What is quantum mechanics really all about?

Quantum mechanics is perhaps the most misunderstood of modern physics topics, with many counterintuitive concepts like cats being both alive and dead and with claims that something doesn’t exist until a human looks at it. In this video, Fermilab’s Dr. Don Lincoln boils quantum mechanics down to its essence and demystifies this mystifying theory.

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[tanmoydutta]

I like the way how Prof.Don Lincoln explains Physics in a very simple, yet precise and concise manner

[ryanlyle]

I’ve seen so many videos over this topic over the years, and honestly this is the best 10 minute overview of QM I’ve ever seen.

[Docinaplane]

Quantum mechanics and thermodynamics was a class I took my first semester in college. I'm not sure if I have recovered from that trauma to this day!

[kennethkogut]

Dr Don, you are the type of son my parents always wanted

[jamesc]

Dr. Lincoln does an outstanding job of breaking down some of the nebulous aspects of Quantum Mechanics without talking over his audience or interjecting silly metaphors. He even presents the various main theories on the collapse of the wave function probabilities without imposing his opinion on the discussion. A lot of grade 12 physics and first year university instructors could really use a refresher on how to introduce this subject matter. They could help students start to understand the concepts without scaring so many of them out of the field before they even really get started. Worth a look. Good for Fermilab for bringing science education out of the lab and classroom to the masses.

[Ballr]

This is why quantum mechanics will never tell me how much they're going to charge to work on my photon-powered car until they actually do the work.

[jonahansen]

"Each idea has burrowed its way into the subconscious of people who are interested in science, but aren't quantum experts." - I love that wording, for some reason....

[peterlobzhanidze]

I HAVE NEVER HEARD such a simple explanation of quantum mechanics, thanks a lot

[physicsguy]

I'm currently studying physics as a grad student, so I should probably not be typing this and get back to work, but I want to humbly say that I don't think this video does quantum mechanics justice.

First of all, I think saying the particle is in "many places" before being measured is problematic. The particle is not in many places, and it is not somewhere but we just don't know where. It is in a superposition represented by the wave function. This concept simply does not have any classical analog. It's a new thing that must be taken on its own terms.

Much more importantly, it is not even the wave-function alone that makes quantum mechanics a weird and rich subject. Think about the dice example Don brought up. If they are "quantum" dice, then you can never predict what side they'll land on and all you can say is that each side shows up 1/6 of the time (fair dice). If they are normal - "classical" - dice, then you *can* know with certainty what side they'll land if you study how you throw them closely enough, but it is still the case that the dice fall on each side with frequency 1/6 over many rolls. Now tell me, what is the difference between these two scenarios? At the level of dice rolling, nothing really. Both sets of dice roll with 1/6 probability per side. If this were all there was to quantum mechanics, quantum mechanics would just be a very pompous way of saying that no matter how careful experimenters are, they can never predict anything with certainty, even in principle.

The heart of quantum mechanics is *not* probability, but *interference*. Let me explain. Quantum mechanics says that for each way an event can happen, there is a special number called an *amplitude*. The amplitude is represented by something called a complex number. Don't mind what that is if you don't already know. What's important is that, unlike probabilities, which are always positive, complex numbers can cancel each other out when added together. To get the probability of the event happening, you add the amplitudes for each way the event could happen and then square the result. Because of this property, you can have situations where there is more than one way for an event to happen, but one way contributes a positive amplitude and the other a negative amplitude, so the amplitudes cancel out and the event never happens! This is exactly what's going on in the famed double slit experiment. With each slit open individually, you can find a particle at a specific location on the screen, but with both slits open, you never find the particle there (the dark bands). You've increased the number of ways the particle can get to the location, but instead of the particle getting there twice as often say, it never gets there. This canceling of amplitudes is interference, and it is essential to any discussion of what quantum mechanics really is all about.

[Vikezupa]

Don is one of the few physicists who brings me back down to ... reality.
Thanks, Don!

[quill]

"I swear, that train 0:24 went right through a STOP sign, Officer!" - j q t -

[delstanley]

I didn't realize at the time, and that was a long time ago that I guess I was sort of introduced to "quanta." Must have been in the 2nd or 3rd grade. It was a poem that went like this:


Little drops of water
Little grains of sand
Make the mighty ocean
And the pleasant land
Thus the little minutes
Humble though they be
Make the mighty ages
Of eternity.

[kennylex]

Noooo, do not kill Schrodinger's cat, let curiosity do that.

[bvbk]

Minor nit pick: visualizing the square of the wave function - it's wrong. Square of a (sine) wave is continuous and differentiable. It doesn't have much bearing on the rest of the content presented.

[nmagko]

I always hated Schrodinger's cat story. Thanks Don to clarify and get rid all those false ideas of quantum world. Excellent video.

[marcobiagini]

In quantum mechanics the physical system is described through a wave function whose evolution over time is determined by the Schrodinger equation. The wave function represents infinite different possible results for the physical quantities related to the system, but when we take a measurement, only one of these infinite possibilities becomes real; after the measurement, we must therefore modify the wave function “by hand” to eliminate all other possible results, and this modification is called the “collapse” of the wave function.

The fundamental problem with quantum mechanics is that interactions among particles are already included in the Schrodinger equation and such equation does not predict any collapse. The collapse of the wave function is a violation of the Schrodinger equation, i.e. a violation of the most fundamental laws of physics and therefore the cause of the collapse cannot be determined by the same laws of physics, in particular, it cannot be determined by the interactions already included in the Schrodinger equation. The Schrodinger equation is what allows us to make quantitative predictions about the outcomes of future measurements; everytime we make a measurement, we receive new information about the system, and we need to "update" our wave function, i.e. to collapse it, otherwise the Schrodinger equation would provides wrong predictions relative to successive measurements. After one century of debates, the problem of measurement in quantum mechanics is still open and still represents the crucial problem for all interpretations of quantum mechanics. In fact, on the one hand it represents a violation of the Schrodinger equation, that is, a violation of the fundamental laws of physics. On the other hand, it is necessary for the laws of quantum physics to make sense, and to be applied in the interpretation and prediction of the phenomena we observe. This is the inescapable contradiction against which, all attempts to reconcile quantum physics with realism, break.

Quantum mechanics is incompatible with realism (that's why Einstein never accepted quantum mechanics); all alleged attempts to reconcile quantum mechanics with realism are flawed. Quantum mechanics implies that physical reality (the universe) consists of the collection of all observed phenomena and such phenomena do not exist independently of consciousness. In fact, the properties of a physical system are determined only after the collapse of the wave function; when the properties of the system are not yet determined, the system is not real, but only an idea, a hypothesis. Only when collapse occurs do properties become real because they take on a definite value. It makes no sense to assume that the system exists but its properties are indeterminate, because properties are an intrinsic aspect of the system itself. The collapse represents the transition from a hypothetical system to an actual system.

The collapse of the wave function represents a non-physical event, since it violates the fundamental laws of physics, and can be associated with the only non-physical event we know of, consciousness. Therefore, the only consistent rational explanation of the collapse is that it occurs because consciousness is involved in the process. However, the fact that properties are created when a conscious mind observes the system in no way implies that it is the observer or his mind that creates those properties and causes the collapse; I regard this hypothesis as totally unreasonable (by the way, the universe is supposed to have existed even before the existence of humans). The point is that there must be a correlation between the collapse of the wave function (=violation of the physical laws) and the interaction with a non-physical agent (the human mind); however, correlation does not mean causation because the concomitance of two events does not imply a causal link. The consciousness that causes the collapse of the wave function must be an eternal consciousness, that is, a conscious God. This is the idealistic perspective, which implies that physical reality exists as a concept in the mind of God who directly creates the phenomena we observe, according to the matematical models through which He conceived the universe (the laws of physics); the collapse of the wave function is a representation of the moment when God creates the observed phenomenon. This is essentially the view of the Irish philosopher George Berkeley, and in this view God is not only the Creator, but also the Sustainer of the universe. Idealism provides the only logically consistent interpretation of quantum mechanics, but most physicists do not accept idealism because it contradicts their personal beliefs, so they prefer an objectively wrong interpretation that gives them the illusion that quantum mechanics is compatible with realism.

[cedricveinstein]

Don Lincoln is one of the *very* few in YouTube who has both the required knowledge of the complex physics subject at hand and the skill&patience to teach it to us "less physics savvy".. at least to the level that we can use it to further build our understanding about that very subject

[kagannasuhbeyoglu]

Many thanks to everyone who contributed to this channel. That must be the Fermilab difference. Thank you Professor Don Lincoln.

[HallsteinI]

Fermilab and PBS Spacetime uploaded on the same say? Christmas had come early'

[ciprianstanescu]

About what I thought as well: what was proved was that the Q.M. probabilistic equations are always correct in all tests, and all the others theories like "in both states at the same time/superposition" have no solid proof yet (like many tv and youtube documentaries would have you believe). Thanks Dr. Lincoln for clarifying this.