MIT 8.04 Quantum Physics I, Spring 2016 View the complete course: ocw.mit.edu/8-04S16 Instructor: Barton Zwiebach License: Creative Commons BY-NC-SA More information at ocw.mit.edu/terms More courses at ocw.mit.edu
Absolutely brilliant explanation. Wondering superposition being so easy a concept, lies the fact beneath it is not the case for QM. Must thank you Prof.
His Inca name is Amaru Qapac, which means “Chief Dragon”, and is an Alumni of the Peruvian National University of Engineering (UNI). He is our champion!
The reason quantum mechanics is so puzzling is that the experimental results themselves are intrinsically puzzling. Quantization is counter-intuitive. Turning everything into waves of probability works as a mathematical model of the physical phenomena, but waves of probability don't really make sense to us. We have perfectly good mathematical models that predict results to stunning accuracy, but we just can't fully accept what the math is telling us because the way it works seems fundamentally un-physical to our limited minds.
That is true.. Maybe it seems counter-intuitive because of our perception of the physical reality.. Perhaps the un-physicality, if I may say so, is because we cannot perceive an electron.. However, an ammeter will give you the rate of flow of electrons pretty accurately.. The good mathematical models have been designed to achieve just that.. Amazing how the human mind finds a way to uncover truths..
@@arjuncalidas2736 but I think Ralph's point goes beyond the simple inability to 'see' or perceive an electron, it is the nature of the electron (being described mathemetically as a particle and a wave in different cases) which is difficult to comprehend. I simply can't imagine or visualize much of the concepts of quantum physics, which are often relegated to rote memory or mathematical reasoning alone. Even Einstein referred to 'spooky action at a distance' . I agree that it is impressive what science has revealed, it is so far beyond our everyday experience, we truly do stand on the shoulders of our predecessors.
@@dcamron46 This could very well be true if we haven't developed methods to experimentally verify treat concepts. But we have. The dual nature of particles are not only defined mathematically. We can very well project these mathematical models to yield results in physics and chemistry. This is why it is very important to not do science, especially quantum mechanics, based only on our physical perception. If we can theorise physical behaviour of a particle, we should be able to verify the said behaviour experimentally. And for the most part, we are able to.
Perhaps we're too indoctrinated with classical ontology. An object either exists or it doesn't exist. We don't ascribe any reality to probabilities, to "might-bes". But maybe the universe is telling us we're wrong. Actually, no: the universe is definitely telling us we're up shit creek. I think the whole of Western philosophy - ontology, phenomenology, epistemology - needs a massive overhaul tbh. It's getting to be embarrassing.
@@duprie37 That sounds like the right idea to me. We ourselves are stochastic creatures with stochastic brains hosting stochastic minds. Only the solar system appears deterministic, at first, which is perhaps why all of science acquired such a clockwork personality post Newton, pre Planck.
That's the moment when he tells you that he doesn't understand quantum mechanics. Photons aren't interfering with each other. Technically not even classical waves are interfering. Interference is the term that describes the absence of self-interaction in linear media and the fields of the physical vacuum. We try to teach this correctly at the high school level but most people never understand the reality behind that learning unit. That, unfortunately, includes many physics professors.
It depends. If you entangle the particles and get them through a detector at different angles, you'll observe that only specific angles will give you "right" proportions for detection (fractions) when you measure spin.
The problem here is the "existence" of the photon between the source and the detector. The source can infer the photons existence, the detector the transform of the photon. The path/paths are assumed.
At 14:30 - wow. Each photon must interfere with itself... which makes sense given the set-up and the speed of light (different quanta could not interfere with each other as they arrive at different times). But this says something very deep about the underlying construction of the universe, and things like three polarising filters allowing light through, where two polarising filters do not, seems to be the same thing. But this implies directly that the holographic principle is correct - that the universe of our experience is actually a secondary phenomenon riding on the back of something more fundamental, and that Schrodinger's equation just catches that main idea.
These lectures really are wonderful and I can't help but think about harmonics and waves when the Professor is erasing things. No, seriously. The audio capture and reproduction technologies nowadays are really impressive, you can really feel it! Squeaksqueaksqueaksqueak! 😬 It must be that dreaded 7th harmonic (maybe)
About superposition : as I understood, a photon can be either in A or B state because thats the only way to explain results of experiments. How reliable these experiments were (blasphemy!), but curious to know what was done to validate experiments and design them. Also, are there alternate explanations.
Hi! How can a photon interfere with itself? Isn't also the energy conservation law being violated? Because we have something like: photon -> 0 (for destructive) and photon -> 2*photon (for constructive). In these equations photons are monochromatic.
After watching this lecture, I got 2 questions in my mind. 1. Why we use States as a vector in Dirac notation? 2. How would Two photons interact with each other, because Photons are the interaction particle for electromagnetic waves, so how would they interact with themselves?
Excellent lecture ! But I m not able to understand if a photon is indivisible how it can go in two directional simultaneously as sir said just nearly 6:30
Is quantum mechanics useful if we wanna go in aerospace propulsion research???? Please someone tell....I wanna make my career in it and I don't know whether to study it or not...
Does this mean that if a quantum computer is to work many repeated ‘calculations’ must be done? If so, how is this any different to an analogue computer?
Just because we see an interference pattern, doesn’t mean that photon is going both ways. That may be one way of explain it, but it’s more of a metaphor than rigorously proven fact, am I wrong?
The correct explanation is that photons aren't going anywhere. Photons are the equivalent of the outcomes of a dice toss, i.e. like the numbers 1 to 6. Obviously none of these numbers have any meaning before the dice have come to rest. Neither does the physical term "photon" have a meaning before the energy (the photon) has been irreversibly removed from the electromagnetic field. So, no. Photons aren't going anywhere. There is energy in the field, but it is not localized in some "thing" called a photon. This is where one has to swallow hard and accept that physics is a system theory. It can not (and could never, not even in classical mechanics) atomize systems to determine "where" the energy in a system is. The energy of a system belongs to the system. It does not belong to any one specific part of the system.
@@Filip-ci3ng Physics, at least in my personal view, has at least three facets. There is the phenomenology, the observations we make. There is the theory that describes these observations and finally there is the intuition that we develop to translate between the theory and the observations. The phenomenology is unchangeable. It is dictated by nature. The mathematics we use in our theories to describe that phenomenology has a few degrees of freedom, but at the end of the day it has to describe the phenomenology well enough. That leaves the intuitive part, which everybody has to develop on their own. These are the stories we tell ourselves and others about how theory relates to phenomenology. These stories are fluid. They have been changing over time and are still in flux. The hard part for humans is to keep them in flux. We prefer absolute, ultimate answers. That, unfortunately, is not how science works. It requires continuous reflection about the quality of out intuition. In some areas like classical mechanics the great shifts have happened centuries ago (Newton to Laplace and Hamilton). In quantum mechanics, unfortunately, many people haven't even caught up with the state of the art a century ago.
Can photons be sent one by one? If it were that way, the size of a photon could be determined by moving the array of mirrors and when no signal is found it is because the photon has not yet arrived.
I think there is some information being lost in his description of what is interesting about the interferometer. When he says that it can be arranged such that one detector or the other sees all of the intensity it is easy to imagine a scenario where each beam splitter passes either everything or nothing. All of the outcomes to that setup (where each splitter passes all or nothing) are that all of the intensity lands on one detector. I think that what is unique to quantum mechanics is that you can separate the beams with both beam splitter and still get all of the light intensity on one detector or the other.
Quick question: how does the double slit experiment work with photons if supposedly they don't interact with one another? Just trying to understand the nature of the photon first before complicating it with mirrors and all the rest...
It cannot be that each photon interacts with itself only. If that were the case, then slight differences in length of the two paths would prevent the photon to coincide in space with itself and no interference of any kind would be observed.
It's much more simple than that. Photons, electrons etc. don't even exist until they are being measured. What "creates" what we call reality is the measurement.
That's a nice thinking, but honestly it depends how much can you direct the beam of light to a particular direction as far it goes it's intensity will become less. And to your query how many times we actually can do it probably infinty just keep the energy conserved cause we thought of it as in a single photon travels through both the path, so perhaps we can even say it can also travel through 3 path or so on but honestly I don't know it. Well it's been 3 years I don't know how much knowledgeable you might be now.
I don't see what prevents superposition in QM to be explained with classical superposition of waves. (and by the way, cube beamsplitters are positionned so that the incoming beam is 90 deg to the cube surface)
Precisely. Each posible outcomeof the experiment corresponds to a unique basis vector, in your case |A> and |B> . They are not only orthogonal but also unit vectors which makes ortholinear.
Why he didnt use the double slit experiment to show the nature of superposition ? Does the Mach Zehnder interferometer shows it more explicitly. I didnt understand that.
He wanted to explicitly point out the nature of superposition in QM when you've a split beam of photons. Since they are elementary particles and since you can easily build the story regarding the amplitude and frequency it's also easy to explain what it means to think of the beam as a quanta or a wave. The drawing also made it easy to understand. All of this to make it clear (how strange QM really is!): with two separate beams, the photons will interfere with themselves and not each other!
Words don't make sense. Senses.., interpret the interactive shaped occurrence of probability, for which we have (inadequate-incomplete) words, but this a good Methodology of illustration/teaching.
It doesn't seem plausible to carry out the experiment with one single photon. I don't think there is difference between the quantum and EM superposition.
Since you brought up linearity in the beginning and claimed that the SE is linear. These wave functions are solutions to the equation, then how come you can have such superposition
You simply reduce the beam intensity. The photons can be individually counted using a photo-electric detector and you can ensure that say one photon about every 10 seconds is passing through.
4:35 is the nail in the coffin for classical wave theory. "A bunch of energy canceling each out into nothing" is impossible by laws of conservation of energy that classical theory champions. Just shows how broken classical theory was.
Energy (or better, power density/intensity) doesn't cancel out in classical wave theory. Amplitude does. Amplitude is an easily observable phenomenon at the classical level. It is correct that it doesn't exist at the quantum mechanical level and that it is NOT trivial to reconstruct it with field operators. That's because it doesn't exist in general. It only exists for certain classical field states and there are far more non-classical states than classical ones in quantum mechanics. That's just one of those non-trivial complications that you would have to learn about, if you were serious about quantum field theory. It's well explained... in 1000 page QFT books. That's how much it takes to get a handle on what is going on here. If you are not willing to learn all that, well... then you are never going to understand it.
Sir 1.how energy is conserved when a photon superimposes with itself? 2. Why in QM only a or b and why not a+b ..because QM is probabilistic in nature? Please reply as soon as possible.
That's not even Dirac and it is the absolutely worst explanation one can give. Like most professors who are teaching QM 101 he didn't think it through because he doesn't care about teaching the class enough. You are hearing these nonsensical phrases because four or five generations of physics professors have been teaching this class in sleep walking mode since the 1930s.
The struggle of professors and teachers;They have to repeat the same thing over and over again for at least 30 years,and that comes with many challenges and risk of losing their jobs because their livelihood depends on it.Basically,every jobs are like this,you must repeat the same texts again and again,and they thought this is a career when its only a job,if it were a career they would go on progressing but its only a job so it gets boring as every job is.But quantum physics is different.
Arguments sure look deeply flawed to me. If a photon has a frequency (E=hf) and yet has a limited spatial extent in order to be particle like then the frequency becomes undefined! Frequency is events per time interval. To be a ‘particle’ the photon would be way to short to allow vibrations of its electric field so that it could be said to HAVE a frequency.
6:20 Wait, if a photon is the smallest energy for light, and you're telling me you can split it, then it's not possible that a photon is the smallest Quantum of light, the half Photon must be. What are the energies of the to split halves? They can't both be one quanta of energy. Sounds to me like some physicists are not doing their proper due diligence.
Democrats killed America and I stood by & watched After that, he says the to reflected or transmitted photons are the same . Another words, when it hits the BS, it creates another copy and that’s why called Super position!
Similar to what the other guy said. When it hits the beam, it isn't literally splitting. It creates a copy of itself in either direction, and these represent possibilities of where it went. We represent the superposition as some probability of it reflecting plus some probability of it being transmitted. The notion of the superposition is one of probability.
It doesn't create a copy of itself. 🤦🏻♂️ It means you'd have twice the energy and you'd violate law of conservation of energy/momentum (a.big no no). What it means is that the photons must be existing in both places at the same time. It means: they are a wave.
how about if each is half the intensity, or energy, that is to say, both take half the energy along their individual paths and than recombine at the detector.
There's something I'm not getting here. I've heard different word choice, namely: "a photon is travellig in both beams with probability XXX for each beam", and it made sense. But this? How can a photon *just* be in two different beams, without further specification?
@@maraudeuse952 Not until you measure the photon directly, it is always in a superposition state. Therefore, a photon technically "looks like" it is traveling in two different beams since you are seeing the interference result.
Great lecture, but unfortunately a lousy student. I think I'm condemned to only understand physics as it was till about the time of Rutherford and Maxwell.
This is not about the nature of superposition, at all. It's about the nature of maths on measurements of superposition. No description of what superposition physically is. All the way from the school definition of momentum as "product of mass × velocity", as if that piece of abstract maths is what it physically is, this is always the con in pure physics. It's never actually a physical description of anything's physical nature.
The problem is that your perception of what is real is less objective than mathematics and your brain is not very scientific in its perceptions of what's around it. You look at a thing whacking another thing but this is not a detailed view of what's happening at a microscopic or atomic or subatomic level within the thing. You need maths for it and the concepts will all be abstract. To pursue understanding behind that you need mathematics and if you try to connect it to reality you have to do so through experimentation. This is why physicists can send things to the moon and you can't
@@-BuddyGuy Xcyooz me: none of them acting alone can send things to the moon, only a team can. Sending things to the moon is applied physics, while my point was on pure physics. Whether it describes physical reality to the exclusion of other approaches to describing it. In Newton era physics, maths representing physical reality starts from a physical reasoning that what is being represented can be. In the century since quantum theory began, they have changed to willingness to believe surreal physical illogicalities if the maths leads to them. e.g. they believe in singularities in black holes, because the maths does not discover any force to halt the compression. But there is no way to express in the maths this bit of physical reason. Gravity is physically exerted through space by a mass, hence only be exerted by masses that still have any physical existence, onto what is spatially around them. Mass within a zero-sized point exerting it on other mass within the same point, across no space and with no space to exert it through, is nonsensical. So the compression must stop at a finite maximum level of compression of matter for matter still to exist with spatial extension hence to exert gravity. When you use an x^2 equation, that has 2 solutions, on a real situation, you don't say that both solutions somehow surreally must be true. You use physical reasoning to discard one solution, because it either is negative or is the wrong order of magnitude. The discarded solution was an artifice of the maths + it represented nothing real.
OMG, I know it's like the least important thing here, but the way he writes dots and dashes and how he (like in 1:02) can even mistake a dot and a dash is just so funny (good funny - don't you judge me, I so love these kind of things!)
Tbh when you teach something really you don't care about how much accurately you write, what's in your mind is you tell each and every thing to the other person in all the possible ways so that he can understand. After gaining a lot knowledge probably someday you might realise nothing will happen keeping all that in me, I'll die one day and so these ideas, understandings, clarity everthing and that's the moment where you might rush to teach everything to every possible mind that can understand it. Even I don't care what I am writing like the calculation in my head is going right suppose it's some answer in root and i just don't write root. Even when I explain any theory in my exam paper I something while writing in English switch to my mother tongue and I'm like what the hell I just wrote then I again start it.
Every reflection in some way is refraction! Everything passes through airborne particulates, frequencies, etc. therefore altered from its origination, no true rendering. The returned image, whether it be the eye or heat, light, etc. is casted, meaning the defused in this case, using your splitter, the primary chamber creates chaos and loses the fullness of the beam, this is refraction if you will, this furor is only partially retrieved and sent to the secondary! The primary continues to collect what was lost in the initial reflection "actually refraction*... This increases the strength of the primary output! Focusing to a desired point with disproportionate energy offsets your calculations, therefore your rendering is inconclusive. You yourself said you did not fully understand in your video! The manipulation of digits to statistics are no more than self proclaimed, not unlike the perimeters of an aquarium. Is this not a form of matrix?! Human "thrives" due to the ability to reverse engineer what is observed. I did not say "flourish"... The engineering is no more than claiming the understanding. One cannot re-create what is not understood this is why your practices simply unsustainable. You will never understand creation nor creator. Just because human can recreate life, you have overlooked the variables and your outcome is tribulation... We are electromagnetic ephemeral Tellurians, nothing more than synthesis to Genesis. The manipulation to fit the rendering is a farce. This is a matrix within a matrix within a matrix so on and so forth. It is time to step back, cogitate, ruminate on your direction. For your direction is our direction and forced fed! Many of us live with bird in the hand, proof and direction too... You take us away from reality in a already distorted world. What I have stated here is provable by every human being that is not disinclined, but your offer is limited to your thesis, your calculated outcome, "calculated" in every sense of the word is misconception due to leaving out the variables. This is why more than not, a creation of something is more harmful than anticipated. For example try to contain every heat wave that comes from the surface, roads, rocks, etc. you cannot contain all, meaning you have not accounted for all of the variables and the outcome, the rendering is false!!! Love to ALL
Dude, you cannot replace Science with handwaving nonsense. Stop this RU-vid trend, plenty of people like you thinking you understand something just because you can spew a chain of words like "chaos", "cogitate", and what the heck are Tellurians?? Admit your ignorance and study some science or shut up for good. You guys are embarrassing
