# B Length contraction of a pair of electrons

#### DaveC426913

Gold Member
This thread has left me shaken and humbled.

... you shouldn't expect to immediately understand.
I thought I already knew.

I sincerely thought I understood this subject better than I actually do. Bell's Spaceship Paradox, relativity of simultaneity, Lorentz contraction were all things I was comfortable with ...

It might be that I understood it a lot better 10-20 years ago, when I read up on physics heavily, but that, over time, my knowledge has rotted from the inside out. One would think, once you've learned how to ride a bike, you never "unlearn" it, but I guess use it or lose it.

Dunning-Kruger, ftw. Or onset of senility...

Thanks for being so patient, everyone.

Last edited:

#### 1977ub

A stick might have clocks sitting at each end - initially synchronized. As it moves along its axis, and shortens in the lab frame, the clocks are no longer found to be synchronized by lab observers, though they are still found to be synchronized by observers at each end of the stick. "Shortening" can just be restated as desynchronization.

#### metastable

If we fire two electrons at the same time, one metre apart, should they length contract till they're less than a metre apart?
If you force the electron acceleration profiles fo be simultaneous in the lab frame, the the string will have been stretched
I'm confused on this point. How is the electrons being "fired" in the lab frame any different than an observer being fired away from the electrons? If the 2 scenarios are equivalent, why would the string between the electrons have any tension (ignoring the electric repulsion)?

#### Ibix

Science Advisor
If the 2 scenarios are equivalent
They aren't. Who/what is being acted on by a force is different.

#### Nugatory

Mentor
I'm confused on this point. How is the electrons being "fired" in the lab frame any different than an observer being fired away from the electrons? If the 2 scenarios are equivalent, why would the string between the electrons have any tension (ignoring the electric repulsion)?
They aren’t equivalent.

In one case there are inertial frames in which the lab, observer, and electron guns are moving with constant velocity throughout (the frame in which they are at rest is one of these, with the “constant velocity” being zero). There is no inertial frame in which the electrons move at constant velocity throughout.

The other case is the exact opposite. If we fire the lab and observers away from the electrons, then there are inertial frames in which the electrons are moving with a constant velocity (possibly zero) throughout there is no inertial frame in which the lab moves with constant velocity throughout.

### Want to reply to this thread?

"Length contraction of a pair of electrons"

### 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