Rod shortening - physical reality?

[Pierre007080]

Hi Guys,
I seem to have gone in circles YET AGAIN! When discussing the expansion of the Universe I was told by friends that masses and objects do not expand with space because the forces within the mass are so much stronger than the inflationary force. When discussing General relativity we decided that the rod shortening WAS a physical reality. Now the debate has turned to Special Relativity and the question is: Is the rod shortening in Special Relativity physically real or only "observed"

 

[starthaus]

Hi Guys,
I seem to have gone in circles YET AGAIN! When discussing the expansion of the Universe I was told by friends that masses and objects do not expand with space because the forces within the mass are so much stronger than the inflationary force. When discussing General relativity we decided that the rod shortening WAS a physical reality. Now the debate has turned to Special Relativity and the question is: Is the rod shortening in Special Relativity physically real or only "observed"

With the risk of confusing you, the fact that shortening is "observed" (from a moving frame) means that it (the shortening) is "physically real". To further confuse you, the shortening is:

-dependent on the relative speed between the frame comoving with the object and the observer frame (so, it "varies")

-strain-free (it does not induce any strain in the rod)

 

[jtbell]

It depends on your definition of "physical reality."

The energy and momentum of an object also depend on the reference frame. Do you consider this to be "physically real" or "only observed?" Whichever answer you choose, you should apply it to the length of an object also, for consistency.

 

[starthaus]

It depends on your definition of "physical reality."

The energy and momentum of an object also depend on the reference frame. Do you consider this to be "physically real" or "only observed?" Whichever answer you choose, you should apply it to the length of an object also, for consistency.

Very good point.

 

[GRDixon]

Hi Guys,
Now the debate has turned to Special Relativity and the question is: Is the rod shortening in Special Relativity physically real or only "observed"

Consider a macroscopic version of the Bohr atom, with a central, fixed (massive), positively charged body and a circularly orbiting, very light, negative satellite. Adjust the satellite orbital parameters so that it goes in a circle, under the influence of the central body's electrostatic field. Now view the system from a frame in which the central body translates to the left at speed v equal to the electron's speed in the first frame. (In this second frame the central body of course engenders a magnetic field, as well as an electric field.) Compute the electron's trajectory in this second frame. You'll find out that it isn't a true cycloid; the electron cuts "in front of" and "behind" the central body at a length-contracted distance. Also, viewed from the second frame, the time for one complete cycle of the electron is time dilated, quite as SRT predicts. To the extent such results pertain to actual atoms, I would say that length contraction and time dilation are "real".

 

[Dale]

It depends on your definition of "physical reality."

I agree completely.

To the OP, how would you experimentally distinguish between something that was "physically real" and something that was not?

 

[Pierre007080]

Thanks for the input Guys. "physically real" would be the aspect of something that does not change with or depend on the observer's relative movement.You have managed to confuse me further, but I still maintain that the shortening caused by GR is depends primarily on the gravitational potential.Yes, the observers relative speed would affect the shortening further but surely the GR shortening aspect cannot be viewed as not "physically real"

 

[bcrowell]

When discussing the expansion of the Universe I was told by friends that masses and objects do not expand with space because the forces within the mass are so much stronger than the inflationary force.

There are no forces involved in either case. Here is a discussion of the issue of whether space actually expands cosmologically: http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html#Section8.2 (subsection 8.2.5)

When discussing General relativity we decided that the rod shortening WAS a physical reality.

"physically real" would be the aspect of something that does not change with or depend on the observer's relative movement.

Putting these two statements together results in something incorrect. When you say "rod shortening," it sounds like you're talking about special-relativistic length contraction. The description of SR length contraction doesn't change when you discuss it in the context of GR, and in GR it's still frame-dependent.

On the other hand, if you have cosmological expansion in mind, then there are two problems with this: (1) it's an expansion, not a contraction; (2) it doesn't affect a measuring rod by any realistically detectable amount.

If you want to make people understand you, I'd suggest you change the terms you're using. Since you say you're defining "physically real" as "frame-independent," please use the latter term, and people here will know what you mean. Also, I'd suggest that you not use the term "rod shortening" to refer to cosmological effects in GR, for the reasons given above.

 

[Pierre007080]

Hi bcrowell,
Thanks for your assistance. I suppose "frame independent" explains more accurately my question. Can I rephrase the question because no one has actually attempted to answer it. Is the distance contraction (rod shortening) of GR frame independent?

 

[yuiop]

Thanks for the input Guys. "physically real" would be the aspect of something that does not change with or depend on the observer's relative movement.You have managed to confuse me further, but I still maintain that the shortening caused by GR is depends primarily on the gravitational potential.Yes, the observers relative speed would affect the shortening further but surely the GR shortening aspect cannot be viewed as not "physically real"

By your definition of "physically real", the length contraction of a rod observed in both SR and GR is not real because the measurement is observer dependent. If you define "physically real" as "that which is measured by an observer" then the length contraction in SR due to relative motion and the length contraction in GR due to relative gravitational potential are both physically real.

As jtbell said, "It depends on your definition of 'physical reality.'"

Either way, the physical reality of length contraction in either SR or GR stand or fall by the same criteria.

 

[Vanadium 50]

"physically real" would be the aspect of something that does not change with or depend on the observer's relative movement.

As mentioned before "frame independence" is a better name for this.

Note that a lot of things fail your criteria for being physically real (even in classical physics): energy, momentum, frequency, velocity...

 

[bcrowell]

I suppose "frame independent" explains more accurately my question. Can I rephrase the question because no one has actually attempted to answer it. Is the distance contraction (rod shortening) of GR frame independent?

A whole bunch of people have tried to answer your question, but you haven't been making it easy, because you've been ignoring our attempts to get you to phrase the question in such a way that we can make sure we're answering it. Please re-read #8, and see if you can state the question in such a way that we can understand what you mean by "rod shortening."

 

[yuiop]

As mentioned before "frame independence" is a better name for this.

Note that a lot of things fail your criteria for being physically real (even in classical physics): energy, momentum, frequency, velocity...

Very good point Vanadium.

 

[JulianM]

OK, so the guy is having trouble anticipating what specific terminology is being asked for, but I still think that the question is worth an answer.

The original question was:
Is the rod shortening in Special Relativity physically real or only "observed"

To help the experts let's clarify that as
Is the rod shortening in Special Relativity frame independent or only "observed" (e.g. has a specific length measured by an observer who is peculiar to only one specific uniformly moving, inertial, non-rotating frame)

It's a great question !
It also leads to other questions like:

What length is the rod when it is not being "observed" ?
What length is it if there are multiple observers ?
What happens to the rod if the observer blinks rapidly ?

 

[Tantalos]

If two observers from two reference frames can see the same rod having at the same time two diffrent lengths, then can it be real?

 

[Doc Al]

To help the experts let's clarify that as
Is the rod shortening in Special Relativity frame independent or only "observed" (e.g. has a specific length measured by an observer who is peculiar to only one specific uniformly moving, inertial, non-rotating frame)

That's already been answered: Length of a rod is a frame-dependent quantity. (This, of course, is one of the surprises of special relativity. Who knew?)

It's a great question !
It also leads to other questions like:

What length is the rod when it is not being "observed" ?
What length is it if there are multiple observers ?
What happens to the rod if the observer blinks rapidly ?

Don't get hung up on the idea of "observation". There is a prescription for defining the length of an object. Each frame gets its own answer, which is independent of the existence of an actual "observer". The length of a rod might be frame dependent, but it's not dependent on someone actually "looking" at it! (Relativity is not that crazy. )

If two observers from two reference frames can see the same rod having at the same time two diffrent lengths, then can it be real?

This thread seems to be going around in circles.

 

[bcrowell]

OK, so the guy is having trouble anticipating what specific terminology is being asked for, but I still think that the question is worth an answer.

When you say "anticipating," you make it sound as if we expected him to read our minds. There is a standard terminology used in the field. It's widely known and accepted. It's described in all the standard textbooks. It's not a big deal if a newbie makes mistakes in terminology, but when that's pointed out to him, he needs to pay attention to fixing it. We can't have a discussion unless we can use words that have a mutually understood meaning.

Is the rod shortening in Special Relativity frame independent or only "observed" (e.g. has a specific length measured by an observer who is peculiar to only one specific uniformly moving, inertial, non-rotating frame)

The answer is yes.

It's a great question !
It also leads to other questions like:

What length is the rod when it is not being "observed" ?
What length is it if there are multiple observers ?
What happens to the rod if the observer blinks rapidly ?

These are all meaningless questions. If two different people propose two different answers to one of these questions, there is no way of settling the disagreement by performing experiments.

 

[JulianM]

These are all meaningless questions. If two different people propose two different answers to one of these questions, there is no way of settling the disagreement by performing experiments.

So if you propose it is shortened, and I propose it just looks like it is shorter, then there is no experiment that can prove/dis-prove either of us ?

I am going to propose instead, that from an investigate position there are no meaningless questions - just meaningless answers

 

[matheinste]

These are all meaningless questions. If two different people propose two different answers to one of these questions, there is no way of settling the disagreement by performing experiments.

So if you propose it is shortened, and I propose it just looks like it is shorter, then there is no experiment that can prove/dis-prove either of us ?

I am going to propose instead, that from an investigate position there are no meaningless questions - just meaningless answers

Here is a proposed demonstration by Rindler and others. Unfortunately I cannot draw the diagram. This would save a lot of words. I hope I have the wording correct because I am doing it from memory.

Take three identical rods. Let one, for the present purposes, be the stationary rod. Let the two other rods be moving at equal speeds but opposite directions towards and along the length of the stationary rod. Arrange for the left hand end of the rod moving to the right to coincide with the left hand end of the stationary rod, and the opposite for the rod moving to the left, at the same time in the rest frame of the stationary rod. Each of the two moving rods will appear shorter than the staionary rod to an observer at rest relative to the stationary rod.

But once again, do we describe this as "real" or not.

Matheinste.

 

[Ich]

I think it's necessary to get one step more abstract, towards the spacetime view of SR:
A rod is not something three dimeansional, it is something four dimensional. It is extended in time also.
If it were three dimensional, it would make sense to discuss whether it's really changing or not.
But it is fourdimensional.
Different observers will call a different 3-D slice of it "the rod", so yes, the 3-D rods all really are different.
But the real rod, the 4-D entity, exists only once, unimpressed by observers. It doesn't change upon observation.

 

[JulianM]

How can the 4-D entity be viewed, or maybe I mean from where, or something.

 

[Ich]

The fourth dimension is not something weird. It's simply time. Every object is extended in the spatial dimensions, and also in time. If it existed just at a single instant, you couldn't see it at all. It exists for a certain period of time (preferrably at least as long as your experiment lasts). You measure its length by noting the positions of its endpoints at a certain moment(that's what I meant by taking a 3D-slice through the object, you get rid of the extension in time), and calculating their distance. Different observers have different notions of simultaneity, so it's not strange that the results are different.

How can the 4-D entity be viewed

Well, you see its 2-D surface. Just try it, all objects in your room are 4D.

 

[yuiop]

Here is a thought experiment that might shed some light on the "reality" of rod shortening.

Imagine we have two hollow rings with identical dimensions. Inside the cavity of one ring we place 1000 identical short rods end to end so that they are touching but not compressed or stretched and form a complete ring that comfortably fills the inside of the hollow ring. In the second hollow ring we inject identical rods at 0.8666c and by careful timing we manage to inject 2000 identical rods so that they comfortably touch end to end and form a complete ring circulating inside the second ring without tension or compression. Is that an just an illusion brought about by differences in opinion in simultaneity or is there something more tangible going on there?

1000 rods comfortably filling one hollow ring and 2000 identical short rods comfortably filling another identical hollow ring, where the only difference is the motion of the rods.

If the rods are magnetic and the 2000 short rods inside the second hollow ring are slowed down using an electromagnetic field, the hollow ring would eventually explode as it is no longer able to contain the 2000 short rods when they slow down, because their combined rest length is longer than the circumference of the hollow ring they are inside of.

I am not going to offer an opinion of whether that is a demonstration of the "reality" of rod shortening or not, because "reality" is a philosophical notion, but it is certainly food for thought.