Why No One Has Measured The Speed Of Light

The script was written in consultation with subject matter experts:
Prof. Emeritus Allen Janis, University of Pittsburgh
The stuff that's correct is theirs. Any errors are mine.

References:
Einstein, A. (1905). On the electrodynamics of moving bodies. Annalen der physik, 17(10), 891-921.

Finkelstein, J. (2009). One-way speed of light?. arXiv, arXiv-0911.

The Philosophy of Space and Time - Reichenbach, H. (2012). Courier Corporation.

Research and writing by Derek Muller and Petr Lebedev
Animations by Ivy Tello
VFX, music, and space animations by Jonny Hyman
Filmed by Raquel Nuno

Special thanks for reviewing earlier drafts of this video to:
My Patreon supporters

[brstrom]

My bank uses the same theory, but vice versa. When the money leaves my debit card, it goes really fast. When something is to be repaid, it takes much longer.

[smartereveryday]

This was a very fun present to unwrap. When you called me and told me to turn the camera on I knew something weird was going to happen and you certainly delivered. As long as I’ve known you Derek you’ve been destroying assumptions. Thank you for this friendship. It’s certainly enjoyable from my perspective.

[vibhavaggarwal]

I am an engineer and was working on time synchronization between two devices on the same network. We encountered cases where network delay from A to B is higher than from B to A and such cases resulted in inaccurate synchronization. I was working on solving this problem and after watching your video I realized it’s essentially the same concept- sending signals from earth to mars is like sending network packets from A to B. So the conclusion was, without the help of any external device it’s impossible to accurately sync time! Thanks a lot for your video :))

[MichaelPenick]

This is one of my favorite videos you've ever done because it reminds us that we must never forget to question everything, even the things we think we can take for granted.

[CGPGrey]

Great video. Despite getting a physics degree and teaching physics for years, I never came across this or thought about it. I was treating the video mostly as a 'fun to think about' sort of video, but your point at the end is really intriguing.

[rigel]

Light: "My speed is immeasurable, and my time is ruined"

[michalgno]

Derek, you have blowed my mind today. Great video and even greater attitude into science mystery.

[DevinHenkel]

Wouldn’t red shift be different in different directions if light traveled faster in one direction than another? Unless we make another huge assumption that the universe is lopsided?

[TheRealGuywithoutaMustache]

I swear this channel is a gold mine for educational and entertaining content

[priyathgregory]

Should have wrote this in my physics exams, "It is neither a supposition, nor a hypothesis, but a stipulation that I can make of my own free will"

[slimo-quelquun]

What about :
-Sending both a light signal in a close-to-totally empty tube, and a sound signal (which we mesured the speed before) in one direction, and mesuring precisely the time difference between those two signals ? And trying that in different directions should give :
The same difference if c is constant
A different difference if c is direction-dependant

[emappingvb]

I can’t believe you sponsored a sponsor that I like. I have been getting kiwico crates since I was 8 and I actually have the infinity mirror. So Thanks.

[markm]

My real takeaway is that two clocks, regardless of precision, will never be truly synchronized. This explains why I am frequently late.

[bent.]

"So someone has measured the speed of light...or have they?"

Huge Vsauce moment right there

[medigoomnis]

Wow, those flies really love your hair...almost as much as I'd love seeing a supernova in real time. I doubt infinite velocity is the answer but the points you bring up are great.

[bertheinrich]

Fascinating as are all of your videos even though I am no Physicist but just a humble sailor and safari guide. I think I better have a drink now or sleep is out of the question tonight. Thank you

[v]

I love how he called it right off the bat, “oh you’re talking relativity, you’re gonna something weird aren’t you?”

[NitePHX]

Destin was probably having a perfectly fine, normal day and then the phone rings. Now he has a broken brain.

[pradyuman]

I am eagerly waiting on the CMBR dipole moment video you teased in between

[Aesir]

This guy is such a gifted narrator and his topics are so interesting what a combo