Spacetime Diagrams | Special Relativity Would Headlights Work at Light 1 year ago   14:31

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This video is chapter 2 in my series on special relativity, and it covers spacetime diagrams, rotational and translational symmetry of both time and space, how certain transformations preserve distances (measured in terms of a reference like a meter or second), and so on. We'll wait until the next video to talk about Lorentz transformations, relativity of velocity, minkowski diagrams, and the speed of light.

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Created by Henry Reich

Comments 1275 Comments

Michael Darrow
I have a rotation space time project on and it has time as horizontal axis. Also, carykh has proper space time with time as horizontal axis.
According to minkowski spacetime diagram if I want to Time Travel why I cannot choose a straight along time axis path from future to past or past to future or any curved in the light cone and why it is necessary to choose a curved path out of the light cone for time travelling??????
Jack Doe
and we are absolutely hurdling through space... as we sit "stationary" we are rotating at just shy of 1,700 kilometers per hour and the actual planet is booking it at 67,000 miles per hour (thats miles, 107,000 kilometers)... stationary is in fact an idiots term... i wonder... is there anything in the entire universe that is actually stationary?... considering the expansion...
Birdie Holtby
Physics is just abuse of innocent cats
Zen Chen
how about using a 3D video with long exposure...will that work?
to plot a 4D...i mean...well nevermind
So what is relativity used to solve?
Katie H
>due to convention, we put time on the y-axis, even though it is usually on the x-axis

>oh also, we still make time the first coordinate in a coordinate pair, even though it's on the y-axis

Physicists must be stopped
Jason Wilton
Why have I only now found this as I'm going into second year and sat exams on this shiz.
¡! N0ot !¡
Im only 12 years old, but i understand this so much more than the simple math i learn at school
Does anyone know about a series similar to this one but telling us about General Relativity?
Cats, unified by way of a bundle of sticks
"my son is 18."
"really, does he have a driver's license?"
"wait...18-month-olds can drive now?
Jonathan Perry
The best way relativity was ever explain to me was like this. let's say you're in a car driving 60 miles an hour and you have a tennis ball and you throw it up in the air 6 in and then down and catch it it moves 12 in to you. But if you're standing outside the car that tennis ball is traveled on this huge Arc that is really long compared to what you experience in the car. So the only way to explain how the tennis ball can travel to different distances at the same time is it time must be different relative to your position because the one person the ball travels a huge distance into the other person it travels a small distance in the same amount of time that they experience. I hope that makes any sense I wouldn't don't have any fancy animation to show it I can do it on a napkin for you
7:15 so that's exactly why any number with no unit in physics isn't a valid useable value by itself
Kareem Tk
"universal truths" for someone who has an entire channel about physics you're not really scientific
A succession of sequences (24 im/sec) with a conscience thinking about. That movie is an exemple of 4D. Human level is even a 5D with discernment.
Ron Wilson
The fact the actual distance between points is invariant (even those he perceived distances is not) seems to indicate that there is in fact some absolute reference, in this case the sheet of paper that you are drawing the diagrams.

Now if the universe is expanding does that mean the paper is expanding as well? And if so (and the distance between two points increases by that amount of expansion) seems to indicate that there may be two sheets of paper, one in which the drawing is own and the other that has a fixed grid from which one can measure that increase in distance.

Hence their seems to be three different kinds of measurements.

1. The sensed measurements as defined by the observer's frame of reference (ala Relativity)

2. The expanding universe sheet of paper that is stretched (with the drawings stretched as well that causes even two non moving world lines to change in distance due to that expansion where the grid lines on that paper are not fixed by stretch with the paper.

3. a fixed under layer of paper that determines absolute distances with a fixed grid.

It seems we can sense 1 directly, infer 2 by Doppler shifts and the like but 3 is not directly observable other than we can infer it in that actual change of distances (be it from motion or expansion) is invariant and hence the tic marks that one measure that change in actual distance seem to be invariant as well.

Or to use your relativity machine there is 1. the pieces that move on your machine,
2. the machine itself that say if it were made of rubber could be stretched and
3. the table the machine sets on that if fixed from which you can measure the distance due to the pieces moving and the machine being stretched.
Crazzy Flynn
Loving this series
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Would Headlights Work at Light Spacetime Diagrams | Special Relativity 1 year ago   12:23

If you were driving at the speed of light and turned on your headlights, what would happen?
Sources and extras below!!

Thanks to Jude for asking me this question on twitter!

“The Life of the Cosmos” by Lee Smolin:
“In Search of the Multiverse” by John Gribbin:
“The Pig That Wants to be Eaten: by Julian Baggini:

Relevant MinutePhysics videos:
light slowing down in glass:
a different perspective from Sixty Symbols about light slowing down:

Why is “c” the speed of light?

Car at light speed:

Light speed is the same in all inertial frames of reference:

Relativistic addition of velocities:

doppler shift:


Light speed and catching-up to light.

Other good relativity resources:

Light echo footage:

“fine-tuned” universe:

Irrational numbers:

a million digits of pi:



animated drawings by Guy Larsen
rocketship animation by Eric Langay