Is there a gravitational effect on spatial measurements

[DaTario]

TL;DR

Hi All,

We often read that the gravitational field acts by slowing down clocks. Is there any statement that, with the same simplicity, reports on the action of the gravitational field on lengths measured by observers which are close or far from Earth, for example?

Hi All,

We often read that the gravitational field acts by slowing down clocks. Is there any statement that, with the same simplicity, reports on the action of the gravitational field on lengths measured by observers which are close or far from Earth, for example?

Best wishes,

DaTario

 

[Ibix]

We often read that the gravitational field acts by slowing down clocks.

This isn't really true, and I suspect you're misunderstanding something. Where did you read this?

Is there any statement that, with the same simplicity, reports on the action of the gravitational field on lengths measured by observers which are close or far from Earth, for example?

Assuming you mean something like "in special relativity there is time dilation and length contraction, and there's time dilation in gravitational fields - is there also length contraction?" then the answer is no. There's a well-defined notion of what it means to compare the rates of two spatially separated clocks (at least in some cases). But it's not at all clear what you mean by comparing the lengths of two rulers that aren't next to each other. So special relativistic length contraction doesn't have a meaningful analog here.

 

[DaTario]

Hi Ibix,

https://pubs.aip.org/aapt/ajp/artic...-free-fall-and-matter?redirectedFrom=fulltext

for instance.

 

[DaTario]

This isn't really true, and I suspect you're misunderstanding something. Where did you read this?

Assuming you mean something like "in special relativity there is time dilation and length contraction, and there's time dilation in gravitational fields - is there also length contraction?" then the answer is no. There's a well-defined notion of what it means to compare the rates of two spatially separated clocks (at least in some cases). But it's not at all clear what you mean by comparing the lengths of two rulers that aren't next to each other. So special relativistic length contraction doesn't have a meaningful analog here.

Dear Ibix,

Shouldn't the physical effect of gravity have been described as an effect on spacetime?
Thus, it seems to me that it would be natural to find some application instances where we would see markedly temporal effects and others where we would see markedly spatial effects.

 

[Ibix]

https://pubs.aip.org/aapt/ajp/artic...-free-fall-and-matter?redirectedFrom=fulltext

Full paper at arxiv. I don't think this explanation can be generally correct because it cannot account for point particles, which also fall in GR. It may work in some cases, but it cannot be the full story.

Thus, it seems to me that it would be natural to find some application instances where we would see markedly temporal effects and others where we would see markedly spatial effects.

I know what it means to compare two clocks that are not in the same place: I check that the distance between them is constant and then I simply compare their tick rates. How are you planning on comparing the lengths of two rulers that are not in the same place?

 

[DaTario]

Ok,

https://arxiv.org/abs/2007.13851

 

[DaTario]

How are you planning on comparing the lengths of two rulers that are not in the same place?

I don't know. Perhaps there is some observable change in some physical effect, such as the Casimir effect, and this change may be attributable to the effect of gravity on distance measurements.
I am aware, however, that an effect on space will possibly act on both the objects to be measured and the ruler itself, making the effect impossible to demonstrate.

Could it be that the constancy of the speed of light cannot be used to produce a consequence on spatial measurements from the temporal effect?

 

[PeterDonis]

https://arxiv.org/abs/2007.13851

This is a speculative proposal that is not mainstream GR (in fact it is a mismash of classical GR and a particular interpretation of QM, which is itself not mainstream). You should not be relying on it to build an understanding of mainstream GR.

 

[PeterDonis]

Shouldn't the physical effect of gravity have been described as an effect on spacetime?

In mainstream GR, it is: it is the curvature of spacetime.

I suggest taking the time to work through a GR textbook. Two good ones available for free online are Sean Carroll's lecture notes on GR and Taylor & Wheeler's Spacetime Physics.

 

[PeterDonis]

I don't know.

This should be a huge red flag to you that the question you are asking is not well-defined.

@Ibix's response in post #2 is correct.

 

[Ibix]

I don't know.

And that is exactly the point. There isn't a meaningful way to do it.

You might try direct optical comparison, or you might drop plumb lines from each end of the distant ruler to the one below. The answers will be different. You might build a rigid rectangular frame and lower it into the gravitational field and try to compare the lengths, but the result will depend on what material you build your frame from.

All the different ways give different answers that depend more on the method you use to compare your measurements than on anything generally meaningful.

 

[DaTario]

This should be a huge red flag to you that the question you are asking is not well-defined.

Ok. But from the point of view of someone who does not have the knowledge, it seems to me that there is a legitimate doubt whether the bad definition is from the concept in my internal system of organizing knowledge and understandings or from the concept within the body of the theory itself. Hence the need to ask.

 

[Orodruin]

Hence the need to ask.

Hence the need for a standardized textbook rather than fragmented knowledge.

 

[Frabjous]

This is a speculative proposal that is not mainstream GR (in fact it is a mismash of classical GR and a particular interpretation of QM, which is itself not mainstream). You should not be relying on it to build an understanding of mainstream GR.

Independent of the merits of this particular paper, I was of the opinion that the American Journal of Physics was a “safe” source for students. Do you have a different opinion?

 

[DaTario]

Hence the need for a standardized textbook rather than fragmented knowledge.

Dear Orodruin,

You seem to forget that people in physics specialize and there is a dimension in forums where people who want to talk can meet, which doesn't seem to be your case. In your case, if you don't want to talk about such fragmented knowledge, don't respond. It's optional.

 

[PeterDonis]

from the point of view of someone who does not have the knowledge

From that point of view the best way to answer your question is to get the knowledge--by studying a textbook.

it seems to me that there is a legitimate doubt whether the bad definition is from the concept in my internal system of organizing knowledge and understandings or from the concept within the body of the theory itself.

Since you admit you don't have the knowledge, the Bayesian prior for this is simple: the problem is with your knowledge, not with the theory. That should always be your assumption if you have not actually studied the field.

 

[PeterDonis]

You seem to forget that people in physics specialize and there is a dimension in forums where people who want to talk can meet, which doesn't seem to be your case. In your case, if you don't want to talk about such fragmented knowledge, don't respond.

This is uncalled for. @Orodruin's response is perfectly appropriate.

 

[PeterDonis]

The OP question has been answered. Thread closed.

 

[PeterDonis]

As a brief follow-up for the benefit of readers:

I was of the opinion that the American Journal of Physics was a “safe” source for students.

No paper is a "safe" source for anyone. You always have to critically evaluate what you read.

For students, I would say reading any paper at all is much less likely to be helpful than reading a textbook. Textbooks have students as their intended audience. Papers have other experts in the field as their intended audience. So even a good paper is much less likely to be helpful to a student than an textbook.