Why light has energy, but no mass? (Understanding E = mc2)

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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[Mahesh_Shenoy]

You will also support my channel. Thanks in advance!

[spate]

Even as a high school physics student myself, your explanations are never inaccessible and don't assume a heavy background of your viewers, that's quite tricky I'd imagine and admirable! Keep up the great work, you've earned a new subscriber!

[barmalini]

What a great lecture Mahesh, now I know I'm not overweight, I've just become fuller of energy.

[drsamliveriadis]

At 64 I fell in love with Physics Relativity and Cosmology. Your simplified explanations really makes it easy to grasp some very complex and yet fundamentally simple ideas that the Titans of Physics spend a lifetime to discover and formulate. Thank you

[danieltdp]

I haven't seen this kind of content for 25 years, since I left physics. Really cool to be taken back by this very clear explanation!

[rafaelfcf]

I'm a Brazilian learning physics invented by an old German guy in English from a guy I presume (I'm sorry if I'm mistaken) is Indian. The internet can be such an amazing place for humanity. Thanks for sharing and making it easier to grasp.

[klosnj]

You know it is a good explaination when you get to minute 6, haven't learned anything you didnt already know, but still have that eureka moment where the puzzel pieces fall together and you intuit the answer before it is given.

Well freaking done, good sir!!

[zosiunias]

5:51 - "This is it, this is the climax" - *Ad pops up*

[transformations]

This is one of the best explanations for E=MC² come across in 15yrs, simply mindblowing.
Your explanation made E=MC² equation much more intuitive and likewise concept of Mass.

You could expand further on the "speed of causality" part, that deserves an entire video and somewhat rushed at the end.

Without getting too complicated, you could also introduce the idea of Markov blankets and the "inside" and "outside" distinction for the reference frames. I think this is very important today, since many people introduce mysticism and all sorts of nonesense about "everything being energy and One" - which is kind of true in a superficial truism, but ignores the inside/outside Markov blankets - which applies to literally everything from atoms, galaxies, minds and people.
(in some respects a refutation of idealism in metaphysics)

Excellent work Mahesh !🙂

[mbchrono]

I love how you have conversations with Einstein to explain these complex topics! It puts things into a certain perspective unlike that of other educators on Youtube. Great video!

[k.vinay.]

For 18 years, I have been trying to understand this. Now, I have gained some real insights into the nature. Thank you; your efforts are greatly appreciated.

[Luis_]

Greetings, I am a 16-year-old residing in Germany. Recently, during my astronomy class, we delved into discussions regarding Newtonian principles and the nature of light. Intrigued by the concept of photons lacking mass, I sought clarification from my teacher, only to receive the response that the explanation would delve too deeply into the realm of quantum physics, beyond my current grasp. Undeterred, I turned to YouTube and found the topic elucidated in a manner that was remarkably comprehensible, even for someone of my age. Kudos to the creators for their adept explanation.

[lucialaw]

I watch a lot of educational content on youtube, especially math and physics stuff. I haven't encountered someone as exited and engaging as you. The way you ask questions and go about investigating them works so well for my understanding of the topic. Keep up the great work!

[thomasendicott]

Mahesh, I’ve only seen a few of your videos, and I rarely comment on any video I watch, and I have to say… the energy and enthusiasm you bring to the topics you lecture on are unrivalled. You remind me of my program head during my stint in robotics back in the day. There is an obvious excitement and deep respect for the universe, how it works, the fundamental strangeness and the absolute WONDER to be felt given the slightest understanding of these phenomena.

If there is a future I could wish for, it is that all else false aside to truly grasping how incredible this reality we exist in is at a near-fundamental level, and you are making large steps to that reality. I am so excited to see this channel grow, and I truly wish you the best. Cheers.

[quoenix]

You really are the dream explainer/teacher I've always wished for. It's just way too intuitive and interesting to watch your explanations. I've watched your videos on Khan Academy before, to study for 12th exams, and honestly I so wished I'd find you somewhere else just for random science stuff other than studies. I'm so excited to find you here on YouTube now, and I can't stop binge watching all of your videos and shorts. Please never stop making these, and thank you!

[aresjupiter]

it's amazing how an equation/formula of 3 letters is so complex in details, that someone needs almost 22 minutes to explain it. and it's also amazing how I, someone who just finished HS, someone who just loves physics, came here and watched it all, and actually understood. your explanation is truly captivating and understanding, even for some who doesn't have a degree on physics or something like that. thanks for being and talking so accessible to anyone who just wants to learn something more/new.

[thelordnelson]

I love the style that you have to explain difficult concepts. Not only does you passion and excitement for the subject inspire you challenge the why rather than just replay the theory in an easier language. For everything I learn I come away with more questions than I started with, and you too always pose those questions that people are no doubt asking themselves. That helps to solidify understanding. Thank you. Love it.

[Cy_Guy]

You're the only youtuber that's been able to explain this better than my astronomy teacher... who could respond to my in person questions.

[amaze]

The algorithm sent me here. All I want to say is what a phenomenal educator/presenter you are. That was truly something special. Now excuse me while I go check out the rest of your videos.

[Tom-zyke]

The single best lecture I have heard, probably ever (and I'm quite old). Now all I need to do is watch it a few more times to "inwardly digest" it!