I never understood why light has ENERGY but NO MASS... until now!

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Chapters:
00:00 Why photons have no mass (vague "explanations")
01:19 What is mass, exactly?
04:33 Understanding E = mc2
05:50 Does kinetic energy add more mass?
07:35 Total energy
09:15 Finding mass of light
10:05 Sponsor shoutout
1107 Photons have no rest frame (hence no mass)
12:32 Relativistic mass?
14:05 Why we don't use relativistic mass anymore
17:48 Speed of causality and massive photons?
20:45 Summarising in 2 lines

Photons have energy. According to Einstein's E = mc^2, they should have mass too. But, photon's are massless. Why? Why E = mc^2 doesn't apply to photons? Why massless particles always travel at the speed of 'c'?
This videos hopes to connect all the pieces together to provide a much more profound understanding of this topic. By the end of the video, you should have clear idea why we say light (or photons) are massless and why any massless particles should always travel at c. We will also see that c is actually the speed of causality.

This video was sponsored by Brilliant
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