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How Going Fast Taught Us New Physics — Transcript · YTtoTranscript

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How Going Fast Taught Us New Physics

StarTalk · 2,821 words · 14 min read · EN-ORIG

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Below is the complete, readable transcript of How Going Fast Taught Us New Physics by StarTalk on YouTube. Read the full text, copy any part you need, or generate a transcript for any video with our free tool.

Another explainer. >> Yes. >> Oh, yeah. >> Yes. >> Uh we might have done this long ago, but but I'm old and I don't remember. >> [laughter] >> Well, now that's the blind leading the blind. >> This is the Doppler effect. >> Oh, sweet. >> Let's go back to the 15th century, 16th

century, 17th century, 18th century. >> That's a pretty long period of time. >> Nothing [snorts] moved fast enough for anyone to even think there's such a thing as a Doppler effect. Consider that. >> That's really cool. >> Okay. >> don't get a Doppler from a horse. >> No, you don't. [laughter] >> YOU AIN'T

>> YEAH, IT'S JUST not happening, okay? Good thing curious people live among us because when a new phenomenon happens, it's like, "Why did that happen? How did that? Why?" Let me get into that. Okay, so when railroads started being built, okay? Early 1800s. >> Okay. >> People noticed that the train whistle, when it was

coming towards you, had a different pitch than when the train whistle was receding from you. >> Okay. >> Okay. >> Mhm. >> It was a higher pitch. It's the same whistle. >> Same whistle. >> The the conductor didn't switch whistles just cuz it passed in front of you. >> Right. >> Okay. >> Yeah.

>> Cuz that could have that could have happened. >> That could. >> Uh if you're a scientist, you have to check all possibilities. >> Yeah. And by the way, that's it's a pretty fancy train. >> [laughter] >> So, that was unlikely and of course was not actually happening. >> Right. >> So, uh there's a fellow named Christian

Doppler. >> Christian Doppler. >> Doppler. Austrian mathematician, physicist. >> Okay. >> The early to mid-1800s, and he noticed this. He said, "This needs some explaining." >> Nice. >> So, he studies it, and we knew that sound propagated in waves, and so if you have a train whistle, there's a wave coming out. And a wave has

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wavelength. And that's the distance between what and what? >> Top and bottom. >> No. >> Peak and >> There'd be half a wavelength. >> I'm sorry. >> Peak, trough, peak. >> Peak to peak. >> Right. >> There you go. It's got to repeat. >> Yeah. >> The whole thing >> got to go.

>> repeat. That's the wavelength. >> peak. >> Right. >> There you go. >> So, if the train is sitting there and the sound is emanating, then whatever that frequency, an exact identical word to frequency is pitch. Okay? >> Right. >> The the musician might say pitch. >> Well, >> The physicist says frequency. We're

talking about the same thing. >> Okay. >> Okay. Cool. >> And and frequency is a little more descriptive because if you're standing there timing it, you can ask how many of these go by you per second. >> Right. >> So, the the higher the frequency, the more of these go by per second.

>> Makes sense. >> The only way that can happen is if the wavelength is shorter. We agree? >> Yes. I mean I mean it's That's the only way it can >> That's the only way it can work. >> It can't work any other way. >> That's the only way you can get more of

these things. >> You get a higher frequency. >> Put them together. >> You get a higher frequency. >> Higher pitch. >> Okay. >> Combine the idea that the sound has a frequency with the fact that the train is moving. So, I'm squeezing the wavelengths as they come out in front of me. >> Exactly.

>> And and the opposite of that is happening on the other side. >> Right. >> As I >> Pulling the Pulling it Stretching it out. >> it out. Stretching it out. Exactly. [music] >> Right. >> So, the train whistle coming towards you should then have a higher frequency >> Right. >> than is native for that whistle. And as

it recedes from you, the frequency should be lower. But, that's precisely what is observed, what is heard in a train whistle. And what would the frequency be when it's right in front of you? >> Directly in front of you? >> Yeah, cuz it's moving past you. Now, it's right in front of you.

>> Um Hold on. I'm trying to think here because the waves are being pushed in >> it. >> Okay, don't. >> If the frequency is higher than normal >> Oh, because you're pushing this way, right? >> and lower than normal over here. >> Well, then this >> a neutral >> Thank you. >> It's just neutral.

>> At some point it's got to go through the >> Through the through the through the actual actual frequency. >> And when's that going to happen? >> Right there in front of you. >> Because when it's right in front of you >> Right. >> it's neither moving towards you >> or away from you.

>> The sound is moving >> Mhm. >> at what speed? >> Well, it's the speed of sound. >> Thank you. >> [laughter] >> See? >> Hey. >> Chuck, I ask easy questions. Don't overthink >> Don't overthink. >> my situation. >> There you go. >> Okay? >> Right. >> Okay? >> Right. >> It's moving the speed of sound.

>> Of course. >> And that's true >> no matter where. >> Okay? >> Yeah. >> The speed of sound in a medium can be different. But for this experiment >> Right. >> the speed of sound is one value. >> Okay. >> So >> That's >> That's great. >> It's it's simple. >> Yeah.

>> Okay? So he wrote a formula to calculate this. >> That is not simple. It's [laughter] the Doppler formula. >> Okay? What he discovered is that the ratio of the speed of the train >> Mhm. >> to the speed of sound equals the ratio of the new frequency of the sound to the

native frequency of the sound. >> Okay. >> The speed of sound is the same. >> Yes. >> But the train is moving. It's pushing [snorts] the sound. >> Squeezing it. >> Squeezing it. >> Squeezing the wavelength. >> The wavelength itself. >> But notice it's not making the sound go faster. >> No, it can't make the sound go faster.

>> But that was not obvious. >> No, it's not. >> That's what I'm saying. >> Yeah. >> If I if I'm got a fast moving train >> Right. >> am I pushing my sound forward? No, you're not. >> Uh and this, by the way, is his observation. Because he's like, if you intuit this, you're like, "Oh, it's

pushing the sound, so the sound just gets pushed." >> And the speed of the sound you think would be the speed of sound plus the speed of the train plus the speed of the train. >> But it's not. >> But it's not. >> It's not. >> And I'm still amazed we could have

figured this out centuries before >> with no problem. >> We had no occasion to even >> reason. >> No reason >> No reason to do >> No reason to do it. >> Right. Okay. So, built into the formula is the fact that as the train moves faster >> Right. >> there's an even higher shift in the

frequency. >> Right. >> Okay. That makes sense. >> That makes complete sense. >> Both coming towards you and and going away. So, it's got to relate the original frequency to the frequency you hear >> Right. >> to the speed of the train and the speed of sound. >> And the speed of sound.

>> In whatever is the medium, air at a given temperature, it'll be some number. >> to change depending if it's water, if it's air, if it's vacuum of space. >> Well, there's no sound. >> sound at all. >> No sound at all. >> [laughter] >> Right. Right. Right. >> UNLESS YOU'RE IN STAR WARS.

>> [laughter] >> NOW, HERE'S SOMETHING COOL. LET'S MAKE THE TRAIN GO faster and faster and faster. The pitch up front is going to get higher and higher and higher. Okay? >> Right, cuz it's getting [music] squeezed and squeezed and squeezed. >> Okay. THERE'S A POINT. >> [laughter]

>> OKAY. THERE'S A POINT WHERE THE TRAIN IS MOVING so fast >> Yes. >> that the end of the wave lands on top of the beginning of the wave. >> Yes. >> And it never moves forward from the train itself or the the thing >> Like whatever it's pushing. >> Whatever it's pushing it. Okay?

>> Dude, that's amazing. >> That's when the train We don't have trains that do this, but So, when when when >> Well, THEY DO IN CHINA. >> [laughter] >> WHEN THE PLANE >> [snorts] >> HITS THE speed of sound >> Right. >> then the sound never gets ahead of the the plane. >> Right.

>> Cuz all crests are piled on top of each other. >> They're like instead of this, they're like this. >> Every single crest >> It's a sand-It's a sound sandwich. >> right at the front of the thing. Okay? And all that sound is now magnified. >> Yes. >> We have a term for this.

>> Say it. SAY IT. >> [laughter] >> A SONIC BOOM. >> DUDE. >> THAT'S HOW YOU GET A SONIC BOOM. >> IT. >> THAT'S THAT'S HOW you get it. It's a Doppler shift equation gone rogue. >> gone wild. >> chain. >> That's on steroids. I love it. >> chain. >> That's crazy. >> Yeah.

>> That is amazing. >> And And like I said, in front of the the the plane, you don't hear it at all. >> Right. >> Be- Well, you got to be moving, you know, it'll catch up with you, but >> Right. >> if the plane is coming towards you and it's going the speed of sound or faster,

it is silent movie >> until it passes you. Until it passes you. That's really cool. >> Yeah. >> That is why when these meteorites are big enough and they come through the atmosphere and they're going so fast, you see them go a streak of light and then all the windows go >> Yeah, it's a time delay. Definitely a

time delay. Yes. >> Why? >> Yeah. So, consider if you are at an angle to the train as it comes towards you, you're not getting the full brunt of the Doppler shift from it. You're getting a diminished fraction of the Doppler shift. >> Okay. >> Because the train is moving sideways to you, not directly

at you. >> Right. Okay. >> Okay? So, the the formula works if you're standing on the track. If you're standing off the track, then there's an angle that you make with the train and there's a cosine in there, remember your trigonometry, which will diminish the effect of the whistle for you. >> Okay.

>> Okay? >> I In In a way, I'm picturing it as I'm trying to picture the sound wave and it's almost like it's a glancing blow as opposed to a head-on. >> Yeah, head-on that you get the full effect, >> Right. >> Okay, the more you come off the angle, off that axis,

>> Right. >> then the train is not coming straight at you. The rate at which it's coming towards you is not as fast as the rate at which it's coming towards you if you're on the track itself. >> There you go. Okay. >> Okay, that's all. It's just a geometry thing. Okay? And so what that means is

if you get a speeding ticket because the radar gun is bouncing waves off of you and inside the radar gun the software is calculating the Doppler shift of the light waves. It's It's microwaves. >> Right. >> Of the microwaves, cuz it's just waves, Doppler shift applies to the microwaves. Wave is wave is wave. Okay? Okay?

>> The Doppler shift on your moving car the only way they can know your actual speed is if they're standing right in front of your car as you drive towards them. But they're never right in front of you. >> No. >> They're in the bushes. >> They're always off somewhere. >> Off somewhere.

>> Your honor, I'm going to need to see the calibration of that radar gun. >> So, what it means is they will never read, unless they're on a turn and they get you >> Oh. >> you know >> Right. >> come you know >> Yeah, cuz you're coming around, but that puts them directly in front of you.

>> Directly in front of you as you come around, okay? But in a normal straightaway they'd have to be in the middle of the road standing right in front of your speeding ass car to get the proper speed. >> If they're off axis they will never get the proper speed. They will get less than your proper

speed. >> Oh, damn. >> This is my point. >> Oh, man. >> That's my point. >> that's why that's why I'm ALWAYS LOSING COURT. >> [laughter] >> THAT'S MY POINT. SO >> CUZ THEY'RE GETTING YOU ON A on a glancing blow. >> They're getting you on the lesser >> And if that broke the speed limit

>> Yeah, you you definitely would have broken the speed >> Yeah. >> If they say I got clocked you >> you at 114, what they really meant was you need to go to prison. >> [laughter] >> Now, they are relying on a reflected signal. >> Right. >> Because the signal they send out will be

a certain frequency. >> And then it bounces back. >> Bounces back >> Right. >> with the speed of the object. >> Right. >> And so they'll You can do that in in the software. If you have a car that absorbs microwaves. >> They turn on the radar gun, they get no signal back.

>> All right. >> And they can't know how fast you were you were traveling. >> I want that car. >> [laughter] >> But it also means because the full up Doppler formula has the speed of the observer built in. >> A police car can get your speed even though it is moving. >> That's

pretty dope. >> Okay. Now, the obvious one is it's just trailing you as it matches your speed. >> your speed. >> But that's easy. >> BUT IF IT'S COMING TOWARDS you >> Right. >> and >> pop you while he's and like >> Got you. >> Got you. Got you coming straight at you

and it's going to get the right number. >> Wow. >> Because of Christian Doppler. >> Wow. You know what? >> F you, Christian Doppler. [laughter] F you, buddy. >> And of course we use the Doppler shift in a slightly different way getting the speed of stars that come towards us or away from us in our galaxy.

>> And the light shifts. >> Yeah, the light shifts. So So we know what the frequency of light would be in a laboratory. So it's it's usually spectral features that [snorts] only exist in one place in the spectrum. And you see it shifted. >> Ah. >> Then you say, "Oh, I know exactly how

fast you going." >> Look at that. >> Okay. And if it's shifted one way or the other, you coming towards me or going away. >> All right. But if it's going sideways to me, it's hard. >> Not much you can Not much you can do that way. >> It's going to show up in a different

place. That takes a long time and but forward or away from us, we got it. By the way, you can sit there at the side of a of the freeway and the cars will just make these sounds. We are so accustomed to that, you're not even thinking. You're not even saying, why is that?

>> Yeah. You got to be curious about that and not everyone is and fortunately Doppler was. >> It's so common though nobody is curious. >> Nobody's curious anymore. >> Yeah and I mean even like NASCAR and Formula One is like that's how they come by you. >> Okay so we're done here. >> This was great.

>> bit Doppler. >> That was fun. That was really fun. >> This has been another explainer. Neil deGrasse Tyson. Thanks Chuck. >> It's always a pleasure. >> Keep looking up. >> [music]