# Is there length contraction?

Discussion in 'Physics & Math' started by phyti, Mar 22, 2010.

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1. ### phytiRegistered Senior Member

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627
F is a static frame
A & B are space monkeys
s = 10 = length of B's ship in F
v = speed in x direction (in c units)
lc: length contraction

case 1 according to F:
vA = 0
vB = .2
s = 9.8
case 2 according to F:
vA = .6
vB = .8
s = 6.0
case 1 according to A:
vB = .2
s = 9.8
case 2 according to A:
vB = .39
s = 5.5 if lc
s = .9 if no lc

The relative speed is .2 in the F frame in both cases,
but A measures different lengths in each case,
and measures different lengths in case 2 depending on lc.

3. ### NeverflyBannedBanned

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3,576
Anal & Banana?

Coming to a Saturday Morning Cartoon channel near you.

5. ### James RJust this guy, you know?Staff Member

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37,189
I don't understand the opening post. You'll need to expand on what you've written.

7. ### phytiRegistered Senior Member

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627
Is there length contraction ?

F is a static frame
A & B are farmers
s = 10 units = length of B's ship in F
v = speed, c= speed of light
h = 1/gamma for B
lc means length contraction

according to F:
vA = .6c
vB = .8c
h = .60, so s = 6.0
according to A:
vB = .38c
h = .93
case 1. s = .93*(6.0) = 5.6 if lc
case 2. s = .93*(10) = 9.3 if no lc

If A remained in F, vB would have to = .83c
for h to = .56, eliminating case 1.
The lc of s = 6.0 in F is the result of the measurement process,
and not a physical change in the ship dimension.

8. ### AlphaNumericFully ionizedRegistered Senior Member

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6,702
Was there something about James's request to explain yourself that you didn't understand? Or are you unable to form a coherent description of your thoughts?

9. ### phytiRegistered Senior Member

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627
The terms in the 2nd post are common to anyone familiar with the basics of SR. I could have questioned his ability to read, but that's something you would do. It's definitely not constructive criticism.

10. ### Jack_BannedBanned

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1,383

Here, you said:

according to F:
vA = .6c
vB = .8c
according to A:
vB = .38c

What is the direction of motion of A and B relative to each other to conclude vB = .38c?

You need to spell all this out.

I think folks are just asking for you to be more specific with your thought experiment which will end up being the barn and pole "paradox".

11. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,167
In the rest frame of A, the velocity of B is 0.38c, as you calculated, and the length of B's spaceship measured in the rest frame of A is 9.2.

The length of B's ship in the rest frame of F is irrelevant, unless A uses measuring tools that are at rest in F.

I won't argue with that, as it stands.

Here's some questions for you:
How many different processes can you think of for measuring the length of a moving object?
Which processes do you think are better, and why?

12. ### phytiRegistered Senior Member

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627
You get it.
When the ship passes A, is it physically contracted, as F claims? That's the question, and the answer is no. If it was, A would measure a shorter ship, even though the relative speed of A and B is less than that of F and B.
It's just another argument against the literal interpretation of length contraction.
Einstein discussed length measurement, and never mentioned or implied a physical process that alters an object.

For local measurements, radar methods, i.e., probing space with light, with its high precision. A recent example was measuring the distance to the moon within an inch or so using laser light.
For astronomical purposes it seems to be doppler shift and light intensity, but these have large margins of error.

13. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,167
F is a reference frame, it can't claim anything.
If the ship is measured using instruments at rest in F, the measurement is shorter than the ship's proper length.

Whether you call that a physical contraction depends on what you mean by "physical contraction".

No, that doesn't follow. The measurement of the ship in F is completely irrelevant to measurement of the ship in the rest frame of A.

That's measuring a distance to an object, not measuring the size of an object.
Try a simpler, practical example.
Say you're watching a train pass by. How would you measure the length of that train?

14. ### phytiRegistered Senior Member

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627
You can't associate an observer with a frame of reference making the measurements?

What it's meant for the last 100 years.

What are doing when measuring the dimension of an object, if not measuring the distance between two ponits in space?

We have moved on from trains to space craft.

My mistake, you don't get it.

15. ### CptBorkValued Senior Member

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6,411
Just for those who might be interested, I realized some time after MacM's thread came and went, that there's a nice simple experimental demonstration of length contraction which was staring me in the face all along. Just look at the angular statistical distribution of scattered particles, and how the scattering angles change as you adjust the energies of the incoming beams (where the energy adjustments correspond to the same effects as boosting). The angular changes match precisely with what the Lorentz spatial contractions predict.

16. ### PeteIt's not rocket surgeryRegistered Senior Member

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10,167
You can if you want to, but you should be clear about it if you do.

Not informative. Whether you call it a physical contraction depends on what you mean by "physical contraction".

Not quite the same thing. The "points" must be moving markers, at rest with respect to each other. The two markers would generally be the two ends of the object of interest.

What's the difference?
Come on, phyti, it's a simple question - how would you measure the length of a train as it passes by? There are a few easy methods. Pick one, and then we can talk about whether the measurement is affected by length contraction.

Last edited: Mar 27, 2010
17. ### Uno HooRegistered Senior Member

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383
If CANGAS or MacM or Geistkessel had authored a thread on this theme, it would already be in the cesspool archive. Due to the unwavering unjust bias of site overlord(s).

Why is "phyti" being granted such unusual slack?

"(Not that there's anything wrong with that)".

18. ### CptBorkValued Senior Member

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6,411
If the arguments start getting repetitive and the same examples keep getting knocked down and built up in circles, this thread will almost certainly find its way to pseudoscience, and if things get nasty it'll go from there to the cesspool.

19. ### phytiRegistered Senior Member

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627
I'll get off the bus here, and it won't appear as favored treatment.

20. ### Farsight

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3,492
What phyti is saying is correct. Length contraction is an aspect of measurement rather than a physical effect. If I can restate this in another way:

You're standing on a 2m pole holding a 1m butterfly net. I am too, and we pass each other at a relativistic speed. We can't swoop our nets down and catch each other.

21. ### rpennerFully WiredValued Senior Member

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4,833
That's a difference without meaning. All of physics is in terms of what we measure. If it was in terms of things we could not measure, it would be metaphysics.

phyti already asked this question in another recent thread where a specific case of the general Lorentzian truth: Even if all inertial observers agree the distance between two world lines is constant, an observer for whom both worldlines have a component of motion radially towards or away from that observer, will measure a smaller distance than another observer for whom there is no such motion.

This is physical, because it is part and parcel of:
1) We all live in the same universe. We may be more or less ignorant of it, but we agree on facts when we aren't prone to making up fabulous nonsense.
2) That universe has a history. Concrete events happen at concrete places and times.
3) We all assign the same history to events where no two of them are separated by more space than could be covered by the passage of light in vacuum. Thus the signing of the Magna Carta always comes before the invention of television, but after the formation of the moon.*
4) The natural order of the universe, to the limits of measurement, does not respect one inertial frame more than another.
5) The measured speed of light does not depend on the motion of the source or detector.

But a length, when it is measured between two worldlines of constant separation is the distance between any pair of events on those worldlines at the same time. So the absoluteness of that measurement is not only not guaranteed by those 5 principles, it is required to be different for different observers so that this consistency of the history of phyti's four events works for both observers.

If there was no length contraction then both observers could not both observe the coordination of mirrors, flashes and reflections.

22. ### Farsight

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3,492
Think it through, rpenner. When I measure you from a distance using the ruler in my hand, I am fully aware of the effects of perspective. I know that you are not three inches tall because I have a deeper knowledge and understanding. It's similar for length contraction.

As I said, length contraction is an aspect of measurement. Nobody doubts that those measurements will be different, the issue is whether they confer reality.

Consider yourself in a spaceship travelling towards a star. You put your foot on the accelerator pedal and reach a relativistic speed, whereupon the star will appear to be flattened into a discoid shape. When you accelerate more, the star appears to be even more flattened. Did it really change shape? No. Your measurement of that star was altered by your change in motion, that's all. To investigate further you swoop past it and approach it from another direction. Lo! This star is magical, it's flat from all angles! It's even flat from my angle of approach in another spaceship!

No, rpenner, we have that deeper knowledge and understanding, we know that our contradictory measurements are merely the result of our different relative motion. We know that a star doesn't really flatten when we step on the gas. Just as we know that we can't both catch each other with those butterfly nets.

23. ### AlphaNumericFully ionizedRegistered Senior Member

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6,702
One must not forget The Terrell Rotation when considering what an object will look like.

When you boost to a frame which is moving close to the speed of light relate to a spherical star you might think it'll look like a disk or squashed sphere but in fact it'll look like a sphere still. The notion of it looking like a disk doesn't account for the fact you're viewing the star from the light it gives off and display differences due to different parts being at different distances from your eyes becomes important.