Deriving Length Contraction: Is There Another Way?

In summary: On page two there is a derivation with x' =0 and x = v/c(ct).Web.bryant.edu/~bblais/pdf/relativity.pdf
  • #1
bernhard.rothenstein
991
1
Textbooks I know derive the formula that accounts for the time dilation by measuring simultaneously the space coordinates of a moving rod.
Is there another way to derive it?
 
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  • #2
bernhard.rothenstein said:
Textbooks I know derive the formula that accounts for the time dilation by measuring simultaneously the space coordinates of a moving rod.
Is there another way to derive it?

I am guessing you meant "lenght" instead of time in your first sentence. And I am assuming that you mean "deriving from the Lorentz transformations".

That's the most direct way to measure the length of a moving object: measuring the positions of its extremities at the same time in the frame of the measurement. Butthis requires two observers (it could be done with a single observer but then th eobserver is not local to the two events and must take into account the finite speed of light and that makes things much more complicated).

A second way requiring only a single observer is to do the following: Notice the time it takes for the moving object to pass in front of you (i.e. measure the time elapsed between the front of the object being aligned with you and the back of the object being aligned with you. Use the fact that this time you measured must be equal to the length of the object (in your frame) divided by the speed of the object. The speed and the lengths are two unknowns. Now use this in the Lorentz transformations and you can solve for the lenth contraction formula.
 
  • #3
bernhard.rothenstein said:
Textbooks I know derive the formula that accounts for the time dilation by measuring simultaneously the space coordinates of a moving rod.
Is there another way to derive it?
[tex]ds^2=dt^2-dx^2[/tex] is an invariant.
Choosing dx=vdt in one frame and dx=0 in another gives the TD formula.
 
  • #4
length contraction

clem said:
[tex]ds^2=dt^2-dx^2[/tex] is an invariant.
Choosing dx=vdt in one frame and dx=0 in another gives the TD formula.
Please tell me if that can be done in the case of length contraction?
 
  • #5
length contraction time dilation

kdv said:
I am guessing you meant "lenght" instead of time in your first sentence. And I am assuming that you mean "deriving from the Lorentz transformations".

That's the most direct way to measure the length of a moving object: measuring the positions of its extremities at the same time in the frame of the measurement. Butthis requires two observers (it could be done with a single observer but then th eobserver is not local to the two events and must take into account the finite speed of light and that makes things much more complicated).

A second way requiring only a single observer is to do the following: Notice the time it takes for the moving object to pass in front of you (i.e. measure the time elapsed between the front of the object being aligned with you and the back of the object being aligned with you. Use the fact that this time you measured must be equal to the length of the object (in your frame) divided by the speed of the object. The speed and the lengths are two unknowns. Now use this in the Lorentz transformations and you can solve for the lenth contraction formula.

I think there are to complementary experiments. In the first one in I we use a stationary rod of proper length L(0) and two synchronized at its ends. We measure the speed of a clock moving between its two ends as as a quotient between a proper length L(0) and a coordinate time interval T
V=L(0)/T. (1) Reverse the situation considering that an observer using the moving clock as a wrist watch measure the speed of the rod used in the previous experiment. He measures a proper time interval T(0) but a distorted(?) length L and considers that the speed of the rod is
V=L/T(0) . (2)

The result is
L(0)/T=L/T(0) (3)
Is it correct to conclude that if we have length contraction in our hand we have time silation as well and viceversa?
 
  • #6
bernhard.rothenstein said:
Please tell me if that can be done in the case of length contraction?

On page two there is a derivation with x' =0 and x = v/c(ct).
web.bryant.edu/~bblais/pdf/relativity.pdf
 
  • #7
bernhard.rothenstein said:
Please tell me if that can be done in the case of length contraction?
No. [tex]dx'^2-dt'^2=dx^2-dt^2[/tex].
In the moving frame, classical intuition is used to assume that setting dt'=0 gives the correct length of a moving rod. No formula results from the equation because dt can be anything in the rest system. The Lorentz transformation is needed to discuss length.
 
  • #8
pam said:
No. [tex]dx'^2-dt'^2=dx^2-dt^2[/tex].
In the moving frame, classical intuition is used to assume that setting dt'=0 gives the correct length of a moving rod. No formula results from the equation because dt can be anything in the rest system. The Lorentz transformation is needed to discuss length.

Very nice proof. The above may also be a good explanation why there are no experimental tests of length contraction to date: http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html
 
  • #9
length contraction

pam said:
No. [tex]dx'^2-dt'^2=dx^2-dt^2[/tex].
In the moving frame, classical intuition is used to assume that setting dt'=0 gives the correct length of a moving rod. No formula results from the equation because dt can be anything in the rest system. The Lorentz transformation is needed to discuss length.

I think there are two complementary experiments. In the first one in I we use a stationary rod of proper length L(0) and two synchronized clocks at its ends. We measure the speed V of a clock moving between its two ends as as a quotient between a proper length L(0) and a coordinate time interval T
V=L(0)/T. (1) Reverse the situation considering from I' that an observer using the moving clock as a wrist watch measures the speed of the rod used in the previous experiment. He measures a proper time interval T(0) but a distorted(coordinate) length L and considers that the speed of the rod is
V=L/T(0) . (2)

The result is
L(0)/T=L/T(0) (3)
Consider the invariance of the space-time interval in the conditions in which in I' we measure a proper time interval T(0) i.e.
VVTT-ccTT=-ccT(0)T(0) (4)
or
VV-cc=-cc[T(0)/T]^2=-cc[L/L(0)] (5)
where we have taken into account (3)
where from the formula that accounts for the length contraction
L=L(0)sqrt(1-VV/cc) (6)
Do you discover a flow in the lines above?
Thanks for your participation on the thread I started.
 

What is length contraction?

Length contraction is a phenomenon in which an object's length appears to decrease when it is moving at a high speed relative to an observer. This is a consequence of Einstein's theory of special relativity.

What is the formula for length contraction?

The formula for length contraction is L' = L * √(1-v^2/c^2), where L' is the contracted length, L is the original length, v is the velocity of the object, and c is the speed of light.

How is length contraction derived?

Length contraction can be derived mathematically using the principles of special relativity. It involves using the Lorentz transformation equations to transform the coordinates of an object in one frame of reference to the coordinates in another frame of reference that is moving at a constant velocity relative to the first frame.

Is length contraction a real phenomenon?

Yes, length contraction has been observed and confirmed through various experiments and observations. It is a fundamental aspect of special relativity and has been validated by numerous scientific studies.

What are some real-life applications of length contraction?

Length contraction has important implications in fields such as particle physics, where it helps explain the behavior of subatomic particles moving at high speeds. It is also a crucial factor in the design of technologies such as particle accelerators and high-speed spacecrafts.

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