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

[Herman Trivilino]

Also, why would suggesting that one of the properties of space is motion lead to a conspiracy theory and what kind of conspiracy theory would that be?

Because there is no evidence to support the notion that space is moving. It's like postulating the existence of ether and then claiming that one of its properties is that it can't be detected. Or more precisely, that ether has several properties and together they "conspire" to make ether undetectable.

There's another recently-active thread where the folly of the very notion of expanding space was discussed at length. There are some very prominent physicists (cosmologists) warning us that the very idea that space could expand is nonsense.

 

[Dale]

But if a distant galaxy (or any photon in that area) are moving away from us due to expansion, then doesn't this mean this is a successful experiment to show that space (or I should say its math model) are also moving?

I see an airplane moving away from me too. I don't take that to mean that space is moving. I see a car moving towards me. I also don't take that to mean that space is moving.

What kind of experiment could one do to measure the "motion" of space?

Any experiment where you look to see if the laws of physics depend on speed or direction. The Michelson Morely experiment is the most famous, but there are many others. The standard model extension is a test theory that can be used to design experiments probing this motion, but so far every attempt measures 0.

Also, why would suggesting that one of the properties of space is motion lead to a conspiracy theory and what kind of conspiracy theory would that be?

Conspiracies are about different actors working together to keep a secret hidden. In this case it would be the various laws of physics all working together to keep the motion of space a hidden secret.

 

[Buckethead]

Because there is no evidence to support the notion that space is moving. It's like postulating the existence of ether and then claiming that one of its properties is that it can't be detected. Or more precisely, that ether has several properties and together they "conspire" to make ether undetectable.

There's another recently-active thread where the folly of the very notion of expanding space was discussed at length. There are some very prominent physicists (cosmologists) warning us that the very idea that space could expand is nonsense.

This is very interesting. Can you point me to this thread? If this is true, then this means that accelerating galaxies are being acted on directly. Isn't this massive heresy?

 

[Buckethead]

I see an airplane moving away from me too. I don't take that to mean that space is moving. I see a car moving towards me. I also don't take that to mean that space is moving.

But you will never see that plane or car moving faster than light and they are also being propelled by their own power. 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?

 

[Nugatory]

And coordinate systems change when velocities change?

A coordinate systems is just a rule for assigning numbers (called "coordinates") to points. We usually choose a coordinate system because it makes sense for whatever problem we're considering; and when we move on to a different problem we choose a different coordinate system. Often we don't even notice that we've changed coordinate systems. For example, you will say in one breath that the car driving down the road in front of your house is moving at 60 km/sec; and in the next breath you will say that Alpha Centauri is 4.3 light-years away and moving at about 15 km/sec... 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 switched to a different coordinate system as you switched from thinking about the car to thinking about Alpha Centauri. So coordinate systems change pretty much whenever our thinking does, and the numbers we calculate from them have only limited physical significance. Realizing that these limits exist and understanding exactly what they are for any given coordinate system is probably the single biggest mental hurdle to understanding general relativity.

Can I say there is one specific coordinate system when looking out into space from the viewpoint of Earth? Can I say that from Earth there is a physical distance between us an a distant galaxy?

If we lived in the flat spacetime of special relativity or the Euclidean space of Newtonian physics, there would be such a "one specific" coordinate system, and we could use it to assign distances. But we don't, so there isn't... unless the galaxy in question is close enough to us that we can treat the spacetime between it and us as flat, and obviously that breaks down at a cosmological scale.

 

[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.

 

[Herman Trivilino]

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!

 

[Herman Trivilino]

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.