Who is measuring proper time and proper length?

If two observers are travelling at a high relative speed to each other, they will measure different times and lengths. The effect is described in special relativity as time dilation and length contraction. But who is measuring the dilated, longer, time, and who is measuring the proper time? Similarly, who is measuring the contracted, the shorter length, and who is measuring the proper length? The goal of this video is to help you figure it out.

[cjjoer-]

My notes are now written in terms of Bob. Very simple straightforward explanation, thank you!

[devonbotney]

Taking my final and this is such an easy way to understand. Thanks!

[thomasgogoyaya]

thanks help me to figure out how to mesure the proper time and proper length

[-_Nuke_-]

So essentially someone can travel at any proper velocity he / she wants right?

For example I can travel to the Andromeda galaxy 2.5 million light years away by having an insane proper velocity millions of times faster than c which doesn't brake relativity because when I eventually do arrive there, I will also be millions and millions and millions of years into the future...

Because even though I was "frozen" in time for all those millions of years of my travel, I only experienced 1 minute of those (because of my insane proper velocity) so if I devide my dx which is going to be not the distance that I measured (which is going to be contracted by a huge factor) but the "true" 2.5 mly distance over my proper time, which was just 1 minute, I will have a huge proper velocity many times faster than light actually 2.5 mly/minute fast!

That's crazy fast, but still like I said doesn't make Einstein mad, cuz I will also arrive there at the year who knows... 2.500.000 AD or something!

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[NachoWizard]

When you say that proper time is when the object is stationary in your ref frame, does that mean that the object can still be traveling a certain speed if you are traveling the same speed as well?
If I wanted to record an event that is going 0.5c and I am traveling 0.5c as well in the same direction, then would I be timing the proper time since in my reference frame, there is no change in speed between me and the event?

[anonymousperson]

the problem is bob is so cute I can't focus....use matchbox next time

[alchemy]

How about no rest frame. Two objects only in empty space?

[geronimomiles]

No, proper length is determined only for a particular frame of reference, which is arbitrarily chosen.
Bob sees his length as proper because he is not moving relative to himself, even if he is spinning.

[muks]

You said Dialated Time, I heard Delayed Time ⁉️😕😵🤔

[everythingisalllies]

Hi Stefan, sorry but rulers and time doesn't change with motion. Why would you think it could? Oh, yes, you probably were naive enough to believe Einsteins nonsense.

[SamsungGalaxy-vwgy]

You are NOT clear at all.
First of all, you don't give definitions.
Secondly, you seem to be confused and unsure.