In what direction does length contraction occur?

In summary: Not following you. :confused:In summary, if three observers are moving at different speeds relative to a train, then the observers will disagree on when the train reaches a line. However, the railway company can draw 1000 numbered lines on the ground, and the observers can all agree on the time that a particular number on the train crosses the line.
  • #1
Flexo
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Consider a train traveling at a relativistic velocity, with three observers. One inside the train, and two outside the train traveling at different velocities with respect to it. Say there is a finish line of sorts at a certain point. All observers will agree when the train reaches the line.

Does this mean that length contraction occurs, in layman's terms, with the back of the object contracting toward the front?
 
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  • #2
Flexo said:
Consider a train traveling at a relativistic velocity, with three observers. One inside the train, and two outside the train traveling at different velocities with respect to it. Say there is a finish line of sorts at a certain point. All observers will agree when the train reaches the line.

Does this mean that length contraction occurs, in layman's terms, with the back of the object contracting toward the front?

Hi Flexo! :smile:

When the question says "All observers will agree when the train reaches the line", it means that they set their watches to the same time (probably zero) when the front of the train crosses the line.

They could equally well set their watches to the same time when the back crosses the line.

Or when the middle crosses the line.

Or the train could be extended, with the extra portion at the front.

There is nothing special about the front of the train.

In layman's terms, the length is observed to contract all over. :smile:
 
  • #3
tiny-tim said:
Hi Flexo! :smile:

When the question says "All observers will agree when the train reaches the line", it means that they set their watches to the same time (probably zero) when the front of the train crosses the line.

They could equally well set their watches to the same time when the back crosses the line.

Or when the middle crosses the line.

Or the train could be extended, with the extra portion at the front.

There is nothing special about the front of the train.

In layman's terms, the length is observed to contract all over. :smile:
It seems to me that the two moving observers would set their watches to zero at different times if the train contracted from both directions. Is this just a problem with my visualization?
 
  • #4
Flexo said:
It seems to me that the two moving observers would set their watches to zero at different times if the train contracted from both directions. Is this just a problem with my visualization?

Sorry … yes it is! :smile:

They can set their watches to zero at whatever times they choose.

If they're moving at different speeds relative to the train, then there will be a different time, t = 1, say, when the back crosses the same line (another line) for both of them.

So if they use another pair of watches, set 1 unit behind the first pair, then by your same argument, the contraction is now towards the back! :smile:
 
  • #5
tiny-tim said:
Sorry … yes it is! :smile:

They can set their watches to zero at whatever times they choose.

If they're moving at different speeds relative to the train, then there will be a different time, t = 1, say, when the back crosses the same line (another line) for both of them.

So if they use another pair of watches, set 1 unit behind the first pair, then by your same argument, the contraction is now towards the back! :smile:

It seems strange that observers will agree on a time that any point on the train reaches the line.
Say the train has numbers from 1-1000 in large print on either side of it. Each of these numbers is placed such that it reaches the finish line 1 nanosecond after the last. Both observers set their clocks to t=0 when 1 crosses the finish line. Are you saying that both observers will agree on the time that any given number crosses?
 
  • #6
Flexo said:
Are you saying that both observers will agree on the time that any given number crosses?

No, I'm saying that the railway company can draw 1000 numbered lines on the ground, and both observers can re-set their watches so as to agree on the time that any particular number n on the train crosses line n. :smile:
 
  • #7
tiny-tim said:
No, I'm saying that the railway company can draw 1000 numbered lines on the ground, and both observers can re-set their watches so as to agree on the time that any particular number n on the train crosses line n. :smile:

It seems to me that no contraction is occurring in this scenario, because the observer seeing the train as going faster would observe less time between number n crossing line n.
 
  • #8
Flexo said:
It seems to me that no contraction is occurring in this scenario, because the observer seeing the train as going faster would observe less time between number n crossing line n.

Not following you. :confused:

There is always an observed contraction of an object moving relative to the observer.
 

1. How does length contraction occur?

Length contraction occurs when an object moves at high speeds relative to an observer. As the object's velocity increases, its length in the direction of motion appears to decrease from the perspective of the observer.

2. What is the cause of length contraction?

The cause of length contraction is the phenomenon of time dilation, which occurs as a result of Einstein's theory of relativity. As an object's velocity increases, time slows down for the object, causing its length to appear shorter to an outside observer.

3. Does length contraction only occur in one direction?

Length contraction only occurs in the direction of an object's motion. This means that an object moving towards an observer will appear shorter, while an object moving away from an observer will not experience length contraction.

4. Is length contraction a real physical effect?

Yes, length contraction is a real physical effect that has been confirmed through numerous experiments and observations. It is a fundamental aspect of Einstein's theory of relativity and has significant implications for our understanding of the universe.

5. Can length contraction be observed in everyday life?

No, length contraction is only noticeable at extremely high speeds, close to the speed of light. This is why it is not something that can be observed in our everyday lives. However, it has been observed and measured in particle accelerators and other experiments involving high-speed particles.

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