Frames of Reference

  • Thread starter H0T_S0UP
  • Start date
rbj
2,221
7
No, you've got it backwards, it wouldn't appear to contract because light takes time to reach an observer, even though it "really" would have contracted in your frame. Like I said, if its rest length was 2 ft, then if it's moving at 0.6c in your frame, then its "real" length in your frame would be 1.6 ft because of Lorentz contraction, but visually it would still appear to be 2 ft long because of the Penrose-Terrell effect.
now i'm confused. you are saying that the Penrose-Terrell effect means that length-contracted rods do not appear length-contracted??:confused:

i thought the whole point of length contraction in SR is that a rod moving in the same direction as its orientation (along the long dimension of the rod) at relativistic speeds relative to some observer appears shorter to that observer than it would to an observer who was moving along with the rod (to whom the rod appears stationary).
 
JesseM
Science Advisor
8,492
12
So it is measured to be shorter but visually it is not? That does not make any sense.
You probably meant to write observed to be shorter.
"Measured" and "observed" are usually synonymous in SR, I think. The measurement would involve the standard notion of a network of rigid rulers and clocks all sharing the same inertial rest frame and with the clocks synchronized according to the Einstein synchronization convention, with events assigned coordinates based solely on local readings in this system. So, for example, if a rod is moving relative to my network of rulers and clocks, and at the moment the back end of the rod passes the 15-meter mark on my x-axis ruler, the clock at the 15-meter mark reads a time of 12 seconds, then I say that at t=12 seconds in my frame the back end of the rod was at position x=15 meters. Likewise, if at the moment the front end of the rod passes the 10-meter mark on my x-axis ruler, the clock at the 10-meter mark reads a time of 12 seconds, then I say that at t=12 seconds in my frame the front end of the rod was at position x=10 meters. Are these not measurements? And since these measurements are simultaneous in my frame, that must mean that in my frame the rod is measured to be 15-10 = 5 meters long.
 
Last edited:
JesseM
Science Advisor
8,492
12
now i'm confused. you are saying that the Penrose-Terrell effect means that length-contracted rods do not appear length-contracted??:confused:
Visually, no they don't appear to be contracted, although they appear distorted in other ways. See this thread.
rbj said:
i thought the whole point of length contraction in SR is that a rod moving in the same direction as its orientation (along the long dimension of the rod) at relativistic speeds relative to some observer appears shorter to that observer than it would to an observer who was moving along with the rod (to whom the rod appears stationary).
This is a common misunderstanding--length contraction and time dilation are not based on visual perceptions, rather they are based on coordinates assigned to events which factor out the problem of light delays, either by backdating every perceived event based on its distance and the assumption that the light from the event to you was moving at c in your frame (so if I see an event 10 light-seconds away when my clock reads t=30 seconds, I'll assign the event a time-coordinate of t=20 seconds in my frame), or more commonly by assuming that all measurements are local ones made on the type of grid of rulers and clocks I talked about in my last post to MeJennifer. See this thread for more on the importance of factoring out light delays in SR.
 
1,986
5
Perhaps Jesse, we should specifically request if any of the statements you make on this forum pertain to physical measurements done by scientific instruments (or is that still too vague for you?) or that they are just inferences by imagining a plane of simultaneity?

In the past you have made a clear distinction between what is measured and what is observed now you seem to be backtracking from that, or is it perhaps that you have trouble admitting that you are wrong once in a while?
 
JesseM
Science Advisor
8,492
12
Perhaps Jesse, we should specifically request if any of the statements you make on this forum pertain to physical measurements done by scientific instruments (or is that still too vague for you?) or that they are just inferences by imagining a plane of simultaneity?
What, local measurements on actual physical clocks are not "physical measurements done by scientific instruments"? Of course the resulting length measurement is frame-dependent (and dependent on using the Einstein synchronization convention to synchronize the clocks), but it's still a type of "measurement" as I would understand the term.
MeJennifer said:
In the past you have made a clear distinction between what is measured and what is observed
Really? What specific post are you thinking of? Anyway, this is just semantic quibbling on your part, I think it is standard to define "measurement" as synonymous with "observation" in SR, but if you choose to reserve "measurement" solely for frame-independent quantities, no one is going to stop you, as long as you spell out your definition and don't play gotcha games of saying people are wrong for using the term differently (unless you can show that your usage is the standard accepted one among physicists).
MeJennifer said:
now you seem to be backtracking from that, or is it perhaps that you have trouble admitting that you are wrong once in a while?
That's pretty rich coming from you, who has made all sorts of silly claims that you simply stopped discussing after I pressed you on them and pointed to statements from mainstream physicists that contradicted them, like the claim that black holes can't form in closed universes, or the claim that there is a single "true" distance between two objects defined (in some manner you refused to actually spell out) by light travel time.
 
32
0
Ok about this length contraction, what is actually happening to the rod that causes it to lose volume (relative to my frame)? If I were to run past the rod at twice its speed after I just watched the rod zoom past me when I was motionless would I see an increase in size?
 
JesseM
Science Advisor
8,492
12
Ok about this length contraction, what is actually happening to the rod that causes it to lose volume (relative to my frame)? If I were to run past the rod at twice its speed after I just watched the rod zoom past me when I was motionless would I see an increase in size?
Again, all motion is relative motion. As long as you are moving inertially, it is always possible to find an inertial frame where you are at rest, and in this frame it will always be other objects in motion relative to you that are shrunk relative to you. Nothing ever expands to a length greater than its rest length in any inertial frame.
 
paw
169
0
Ok about this length contraction, what is actually happening to the rod that causes it to lose volume (relative to my frame)? If I were to run past the rod at twice its speed after I just watched the rod zoom past me when I was motionless would I see an increase in size?
Try thinking of it this way. Nothing is really happening to the rod. No forces are acting on it. It's not being crushed or anything like that.

What is changing is the result you get when you measure it. If the rod goes past you at some speed v you will measure the rod to be shorter than if it was stationary from your point of view. If someone was riding on the rod they won't see the rod change in any way .

Now if you accelerate and pass the rod so it appears to have reversed direction and is now travelling some other speed, say -2v from your new frame of reference, the rod appears even shorter than it did before when it appeared to be doing speed v. Notice the direction doesn't matter, just the relative speed.

Say you now slow down and let the rod catch up with you then match speed with it. From your original frame both you and the rod are now travelling at v but it is easier to define a new frame where both you and the rod are not moving. In this case you measure the rod to be the same as its original length which is exactly what you'd expect.

The reason this works is there's no way to decide whether you, the rod or both are moving in any absolute sense. Because of this we are free to use whatever inertial frame (point of view) we want.

So it's all in how you look at it, from what frame of reference you are measuring the rod. This doesn't make length contraction any less real. It is a real effect. But hopefully this will help you understand that nothing is really happening to the rod.
 
1,986
5
Try thinking of it this way. Nothing is really happening to the rod. No forces are acting on it. It's not being crushed or anything like that.
That is correct.

All that is happening is that some people want to "explain" relativity by creating 3-planes of simultaneity that gives an enormous source of confusion. Such 3-planes are simply mental constructs as there is nothing physical about them.

A far better description of what is really happening, e.g what is actually measured instead of inferred by such measurements is to use Bondi k-calculus.
 
303
0
All that is happening is that some people want to "explain" relativity by creating 3-planes of simultaneity that gives an enormous source of confusion. Such 3-planes are simply mental constructs as there is nothing physical about them.
I never really thought of them as 3-planes, but I can see why you refer to them as such. However confusing they may be, I don't see them as inherently inaccurate if applied appropriately.

A far better description of what is really happening, e.g what is actually measured instead of inferred by such measurements is to use Bondi k-calculus.
I suppose this begs the question of whether an inference made by one technique agrees with calculation based upon another approach to the same problem.

Regards,

Bill
 
JesseM
Science Advisor
8,492
12
That is correct.

All that is happening is that some people want to "explain" relativity by creating 3-planes of simultaneity that gives an enormous source of confusion. Such 3-planes are simply mental constructs as there is nothing physical about them.

A far better description of what is really happening, e.g what is actually measured instead of inferred by such measurements is to use Bondi k-calculus.
This is a pedagogical opinion, one on which all textbook authors seem to disagree with you since they all start out by introducing the notion of inertial frames (and as a matter of pedagogy, aren't the algebraic equations of SR in inertial coordinate systems a bit easier for beginning students than the Bondi k-calculus?) And can you express the idea that the laws of physics must be "Lorentz-invariant" without referring to the notion of inertial coordinate systems constructed in the standard way? If your answer is yes, please provide a reference. If not, that's a major weakness of your approach to thinking about relativity, since Lorentz-invariance is a very important symmetry in physics.
 
1,986
5
And can you express the idea that the laws of physics must be "Lorentz-invariant" without referring to the notion of inertial coordinate systems constructed in the standard way?
You do realize that Lorentz transformations are coordinate transformations right?
 
JesseM
Science Advisor
8,492
12
You do realize that Lorentz transformations are coordinate transformations right?
Sure, but the fact that the equations of the laws of physics are invariant under this transformation and not some other (such as the Galilei transformation) is a real physical symmetry of the laws of nature, just like spatial translation symmetry and time translation symmetry. See Lorentz covariance.
 

Related Threads for: Frames of Reference

  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
10
Views
2K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
6
Views
882
  • Last Post
3
Replies
69
Views
4K
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
20
Views
8K
  • Last Post
Replies
5
Views
2K
Top