Take on Length Contraction at relativistic speeds

  • #26
Ibix
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Isn't Alpha Centauri 4.37 light-years from Earth? I don't see any "trick," unless you meant to ask how far Alpha Centauri is from the ship. An observer on the ship will measure a contracted distance, correct?
Furthermore, neither 4.37ly nor the contracted distance is any more correct than the other. They are just measures of different things.
 
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  • #30
PeroK
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It won't. Length contraction is invisible to detectors due to simultaneity. A picture of a moving object will not appear contracted to any detector(s).
https://journals.aps.org/pr/abstract/10.1103/PhysRev.116.1041 (Invisibility of length contraction)
We are talking about measurements of length, not the optical effects when an object is travelling at near light speed. The arrangement of a linear series of detectors in this thread does not lead to the Terrell effects you have linked to.

Length contraction is also invisbile if you are in a darkened room and you detect the movement of an object by physical contact with detectors. So, you can measure length contraction, even if you never see the object.
 
  • #31
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It won't. Length contraction is invisible to detectors due to simultaneity. A picture of a moving object will not appear contracted to any detector.
https://journals.aps.org/pr/abstract/10.1103/PhysRev.116.1041 (Invisibility of length contraction)
Right, a picture won't show anything That's why we need multiple detectors, one at the event "nose was here at at time T" and the other at the event "tail was here at that time T". Now we don't have a picture but we do have two points where the two ends of the object were at the same; the distance between these points is the ocntracted length.
 
  • #32
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To observe, you have to take a picture.
You do not need to take a picture.

Much of the discussion above about multiple detectors is based on an idea in one of the early chapters of Taylor and Wheeler's "Spacetime Physics": A reference frame is modeled as as an infinite lattice of detectors with synchronized clocks, each recording only what happens at the point where they are. Measurements are carried out by collecting these recordings at our leisure and then analyzing them after the fact; there are no light travel delays to correct for.
 
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  • #33
You do not need to take a picture.
You may not need a picture but you need some EM signal to activate a measurement. How do you initiate a measurement?
 
  • #34
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You may not need a picture but you need some EM signal to activate a measurement. How do you initiate a measurement?
You could do it with sound. You could do it with a physical probe: measuring the oil in your car. Something could flick a switch. In fact, the detectors in this thread could be switches that the object collides with and stops a clock.

Also, in physics, a measurement generally (as opposed to a "raw observation") takes into account factors such as travel time of the signal. If you say: event X took place at time ##t##. you do not mean that you received a signal about event X at time t. That's actually fundamental to classical physics. Although a lot of people believe that it's the basis of SR!
 
  • #35
You could do it with a physical probe:
Say you have two probes at each end of a moving stick, the flick will be simultaneous in the stick frame. Non-simultaneous for someone not on the stick. Therefore, the lengths are the same in both frames. Ergo you can't detect length contraction.
 
  • #36
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Say you have two probes at each end of a moving stick, the flick will be simultaneous in the stick frame. Non-simultaneous for someone not on the stick. Therefore, the lengths are the same in both frames. Ergo you can't detect length contraction.
There are a lot of ways to do it. A simple, if slightly clunky approach is:

The object has something at the front and back that flicks a series of switches. The switches record the time they are impacted by the front and the rear. Two time measurements for each switch.

That is then an elementary record of time that the front and rear pass each switch/detector.

If two switches are hit simultaneously in the lab frame, then that represents a simultaneous measurement in that frame, hence a measurement of length in the frame. The non-simultaneity in the object's frame is not an issue in the lab frame.

If two switches are hit simultaneously in the object's frame, then that represents a measurement of the distance between the detectors in the object's frame.
 
  • #37
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You may not need a picture but you need some EM signal to activate a measurement.
Not necessarily, although most practical detector designs will have some electronics in them somewhere. However, this is all besides the point because the essential thing is that we are using different detectors at the nose and at the tail so are not relying on any transmission between the two.
How do you initiate a measurement?
Here's one way of going about it.
Both detectors are standard interrupted-beam obstacle sensors, similar to the one that I installed myself on my automatic garage door. The detectors are constructed identically and placed across the path of the moving object: a light source on one side of the path of the moving object sends a narrow light beam perpendicular to that path. A photodetector on the other side of the path will receive that signal as long as the object is not in the way; and whenever the light appears or disappears will print out a slip of paper with a timestamp and either the word "ON" or "OFF" according to whether the light appeared or disappeared. We can gather up these slips of paper after the object has passed and see what they tell us.

To measure the length of the object I position my detectors in such a way that one detector records an OFF event with the same timestamp as an ON event recorded by the other detector. That gives me my simultaneous measurement of the position of the two ends of the object, and the distance between the detectors is the measured length of the moving object in the frame in which the detectors are at rest.

(It's actually possible to do this measurement with a single detector, but that involves calculating where the front of the object is at the same time that the back of the object is detected. This two-detector setup makes it very clear that we are directly measuring the distance between where the ends of the object are at the same time).

(The light delay for the signal moving between the path of the object and the detector is irrelevant for two reasons: first, in principle it can be made arbitrarily small; and second, the delay is the same at both detectors and independent of the speed or length of the measured object. It certainly does not involve any of the complexities discussed in the APS article you cited above).
 
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  • #38
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PS everyone is going to agree that you don't see "raw" time dilation if a clock is moving away from you or towards you. This is simply because you have the Doppler effect in addition to time dilation. But, you can measure the dilation of a moving clock with two observers - or by taking the light travel time into account.

Likewise, what one observer sees directly of a fast moving object is complicated by light signal travel times. In that respect, no one observer will ever "see" pure length contraction.

But, experimental physics and the concept of measuremnt goes far beyond the raw observations from a single source.
 
  • #39
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Mentors' note: An off-topic digression on exactly what counts as a "measurement" or "observation" of time dilation and lebgth contraction has been removed from this thread. Please don't reopen that particular rathole - it's unhelpful and unwinding it makes unnecessary work for the mentors.
 
  • #40
That gives me my simultaneous measurement of the position of the two ends of the object, and the distance between the detectors is the measured length of the moving object in the frame in which the detectors are at rest.
A simultaneous measurement is not simultaneous to a moving frame. The length measured will be the same. You cannot detect length contraction due to relativity of simultaneity.
 
  • #41
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A simultaneous measurement is not simultaneous to a moving frame. The length measured will be the same. You cannot detect length contraction due to relativity of simultaneity.
It is precisely because of the relativity of simultaneity that the technique I describe yields the contracted length. The two measurements are simultaneous in the frame in which the detectors are at rest, and therefore identify where the ends of the object are at the same time in that frame. The two measurements are not simultaneous in the frame in which the object is at rest and the detectors are moving (in that frame the nose detector is not unmasked until after the tail detector is masked).
 
  • #42
Ibix
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A simultaneous measurement is not simultaneous to a moving frame.
True, but irrelevant. If I want to measure the rest length of the rod, this is relevant. But we're deliberately measuring a non-rest length.
The length measured will be the same.
Can I suggest that you actually do the maths for an array of interrupted-beam sensors as Nugatory describes? Actually, a plane light source at y=+δ that emits a flash of light at t=-δ/c and a large photographic film at y=-δ, where δ is very very small, is easiest to model. The length measured this way will, indeed, be length contracted.
You cannot detect length contraction due to relativity of simultaneity.
In the rod's rest frame, no you can't. In any other frame, yes you can.
 
  • #43
It is precisely because of the relativity of simultaneity that the technique I describe yields the contracted length.
Your technique is omitting something. I will take some time to think about it.
 
  • #44
PeroK
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Your technique is omitting something. I will take some time to think about it.
There is a fundamental flaw with your argument. If you really could never measure length contraction, then it literally would not exist. Physics and SR in particular deal with what you can and do measure. The theory predicts it and a measurement must support it and show the predicted result.

Length contraction in SR is not some abstract mathematical function in the background that has no direct bearing on reality. The contracted length is literally what you measure in that frame.

If you measure the same length as in the rest frame, then there is no length contraction - by definition. Length is what you measure. It's not only an abstract concept in the theory.

Either, therefore, you are claiming that length contraction is wrong; or you are claiming that physics generally does not have a direct relation between the theory and the experiment?

Can you confirm your stated position?
 
  • #45
Can you confirm your stated position?
My position is that to measure the length of an object you need your detector to be on the object you intend to measure. I am still thinking about his technique. I will respond to it later.
 
  • #46
Ibix
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My position is that to measure the length of an object you need your detector to be on the object you intend to measure.
Huh? Why on earth would you need that? How could you even define the length of a gap between two objects (possibly in motion with respect to each other) if that were the case?
 
  • #47
Why on earth would you need that?
For it to be a direct measurement.
 
  • #48
Ibix
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For it to be a direct measurement.
Would you mind answering the other question in my post:
How could you even define the length of a gap between two objects (possibly in motion with respect to each other) if that were the case?
 
  • #49
Would you mind answering the other question in my post:
I meant be in the frame as the frame you wish to measure the length of.
 
  • #50
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My position is that to measure the length of an object you need your detector to be on the object you intend to measure.
That's how you measure the rest length - that is, a measurement the distance between where the two ends are using a frame in which the object is at rest.
 

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