The Biggest Clue that the Speed of Light Is Constant | Relativity and Electromagnetism (Parth G)
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How Mass WARPS SpaceTime: Einstein's Field Equations in Gen. Relativity | Physics for Beginners
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Spacetime Diagrams: An Easy Way to Visualize Special Relativity (Physics by Parth G)
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- Опубліковано 20 січ 2025
Hi everyone, thanks for watching! Please check out this playlist for more Relativity Videos: ua-cam.com/video/qtqTPCAw7Fo/v-deo.html
And as always, let me know what other topics to cover in future videos :)
PBS tried to combine Lagrangians with relativity in an interesting way but I don't think any one could understand what he meant. Please could your use your clear explanation skills to make a better version?
_"If the Principle of Least Action is really a fundamental law, then there should be a relativistic version of the action that we can minimize to find Mercury’s orbit. After all, planets trade between PE & KE as they move around their orbits, just like the ball. It turns out that we only need to make a couple of relativistic corrections to do this. The most important is writing the Lagrangian as a function of something called 'proper time'" ... "If you plug the relativistic Lagrangian ..."_
→
L = ̲ ̲-̲ ̲m̲c̲²̲ - V( r )
→
γ ( r )
_"... into the Euler-Lagrange equation, you get equations of motion correctly describing the orbit of Mercury around the Sun."_
τᵳ
⎧
S = - mc² | dτ
⎭
τᵢ
̲ ̲d̲ ⎧ ̲ ̲∂̲L̲ ⎫ - ̲ ̲∂̲L̲ = Γ² ⎧ ̲ ̲∂̲L̲ ̲ ⎫ ̲ ̲d̲L̲
dt ⎩ ∂ν ⁱ ⎭ ∂x ⁱ ⎩ ∂ν ⁱ ⎭ dt
_"This shows us that the Principle of Least Action holds up in GR - but it reveals a lot more. 1st, it means that Lagrangian mechanics based on this principle can be made to be “Lorentz invariant” - it gives equations of motion that are valid even in the shifting frames of Einstein’s relativity. And we also start to learn something about the nature of the Action. Once we apply the relativistic corrections, the Action simplifies into just an integral over proper time."_
τᵳ
⎧
S = -mc² | dτ
⎭
τᵢ
_"Let's take a moment to think about that. Before, Action was a somewhat vague notion of change in energy, but working with a more precise model of the universe we see Action is much easier to understand. Action is now just how much time is perceived by an observer in their own frame of reference. All objects moving through spacetime, move through paths that minimize the time measured on that path. Remember that Fermat figured out that light always travels the path that minimizes its travel time: the principle of least time. Well it turns out that this is just a special case of a Principle of Least Proper Time. The reason for this is that in the classical limit proper time becomes KE - PE for objects with mass, ..."_
τᵳ t ᵳ
⎧ ⎧
S = -mc² | dτ → S = | (KE - PE) dt
⎭ ⎭
τᵢ tᵢ
_" ... but for objects without mass, like light, proper time & time are the same."_
(from 6:21 of the PBS Space Time's YT vid 'Is ACTION The Most Fundamental Property in Physics?')
We need a video about the density of states please
Sir I've sent an email that you have not responded to.
You are amazing
Yes, please do a part 2 on this and take it to the next level.
You are such a great teacher. You have a gift.
People really need someone like parth to understand complex concepts like this. Good job parth! Keep going!
Most useful thing about spacetime diagrams is that using them you can calculate a solution to basically any relativiatic problem graphically and intuitively.
I'm a physics tutor, and when explaining relativity I always start with diagrams, deriving all equasions from them. I have no idea why this is not the default route since it is so much more effective
Hello , I think this may interest you, a type of space-time diagram better than Minkowsky's because on both axes the scales are the same.
ua-cam.com/video/o4kKeG8PyyM/v-deo.html
In Argentina we use it a lot, and I think its dissemination is well worth it.
I’m horrible at math, I feel like I can understand it the way you teach. These videos are invaluable to me, you’ve made me a better person so thank you.
And of course Parth! I literally love your physics videos! Explain this concept in much much more detail in the next video!
Thanks. I finally have a real intuitive understanding of Spacetime Diagrams now. Before, I understood how to transform between the frames but I lacked the intuition to really understand it. Like Sal Khan says: if you don't have a real good intuition then you will never really understand the subject matter. You have a way of breaking it down for dummies like me who really want to learn. May Yahweh (Allah) bless you.
Iva, SC USA
Thanks for explaining this extremely convoluted toipc in such a simple fashion. Much appreciated!
Very interesting to consider that different events can appear to occur at the same point in space to different observers.
This was an amazing explanation. I would love to see more detailed videos of the same topic. I just wanna know that why is the speed of light constant for any given reference frame. Why can't it change.
That's a very good question. i dunno but I'm thinking that:
Normally we're moving through the time dimension. If we weren't we'd never reach lunch time = disaster. So even when we are stationary by our computer we have a vertical world line because we're moving through the time dimension.
If you increased your speed to close to the speed of light, c then you'd still see yourself as stationary & you'd see everyone else moving - they'd appear to be moving at close to c. So the the accelerator pedal in our car makes the whole universe accelerate.
& therefore because they're moving at close to c, time dilation happens & so you'd see their clocks ticking slowly. You'd see them running about in slow motion, or even frozen in time. So therefore it seems to you that they aren't moving through the time dimension as much.
It seems to you that they have changed from
1) being stationary by their computer + moving through time ----> to 2) moving through space + not moving through time much
It's like they've been rotated.
But light is different because it's 1) moving through space at the speed of light before ----> & 2) moving through space at the speed of light after
It just ignores our accelerator pedal. It doesn't get rotated
If we are in a car & throw a tennis ball then the tennis ball is going to move at the car's speed + our throwing speed = fast like maybe 170 mph
99% of mass is due to massless gluons bouncing around inside every neutron & proton. momentum = mass x velocity. So when the car has momentum it's due to these gluons. & then we throw the tennis ball & the effect of the gluons gets added to the tennis ball making it go at 170 mph
But if we shine a torch the torchlight is just going to move at c. It will ignore the car's speed. The car's gluons don't affect the photons getting emitted by the torch.
So somehow there's a connection between not getting rotated & ignoring the gluons. I dunno what that connection is. Basically you asked an awesome question.
Thanks! The speed of light being constant is a postulate / assumption we make in Special Relativity, because some of the mathematics of Electromagnetism that came before this theory suggested the speed of light may be constant (without indicating any reference frame). So Einstein took this idea and worked on Special Relativity, which was later tested experimentally.
@@alwaysdisputin9930 that is an amazing breakdown.
@@ParthGChannel I'm aware that the Maxwell equations give the velocity of the electromagnetic waves and it simply depends upon the permittivity and permeability of electric and magnetic fields respectively, which has nothing to do with reference frames and motion altogether but just by how the material is (even free space or vacuum). But this is what baffles me, that even in vacuum we can have an acceleration in vacuum. Light doesn't. It maintains it's speed no matter what. We are somehow independent on the material nature of free space where as light is not.
@@alwaysdisputin9930 so does that mean that the gluons do not interact with electromagnetic fields or electromagnetism in general bcx light simply chooses to ignore it. Gluons don't have rest mass like photons, so would it also mean that photons/light only has interaction with stuff that have rest mass. I'm asking this cz in refraction other than bending light does apear to slow down as well bcz of present atoms inside.
I was waiting for this topic for a long time. Wonderful explanation. Waiting for the follow-up video
Great !!! thank you soo much 😊😊
The fact that two events happening at different times can actually occur at the same time in some other reference frame is actually shown in Nat Geo's series "Genius" in the train scene that two lightning bolts can be seen simultaneously when you're stationary while they occur independently if you are on the train.
This is the best explanation of light cones I have come across on the Internet. The videos I watched earlier, were not quite clear in explanation but thanks to Parth G, I now understand this concept very well. Liked and subbed!
I have recently become obsessed with trying to understand both of the theories of relativity. I have consumed a ton of content online and read a few books on the subject (including Einstein's actual paper). I think this video does an excellent job explaining special relativity.
I just found your channel and I am so glad that I did! As a physics major, it's been hard finding good explanations for some of these concepts and your videos are INCREDIBLY helpful! Thank you so much for sharing content on here-- you remind me of why I wanted to study physics in the first place!
haha same there is an saying we dont believe teachers on youtube unless he is of indian orgin
Would love to see a follow up vid with more in depth info about these diagrams and their interpretation. Thx!
An excellent video! Now I see some things I've been trying to understand for years. Thanks, Parth.
What a descriptive and concise explanation of spacetime Minkowski diagrams. I've read and seen alot of videos on same topic but this is probably the best I've seen 👏👏👏
Thank You Parth for the excellent short explanation ! Super Useful !
Dang! Professor Parth! You dumbed this down so much it made me feel like Minkowski and Einstein at the same time!!!😊
Job well done, you saved this Secondary School Physics- tutor!
Finally the first to comment. Your videos are really dense with information and its awesome to listen to . Keep up the good work
Very nice and simplified explanation for high school students and more like me. Keep it up man!!
Beautifully explained. I used this to help me understand a section on Black Holes i am reading and it crystallised the concept for me. Thanks!
Very well explained. I tried to understand this for some time now and now I got it. Thank you
Very good video. You explain really calm and good! Thank you!
I'm so thankful that you made a video about minkowski diagramms in STR. When I first had to calculate with themI got confused at how the y-Axis is not distance but time, since technically ct would cancel the time units out and it could be a distance,distance diagramm.
Thank you so much for explaining the Minkowski space! And yes I will be checking out your Ch. 2 on Special Relativity!
This is the video that made me really understand causality and light cones. Thank you!
Parth , talk about the pauli exclusion principle and information in the cosmos .
I hope you complete and post the podcast you mentioned following on from your first excellent podcast on Spacetime diagrams … great work
ive watched a few videos and this is by far the best explanation
Thank youuu I was having trouble understanding this concept when reading the textbook but you made it really interesting and simple to understand
The way I look at the graph you used is that "x" is a distance from the origin. The exact coordinates of the location are unimportant - the one dimensional value of distance is all that matters.
This video is the best explanation i’ve ever heard about future light cone 🎉🎉🎉.
thanks for an amazing explaiantion, had lot of fun learning!
simply best. i am glad i am a teacher and student of physics.
Hi Parth, I've just finished reading "The Ascent of Gravity" and have started "Why does E=mc2?" Chapter 4 is on spacetime and although the author does break it down to a very simple level, I still could not grasp the message or purpose of The Minkowski diagrams the author used. From watching your video it makes a lot more sense so thank you very much (although i've much reading and research left to do to even begin to understand, fully)
This is the best explanation I've seen
I appreciate quality content like this
9:57 Events that are in the yellow shaded region which are causally connected are said to be time-like. r^2 < (cdt)^2. r is the distance between the two events.
Events in the blue shaded region that are not causally connected are said to be space-like. r^2 > (cdt)^2.
Events that are on a 45-degree light line, are called light-like. r^2 = (cdt)^2.
Great video. Really liked the explanation, please continue this topic.
Thanks for your effort
Bhaai bahut sahi video banaai hai! Zabardast!
2:00 Can you measure horizontal distance also with ct parameters? Making it cty and ctx and ctz for R3?
Thanks!! This was such a great explanation of the concept!
it would actually be a really good idea to do one more video in more detail and then two others about general relativity
Nice illustration! Now if we can get people to see that there is little to no gravity to slow down light between galaxies until the light enters into a galaxy where time slows down and distance contracts, slowing down the speed of light. This is the reason superluminal motion is perceived to be seven times the speed of light since our measures of time and distance are being projected into outer space where time runs faster and the measure of distance is much larger relative to ours. Of course nothing ever exceeds the speed of light. It’s that the assumed measures of time and distance are off.
Another little secret of general relativity is that the speed of light is NOT constant because the measures of time and distance are not constant throughout the universe. Looking at a galaxy is not like looking at a cat or dog where the measures of time and distance are constant because they are not constant from here to outer space between galaxies. The measures of time and distance appear to be constant or flat to us locally but they are not flat over great distances, especially outside of galaxies and in between galaxies.
You explain brilliantly!!!!😊
Thanks a lot for this explanation.
Proud to be a physics student and your subscriber..🙏
I had only seen the space time diagram, now almost all of my doubts regarding this particular topic is clear. Want to watch video on how gravity is just the wrapping of space time. Just want to visualise on my mind the curvature of space-time as gravity graphically.
Please do make a video on that.
This was so clear! Thank you so much!
I prefer my space-time diagram. Putting it to use, as shown in my YT videos, allows you derive the SR mathematical equations in mere minutes.
Excellent video, keep up the great work
11:54 why the minkowski diagram axes are oblique?
yes, we want to continue with this topic :-) thanks for your videos
your way of explaining is amazing.
So clear and helpful. Thanks so much.
Thank you so much for this video! It was really helpful
Thank you for making this!
Ya please continue this thanks 👍
Really enjoyed video... yes more like this please :-)
10:20 does this assumes that spacetime is flat?
Why is the coordinates (t1,x1) when the vertical axis is ct not x?
Beautiful explanation, thank you!
I need to watch more of your videos so i can explain something to you over the phone and my reasons why space and time need to be separated. We are making a huge mistake because of this. I need help advancing my garage theory of quantum mechanics and space.
Did the follow-up video ever get made?
very interesting video, thank you for the explanations
Very nicely explained! 🙌
Amazing! Please more about this.
I have a very important question. If in all reference frames in the space-time diagram , light travels at 45-degree angles in an up-sloping direction , And backwards time-travel means a motion path that decreases on the cT axis , then for sending a prohibited by physics ..signal traveling at just BARELY above light speed, how does this tiny deviation lead to paradox if it’s still upsloping? So not a horizontal instant FTL signal , a BARELY above c signal like 1.0001 of C.
Is it that …. It’s Earth’s ability to exploit the past of the fast-moving object whose X’ and CT’ prime lines are almost both squished on the 45-degree line relative to our X and CT axis , that if we go a little faster than light , we easily break being above or parallel to THAT X’ prime axis ?
Thank you for the video! This video is really well done and is really easy to understand.
I have a question, however: is it possible for 2 events to occur outside of the world line for observer 1 but occur inside the world line for observer 2 essentially making something possible for one observer but impossible for another observer?
8:34 Quantum Entanglement?
@@Zodiaczero2 so can we say that spacetime diagrams don't really work in same way at quantum level as they do at classical level
@@Zodiaczero2 NOT EXACTLY.
In entanglement you can't even identify individually that did A caused B or B caused A. There is no way of keeping track of which triggered which. That is why it is "WEIRD" and does not have any classical explanation.
Its somewhat like this counterintuitive statement. "Knowledge of parts does not tell you about the whole but the other way around!". Where as classically u would always take it from "Common Sense" that if you have knowledge about the constituents then u have the knowledge about the whole system comprised of them.
But in case of Entanglement that does not hold. You shouldn't even talk about particle A and Particle B but together the whole A+B system is what makes sense and is observed.
And they together do not breakdown Causality nor any Principal of Relativity as they obey the same fundamental equations of Quantum mechanics (Namely Dirac's Equations) which are consistent with Special Relativity.
@@yashdadhwal3034 They do within the context of Quantum Field Theory.
@@dhritimanroyghatak2408 yeah but let's just say if they are a light year far then affecting each other I mean wave function getting collapse simultaneously seems pretty weird.
@@yashdadhwal3034 well what gives you the impression that under normal circumstances wavefucntion's don't collapse instantaneously?
Even if you the do the classic double slit experiment with a single electron there too when a measurement is maid the wavefunction collapses instantaneously, same as in entanglement.
The problem arises when u start thinking about the entire system in terms interactions between the wavefunctions of individual constituents. So its not that the wavefunction of B light years away collapses as measurement on A is made. THEY R THE SAME WAVEFUNCTION WHICH COLLAPSES INTO A SINGLE STATE.
That single state cannot be separated and treated as two separate states.
That's basically THE DEFINITION OF AN ENTANGLED STATE.
One of the major mystery behind this perhaps lies behind the mechanism via which a wavefunction "COLLAPSES" when measurement is made which have boggled the likes of Bohr, Einstein, Heisenberg, Schrodinger, Dirac, Feynman, others but as of yet all we know is that it happens in that strange way and not how or why!.
I do agree the entire phenomena seems pretty weird which it is.
13:06 this also means that the event that happen simultaneously for me might be at different time for someone else, isn't it ?
Can u switch topics from like this space and quantum and those theory to like more day to day never seen hidden physics things
I had a question. When both the observers measured the speed of light, did they both measure it in the x-dimension i.e the dimension in which the flashlight was moving, or did the stationary observer measure it along a diagonal drawn from the moving flashlight to the stationary observer?
I have some issues with these diagrams, and is that it would make you think that pitagoras theorem applies in space-time, and it doesn't.
So it would be always necessary to point out that the transformations between reference frames have different invariants than the ones we see in euclidean spaces.
8:22 for me there is BIG difference between "causally connected" and "possibility to be causally connected"
many things are possible but are not happening because they are possible, possibility is not same as cause
simply amazing!!
Hello! Can you do the same video but withought CT? And put just T for time, so that we can see how it looks like withought multiplying t with c?
Or can you link to us a video where spacetime is x and t rather than x and ct?
Thanks!
Does this mean that in the reference frame of a speed of light particle, every event happens at the same time (at the world line origin)? Even if an outside observer sees different events happening to it at different times and in a particular order, none of that is actually experienced by the particle? What does the event order mean, if anything, to the particle then?
Like for example, a photon is observed being emitted from an electron, and shortly after being absorbed by the electron system of a neighboring atom. These events would happen at the same time in an instant from the photon's point of view; no time passes between its origin and end? I find this confusing because the event times and order are relevant to an outside observer, but the particle itself sees none of it? It seems like the photon cannot access the information of how and when events happen in the "sub light speed" universe, but these events determine its origin life and ultimate fate...
Very good explanation
Respected Parth, rayleigh jeans law failed due to usage of equipartition theorem in it ....
does it mean equipartition theorem is wrong? If it is wrong why do we learn equipartition theorem in thermodynamics?
Thank you
It isn't completely "WRONG" as per say but more like a special case scenario of something much more general and that "Something more general" is what Max Planck discovered when he improved the Equipartition Theorem via his postulation of Energy Quantization thus giving rise to the new revolutionary domain of "QUANTUM MECHANICS".
As to why one learns the good old Equipartition Theorem in Thermodynamics or Statistical Mechanics is for the same reason why one learns Newtonian mechanics/Classical Electrodynamics before jumping onto any advanced topics.
1) Firstly a lot of the phenomena in every day life do not need those improved version of the theorem and can simply be explained by the classical model which still have immense utility in various Engineering and application fields, for eg:- The speed distribution curves of gases made of macro particles, Understanding Heat Capacities at higher Temperatures etc all can be done without resorting to the improved model.
2) Secondly to be able to understand the advanced topics u first need to thoroughly understand its predecessor out of which it emerged and basically carefully analyze the classical models failure and for what reasons.
For these two main reasons one is always introduced 1st to the classical topics and then is slowly and systematically guided to advanced topics so that it all follows a progressive theme.
loved this!
Nice video.. Thank you
Thank you so much!
Please make another detailed video
How is causality maintained in relativity? If two events are causally connected in one reference frame, is it possible to find another reference frame where they happen simultaneously? That does not make sense.
no, the cause always happens to be in before to the effect.
If a guy shoot two laser beams opposite toward two bombs.
Observers can disagrees which one has been exploded first, but they will always agreed that the guy did shoot the beam before both bombs explodes.
If the moment the guy shoot the beam and both bombs exploded simultaneously then it means the laser beam was traveled faster than light or instantaneously to both bombs.
@@Zodiaczero2 From the perspective of the laser which is traveling at the speed of light, doesn't everything in the universe happen all at once? Since no time is passing on the photon.
what u r mixing is simultaneity with causality I suppose. If two events are simultaneous in one reference frames then it is possible to find other frames of references where they are not simultaneous. For eg if I detonate two bombs at exactly the same moment in my frame (i.e making them simultaneous for me) a person travelling w.r.t me will see not observe those 2 events simultaneously.
Whereas let's say u have a event 1 which trigger's an event 2 (thus making them causally connected) then every frame of reference will agree that event 1 occurred before event 2 and caused it even tho they might not agree with you on the exact starting and stopping time of event 1 and event 2 but they will always be causally connected for them as well.
I just elaborated what @Mei Aihara already summarized.
@@niloymondal In relativity "FROM PERSPECTIVE" of Laser itself is meaningless because anything moving with speed c cannot be a reference frame to begin with due to the various Paradoxes that emerges (such as one u just mentioned). U can be imaginative an conjure up such scenarios but they lead to various infinities when put into the equations.
All of the equations in relativity only makes sense (TO BEST OF OUR KNOWLEDGE) for speeds less than c only.
Thank you for the video.
Does anyone know if he actually made the video he promised at 12:12 ??
Yes I want more explanation with physical examples.
Is it possible to find two reference frames - one where two events are not causally linked, and one where they could be causally linked??
Yes and that's precisely what an "EVENT HORIZON" is (assuming u've heard that term) so yeah regions beyond the Event horizon of a Blackhole are not Causally linked to regions outside it.
As to causally linked events they are our everyday phenomena where nobody is travelling faster than the speed of light and reality behaves by our good old laws of nature. So like you doing an experiment on IST and someone on ground are causally linked.
explain the standard model equation
My brain still struggles to comprehend the fact that the light from any frame of reference travels at the same speed C. 🙃 what about the speed of light from a reference point of a photon? Do photons "see" each other as being still or as travelling at speed of light?
Hello sir ,I want to know what #education you took after +2 ?
Are you from India or abroad ??
Pls answer sir !!
Because we rarely travel anywhere near the speed of light the world line of anybody who has ever lived will look like a completely vertical straight line
Yes we want another video very good explanation. But 1 doubt but according to quantum entanglement information travels faster than light or things do change they are affected very quickly so you will get point on the other side which is impossible according to relativity
Yeah i was thinking the same thing. Measurement on Earth on 1 of the entangled particles measures spin up, instantly the wave function collapses & 2nd entangled particle near Alpha Centauri becomes spin down. It seems to be FTL cause & effect?
Both entangled particles are described by the same wave function
@@limerence18 Yes. So do you agree it's FTL cause & effect?
Umm the thing is that quantum entanglement is not breaking any laws of Relativity. The reason you are inclined to think that ways is because you are under the impression that in an entangled state the individual pure states themselves are enough to determine the whole setup but that is not true with this peculiar phenomena of Quantum mechanics.
It turns out there is no way that u can talk about whole thing by just talking about its constituents but u have to treat the whole setup via a single entity whose parts are highly corelated to each other to the extent that it doesn't even makes sense to just talk about the individual parts.
That's why entangled states r explained by a single wavefunction (which is a superposition of the pure states) which obeys the principals of relativity.
@@dhritimanroyghatak2408 Even if it's 1 single entity, still the fact remains, you prod it on Earth & FTL, it knee-jerks in Alpha Centuri.
Also you seem to be saying that the system doesn't have 2 particles. On the other hand, if you want to say these parts of the system do exist, then you're wrong to say "it doesn't even make sense to just talk about the individual parts"
Wow, i get it, sort of! Nice Work.
Amazing thank you so much
Respected parth, Does accelerating/oscillating/rotating magnet produce EM WAVES?I ask this because we know that change in magnetic field produce electric field right....
Thank you sir
ct is m/s * s = m. c is NO „scaling factor“ for time. If c = 1 as a dimensionless number than c is a scaling factor for m. So m = m.