## Physics Riddle – Moving Faster Than The Speed Of Light?

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If train A is moving towards train B, and each train moves at 75% the speed of light (relative to the ground), will an observer on train A think that train B is moving at 150% the speed of light? But nothing moves faster than the speed of light, so what is going on? This video presents a formula to add velocities under special relativity. I then offer some justification of the formula by explaining time dilation and the formulas for change of coordinates under the Galilean and Lorentz transformation. From the assumed formulas, I show how to derive the velocity addition formula.

https://einstein.stanford.edu/content/relativity/q1021.html

Time dilation
https://en.wikipedia.org/wiki/Time_dilation

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Richard Ditty says:

Lets say you're a passenger on a train traveling at a speed close to C, your light clocks would run slower, but would you continue to experience time at the same rate. Now, would this problem hold up if you had a clock that counts time using a pendulum, such as a grandfather clock.﻿

Richard Ditty says:

I wonder if this problem has been analyzed using maxwells equations. They show that light can be described as a wave, this illustration views it as a particle. I'm not sure how it could be done, but it is interesting none the less.﻿

Алексей Беляев says:

Nice video, seems like I've finally come to the understanding of what I was taught in physics class :)﻿

maya q says:

Great video, thanks. What I need now is to know how fast and for how long I need to travel in order to slow down time and with that my aging. How can I get one second younger by traveling compared to not traveling?﻿

MrVoayer says:

Contrary to Special Relativity this video greatly shortened the time to learn basics of speed and time algebra when speeds are near the speed of light. Great job!!! Well done!!!﻿

Nate Petersen says:

okay, I loved this, PLEASE DO MORE PHYSICS!﻿

An artist theory on the physics of 'Time' as a physical process. Quantum Atom Theory says:

The light clock only makes objective sense if we have an emergent future unfolding photon by photon. With the Universe being a continuum with energy ∆E equals mass ∆M linked to the Lorentz contraction ˠ of space and time. The Lorentz contraction ˠ represents the time dilation of Einstein’s Theory of Relativity with energy ∆E slowing up the rate that time ∆t flows as a universal process of energy exchange or continuous creation.﻿

Ewan Rawcliffe says:

This is physics curriculum for 16 y/o in Scotland, what about else where?﻿

Titurel says:

at 7:00 you mean 2L/C not 2L*C﻿

TGL SuperStarMan says:

Or you could just do "How long does it take a train to go 50km at 0.75c" since they are both going at the same speed.﻿

Nhan Vu says:

Lol you can't see things that move faster than light. They look
Ike they are moving at the speed of light not faster than the speed of light. How are you supposed to see that?﻿

Ryan Wilson says:

There were a few points in the video where it seems like he understood the math of what is going on, but didn't quite grasp the physical application behind it. Either way, it doesn't completely matter for the explanation. Good video either way﻿

FroSTtuB3 says:

Length contraction was not taken into account.﻿

Louis Lin says:

Thank you for your dedication and effort in making these videos! I really enjoy them!﻿

origami tenkaizu says:

Great video! I have a question. Aren't you technically adding their speeds, not velocities? For example at 0:16, wouldn't the combined velocity be 0, since velocity is a vector? Whereas the combined speed is 100, since speed is scaler, right?﻿