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L F The speed of light in a vacuum being constant is actually a prediction of Maxwell’s equations, which describe electricity and magnetism. Einstein computed the effects of this in Special Relativity. In addition, there was experimental evidence at the time that seemed to indicate this fact about light. A famous example comes from the Michelson-Morley experiment, which tried and failed to detect the ‘aether’, a special medium in which it was thought light travelled. This in no way diminishes Einstein’s achievements, however! To paraphrase Newton, everyone stands on the shoulders of giants.
Dr. Somara, I cannot imagine how mindbogglingly hard it must have been to get through all of this in school and UNI. Thanks for trying to make this palatable for people like me who are fascinated by it, but just not driven enough to take it on! Really cool stuff.
that's the first video on spcial relativity that explained it so well i understood! most people don't include maths in their explanation because they think it'll make things more complicated but it actually made it so much easier to comprehend. thank you for your series!
Man, that is... Mindbending. We're used to thinking time and space are constant, and can't be shaped to our will. But when you realise it's all conatantly warping and stretching it gets... a bit confusing
I just dropped by to tell you I'm only looking for a special recipie to cook for relatives and got subjected to a serious mental injury by clicking this video.
I've watched several videos in attempt to understand special relativity/time dilation, and either (a) all of the previous videos have been slowly adding to my understanding to the point where when I watched this video, it finally made sense, or (b) this video does the best job of explaining special relativity that I've found yet! It's probably a little of both, but either way, great job, and THANK YOU!
nothing gets easier to understand by telling it faster... while I somehow enjoyed crash course history and philosophy, these physics crashes just seem to me like a futile attempt to reduce complex topics and speed them up to make them somehow fit this format - so that the result is only understood by those who alrdeady understood it in the first place. maybe that's just my impression as a physics teacher...but I would take a complete series of videos instead of one to get each of these topics.
j-theory the quantum mechanic I’m reading that right now and I just got done with the general relativity section, and still don’t understand it. I mean I understand that time changes according to the something is going, but I don’t understand why. Maybe my understanding of time is wrong or something. But I watched this video to get another point of view or explanation.
I fully agree with you. This covered the topic about as well as a Hollywood movie, but most likely just scared more people away from ever really learning about it. Most people are going to watch it and say to themselves, "Wow that was a lot, and I understand that time changes based on how fast you're going, but I don't understand the rest, so I guess I just can't understand that and I guess that I am just not smart enough to learn about that." And that's when they decide that they can't do it, and quit trying.
Oh my god, Special Relativity finally makes sense! I've tryed to wrap my head around it several times now, and had it explained to me in several diffrent matters. Yours is the one that finally made it click! Thank you so much! The world is confusig as ever, but at the same time, it makes just a little more sense to me.
Wow, she talks near the speed of light! If only she was traveling near the speed of light, relative to me, then I would observe her time dilate, which would cause her to speak much slower from my point-of-view, so I could more easily take in all that she is explaining.
+kevin randleman And general relativity is as basic as it gets when compared to quantum mechanics. Just because there's something more advanced out there doesn't mean everything else is basic.
Year twelve is I'm assuming like the year after senior year in high school? I'm only in my second year of high school/secondary school and I'm really hoping we do more than just Newtonan physics and math only relating to that.
Hi. I believe that one of your examples is incorrect. At 6:36 you say, ‘We already know that time moves faster for you than for Bob. Whilst 666 nS has passed for Bob, only 577 nS passed for you.’ These two sentences are contradictory. If ‘time moves faster for you than Bob’, then the amount of time elapsed for you (person on the platform) must be greater than for Bob. The time elapsed for the observer on the platform should be 769ns, not 577ns. This is also consistent with the twin’s paradox and the idea that ‘a moving clock runs slow’. From the twins paradox, we expect the observer on the fast moving object (in this case, the train) to age less than the stationary observer (person on platform). Thus, time elapsed for Bob on the train must be less than for the person on the platform. From, ‘a moving clock runs slow’, we also anticipate that Bob’s recorded time will be less than that recorded from the platform. To summarise, if Bob records 666 nS from the train, you record 769 on the platform. Happy to be corrected, but I believe that this is correct.
Einstein was, without a doubt, in a league of his own. The simple fact of light travelling in a constant speed was taken into consideration, yet he brilliantly revolutionized Newtonian physics by applying certain case scenarios which were, back then, impossible to allocate to. Pure genius!!!!!
@Justa Fool what do you mean "proven"? Gravitational waves were proven to exist which were predicted by general relativity, atomic clocks are proving special relativity..should it be something tangible to be considered as proof..no one has seen an atom..but it's existence is proven by experiments.. should I say it doesnt exist because it cant be seen by a microscope or my eye..
@Justa Fool yes Einstein made mistakes..his assertion that universe is static by cosmological constant is false, but the idea that speed of light is absolute is true..been proven again and again..if light speed is absolute, spacetime should be variable and it's a theory..and it has been proven correct..see the article above..how do you refute it? Or do you have any other theory to explain it?
@Justa Fool why is it not rational when it's explaining all the things it was supposed to do so? Black holes, gravitational waves, light bending around massive object's, time dilation, length contraction..all are out of relativity..and are proven correct..what evidence DO you have to prove it false?
@Justa Fool Either way..Prove it wrong then! Thinking it is wrong doesnt make it wrong..PROVE IT WRONG if you think so. Relativity hasnt been proven wrong yet, so come back when that happens.. or stop making assumptions of your own with no backing.. How is LIGO fraud when after it detects gravitational waves and then they focus the telescopes towards that location, they are able to find electromagnetic radiation coming from those events as EMR might travel a bit slower due to gas and dust present in between. You have literally got no footing..
Thank you very much! I'm an Italian student and I can't find videos like his in my Country. The graphic of these videos is superb and the explanations are excellent. Well done!
regarding 4:42: if the distance in the moving reference frame is shorter, it should take the light a shorter period to travel up and down (what you also mention at 4:10). Why is it longer than in the stationary reference frame (according to the formula)? If gamma was 2, then if 10 seconds pass in the stationary rf, 20seconds would have passed in the moving rf. Should it not be the other way around?
Over the past couple years I've heard this and seeing this explained many different ways but with your visual effects and a combination of your method of explaining I just had that " Ah ha !" moment. The lightbulb has just been switched on I completely see and understand now it's so simple. Keep up the good videos
I think you guys made a mistake with labelling the formula at 4:38. Tv is the reference frame of a stationary observer, and To should be the moving reference
That formular just tells how long the same time-duration of stationary frame needs in the moving frame to pass from the view of stationary frame (clock-ticks). E.g.: T0=1sec, v=0,5c, how long do you have to wait/measure, to be sure that T0=1sec has also passed in the moving frame? ==> from perspective of stationary frame it takes a duration of approx. 1,15sec to pass on the stationary clock, to be sure that the T0=1sec-duration in the moving frame has passed. It's a little missleading, but the formular does not mean, that if in stationary frame 1sec has passed, that in the moving frame already 1,15sec have passed...
@@crossfire1403 And everything she said is based on ASSUMPTIONS and therefore I must ASSUME that everything she said is true. Sorry but my world is based on facts not assumptions.
Thank you so much for this episode. Even in my college physics classes, we never got into anything relativistic so I never really understood it. This was much needed. Great work, y'all!
This video came out right when I needed. In a couple of hours my physics teacher is going to give a lecture on this and I'm so exciteddd physics is the best
Not stopping for even a second between sentences to give the viewer a moment to digest what you said while talking about one of the most complicated ideas in phisics is an excellent idea!
4:34 wait a minute, "stationary reference frame" "moving reference frame" ? I thought we established, that it is indistinguishable which one is moving. From the perspective of bob, the platform is moving. So from bobs perspective the time on the platform is dilated the same way. How can that be? That would mean that also t0=yt ( while still t=yt0) so t0=y²t. Something is wrong here. (I used y for /gamma to type it on a normal keybord)
It is indistinguishable because Bob sees the train station move past him. If he is not accelerating he would not be able to tell if it is himself moving or if the train station is moving(like when you're in the backseat of car next to another car moving at a constant speed, and the other car slowly decelerates, you can't tell if you're slowly speeding up or they are slowing down). So noone can actually prove who is moving unless one of them were to be accelerating, but then special relativity wouldn't hold. In Bobs' perspective he sees the world differently(everything has shrunk in size), but the train station sees Bob as if he shrunk in size. This gets more interesting once you investigate the twin paradox and that photons "experience" the world as if time had stopped entirely.
I think you did not understand the question: bobs time is running slower that the stations time the stations time is running slower than bobs time ergo the stations time runs slower than the stations time ??? how does that work?
That is not the case. They perceive these things differently. Imagine that theres a clock on the train station and on the train where Bob is. Bob will see that clock is moving slower on the train station, while his own clock is running perfectly "normal". The train station sees that Bobs clock is running slower, while their own clock is running perfecly "normal".
I am having the same problem. if t = yt0 where y is always positive then t > t0. According to the formula t = time in moving reference frame (clock in train) and t0= time at stationary reference frame (clock at train station) --> Then t>t0 means that more time will have elapsed on the moving train than at the stationary train station. In other words clocks are faster on the moving train... This cannot be true because the whole point is that clocks slow down as you approach the speed of light on a moving train. I suppose if you switched t and t0 like MusikCassette did then the formula would work out. Not sure if t and t0 are improperly labeled in the video or if I am misunderstanding the concept. Anyone help?
Well, that does not help. you just repeaded the thought experiment, the question was a little more specific than that: Imagine the equation t=yt0 Imagine the equation t0=yt now Imagine the conclusion t=y²t and now Imagine me having a problem with that. The Problem is not, lack of imagination, but how to use those equations.
You guys might have saved my life. I'm in the middle of my first exam period of my degree (biomedical engineering), I failed my first physics exam- and need to retake it, on Tuesday. I have seen the two theorems for the time/length thing, and was never sure which was for the stationary system... Your explanations were very clear and helped my understand why I kept getting these questions wrong... I think I love you
Tell me about it. Mostly boring old classical theories in physics at sixth form, and what little of quantum mechanics that's touched upon is left horrendously unexplained ;-;
I came to this video with a competent understanding of the subject, yet found the video hard to follow. It all makes sense because it's all correct and well-scripted, but the editing and fast speech make it difficult to take in what she's explaining. The 'breathing time' between utterances is removed, and that time that it takes to inhale, although short, is just enough to allow us to process each piece of information before the next piece comes. Without it, all of this correct, well-scripted information is a rush of sound. No documentary maker or teacher talks this fast and without pauses between pieces of information, so why do it here?
You couldn't have had a competent understanding of the subject, as this vid is really clear and easy to understand. I had a limited understanding of the subject and understood it perfectly
What about Irish or Scottish, or Welsh? They're British too. I think that's what they may have been meaning.Cause I don't think she has an Irish accent
Hi Crash course team, at 4:38 she says that time in that moving reference frame = time in your reference frame * gamma, plz check it again because I feel there is some error in that equation. it should have been like: time in your reference frame = time in that moving reference frame * gamma
That formular just tells how long the same time-duration of stationary frame needs in the moving frame to pass from the view of stationary frame (clock-ticks). E.g.: T0=1sec, v=0,5c, how long do you have to wait/measure, to be sure that T0=1sec has also passed in the moving frame? ==> from perspective of stationary frame it takes a duration of approx. 1,15sec to pass on the stationary clock, to be sure that the T0=1sec-duration in the moving frame has passed. It's a little missleading, but the formular does not mean, that if in stationary frame 1sec has passed, that in the moving frame already 1,15sec have passed...
If I am in a spaceship flying to alpha Centauri, would the 4.1 light years I need to travel contract if I move at a significant fraction of C? So I would actually travel less distance on my way there from my perspective?
The distance is always the same; the distance never changes, it is only the amount of time that you experience while traveling that distance at a certain speed.
Everyone experiences their OWN time to be normal. So if I were watching someone in a spaceship at 99% c I would see their clock tick slower. Since the people IN the ship are experiencing time normally, doesn't that mean that from their perspective they don't travel the distance they calculated at the start of the trip?
If someone was to be moving at 99% of the speed of light, the distance between the earth and alpha Centauri would be length contracted so much that it would seem only a foot length away in their frame of reference.
secondo92 but the thing is, it doesn't seem like it is 1 foot away. From the perspective of the spaceship, it is objectively, 100%, the actual distance, 1 foot away.
They always talk about how things depends on the point of view. Everything is relative. Or something... But if I fly a rocket at the speed of light. Why is time slowing down for me? Why not the rest of the universe? Who chooses witch one of us is moving. Me in my rocket or everything around my rocket? I specifically remember reading somewhere that it is impossible to ever measure the absolute speed for anything because you don't have any reference point.
You can only measure things through what is called an inertial reference frame, or non accelerating point of view. If this is true, you can measure the other things acceleration, telling you that it moved, not you. This is actually the exact scenario that led Einstein to create general relativity, which takes acceleration into account
Why does Crash Course insist on making its videos so short? It seems like a topic like this could benefit from a 20 or so minute video. 9 really is not enough, in my opinion.
Well, a 20 minute discussion of Special Relativity would inevitably still be a "crash course." And I get that animations require a lot of work to make, but then they could just use more footage of the host talking or whatever with simple animations (showing equations and so forth).
awesome, ads suck, but this is our world so if it's what it takes to get this content out, I'm in! What's killing me though is the Hafele-Keating experiment, cannot get past the paradox of the clocks not having the same change relative to each other (I'll keep reading)
I kinda had an epiphany myself about how the speed of light (in vacuum) remains constant no matter what. Speed is distance divided by time. The speed of light remains constant through the stretching of time (making the taveller's local time slow down).
i’d just like to point out the irony in that last comment... “that’s because we’re used to seeing the world at much much slower speeds than light” 😂😂😂 great vid btw
Allen Dowe I thought the same thing. Not completely sure but I think the logic in relating time dilation to length contraction is flawed, or at least i don’t understand it. How can you maintain that the stationary perspective measured longer in the time dilation example, if in the length contraction example you say that in d = v * t, the moving observer measures longer?
then if you take this scenario to like a year long journey Bob would have aged more than you? as it seems more time has passed for him so he would have thus aged more?? This is not the standard answer to the scenario, where fast movement= less fast ageing.
Hello Robert, I've taken time to calculate this, and the video is, indeed, wrong. If 667 nanoseconds pass for Bob, 770 nanoseconds should have passed for the still reference frame. I believe they have made a mistake when defining t and t0. It should be as followed: t= Time for still reference frame t0= time for moving reference frame
It occurs to me that Shini, when translated using Japanese numeric substitution, is 42 (しに). Which is, as we all know, the answer to Life, Universe and Everything. Also, the number of this episode. Fancy that!
Yes, because according to Einstein’s famous equation, E=mc^2. (Where E is energy, m is mass, and c is the speed of light.) Because of the equivalence of energy and mass, the more energy an object has due to its motion would add up to the mass. (Therefore, as the energy required for motion increases, so does mass.) And that is why mass would increase as you approach the speed of light.
This is actually considered to be incorrect now. According to the OpenStax text book (Volume three, section 5.8), it says under relativistic momentum: "The relativistically correct definition of momentum as p=γmu is sometimes taken to imply that mass varies with velocity: mvar=γm, particularly in older textbooks. However, note that m is the mass of the object as measured by a person at rest relative to the object. Thus, m is defined to be the rest mass, which could be measured at rest, perhaps using gravity. When a mass is moving relative to an observer, the only way that its mass can be determined is through collisions or other means involving momentum. Because the mass of a moving object cannot be determined independently of momentum, the only meaningful mass is rest mass. Therefore, when we use the term 'mass,' assume it to be identical to 'rest mass.' "
I don't understand the statement about simultaneity. I get that bob observes the lighting at two separate moments and the person on the platform sees the lightning at the same time. But that doesn't mean its isnt a simultaneous event. Since the person on the platform is equidistant from both bolts of lightning and the speed of light is constant, it happens at the same time. For bob the distance and time are different for each bolt. This doesn't mean that it wasn't a simultaneous event. If someone claps their hands right next to me and someone else does the same but further away and i observe the sound from the closer clap first it doesnt mean that the claps didnt happen simultaneously. This means that even though bob sees the lightning that is closer first, both bolts still strike simultaneously.
Everything depends on the frame of reference. Simultaneity can only be judged from the single reference point. That is, you observed the claps occurring at different times so not simultaneous to you. At least one of the people clapping heard non simultaneous claps. However, one of the clappers may have heard the claps at the same time so he correctly assessed them to be simultaneous to him. There is no such thing as describing an event as irrefutably simultaneous.
Depreciation theory : We starting with a example: We have 2 cars both of them made at same year,after 10 years ,first car has a 200,000 KM millage and second one has a just 20,000 KM millage. If we compare these cars with together,we can say: second car has a less depreciation or say: passing of the time for second car was slower . As a result less depreciation is like :passing of the time with slower speed According of this theory:The distance of the Electrons passing around of the nucleon of the atom, is like a millage of the car. If we can reduce this distance ,millage reduces and we have less depreciation or passing of the time with less speed .( like atoms at Black Hole that the electron attaches to the nucleon and they are not moving at all,the time is not passing or there is no depreciation) As you know,with increasing the speed of the atoms ( very high speed, fracture of light speed) we can reduces the distance between electron levels of atoms and reduce depreciation or reduce passing the time. As second example we consider 2 twins brothers:One of them is going to the space with high speed,and second one stay at the earth. After for example 50 years ,the first brother comeback to the earth and we can compare these brothers with together: The first one looks younger.why? Because he had high speed for 50 years ,with this speed, the distance between electron levels in his body reduced ( the electrons moved less or they have less millage) and they have less depreciation or the time passed for first brother with less speed. I think the time always passing with the same speed and if Einstein said: passing of the time is the function of speed, we can explain it like this: The high speed is a main reason for reducing the distance between electron level and the electron has a less millage ,less movement and less depreciation, that is equal with slower passing of the time Thanks Sepehr Pour zahed
Not completely sure but I think the logic in relating time dilation to length contraction is flawed, or at least i don’t understand it. How can you maintain that the stationary perspective measured longer in the time dilation example, if in the length contraction example you say that in d = v * t, the moving observer measures longer? Time should be longer for the stationary perspective, hence the twins paradox, so how is the distance for stationary measured shorter if d = v * t? That would imply that the time multiplier is shorter, but it’s not. An solution to my confusion would be greatly appreciated
Also this video gets the lack of simultaneity wrong. That example simply demonstrates light being the fastest means of communication. With telescopes, we can see the early universe because the light has taken that long to reach us, but we still know it happened billions of years ago; it is not out present, we are just seeing it now.
What she failed to mention is that the clocks used for measuring the time it takes the train to pass are BOTH with the stationary observer. Bob sees a certain time pass on a clock at the train station (he concludes that his train is 100 meters long) yet Sarah? measures less time on the same clock as for her Bob's train is length contracted. That really was an oversight in an otherwise pretty good video.
@@ronblaess1 but why does the "moving-frame-observer" measure its own "plattform-pass-by-duration" by using a relative to him moving clock? This outer clock-readout therefore will result in a wrong "time-duration-measurement", because the outer clock is in a different reference-frame from the perspective of the "moving-frame-observer". How can it be right to make time-measurement by using an "outside-the-own-referene-system-clock"?
Questions: 1. If light has no mass, why would it's direction be affected by the trains direction? 2. If it indeed moved a greater distance, why would you assume it took the same amount of time, from the observer on the train's perspective?
First off, it's never too early to start saving for retirement. Second, content creators don't always get to choose who sponsors them. If it's a choice between an insurance company and no sponsorship, you bet your sweet bippy they'll pick the insurance company.
Sameopet I guess it depends on who is actually willing to pay for the slot. Just becouse you are not paying for it, it doesn't mean it's free. But I'm sure there are ways to support the show...
Sameopet By the way, the later you start to save the harder it gets, as you need to put aside more and more to make an actual difference by the time you retire. So do yourself a favoure, and start saving the first day you start to work. You can't imagine how happy and grateful you will be later. This should be part of curriculum, so in a sense it IS educational.
If v^2 is equal to c^2, then you get a case of 1 divided by zero, which is infinity. That is why mass becomes infinite, time stops, and length becomes zero at the speed of light. And theoretically if an object with any mass at all was able to exceed the speed of light, it would have negative mass and length, and it would experience time flowing backwards. That's the hypothesis of tachyons and explains why we will never be able to observe them if they do indeed exist.
Shouldn't time and distance be directly proportional? I mean if "v*t=d" when time increases, the distance should also increase right? Why is it inversely proportional now? Can someone please explain
Ann-K Seraphin - I cover this in chapter 2 of my special relativity series. Feel free to take a look if you want a more in-depth understanding of SR. Here's the short version to hopefully clear things up for you. If you stand still and measure the speed of a light pulse, you would get the speed of light "c". Let's now say that another observer is moving in the direction of that light pulse , at a velocity v, and also measuring it's speed. Intuitively, you would assume that they measure the speed of the pulse to be c-v relative to them. The problem is that, in reality, they also measure the speed of light to be moving at c. So how do you account for this? Since v=d/t, you could accuse them of measuring either distance or time incorrectly. In order for them to increase the velocity from c-v to c, they must have either measured a longer distance or a shorter time... or maybe both. In my chapter 2 video, we find out that the answer is both, and that is the roots of length contraction and time dilation. I hope this helped! Let me know if you are still confused.
Beautifully explained. But I think it's time we ditch the speed of light being a constant. Time is not tangible and cannot be dilated. The speed of light must be shifting
I have to disagree with some people here. As someone more or less new to the topic I found this video highly informative compared to every single one I‘ve seen so far. Thanks, really appreciate it!
The short and simple version: you know distance = speed x time, yes? And you know that in a right triangle, the hypotenuse length C is longer than the base length B, right? So if light travels the same speed in a vacuum no matter what (which it does), then if someone shines a light as they're moving past you, they'll only see the light form a straight baseline, while you'll see it trace out the hypotenuse of a triangle. So if speed is constant and the distance is longer, then time must be changing between you and them--specifically, since you see the light moving a longer distance, it'll seem like their time is slower than yours (since it takes longer to travel). Similarly, if the time something takes to pass you is longer, but it's moving at the same speed for both of you, then it will seem to that object that it's longer than it seems to you--which means from your perspective, it's "shrunk" or contracted.
