# Evidence for length contraction

#### redruM

would some be able to give me an example that shows some evidence of length contraction?

i have looked at the muons example, but it seem to be dependent on the time dilation effect, rather than being evidence of length contraction.

thanks for any help.:)

much appreciated.

btw, feel free to move this thread, if it isnt in its relevant location.

Related Special and General Relativity News on Phys.org

#### Njorl

If I run real fast for a long time, I will get thinner!

Njorl

#### redruM

lol.

hmm...maybe i should try that

#### chroot

Staff Emeritus
Gold Member
Time dilation and length contraction occur concurrently. They are, in effect, two ways to describe the same effect. The Lorentz transformations are the "cause" of both effects.

In the case of the muon, you have two choices: you can consider the muon's clock running more slowly than a stationary muon's clock on earth, due to its high velocity.

You can also think that, to the muon, the earth's atmosphere is length-contracted, making it not so thick and easier to penetrate.

- Warren

#### lightarrow

redruM said:
would some be able to give me an example that shows some evidence of length contraction?
I know, I'm a bit late to answer!
A clear example is doppler red shift of stars light.

#### MeJennifer

lightarrow said:
I know, I'm a bit late to answer!
A clear example is doppler red shift of stars light.
Could you explain how that is evidence for length contraction?
The redshift of starlight is a combination of the regular doppler effect adjusted for relativistic time dilation and gravitational time dilation.

Last edited:

#### jtbell

Mentor
There is no direct experimental evidence for length contraction, as far as I know. That is, no one has managed to measure the length of a fast-moving object, by measuring the locations of its ends simultaneously in the laboratory reference frame, and get a result that differs from the non-relativistic expectation.

As one might expect, the technical obstacles are rather large. Either you have to make a macroscopically sized object move very very fast, and then figure out how to measure it while it's whizzing past you, or you have to be able to make very very precise measurements on somewhat more slowly moving objects.

I don't consider this to be a problem for the validity of relativity, because length contraction is so tightly bound with other phenomena that we can verify. It sure would be nice to do this, though!

#### bernhard.rothenstein

redruM said:
would some be able to give me an example that shows some evidence of length contraction?

i have looked at the muons example, but it seem to be dependent on the time dilation effect, rather than being evidence of length contraction.

thanks for any help.:)

much appreciated.

btw, feel free to move this thread, if it isnt in its relevant location.
Please have a critical look at
arXiv.org > physics > physics/0507016
All papers Titles Authors Abstracts Full text Help pages
Physics, abstract
physics/0507016
From: Bernhard Rothenstein [view email]
Date: Sun, 3 Jul 2005 18:04:26 GMT (224kb)

Length measurement of a moving rod by a single observer without assumptions concerning its magnitude
Authors: Bernhard Rothenstein, Ioan Damian
Subj-class: Physics Education

We extend the results presented by Weinstein concerning the measurement of the length of a moving rod by a single observer, without making assumptions concerning the distance between the moving rod and the observer who measures its length.
Full-text: PDF only

where I try to show that there is a lenght measurement procedure which can lead to length contraction but to lenght dilation as well.

#### Sojourner01

I don't think anyone's ever moved a macroscopic man-made object at anywhere near relativistic speeds.

There is one possibility, though:

The planet Mercury. Mercury is close enough to the sun to show relativistic effects when viewed from earth. The time dilation has been proven; but I don't think anyone's done any research on a corresponding length contraction.

#### Meir Achuz

Homework Helper
Gold Member
jtbell said:
There is no direct experimental evidence for length contraction, as far as I know. That is, no one has managed to measure the length of a fast-moving object, by measuring the locations of its ends simultaneously in the laboratory reference frame, and get a result that differs from the non-relativistic expectation.

As one might expect, the technical obstacles are rather large. Either you have to make a macroscopically sized object move very very fast, and then figure out how to measure it while it's whizzing past you, or you have to be able to make very very precise measurements on somewhat more slowly moving objects.

I don't consider this to be a problem for the validity of relativity, because length contraction is so tightly bound with other phenomena that we can verify. It sure would be nice to do this, though!
This is the best answer to date until the penultimate sentence.
The measured length of a moving object depends on how you measure it.
If you use a pre-relativistic (Galilean) notion that an equal time measurement is appropriate, then you get an equal time measurement.
With a light cone measurement, as is fashionable in parton physics or photography, a moving sphere remains a sphere. If you look at a moving sphere, you see a sphere. Some people would consider this a measurement. ("Seein is believin.") The only unambiguous measurement of the length of an object as an intrinsic propert is in its rest system, if you don't look at it from a rotated angle.

#### lightarrow

MeJennifer said:
Could you explain how that is evidence for length contraction?
c = nu*lambda. The frequency nu decreases because of time contraction, so the wavelenght lambda must increase to keep c at the same value.
This is actually evidence (IMO) for lenght dilation (lambda), instead of contraction, but the principle is the same.
Of course gravitational redshift must also be taken into account, but, after having considered it, what remains is the doppler effect.

#### petm1

lightarrow said:
c = nu*lambda. The frequency nu decreases because of time contraction, so the wavelenght lambda must increase to keep c at the same value.
This is actually evidence (IMO) for lenght dilation (lambda), instead of contraction, but the principle is the same.
Of course gravitational redshift must also be taken into account, but, after having considered it, what remains is the doppler effect.
It would make more sense (to me) out of SR if you did have length dilation, and time dilation when talking about things moving faster than our perferred frame of reference, then the change to the meters per second in the speed of light would be the same for both the meter and the second keeping the speed of light proportional relative to the moving observer.

chroot said:
Time dilation and length contraction occur concurrently. They are, in effect, two ways to describe the same effect. The Lorentz transformations are the "cause" of both effects.

In the case of the muon, you have two choices: you can consider the muon's clock running more slowly than a stationary muon's clock on earth, due to its high velocity.

You can also think that, to the muon, the earth's atmosphere is length-contracted, making it not so thick and easier to penetrate.
You, using length dilation, could think of the muon as increasing in size, which would also explain why the earths atmosphere appears to be length contracted to the muon.

#### pmb_phy

redruM said:
would some be able to give me an example that shows some evidence of length contraction?

i have looked at the muons example, but it seem to be dependent on the time dilation effect, rather than being evidence of length contraction.

thanks for any help.:)

much appreciated.

btw, feel free to move this thread, if it isnt in its relevant location.
If you have a long straight current carrying wire in frame S and you move to a frame S' which is moving parallel to S then the wire will be come charged. This is a direct effect of length contraction.

Pete

#### Meir Achuz

Homework Helper
Gold Member
pmb_phy said:
If you have a long straight current carrying wire in frame S and you move to a frame S' which is moving parallel to S then the wire will be come charged. This is a direct effect of length contraction.

Pete
This effect is because (rho,j) transform as a 4-vector. It has never been observed experimentally as "evidence".

#### Meir Achuz

Homework Helper
Gold Member
redruM said:
would some be able to give me an example that shows some evidence of length contraction?
Although the thread has lengthened, the short answer ot your question is that I do believe the equations, but there is no evidence of length contraction.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving