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If You Don’t Understand Quantum Physics, Try This!

If You Don’t Understand Quantum Physics, Try This!

Quantum physics has a mystique of being complicated
and hard to understand, in fact Richard Feynmann who won the Nobel prize for his work on quantum
electrodynamics said: “If you think you understand quantum physics, you don’t understand
quantum physics”. Which is kind of disheartening for us because if he didn’t understand it,
what chance do the rest of us have? Fortunately this quote is a little misleading.
We do in fact understand quantum physics really well, in fact it is arguably the most successful
scientific theory out there, and has let us invent technologies like computers, digital
cameras, LED screens, lasers and nuclear power plants. And you know, you don’t really want
to build a nuclear power plant if you don’t really understand how it works. So quantum physics is the part of physics
that describes the smallest things in our Universe: molecules, atoms, subatomic particles
thing like that. Things down there don’t quite work the same way that we are used to
up here. This is fascinating because you and everything around you is made from quantum
physics, and so this is really how the whole universe is actually working. I’ve drawn these protons, neutrons and electrons
as particles, but in quantum mechanics we really describe everything as waves. By the
way I’m using quantum physics and quantum mechanics interchangeably, they are the same
thing. So instead of an electron looking like this, it should look something like this.
This is called a wave-function. But this wave-function isn’t a real physical
wave like wave on water or a sounds wave. A quantum wave is an abstract mathematical
description. To get the real world properties like position or momentum of an electron we
have to do mathematical operations on this wave-function, so for the position we take
the amplitude and square it, which for this wave would look something like this. This
gives us a thing called a probability distribution which tells us that you are more likely to
find the electron here than here, and when we actually measure where the electron is,
an electron particle pops up somewhere within this area. So with quantum physics we don’t know anything
with infinite detail, we can only predict probabilities that things will happen, and
it looks like this is a fundamental feature of the Universe which was quite a departure
from the clockwork, deterministic universe in classical physics, the kind of thing Newton
derived. This wave-function model predicts what subatomic
particles will do incredibly well, but weirdly we’ve got no idea if this wave-function is
literally real or not. No one has ever seen a quantum wave because whenever we measure
an electron all we ever see is a point like electron particle. So there is like this hidden
quantum realm where the waves exist, and then the world we can see, which is where all the
waves have turned into particles. And the barrier between these is a measurement. We
say a measurement ‘collapses’ the wave function, but we don’t actually have any
physics to describe how the wave collapses. This is a gap in our knowledge that we have
dubbed the measurement problem, and this is one of the things that Feynmann was referring
to with his quote. Another confusing thing is how exactly to
picture an electron. It seems to be a wave until you measure it, and then it is a particle,
so what actually is it? This is known as particle-wave duality, and here is an example of it in action:
the famous double slit experiment. Imagine spraying a paintball gun at a wall
with two openings in it, you’d expect to see two columns of paint go through and hit
the wall behind. But if you shrink this all down to the size of electrons you see something
quite different. You can fire one electron at a time at the slits and they appear on
the back wall, but as they build up over time you get a whole pattern of stripes, instead
of just two bands, this pattern of stripes is called an interference pattern, something
you only see with waves. The idea is that it is the electron-wave that goes through
both slits at the same time, and then the waves from each slit overlap with each other,
and where the waves add together you have a high probability of the electron popping
up at the wall, but where the waves cancel out the probability is very low. So actually
on the back wall the highest probability of finding the electron is in the middle of the
slits, and then it goes down and up again, and down and up again and this is the interference
pattern. So when you fire one electron after another they follow this probability distribution
and this interference pattern starts building up, and that’s exactly what we see in experiments.
So this shows that electrons behave like waves in this experiment. A question is what actually happens to this
spread-out electron-wave when you do a measurement? It seems like it goes from this spread out
wave to this localised particle, but like I said, there’s nothing in quantum mechanics
that tells us how the wave-function collapses. And this is not only true for electrons, but
for everything in the Universe, so this double slit experiment has huge consequences for
our model of the Universe, and it was very surprising the first time it was done. Physicists
are still grappling with this question today and have come up with many interpretations
of quantum mechanics to try an explain these results, and explain how reality actually
works. Okay lets go back to the wave-function. Now
we can use this picture to explain other features of quantum physics that you may have heard
about. So this is just one possible wave-function
for an electron, but there are many others. Like this one for instance. This says that
the electron has a probability of being over here, and a probability of being over here,
and very little probability of being in the middle. This is perfectly allowable in quantum
physics and this is where the phrase ‘things can be in two places at once’ comes from.
This is known as superposition, which comes from the fact that this wave can be made by
adding, or superimposing these two waves. The word superposition just means the adding
together of waves and we already saw this in the double slit experiment, and is not
really a very special phenomenon. You can even see superposition by dropping two pebbles
into a pond where the ripples overlap. Now for entanglement. Let’s say two electron-waves
meet. Their waves interfere with each other and become mixed up. This means that mathematically
we now have one wave-function that describes everything about both electrons and they are
inextricably linked, even if they move far away from each other. A measurement on one
of the particles, like measuring if it is spin up or down is now correlated with a measurement
on the other, even if they move billions of miles away. Einstein was very uncomfortable
with this idea because if you measure one of the particles here you instantaneously
know what the other will be even if it is billions of miles away, and that’s got a sort
of whiff of faster than light communication, which is not allowed by the theory of relativity.
But it turns out you can’t actually use this to communicate information, because the
measurements give you random results, but the fact that they are correlated means that
somehow there is a link that stretches over that distance. This is called non-locality. Quantum tunnelling. Quantum tunnelling is
where particles have a probability of moving through barriers, essentially allowing things
like electrons to pass through walls. When a wave-function meets a barrier it decays
exponentially in the barrier, but if the barrier is narrow enough the wave-function will exist
on the other side meaning there is a probability of the particle being found there when a measurement
is made. In fact the only reason you are alive is because
of quantum tunnelling in the Sun which make the Sun shine. Protons normally repel each
other, but they have a small probability of quantum tunnelling into each other which is
what turns hydrogen into helium and releases fusion energy. All life on Earth exists because
of energy from the Sun, except for life around hydrothermal vents. Now on to the Heisenberg Uncertainty principle.
I said that the beginning that this wave-function contains all of the information like position
and momentum of the electron, we just have to do some maths on it. The position is given
by the amplitude, or height of the wave, and the momentum is given by the wavelength of
the wave. But for this specific wave the position gives
us a probability distribution, so we don’t know exactly where the electron is. Also there
is an uncertainty in the momentum because this wave is made of many different wavelengths. But we can reduce that uncertainty, let’s
have a wave that only has one wavelength, so a sine wave. Now we know the momentum exactly
because the wavelength has a single value, but look at the position. There is an equal
probability of the electron being found anywhere in the universe. Okay let’s do the opposite
let’s make a wave that has only got one position. Now we know exactly where the electron
is, but what is the wavelength of the wave? Now the wavelength is very uncertain. Basically
only a sine wave gives you a precise momentum, and any wave that isn’t a perfect sine wave,
you have to build out of multiple different sine waves, and each of those multiple different
sine waves has got a different wavelength, and hence you have a range of possible different
values of momentum for the particle. This is Heisenberg’s Uncertainty principle,
you can only know certain things precisely, but not everything. Either you have got a
definite value of momentum, and don’t know anything about position, or you know the position
very well, but don’t know anything about the momentum, or you are in some intermediate
state. And this isn’t a limit of our measuring apparatus, this is a fundamental property
of the Universe! And finally, where does the name ‘quantum’
come from. Well a quanta is a packet of something like a chunk of something, and one of the
first quantum effects people saw were atomic spectra which is where atoms give off light
with specific discrete energies. It works like this. Imagine a string that is tied at
both ends, like a guitar string. If you pluck it, only certain waves can exist because the
ends are tied down, in this situation we say that the wavelengths are quantised to certain
values. The same thing happens if you ties the ends
of the string together because the waves have to match up, they can only vibrate in certain
restricted ways. And this is what is happening to an electron in an atom. The electron-wave
is constrained by the atom and quantised to certain wavelengths, short wavelength have
high energy and long wavelengths have a lower energy. This is why the light emitted by an
atom looks like a barcode because each bar of light corresponds to an electron jumping
from a wave with a high energy to one with a lower energy, and at the same time emitting
a quantised photon of light when it does this. So the light from an atom is quantised to
discrete packets of energy. Okay so that’s all the basics of quantum
physics, here are some technical notes which aren’t essential to know, but pause the
screen now if you are interested in a little more mathematical detail. So to round up. In quantum physics objects
are described with wave-functions, but when we measure them, what we see are particles,
so this leads to particle-wave duality, and also the measurement problem. And the consequence
of these wave-functions are the quantum phenomena of superposition, entanglement, quantum tunnelling,
the Heisenberg uncertainty principle and energy quantisation. So if you understand these things
you have got a good basic understanding of quantum physics. Despite its reputation I think quantum mechanics
isn’t too difficult for most people to get the basics of what is going on. In the past
I have relied upon analogies to try an explain it, but here I have just described what is
actually going on which I think might be more helpful. But if you have more questions I’ll
be on the comments below so ask away. For me the weird thing about quantum physics
is that on the one hand it is incredibly accurate and predictive but also it has got giant holes
in it like the measurement problem which we just don’t understand. So we can wonder,
will we ever actually understand quantum physics, or is it just too abstract for our human brains
to comprehend. Well I hope this video has helped you understand a little more about
how quantum physics works. And thanks to the sponsor of this video brillaint.org,
who have just launched their daily problems which you can dip into if you have a spare
5 minutes each day. Each problem teaches you some interesting facts that you can then use
to solve the problem. And if you enjoy that specific problem there are links to more on
the same topic so you can develop your framework of knowledge around that subject. And as ever
if you are confused and need more guidance, then you can join with the community discussions.
So this is a simple, fun way to keep learning more. If that sounds interesting go to brilliant
dot org slash dos or click on the link in the description below, and the first 200 to
do so will get 20% off the annual subscription which unlocks all of their premium content. Well that’s it from me, thanks for watching
and I’ll see you next time.

100 thoughts on “If You Don’t Understand Quantum Physics, Try This!

  1. Thank you very well, but I have question:- I can't find how quantum physics leaded to the technology we have today. I think that quantum physics is not more than math and philosophy . Could you intoduce me any channel or website that relate it to technology please?

  2. If you think of the "barrier being narrow enough" as a rifle barrel "guide" for the wave function, then it might keep the properties of the wave function ie position and momentum, and allow it to pass through or "tunnel through" the barrier unchanged. If the "wave guide" was an over sized barrel like a large smooth bore shotgun barrel then the "wave function pattern" would be random. Shotgun pellets display random patterns of pellet distribution even when identical pellet size and charge weight are employed. A shotgun "slug" behaves more predictably than an equal charge of shot pellets, but its final impact point some distance from the gun's muzzle ie its accuracy is still not perfectly predictable, but close . There is still a random aspect to the slug compared to the pellets regarding predictability of the outcome. Still the prediction becomes less deviant ie more certain to predict the outcome. Perhaps "narrow barriers" are really "quantum wave barrels" that keep the wave "entire" during its passage through that barrier.It was as if the barrier did not exist. ie no barrier. So barrier is a mis-leading term.The double slit experiment gives the wave a dilemma. I wonder if the confusion is compounded with a DOUBLE double slit barrier for the wave to negociate? It is a fascinating subject for an inventor. Thanks, Rod Mac

  3. In aquagels the pores of the nano structure skeleton is so small that the mean free path is limited so heat cannot get through this material. There is a fundamental constraint on molecular motion and therefore material behaviour. I wonder if "wave barriers" perform a similar control function to any "wavefunction" trying to travel through that barrier?

  4. hmm I think it's more subtle than just disallowing useful information to be exchanged.. the very fact that someone measured is itself a communication of knowledge from one point to another… quantum game theory has instances of strategies for play which although not information transferring are optimal and better than an independent strategy therefore something spooky could be happening 'quantum luck' maybe

  5. Regards the double slit experiment. If the particles have energy of Their Own, and they obviously do since they emit light, could that energy interfere with the expected results? They're going off on their own tangent, and that's why you don't get the double line that you would expect.

    I and just learning quantum physics, so my idea might be totally off base. But if a thing has energy, that energy is going to affect how it moves. And if the energy is on one of several wave lengths, then the particles would move accordingly and create multiple rows instead of just two.

  6. this is a very interesting video and i simply loved it while i watched. thereafter i got confused about what i saw. i know that's confusing, but i guess quantum physics is a little confusing to beginners .

  7. Thought + emotion = matter. If you don't feel it, it won't = matter. It is the emotion of the observer that creates the outcome. All thought affects the quantum field whether observed or not.

  8. We are two physicists who also tried our hand at explaining quantum physics (beginner friendly). Leave us a comment if you like it: https://youtu.be/R2pStpPflsU

  9. oh quantum mechanics?
    fuck college just watch this 12 minute video
    and get affected by Dunning-Kruger effect
    and go lecture all those ignorant people on the internet

  10. Why does a laser light not change color it too has electrons jumping valencies.do the electrons not wobble around like here?

  11. Every different from the clock theory, but we don’t if own theory of the universe works. God made the universe too complex.

  12. And, if you understood this vid you still don't understand QM. lol. The absolute (?) best explanation of QM since I first met it in 1968. I'm old, for sure. Can you please do a vid about Goedel and his impact on philosophy, which is, of course, another word for metaphysics. TY so much.

  13. Knowing that something works, doesn't mean you know how it works. And that's where Richard Feynman was refering to.

  14. Hey guys, I wrote a small article on quantum mechanics for ppl with not much background in the subject (I am myself no expert). Do give it a read to learn more:

  15. All that this experiment and the term "duality" means is that scientist don't know anything about it, but their desire to present a possible thesis makes them come up with some sort of explanation, far from what reality could be….

  16. Well if I understand QP, it means I don't right so I ain't making an effort 🤣😂.. kidding, thanks for the video ..well I didn't understand anything from the Audio 🤣😂

  17. Got more than eight minutes in before I was lost. Not too bad. I understand about two thirds of quantum physics.

  18. Well, why is the wall in front? The obvious about wave disruption or waves are intermingled with alternate slowing effect. Prime ex. Is sunlight. The actual dispersion is too strong for the Earth. And the bend of the setting of space is a contribute. So, that's it. Lol…

  19. So the theoretical waves are energy and matter as one in a higher spatial dimensions without time. In addition, one of the byproducts of these waves is acceleration, angular velocity, and gravity. In physics we experience, the 4th spatial dimension is in the form of time.

  20. Question how much scientists know and understand QM. And when did they start applying it to technology and how relevant to technology is it.

  21. So, do I get it right that if one party has measured one of the entangled particles, the other party has no way of telling whether that has happened? Because despite of the result of the measurement being "random", the fact of collapsing itself is enough information already in my book.
    Also, where does the certainty of ANY measurement absolutely necessarily collapsing the wave function stems from? Maybe we simply haven't discovered more real time and less intrusive methods of doing that yet?

  22. Can you really say that the waves in quantum physics are just mathematical tricks? I mean Planck thought that his constant was just a mathematical trick where as Einstein proposed that light came in quantified packets of at least that size.

  23. Ofc u cant understand quantium phys… Coz they are made up from a genius only genius will umderstand them an find a way to diss Einstain did that happend already?!

  24. All I know is that changing the past doesn't change the future. Think about it, if you travel to the past, that past becomes your future, and you're former present becomes the past, which can't now be changed by your new future.

  25. schrodinger equation has a fatal error. Thus quantum mechanics cannot explain the nature and atomic structure. This is true!!

  26. "Probability", so I should take a quantum physicist to Vegas? I could 'wave' bye-bye' with all their sub- microscopic money. 'Beelyons and beelyons' worth.

  27. Now I understand quantum physics problems
    Very well explained how it break old theory of physic and trying to understand new era of things faster than speed of light
    Thus practically utilise Einstein's theory of relativity
    Where things can done in no time .
    Physic may use this to study intergalactic communication in no time
    Distance dosent bother any more

  28. Quantum tunnelling also happens in every cell of the human body. The more I learn about quantum theory, the more I think we are part of a quantum computer simulation.

  29. Thanks for that. A nice dive into quantum mechanics for a person like me, who heard the words, but never knew what they mean. Now I want to learn a little more, so I can annoy people at work with more amazing science.

  30. lol this video is so funny . did the guy in the video come to this conclusion from trial and error or was it from anothers words . i know what quantum physics is . its a word that is used to try and describe living particles that we normally cant see . everything basically is living and holds energy of some sort and moves in cycles of energy .

  31. i think the fact that our brains, and everything that we interact with on a daily basis is for the most part very linear, that itself sets limitations on what we can understand and perceive. The nature of our brains is the limitation of itself. The fact that there is x amount of memory to understand y amount of knowledge. Whereas things like quantum physics, or inter dimensional knowledge are on an infinite scale, and can't be perceived entirely, but are still super interesting to think about. As edgy or strange as it sounds, I've wanted to die multiple times, to really get a glimpse of reality. Day in and day out we're so caught up in us, us, us. Surviving, maintaining things like emotion and mentality. But in the grand scheme of things what is it for? Our nature is so insignificant yet so valuable at the same time. Lack of ability combined with amazing ability. Everything you live for and 'know' could be completely wrong or if not, completely worthless to know. Yet we continue to live by norms, and what seems safe.

    I've thought multiple times before, humans are like animals, but smarter, but still stupid. Clever and innovative ways to survive, but still selfish and naive.

    But I mean hey, who's to say anything matters. The Andromeda galaxy, supernova's, black holes. Almost everything eventually comes to an end.

  32. I have learned a tiny bit of quantum physics in school, but I never understood black body radiation and why this could only be explained with quantum mechanics. Could you maybe explain that in a video?

  33. You got the beginning right, but then you made the same mistake that most people make. Nobody has ever seen a point-like electron, ever. No such thing can even exist, since it would have infinite self-energy. All people have ever seen are three numbers x, y and z that describe a classical position in a detector where something happened. That there was a point-like electron at those particular coordinates is a figment of human imagination. Philosophically, this is where it's important to take the allegory of the cave seriously. In quantum mechanics the shadows (measurements) are real, but we can't, with any certainty, assign classical objects to them that would have made these shadows in the same way that classical objects make classical shadows. That's the really hard step to make when learning quantum mechanics… to suppress this need to see an objective classical reality behind a physical dynamic that is, with exception of the fundamental symmetries and the conservation laws that follow from them, nothing like classical mechanics. Terms like "particle", "collapse of the wave function", "particle-wave duality" etc. are simply failed philosophical attempts to map quantum mechanical shadows to classical objects. Any modern treatment of the subject should call them out for what they are: 80 year old category mistakes that need to be avoided by the modern student of the field.

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