The physics behind Einstein’s most famous equation - Lindsay DeMarchi and Fabio Pacucci
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Date: 2025-06-25
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Comments and reviews: 20
Ozzah
This video is extremely sub-par from what I would expect from TED-Ed. It is poorly written and highly disorganised. Admittedly, I am not a physicist, but I am a mathematician and have an keen interest in physics a little deeper than pop sci level. To my knowledge, E=MC2 does not tell us anything about C being a speed limit; instead it establishes the relationship between mass and energy (i. e. a little bit of mass is equivalent to a whole lot of energy. If you were going to talk about E=MC2, you missed the opportunity to talk about the full equation E2=(MC2)2(pC)2, which DOES factor in momentum.
If the message of this video is strictly about why C is a universal speed limit, you should have focused entirely on the Lorentz factor. You should have explained more clearly that spacetime can be represented as 31 dimensional space that you're ALWAYS travelling at C, and the faster you go in one dimension, the slower you go in another (i. e. time. You could have explained that the Lorentz transformation is borne out of the need to reconcile Maxwell's equations with what we know about moving charges and magnetic fields; relativistic motion is inconsistent with the law of induction unless you introduce the Lorentz factor.
But even then, the speed limit is the result of the singularity you get as a result of the Lorentz transformation. You have a 1-v2/C2 term in the denominator, which means when v=C you have 1-1=0 in the denominator, which has a limit of infinity. i. e. you would need an infinite amount of energy to travel at EXACTLY C, and you would experience an infinite amount of length contraction and time dilation. However this only applies to v=C exactly. When v>C, the equations are again consistent, but you experience negative time dilation, negative length contraction, and you require negative energy to continue accelerating.
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This video is extremely sub-par from what I would expect from TED-Ed. It is poorly written and highly disorganised. Admittedly, I am not a physicist, but I am a mathematician and have an keen interest in physics a little deeper than pop sci level. To my knowledge, E=MC2 does not tell us anything about C being a speed limit; instead it establishes the relationship between mass and energy (i. e. a little bit of mass is equivalent to a whole lot of energy. If you were going to talk about E=MC2, you missed the opportunity to talk about the full equation E2=(MC2)2(pC)2, which DOES factor in momentum.
If the message of this video is strictly about why C is a universal speed limit, you should have focused entirely on the Lorentz factor. You should have explained more clearly that spacetime can be represented as 31 dimensional space that you're ALWAYS travelling at C, and the faster you go in one dimension, the slower you go in another (i. e. time. You could have explained that the Lorentz transformation is borne out of the need to reconcile Maxwell's equations with what we know about moving charges and magnetic fields; relativistic motion is inconsistent with the law of induction unless you introduce the Lorentz factor.
But even then, the speed limit is the result of the singularity you get as a result of the Lorentz transformation. You have a 1-v2/C2 term in the denominator, which means when v=C you have 1-1=0 in the denominator, which has a limit of infinity. i. e. you would need an infinite amount of energy to travel at EXACTLY C, and you would experience an infinite amount of length contraction and time dilation. However this only applies to v=C exactly. When v>C, the equations are again consistent, but you experience negative time dilation, negative length contraction, and you require negative energy to continue accelerating.
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Aviator27J
I do know the formula rather well for a non-physicist, but I love me some astrophysics, I study it often, and I took a couple courses in it in college (planetary and stellar astrophysics. This was a good video, especially explaining how travel in one axis affects the velocity vector of travel in another axis. There's more interesting stuff to unpack from such a simple formula, but it was a good overview for a 5 minute video. And I agree with the posts below: Cherenkov radiation is a cool spectacle, so consider making a quick video on that! You can see the blue glow in a nuclear reactor and the light emission is usually compared to the sonic booms resulting from an object exceeding the speed of sound. It's not a perfect analogy but it works to explain the basic idea. Also, as an aside, things can move faster than light, but you have to remember that they aren't transmitting information. If you could point a laser at the moon and see the dot on the surface, then shake it really fast side to side, it could move laterally faster than light could traverse that distance, but that's a different situation. If you could get some long stick or rod between, say, earth and Mars, and push it, would that move faster than light No, it would only transmit the motion at the speed of sound.
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I do know the formula rather well for a non-physicist, but I love me some astrophysics, I study it often, and I took a couple courses in it in college (planetary and stellar astrophysics. This was a good video, especially explaining how travel in one axis affects the velocity vector of travel in another axis. There's more interesting stuff to unpack from such a simple formula, but it was a good overview for a 5 minute video. And I agree with the posts below: Cherenkov radiation is a cool spectacle, so consider making a quick video on that! You can see the blue glow in a nuclear reactor and the light emission is usually compared to the sonic booms resulting from an object exceeding the speed of sound. It's not a perfect analogy but it works to explain the basic idea. Also, as an aside, things can move faster than light, but you have to remember that they aren't transmitting information. If you could point a laser at the moon and see the dot on the surface, then shake it really fast side to side, it could move laterally faster than light could traverse that distance, but that's a different situation. If you could get some long stick or rod between, say, earth and Mars, and push it, would that move faster than light No, it would only transmit the motion at the speed of sound.
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todaywas.
I don't know what to do. I have watched countless videos about the special theory of relativity, space time, time dialation and many other physics and science topics. But at the end of it I don't understand anything. I don't know is there something wrong with me or.
Maybe i have just started class 10 and i am lacking something to understand these things.
Watching such videos i never see comments of people not understanding such concepts. But i don't think age is the actual reason after all what will change in the next 3 years or even 5 years. I think i am just not intelligent enough. but if this is true how do i level up myself. how do i increase my intelligence. i don't know. I feel helpless sometimes then i i see many of my friends who don't have any such issues. They are not even trying to learn such concepts but still they are at least not underconfident. I dont know what to do. should i just leave it all forget such things and stick to my school textbooks to get some false confidence i am not sure
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I don't know what to do. I have watched countless videos about the special theory of relativity, space time, time dialation and many other physics and science topics. But at the end of it I don't understand anything. I don't know is there something wrong with me or.
Maybe i have just started class 10 and i am lacking something to understand these things.
Watching such videos i never see comments of people not understanding such concepts. But i don't think age is the actual reason after all what will change in the next 3 years or even 5 years. I think i am just not intelligent enough. but if this is true how do i level up myself. how do i increase my intelligence. i don't know. I feel helpless sometimes then i i see many of my friends who don't have any such issues. They are not even trying to learn such concepts but still they are at least not underconfident. I dont know what to do. should i just leave it all forget such things and stick to my school textbooks to get some false confidence i am not sure
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sushanthkodali
I wasn’t going to post anything here, but something nudged me to. Maybe because I know what it’s like to be scrolling through comments, hoping for something to click some small sign that you’re not crazy for wanting more. For a long time, I was doing all the right things, but still feeling stuck. It’s like I could see the life I wanted, but couldn’t reach it. I didn’t have language for what I was missing until I found The Cancelled Laws of Reality by Selene Veritas. It didn’t force me into some flashy transformation it just helped me see how the laws behind reality actually work. It showed me how to stop forcing and start aligning. If that speaks to where you’re at, maybe this will find you like it found me.
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I wasn’t going to post anything here, but something nudged me to. Maybe because I know what it’s like to be scrolling through comments, hoping for something to click some small sign that you’re not crazy for wanting more. For a long time, I was doing all the right things, but still feeling stuck. It’s like I could see the life I wanted, but couldn’t reach it. I didn’t have language for what I was missing until I found The Cancelled Laws of Reality by Selene Veritas. It didn’t force me into some flashy transformation it just helped me see how the laws behind reality actually work. It showed me how to stop forcing and start aligning. If that speaks to where you’re at, maybe this will find you like it found me.
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saiskanda
Don't know if this is accurate but this was how i was explained that you can glimpse the past by moving faster than light
Assume an alien is looking at you from very far away say one light day away
When they look at you, they are actually seeing your from yesterday
Now assume you start to travel towards the alien at 2x the speed of light, you have now outpaced the light that left you a few minutes back, so if you turn back you will actually see yourself from 2 minutes ago coming towards you.
(Note the 2x, 2 minutes etc are not accurate, just random numbers to explain)
So now none of this is possible because from the video, you will never practically be able to travel at 2x the speed of light
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Don't know if this is accurate but this was how i was explained that you can glimpse the past by moving faster than light
Assume an alien is looking at you from very far away say one light day away
When they look at you, they are actually seeing your from yesterday
Now assume you start to travel towards the alien at 2x the speed of light, you have now outpaced the light that left you a few minutes back, so if you turn back you will actually see yourself from 2 minutes ago coming towards you.
(Note the 2x, 2 minutes etc are not accurate, just random numbers to explain)
So now none of this is possible because from the video, you will never practically be able to travel at 2x the speed of light
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SBDAVINCI
This is a very bad explanation. E=mc2 does not say anything about why objects can’t travel faster than the speed of light. It’s simply an equation that tells us the energy contained in a given mass. Speed of light being a constant for all reference frames is a postulate, in other words an assumption. This assumption allowed Einstein to derive the relativistic time and distance equations. Those equations then explain time dilation and length contraction at different speeds. E=mc2 is simply an outcome of the previously mentioned assumptions and equations. Another funny thing is, E=mc2 can only applied to stationary objects so I really has nothing to do with an objects speed.
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This is a very bad explanation. E=mc2 does not say anything about why objects can’t travel faster than the speed of light. It’s simply an equation that tells us the energy contained in a given mass. Speed of light being a constant for all reference frames is a postulate, in other words an assumption. This assumption allowed Einstein to derive the relativistic time and distance equations. Those equations then explain time dilation and length contraction at different speeds. E=mc2 is simply an outcome of the previously mentioned assumptions and equations. Another funny thing is, E=mc2 can only applied to stationary objects so I really has nothing to do with an objects speed.
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amlesab
Tears are forming while I write this because I never understood why I felt so distant from the life I was working so hard to build. On paper, things looked great a solid job, a supportive partner but something still felt hollow. Like I was playing by rules that didn’t apply to me. Then I read The Cancelled Laws of Reality by Selene Veritas, and I can’t explain how deeply it hit. It’s not just another mindset book it revealed the deeper mechanics of how this universe really responds to us. I feel like I finally stepped into the version of me that always existed under the surface. Selene thank you. You changed everything
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Tears are forming while I write this because I never understood why I felt so distant from the life I was working so hard to build. On paper, things looked great a solid job, a supportive partner but something still felt hollow. Like I was playing by rules that didn’t apply to me. Then I read The Cancelled Laws of Reality by Selene Veritas, and I can’t explain how deeply it hit. It’s not just another mindset book it revealed the deeper mechanics of how this universe really responds to us. I feel like I finally stepped into the version of me that always existed under the surface. Selene thank you. You changed everything
reply
vivek-420
Wrong information! Didn’t expect such mistakes from Ted. Light (photons) actually doesn’t slow down in water. It travels at the c itself. Photons interact with other particles causing emission and absorption thus technically slowing down. This absorption and re-emission process takes a tiny bit of time, causing an effective delay in the wave’s propagation. The slowing is a result of the macroscopic effect of repeated absorption and re-emission, or scattering, as the wavefront propagates but photons always travels at c.
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Wrong information! Didn’t expect such mistakes from Ted. Light (photons) actually doesn’t slow down in water. It travels at the c itself. Photons interact with other particles causing emission and absorption thus technically slowing down. This absorption and re-emission process takes a tiny bit of time, causing an effective delay in the wave’s propagation. The slowing is a result of the macroscopic effect of repeated absorption and re-emission, or scattering, as the wavefront propagates but photons always travels at c.
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ZennExile
Time is just the observation of entropy. Time dilation doesn't need magic to exist. It's simply space being expressed outside of the matter it used to be contained in. Space is a fluid, there is no real vacuum. Mass presses space from between particles of matter. That fluid is expressed outward, but has nowhere to go, it can't push the whole universe out of the way. These dense regions of space cause time dilation. They interfere with causality, which interferes with the observation of entropy. Ergo, time dilation.
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Time is just the observation of entropy. Time dilation doesn't need magic to exist. It's simply space being expressed outside of the matter it used to be contained in. Space is a fluid, there is no real vacuum. Mass presses space from between particles of matter. That fluid is expressed outward, but has nowhere to go, it can't push the whole universe out of the way. These dense regions of space cause time dilation. They interfere with causality, which interferes with the observation of entropy. Ergo, time dilation.
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balboa9439
4: 57
When going over light speed.
I feel like you wouldn't be able to chat with Einstein.
You could have observed the past but not really interact with it.
If you were to observe Earth from far away you would not see the present but past.
Therefore if going faster than light speed you simply could have reached up to the light that bounced back from Earth way in the past.
How can you chat with Einstein when he is sitting in his chair while you are going over 299 792 458 m/s
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4: 57
When going over light speed.
I feel like you wouldn't be able to chat with Einstein.
You could have observed the past but not really interact with it.
If you were to observe Earth from far away you would not see the present but past.
Therefore if going faster than light speed you simply could have reached up to the light that bounced back from Earth way in the past.
How can you chat with Einstein when he is sitting in his chair while you are going over 299 792 458 m/s
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SttmpJohnson
It is incorrect to say that photons have zero mass. They have zero REST mass, and therefore cease to exist at rest. Photons can only travel at the speed of light and of course have a certain amount of energy associated with them depending on their frequency. From Einstein’s equation then these photons have a mass equivalent to E/c2. Therefore photons have a momentum associated with them that allows solar sails to function in space as they do, for example.
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It is incorrect to say that photons have zero mass. They have zero REST mass, and therefore cease to exist at rest. Photons can only travel at the speed of light and of course have a certain amount of energy associated with them depending on their frequency. From Einstein’s equation then these photons have a mass equivalent to E/c2. Therefore photons have a momentum associated with them that allows solar sails to function in space as they do, for example.
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AakritiKaushal-m5y
I'm fascinated by the potential visual effects of high-speed travel! Can you help me understand the theoretical foundations behind the idea that approaching the speed of light could lead to kaleidoscopic vision Given that we've not directly experienced such speeds, what scientific principles and mathematical models inform these predictions I'd love to explore the intersection of relativity and visual perception!
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I'm fascinated by the potential visual effects of high-speed travel! Can you help me understand the theoretical foundations behind the idea that approaching the speed of light could lead to kaleidoscopic vision Given that we've not directly experienced such speeds, what scientific principles and mathematical models inform these predictions I'd love to explore the intersection of relativity and visual perception!
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bakeandmakealex
The nuance at the 3: 34 is incorrect. The missing part that time in complex of time and position slows down for the watching relative to the moving. Like if we look from the watching position on the moving position the position of the moving object, the the time relative to the motion is smaller. So the time is slowing down from the watching position it's not really slowing down.
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The nuance at the 3: 34 is incorrect. The missing part that time in complex of time and position slows down for the watching relative to the moving. Like if we look from the watching position on the moving position the position of the moving object, the the time relative to the motion is smaller. So the time is slowing down from the watching position it's not really slowing down.
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ted_ed
I first thought if we are travelling to say proxima centuri at light speed it will take 4. 25 years as it 4. 25 light years away far away then I realised the time also reduces and becomes near to zero so we reach in a single moment same as photons released from sun's surface. This is very fascinating
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I first thought if we are travelling to say proxima centuri at light speed it will take 4. 25 years as it 4. 25 light years away far away then I realised the time also reduces and becomes near to zero so we reach in a single moment same as photons released from sun's surface. This is very fascinating
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redsparks2025
TED's detailed explanation is still the simplified version. I use to doubt relativity until it was proven by how satellites use relativity equation to sync their experience of time in low gravity to our experience of time in high gravity. No more driving off roads into brick walls. LOL.
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TED's detailed explanation is still the simplified version. I use to doubt relativity until it was proven by how satellites use relativity equation to sync their experience of time in low gravity to our experience of time in high gravity. No more driving off roads into brick walls. LOL.
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anthonylouisi9509
3: 49 This is false, light always travel at a constant speed, the reason light seem to be slower when passing through water is because light particles have to move through the water particles, resulting in a longerpath. Light always move at c, never slow down or speed up
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3: 49 This is false, light always travel at a constant speed, the reason light seem to be slower when passing through water is because light particles have to move through the water particles, resulting in a longerpath. Light always move at c, never slow down or speed up
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abhinavsingh1327
if we somehow exceed light speed while travelling that would destory our eyes and body, The enormous amount of force exerted on you could reach above 100G and since your faster than light then the incoming light would simply blind you or you discover a new spectrum
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if we somehow exceed light speed while travelling that would destory our eyes and body, The enormous amount of force exerted on you could reach above 100G and since your faster than light then the incoming light would simply blind you or you discover a new spectrum
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Lexyvil
Great explanation! But you should also have mentioned that time does flow normally, but for the one that goes at the speed of light. Only the observer supposedly sees the time stop, if it's assume they can keep staring at the moving clock.
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Great explanation! But you should also have mentioned that time does flow normally, but for the one that goes at the speed of light. Only the observer supposedly sees the time stop, if it's assume they can keep staring at the moving clock.
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rasmusx94
I wonder if you could use Newton’s third law of motion, and shoot an amount equal to our own mass of photons behind us, to push us with the same speed in the opposite direction aka forward probably not
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I wonder if you could use Newton’s third law of motion, and shoot an amount equal to our own mass of photons behind us, to push us with the same speed in the opposite direction aka forward probably not
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venkat4167
No you miss everything. If you push mass close to speed of light with energy, the velocity can’t increase further. Which means the remaining energy gets converted to mass given by E=mc2.
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No you miss everything. If you push mass close to speed of light with energy, the velocity can’t increase further. Which means the remaining energy gets converted to mass given by E=mc2.
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