved and this is the same thing as tidal gravity, it is flat in the absence of gravity. It can rotate (Lense-Thirring effect or frame dragging) if it is near a rotating body that either surrounds it or is within it.
And most interestingly if it is rotating then any object stationary relative to it and not necessarily at its center, will feel no centrifugal force.

Did I pass?

One point that makes me squint is that I don't know why it is not also allowed to move. It seems motion is just one step beyond expansion. I understand that local tests (such as the MM test) have detected no motion and I get that, but what about motion between large sectors of space such as between galaxies? Is there anything in SR preventing that? And no, I'm not suggesting a spacetime made of "ponderable matter" as Einstein so eloquently puts it.

And as a P.S. when I say I like to visualize spacetime, the way I do so is to remember that if you hold an object stationary above the Earth it is accelerating through spacetime and if you let it go it immediately becomes an inertial frame in spacetime. I use this to stay grounded :)

 

[Buckethead]

That's roughly how fast Alpha Centauri is moving when it makes a full circle around your house in 24 hours. And before you reject that way of calculating Alpha Centauri's speed as ridiculous, consider that it is exactly how you calculate the speed of an aircraft that you see in the sky: distance to moving object times rate of change of angular position.

The point here is that we define speed as the rate of change of the position coordinates with respect to the time coordinate. If we use different coordinates we'll get different speeds, but none of that has any real physical significance.

OK, that makes sense. Thanks.

 

[PeterDonis]

it seems spacetime is physical in nature and therefore I suppose you can contain it in a boundary using 4 dimensional points

Um, what? I have no idea what you are proposing here. Where are you getting this from?

It also seems you can measure the distance of that boundary using a laser, mirror, and proper time clock.

Same response as above.

if it is rotating then any object stationary relative to it and not necessarily at its center, will feel no centrifugal force

Since this is obviously contradictory to observation, whatever model you are using is evidently wrong. But I still have no idea what model it is.

 

[Buckethead]

Excuse me, but why this is true? Solutions of EFE inside a spherically symmetric dense mass object produce spacetime with rotating geodesics where no acceleration is present.
Even in Newtonian mechanics inside a massive object where the acceleration is directly proportional to the distance R from its center and not the inverse of its squared distance. there is rotation without any acceleration felt!

Yes, but isn't that what I said when I said inertia is relative to a spacetime structure...even if that structure is within a rotating sphere!

 

[Buckethead]

Um, what? I have no idea what you are proposing here. Where are you getting this from?

I was extrapolating. Ooops. My logic is that if spacetime is physical in nature and since spacetime is a combination of 3 spatial coordinates and 1 time coordinate I was supposing that a point is spacetime could be identified by a 4 dimensional point. Not so I take it?

 

[PeterDonis]

I say I like to visualize spacetime, the way I do so is to remember that if you hold an object stationary above the Earth it is accelerating through spacetime

No, it isn't. It is experiencing proper acceleration, but it is not "accelerating through spacetime". There is no meaning to the latter idea.

 

[PeterDonis]

My logic is that if spacetime is physical in nature and since spacetime is a combination of 3 spatial coordinates and 1 time coordinate I was supposing that a point is spacetime could be identified by a 4 dimensional point.

That part is fine. But I don't see how you are getting from that to stuff about a "boundary".

 

[Buckethead]

That part is fine. But I don't see how you are getting from that to stuff about a "boundary".

If you draw a line between 2 of these spacetime points then would you have a line? If so what if you planted 4 points. Wouldn't you have some kind of spacetime tetrahedron? This is what I'm calling an object with a boundry.

 

[Buckethead]

No, it isn't. It is experiencing proper acceleration, but it is not "accelerating through spacetime". There is no meaning to the latter idea.

By proper acceleration do you mean proper acceleration through space?