8.01x - Lect 34 - The Wonderful Quantum World, Breakdown of Classical Mechanics
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- Опубліковано 7 лют 2015
- This Lecture is a MUST - The Wonderful Quantum World - Heisenberg's Uncertainty Principle - Great Demos.
Assignments Lecture 33 and 34: freepdfhosting.com/9abfa7fb57.pdf
Solutions Lecture 33 and 34: freepdfhosting.com/9ba2d369b9.pdf - Наука та технологія
That bizarre feeling when you discover yourself applauding alone at your bedroom after the final speech. Thank you Walter, see you in 8.02.
very cool
@@lecturesbywalterlewin.they9259 I have this question. It has been troubling me for a while. The electron in a box has a wave function. If we square the wave function we get the probability amplitude. Does that mean that the Electron randomly changes its position with accordance to the probability amplitude ? That is most of the time it will be in the place where there is the hills & valleys and never in the nodes? Or does the probability density mean that, the electron is in all the places where there is a non zero probability at the same time ?
I would be really thankful if you can help me out with me.
Thank you.
@@soutrikchakraborty5211 this is covered in every QM course. Both QM 1 and QM 2 are on my channel
@@lecturesbywalterlewin.they9259
I had a doubt that if we take that the slit to be made of 2 different opaque sheets and if we remove one of them , then will the spreading of light be stopped?
@@lecturesbywalterlewin.they9259 I cant find the quantum mechanics lectures can you please provide the link for it..it would be really helpful.
I know this is MIT and students should expect the best but the professor should have been given a standing ovation.
The best possible explanation over the internet.
Professor, I would like to thank you for uploading this wonderful resource to the Internet, I have very much enjoyed "taking" 8.01
:)
watching this from a remote village from Sri Lanka after 5 years
I have a strange feeling as even though there is a great space and time variation,i still enjoyed this lecture from a MIT professor , where I will never be able to see physically,let alone USA.
I am a medical doctor but still enjoyed this as it refreshed my college physics.
Thank you sir!
thanks for your kind words
Love from srilanka
@@lecturesbywalterlewin.they9259 Accept Islam, you will go to heaven.
excellent lecture!
many thanks from a Greek physicist from California!!!
my pleasure
An excellent lecture, thank you for taking the time to make this publicly available
You're very welcome!
Very important lecture explaining Heisenberg principle with great clarity. Fantastic.
:)
One of the most heart touching physics class that one can hear from a great teacher in his whole life! what a fantastic, amazing person you are!!! You are going to live forever! 💖
Wow, thank you!
I would never have understood it so easily before----thanks for this video
I love that presentation. Especially with the Physics book. The energy it takes to understand the content is much higher than the Gravitational energy that can be changed in potential energy. It's the other way round with Poem books. A poem book generates energy.
No words to say just incredible thank you
I feel really fortunate to have have come across this vedio and got to hear this amazing topic by a great physicsist like you whom i may never be able to even see.
Sitting here in india , preparing for neet ,i 'm privileged to learn from a MIT professor from USA
Glad you enjoyed it!
Hello sir ....I am saying this from my heart that you made quantum so easy for me ... really before this lecture i found quantum was so boring but you make it easy to me ...as I am from in india I like only person who teach physics so well which is alakh pandey sir ( physics wallah) and now today I watch your lecture .. really it is so good experience to watch your lecture....hats off to you sir ..❤️😊
It's my pleasure
I have no words to express how much I liked this conference. Thank you Professor Walter Lewin :')
Our pleasure!
Amazing explanation sir
Hats off to you sir
Dear Professor. Your lectures are way more than awesome. I'm a huge fan of yours! I'm subscribed to your UA-cam Channel, I have got your book, I have got an autographed picture of yours.
Thanks a lot for everything you've been doing for all of us!!! We can't thank you enough!!!
Regards from Brazil/Canada!!!
Many thanks!
Such a great teacher . I never be see such type teacher
What a wonderful professor
a great teacher a great person
Best physicist in the whole world.🙏🙏🙏🙏🙏
Thank you for your great way of teaching... Makes physics fun and enjoyable... 🙏🙏🙏🙏🙏
:)
Thank you professor, god bless you
Super duper professor, you are amazing at explaining.
Amazing. Thanks for posting
Thank you very much dear Professor Lewin.
Welcome!
superb explanation 👍
Thanks sir your explanation is such awesome
Professor, please take a class on Thermodynamics. It's okay if it is not in MIT, because all I want is you to teach Thermodynamics. MIT or your home, doesn't matter, sir. Please.
Great lectures, unbelievable that I can just watch them for free. Can I ask what you said in the room when you’ve edited in you saying “energy levels” or “from a higher to a lower energy level” in the video? E.g 7:30
Great teacher
Great content. Regards
46:36 best applause ever...
I think he says "orbit" during the lecture which was later corrected to "energy level", as evidenced by the voiceover and frame change each time the word comes up.
Perfect class...
Full depth explanation...
Thank you sir for this wonderful lecture 😊. You make physics so easy .
It's my pleasure
First time quantum mechanics feel having sense,it never convinced me before
It would be amazing to watch a series of lectures given by Prof Lewin on Modern physics, his approach to QM ,General Relativity, supersymmetry or multiverse, and how gravitatonal waves recent finding can impact.
I retired from lecturing courses at MIT in 2009. My legacy are my 3 MIT course lectures, Help Sessions, colloquia and special talks. They are being viewed daily by about 30k people.
Lectures by Walter Lewin. They will make you ♥ Physics. Your legacy is bigger than you can imagine, you aré a source of inspiration for young minds all over the world, a few people can say that they reach every corner on the planet sharing and spreading love for physics. Respect and love from south america Prof.
thanks for your kind words
I do not solve problems for viewers. I teach Physics. Watch my 8.02 lectures.
Mr. Lewin. Thanks for this lecture.
We can not enter the world of particles therefore their behavior does not come to us intuitively. The laws of physics are working, and god does not need the dice for it.
what the amazing lecture, love it
Glad you enjoyed it!
Why is the video and sound edited whenever you refer to "energy levels" or "energy states"?
In the original video he said "orbits" instead of "energy levels" or "energy states." I assume he changed it to be more accurate as electrons do not move in traditional orbits but instead exist in a probability field based on how much energy they have.
Also, Walter Lewin is a bit of a perfectionist so minor things like that bother him until he actually goes and fixes it.
Using the term “orbits” can give you the wrong idea of what is going on, so he changed it to “energy levels”
Excellent and amazing thank you
Most welcome 😊
I love how every word moves the lecture forward! No digressions or filler words. But I have two concerns regarding the content. It's not as simple that e.g. electrons are sometimes particles and sometimes waves. The probability distribution for the properties of the particle is wave-like, what you actually measure is *always* particle. That's two different levels of abstraction. Secondly, the uncertainty principle is about repeated experiments. If you measure the momentum of the billiard ball many times within the triangle, you will have a large variance in your set of measurements. The principle does not apply to one and the same particle.
That is a common mistake in most introductory classes to quantum mechanics. You didn't get it right either, though. We never measure particles. We can only measure quanta, i.e. small amounts of energy that quantum systems transfer between each other. Quanta do not have coordinates and hence they do not have particle-like behavior. Einstein got this wrong in his 1905 paper on the photoeffect and his mistake has stuck around for 116 years... time to end it.
And yet radio waves are never treated as particles, they always treated as waves even though they are still photons.
Love you sir ❤️❤️ FROM NEPAL
Thank you Sir. 🙏🙏🙏🙏🙏
Excellent lecture Sir. Thanks 🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏
Most welcome
Hello Prof Lewin.
We know that stars are made mostly of hydrogen with other elements present in small fraction. The spectrum of hydrogen gas has discrete wavelengths. For example in the visible region we have 4 wavelengths in Balmer series.
Then why we approximate the radiation coming from the stars as blackbody radiation and use weins law to calculate their surface temperature. At the same time the spectrum of stars also contain some missing lines like the case of discovery of helium element from the spectrum of Sun.
You are correct, the spectrum of stars is not a perfect Planck spectrum. Thank god it's not... if it were, then we couldn't identify the chemical composition of stars from far away! Temperature is, as you are most likely aware of, a thermodynamic equilibrium quantity. Stars are not in exact thermodynamic equilibrium and therefor their surface temperature is an approximation.
Dr. Lewin,
Just a great teaching style.
Sadly, in the era of publish or perish, most professors dont teach as good.
Best,
Joe
Hello Sir, I truly enjoyed this classical mechanics series , I am now going to start part 2 , thank you putting it on here , also If I start learning quantum mechanics which lectures/ books would you recommend?
any college physics book will do
use the web
So it is possible to drive electricity on an element in 80 ways because of QM. Sparks take different directions though electricity is in a single direction. So quantum probability is one by eighty. Or 84×2 root 168. Or 163 and R constant.
Hii sir I'm from Kashmir and i am your a big fan and our teacher is also a fan of yours sir even he shows us your experiments in class and always say that i have learn this from prof walter @physics with Nasir hajni
So nice of you
There is no video on tunnel effect 1-d box and other?
Quantum mechanics is hard to think.As everything is nothing,and absolute nothingness can not be imagined.The absence of matter is referred to as vacuum.But even field like gravity acts there.The universe is not bound by the limits and what is beyond that beyond that often blows my mind up.But you have given me an exemplary effigy what I was supposed to learn.
sir i think the wavelength of the ball is 0.132*10^-34
Sir, its so great to talk easily to an expert like u ... can u plz say how can i cover maths required for general relativity myself parallely with my academics at UG level ? best books covering entire maths required for GR ...
No I cannot
take a course in GR and decide on your own
i don"t have any knowledge for GR , i asked quora but many people are saying different things , i just want to start some maths required for GR in free time now before college , so wanted advice for how to cover some maths from u sir
Does the students where sitting on the left side have the same order of light colors as students sitting on the right side??
Thank you sir!
You are welcome!
I m slightly off from the topic.....I went through principle of least action today( hamilton' s varriational principle) it says particle will take a path such that some integral is minimum.....can there be more than one paths for which the integral is stationary??
Mr. Walter, Classic Physics may fade away but Classic Walter would not!
professor i am a student of class 11... i was searching for the topic atomic structure but i found ur channel and started watching it ... it made me think positive against classical mechanics
thank u sir.... kindly make more videos on this topic
if u can please reply me sir.... regards ur student from india
You are most welcome
Thanks professor for explain such hard topics in intersting way
You are most welcome
With due respect sir , I am from India and I love Quantum Physics and Nuclear Physics from the core of my heart . Kindly suggest me the ways so that i can make my career in these two great fields . Thank you sir .
after high school go to the very best college that will accept you consistent with your highschool grades. Get a bachelor's degree in Physics. After that apply to the very best Universities that may accept you and get a PhD in QM & Nuclear Physics. Take it from there. 3 yr college + 6 yr for your PhD adds up to about 9 years. I was 29 yr old when I got my PhD in nuclear physics.
Dr. Lewin, what is the name of the double slit experiment you illustrated in this course? Please let me know; thanks!
use google
Sir, Suppose I send a polarized beam of light from an infinite distance away from a thin slit, whose thickness is slightly higher than the diameter of the photons in that polarized beam, then as the light passes trough that slit, we have an idea of the photon's position at the time it passes through the slit, and since the light was sent from infinite distance so, the vertical momentum is zero as well as the horizontal momentum is arguably constant( assuming it doesn't collide with other particles in the medium) till the time it passes trough the slit, so at time 't' when it passes trough the slit we know precisely what the position of photons are and the momentum at the same time 't'.
When your light passes through the slit, you know the position in the direction perpendicular to the slit very well. Thus according to HUP the momentum in that direction will be very uncertain and if you look at the screen behind the slit you will SEE THE SPREAD of light in that direction. There is NO way to by-pass HUP. Any Newtonian way of thinking is not valid.
Ok ,Thank you so much for replying! I was just wondering is HUP a fundamental law of nature? or Is it just because of the wave nature of particles?
it's a corner stone of QM. In QM particles and waves are IDENTICAL. You cannot think of them as separate. In Newtonian physics we think of them as separate. To apply QM to a base ball would not be very useful.
calculate the wavelength of a baseball going 80 mph using De Broglie!
Professor Lewin, do you have any courses in quantum mechanics?
no
Wonderful #makemescientific
I have a slight doubt Professor. When an electron is transiting from one energy state to other during that time at any instany won't it occupy an intermediate position just like a ball in gravitational field does. If not , why ?
QM is the way the world ticks
At 0:46 , When you talked about how many atoms put to form one inch , shouldn’t we consider the size of the atom as well? Cause some have one layer or some have two or even more layers
atoms have no layers. **Size: **Atoms have an average radius of about 0.1 nm. About 5 million hydrogen atoms could fit into a pin head. The nucleus of an atom is 10,000 times smaller than the atom. If an atom was the size of Wembley Stadium, then the nucleus would be the size of a garden pea.
37:16 Is that because of the Taylor expansion of the sine?
yup . to be more precise it's the mc laurin expansion . when you substitute angles whose values are closer to zero you get almost the same result if you had substituted zero .
THINK YOU LECTURE
Hi Prof. Lewin
Do buckyballs also display the interference pattern in the two-slit experiment? If 'yes', how is it possible since buckyballs are much bigger than electrons, photons.
(My answer is 'yes' but would like to double check with my concept.)
no, neither do tennis balls and base balls
+Lectures by Walter Lewin. They will make you ♥ Physics. en.m.wikipedia.org/wiki/Double-slit_experiment
In the paragraph 4 & under the 'Variations of the experiment' stated that buckyballs do display this interference pattern.
Yes I now remember. The balls must have been super small. Read this
en.wikipedia.org/wiki/Double-slit_experiment
What does it mean for the uncertainty of the momentum to be greater or equal to hbar/Δx, is there no upper bound? Does that mean that if i'm walking in my room I might all of a sudden reach the speed of light and die? How can a particle have a higher speed and the same energy as another particle?(meaning where do I get the energy to have a higher speed?)
google Heisenberg's Uncertainty Principle
At 41:35 u said the spreading of the light was already explained earlier.....by that did u mean the wave nature of light and the experiments done by young?
yes the double slit interference in 1801 by Young was Proof at the time that lights are waves,
This was before QM. We now know that light is BOTH particles (photons) and waves. A concept that does not belong in Classical Physics - a concept that neither you nor I can understand but it's the way the world ticks. Watch Feynman’s double slit lecture ua-cam.com/video/2mIk3wBJDgE/v-deo.html
But at school they taught us that atoms have some layers with different energy levels which can hold a limited number of electrons according to the formula 2n^2 and I meant that the one with more layers has to have more volume .
The Bohr Model is a simplification. Electrons in artoms have discrete energies but not discrete positions. In QM nothing has dicrete positions.
Sir wave means there is motion of disturbance in a space Like a em wave. Sir in em wave , the electric field and magnetic is oscillating in space.
If Electron is a particle then how it can oscillate in space in various point
use google
If we know the history of an event, can the test probe be used to know the position and change the course of the event?
question unclear
professor,
In matter waves, actually what is the thing that weaving. like in sound waves the particles of the medium osscilate to and fro.
we found out the wavelength for the electron, Is that electron moves in the waveform , having that wavelength.
help me so much confusion and sleepless nights.
all matter has wavelength according to QM (De Broglie). Please use google for more.
How many factors are involved in collision of test probe on an sub atomic particle?
use google
If electrons can exist only in specific or discrete energy levels, how does an electron move from lower to higher energy levels? Would that electron not move through all space in between including area that do not include the allowed energy levels?
you think Newtonian - QM has it own rules. The question how did the electron move from quantum level E1 to E2 is an illegal question in QM.
@@lecturesbywalterlewin.they9259 Thank you very much sir
Professor, why angular displacement, a. Velocity, a. Acciliratit are directed toward axis of rotation
question ill-defined
Exactly at 40:22....why are we seeing this funny shape and after this time where does the green spot, which is going outwards, where has it gone because we can't see it now?
Also some part is separating from the spot around 40:22?
the video is badly out of focus. I suggest you watch this lecture.
ua-cam.com/video/MeK0DV329mU/v-deo.html
It's well in focus there. Watch 57:15 and threafter.
the uncertainty principle is a strong argument against the idea of teleportation since the information of each particle can never be completely determined, is this correct? If it is impossible,why it is still speculating?
correct - that is also an issue with Schrodinger's cat and with an "object" tunneling through a barrier. It lacks the *coherence* for that to happen. Alpha particles can tunnel through a barrier but not a tennis ball.
Is photon separate able? How do we know if one photon is one photon , not a group of mass when we shoot ONE photon every day and to observe the landing on the screen, , can we really measure that accurately?
read up on Photo-electric effect for which Einstein received the Nobel Prize.
Sir so can we say that heating objects emits light as the heat energy sends electrons to higher orbitals and then they go back down to release light
those account only for emission lines, not for the black body radiation. 99% of the Sun's radiation is due to black body radiation. use google
around 10 min in video you made a energy level diagram of an electron and you said that an electron in higher energy state would come to lower energy state by releasing a photon, so here I want to know that how an electron could produce a photon in higher energy state and releases it to come to lower energy state .i.e. how an electron can produce a photon??
When an amount of energy E is available according to QM EM as well as particles can be produced. In the case of EM (visible light or X-ray or gamma ray) It's even possible to draw a parallel with classical physics. The electron does NOT disappear. In class ph any accelerated charge will create EM radiation.
Perhaps I'm missing something, but the Uncertainty Principle doesn't seem all that counter-intuitive. Imagine a crowded sports area. At the end of the event, people start filing out a wide exit door in a very orderly way. For some reason, the door slowly begins to close so people are forced to exit through a continually narrower space. At a certain point, panic ensues and, now individuals are squeezed together at the exit, and some can pass or are pushed through the door in a disorderly sequence and in unpredictable directions. Same thing happens with an adjustable nozzle on a garden hose! Why not photons?
Heisenberg's UP is QM. All QM, including HUP is counter intuitive as we all think classical phyisics (newtonian). *Objects at position P in space can stand still*
@@lecturesbywalterlewin.they9259 So, QM is counter-intuitive with respect to classical physics exclusively and perhaps not with respect to "particle" behavior in other domains such as animal behavior. Granted; but for me being a non-scientist, (but comfortable with the mathematics), thinking of possible analogies is helpful in grasping the ideas. I loved 801 and will be continuing with 802 and 803. Bless you so much for making these available.
Sir light when passes through the slit. It can also be explained through diffraction. Sir than why need QM for this
HUP always applies. It's there also in diffraction
Whether the quantum nature is due to Planck constant, h or quantum number, n? E=nhf
*E=nhf*
sir since Bohr model has many drawbacks then why still its theory of quantisation is still used as a pillar of quantum mechanics?
google "quantization in QM"
Does uncertainty principle and Quantum leaps in quantum physics contradict each other?
no Heisenberg is also dE * dt = hbar/2pi use google
So for Heisenberg principle
We can say that it is only applicable in microscopic world and it is negligible in macroscopic world and hence not applicable.Right?
29:09
No, that is completely wrong. The uncertainty principle is just as valid for sound waves, water waves, classical electromagnetic waves etc..
how can newton be wrong
hahahahahahaha
corpuscules
Sir, can we use entanglement to clone humans or particular part of our body as we can use it for teleportation(maybe a lot latter in the future)? And if yes, then there must be some way to cure diseases like cancer by taking information of each and every particle and then exchange the information of defected organ by the information of a healthy organ and then use entanglement to send this information to some other set of particles to constitute the body back in the way it was.
ask a physician
Lectures by Walter Lewin. They will make you ♥ Physics. Thanks for replying, sir.
Sir as the radious of the hydrogen atom is 1angstrom. Hence there will uncertainty in the velocity of electron up to 1/3 speed of light. but how this prove that electron are confined to the discrete energy level. How this says that electron is confined to atom level
use google
Dear Prof. Lewin where can I get those gratings used in this lecture?
use google
Thanks for your reply. But Prof can you tell me a specific name of those gratings or a particular online store where I can find them.
They are called "diffraction gratings". If you live in the US or Europe I'll send you 2. I have sent over the years several to India and Pakistan they never arrived; they were stolen!
I understand that. I live in Indian and I will find them. Thanks a lot for even thinking of sending them 😊
+Devansh Parikh transmission grating amazon , something like that
why do the lines on the spectra have any thickness at all?
THINKKKKKKK
@@lecturesbywalterlewin.they9259 I know this is late but is it due to the thickness of the light source?
amazing question . did you find the answer . if so please explain
Why would heisenberg choose the uncertainty principle as between position and momentum and not position and velocity? Doest that mean m is not always definite? Or is it just a convention.
HUP can be derived. There are 2 similar results. deltaExdeltat and deltapxdeltax
There is no result deltamxdeltax. But if you know m, you can convert HUP into deltamxdeltax
Thank you for your reply.. i think it take me some hours of head scratching to understand it though! Ive visualised hup as a photograph of a racing car on a track. If the photo is really high shutter speed the picture is very clear but i have no idea if the car is moving fast or slow or backwards. If it's long exposure you see a blur so you get an idea of the speed and movement but you don't have a clue where it is in the blur..
Prof Levin, I know that this question has nothing to do with this video but I didn't find any of your videos about this subject, My doubt is: gravity is the curve of space-time or is a force with graviton as a force exchanging particle? If it is curved space-time why an quantum theory of gravity is necessary and not a quantized space-time? If is a force and the graviton is the force exchanging particle like photon is for electromagnetic force, then why a strong magnetic field does not causes time dilatation like a gravity field do?
I study physics and every professor I make this question and every book I read answers it different according to their point of view.
If you could attach a clock to a photon ,''somehow'', then the clock will show no reading, that's what is meant by ''time stops at the speed of light''. That's the ultimate time dilation. I suggest you consult cosmologists regarding most of your questions. I highly recommend Professors Scott Hughes sahughes@mit.edu, Ed Bertschinger edbert@mit.edu and Alan Guth guth@mit.edu all at MIT.
Thank you for answering me. I understand how photons don't experience time. But my doubt is if a strong enough magnetic field would cause time dilation in particles inside this field. (else the photons itself causing the magnetic field). Why only gravity curves space-time and not all fundamental forces?
Another doubt I have (now in context with the lecture) What if the photon is a wave, only a wave, and the device trying to measure it in the double slit experiment changes the phase of the wave changing the pattern in the detector causing us to believe it is a particle? The photon/electron being only a wave would solve most problems of non locality and collapse of the wave function...
And again, thank you for your patience and all your amazing videos.
I suggest you contact any of the people I suggested in an earlier msg. You can also consider Professor Max Tegmark of MIT. tegmark@mit.edu
36:50 Why is the angle deltap/p?
He either meant sin(Theta) = deltap / p or he approximated it because sin(x) is approximately x when x is very close to 0 (see taylor / maclaurin expansion of sinx). The later calculation is precise anyways, just a wrong variable name.