Length Contraction Explained Simply

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Discussion Overview

The discussion revolves around the concept of length contraction in the context of special relativity, exploring its implications, evidence, and relationship with time dilation. Participants seek to clarify the phenomenon and its experimental support, as well as the interpretations of related historical experiments.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants express difficulty in understanding length contraction and seek simpler explanations.
  • One participant notes that length contraction and time dilation are observed effects depending on the relative motion of observers and their reference frames.
  • Another participant questions the existence of direct evidence for length contraction, suggesting that it may not be proven.
  • Some argue that the Michelson-Morley experiment (MMX) is related to length contraction, while others assert it does not provide direct proof.
  • There are conflicting interpretations regarding how observers perceive lengths and time in different frames of reference, particularly in relation to moving objects.
  • Participants discuss the implications of the isotropy of light speed and how it relates to the null result of the MMX.
  • Some participants clarify that while there may not be direct tests for length contraction, the MMX can be seen as an indirect proof.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the relationship between length contraction and the Michelson-Morley experiment, nor on the existence of direct evidence for length contraction. Multiple competing views remain regarding the interpretations of the phenomena and their implications.

Contextual Notes

There are unresolved questions about the definitions and assumptions underlying length contraction and its experimental verification. The discussion reflects a range of interpretations and the complexity of the concepts involved.

bhsmith
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I'm having a hard time understanding length contraction. can anyone explain it to me in simple terms?
 
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just remember that the length of an object in one reference frame is the distance between the front and back at one simultaneous moment so relativity of simultaneity enters into it.
 
Alright, I understand that. I have been researching Length Contraction and i have found that there is no real evidence to prove length contraction. is this correct?
Also a lot of places state that since time dilation is true then length contraction is true as well because they are concurrent. I don't understand this statement. How are these two phenomena concurrent?
 
Somehow you seem to have missed the point. Length contraction and time dilation are effects that are observed by someone moving relative to some clocks and rulers ( yours, for instance).

So if I cruise by your spaceship in mine, I will see your clocks running slowly and your rulers appear shorter than mine. But nothing has actually happened to any clocks or rulers. Naturally, you will think the same about my clocks and rulers.
 
I think length contraction was initially proven by the 1888 experiment of Michelson-Morley.

Go https://www.physicsforums.com/blog.php?b=1959 for an explanation by starthaus "MMX.pdf."

Later backed up by Lorentz-?Fitzgerald?
 
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Actually, if I got it right: if a train is 100m long at rest in the stationary frame, it will still be 100m long when it starts to move, when observed from the stationary frame. But the observer in the train will experience contracted lengths along the path of movement.
 
stevmg said:
I think length contraction was initially proven by the 1888 experiment of Michelson-Morley.

Go https://www.physicsforums.com/blog.php?b=1959 for an explanation by starthaus "MMX.pdf."

Later backed up by Lorentz-?Fitzgerald?

No, MMX doesn't have anything to do with length contraction. Check https://www.physicsforums.com/showthread.php?t=229034". At this point there are no direct experimental proofs of length contraction.
 
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psmitty said:
No, MMX doesn't have anything to do with length contraction. Check https://www.physicsforums.com/showthread.php?t=229034". At this point there are no direct experimental proofs of length contraction.

The MMX has everything to do with length contraction. Consider an inertial apparatus has two orthogonal arms x and y that are equal length in the rest frame of the apparatus, such that a omnidirectional signal propagating from the origin reflects off mirrors at the ends of the arms and return simultaneously to the origin. In an inertial reference frame moving parallel to the x arm the simultaneous return of the signals to the origin can be explained if the moving observer measures the x arm to be shorter than the y arm, if we insist that the speed of light is the same for all observers and independent of the velocity of the source. Changing the clock rate of a clock co-moving with the origin cannot make the signals return simultaneously. The MMX result can alternatively be explained in terms of time dilation like this. The observer at rest with the apparatus can take the point of view that the clock of the moving observer is time dilated and out of sync and so this causes the moving observer to incorrectly measure the length of the x arm to be shorter.

psmitty said:
Actually, if I got it right: if a train is 100m long at rest in the stationary frame, it will still be 100m long when it starts to move, when observed from the stationary frame. But the observer in the train will experience contracted lengths along the path of movement.
This is not quite right either if I understand you correctly. The observer in the stationary frame (the frame at rest with the track that does that does not accelerate) will measure the train to be shorter than 100m when it has velocity relative to the track. The observer onboard the train will still measure the length of the train to be 100m after it has accelerated and returned to inertial motion and after he has resynchronised all his clocks. The observer on the train after it has accelerated will measure objects at rest with the track to length contracted.
 
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psmitty said:
No, MMX doesn't have anything to do with length contraction. Check https://www.physicsforums.com/showthread.php?t=229034". At this point there are no direct experimental proofs of length contraction.

This is false, in the frame of an observer wrt which the Earth (with the lab and the MMX setup) are moving, length contraction is the correct explanation of the null result.
In the frame of the lab, the isotropy of light speed is the explanation of the null result.
 
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  • #10
psmitty said:
No, MMX doesn't have anything to do with length contraction. Check https://www.physicsforums.com/showthread.php?t=229034". At this point there are no direct experimental proofs of length contraction.

starthaus said:
This is false, in the frame of an observer wrt which the Earth (with the lab and the MMX setup) are moving, length contraction is the correct explanation of the null result.
In the frame of the lab, the isotropy of light speed is the explanation of the null result.

You've been educating me (stevmg) about this for a long time... I assume the false statement from psmitty is the statement that "MMX doesn't have anything to do with length contraction." (in this thread there have been too many negatives so that it becomes convoluted.)

Of course textbook explanations and your blog on MMX referred to above establish this.

stevmg
 
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  • #11
stevmg said:
No, MMX doesn't have anything to do with length contraction.

Sure it does, read my statement again.


Check this thread. At this point there are no direct experimental proofs of length contraction.

There is no DIRECT test for length contraction. This doesn't mean that MMX is not an INDIRECT proof of length contraction.




Of course textbook explanations and your blog on MMX referred to above establish this.

stevmg

My blog (and textbooks) show how length contraction explain the null outcome of MMX.
 
  • #12
starthaus -

I was backing you... Your blog makes that point. My previous post must have been a quote without the
. I understand your MMX.pdf quite well.
 
  • #13
stevmg said:
starthaus -

I was backing you... Your blog makes that point. My previous post must have been a quote without the
. I understand your MMX.pdf quite well.

I see, you can go back and edit your post to fix that.
 
  • #14
SH

Check it out (post 10) again - it looks right now but I am not used to going back before the last post to edit a prior post and the subsequent replies may be based on the "wrong" info in that prior post.

SMG
 
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  • #15
stevmg said:
SH

Check it out (post 10) again - it looks right now but I am not used to going back before the last post to edit a prior post and the subsequent replies may be based on the "wrong" info in the that prior post.

SMG

yes, this makes sense
 

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