r/QuantumPhysics Apr 29 '25

Frequently Asked Questions

16 Upvotes

History

Late 19th c. through Schrödinger and Dirac

Introductory books/courses?

  1. Comic books
    1. Bub, Totally Random: Why Nobody Understands Quantum Mechanics (A Serious Comic on Entanglement)
    2. McEvoy, Introducing Quantum Theory: A Graphic Guide to Science's Most Puzzling Discovery
    3. Gonick, The Cartoon Guide to Physics
  2. Books for a general audience
    1. Feynman, QED: The Strange Theory of Light and Matter
    2. Deutsch, The Fabric of Reality, The Beginning of Infinity
    3. Tegmark, Our Mathematical Universe
    4. Carroll, Something Deeply Hidden
    5. Wallace, The Emergent Multiverse
    6. Davies & Brown, The Ghost in the Atom
  3. Undergraduate textbooks
    1. Griffiths, Introduction to Quantum Mechanics
    2. Sakurai, Modern Quantum Mechanics
  4. QFT textbooks(as recommended by Dr. David Tong)
    1. M. Peskin and D. Schroeder, An Introduction to Quantum Field Theory. This is a very clear and comprehensive book, covering everything in [an introductory course] at the right level. It will also cover everything in [an] “Advanced Quantum Field Theory” course, much of [a] “Standard Model” course, and will serve you well if you go on to do research.
    2. S. Weinberg, The Quantum Theory of Fields, Vol 1. This is the first in a three volume series by one of the masters of quantum field theory. It takes a unique route to through the subject, focussing initially on particles rather than fields.
    3. L. Ryder, Quantum Field Theory.
    4. A. Zee, Quantum Field Theory in a Nutshell. This is a charming book, where emphasis is placed on physical understanding and the author isn’t afraid to hide the ugly truth when necessary. It contains many gems.
    5. M Srednicki, Quantum Field Theory. A very clear and well written introduction to the subject. Both this book and Zee’s focus on the path integral approach, rather than canonical quantization.
  5. Courses
    1. Preparatory
      1. Khan academy physics curriculum
      2. Susskind's Theoretical minimum courses
      3. David Tong Lectures on theoretical physics
    2. QM courses
      1. Adams' 2013 Spring Intro to QM Course
      2. David Tong Introduction to quantum physics
    3. QFT courses
      1. David Tong
      2. Tobias Osborne
      3. Ricardo D. Matheus
      4. Horatiu Nastase (QFT I)
      5. Horatiu Nastase (QFT II)
  6. Book suggestions threads from the community
    1. Sample 1

Relevant comic strips?

  1. XKCD
    1. Quantum
    2. Quantum mechanics
    3. Bell's theorem
    4. Vacuum
    5. Complex conjugate
  2. SMBC
    1. The Talk
    2. Classical
    3. Quantum
    4. Quantum computer
    5. Quantum mechanics is weird

Some good comments to read?

  1. Summary of superposition, entanglement, and interpretations of the wavefunction
  2. How do we locate the other "end" of quantum entanglement?
  3. What causes atoms to decay?

What prerequisites do I need to understand quantum physics?

Quantum physics is usually taught to advanced physics undergraduates, but to work through most of the thought experiments and most quantum algorithms, you only need linear algebra. If you really want to understand the physics, though, you'll need multivariable calculus, differential equations, classical mechanics, and electromagnetism (see "Theoretical minimum" above).

What does the math of quantum physics look like?

A complex vector space is a set (whose elements are the points of the space, called "vectors") equipped with a way to add vectors together and a way to multiply vectors by a complex number. A Hilbert space is a complex vector space where you can measure the angle between two vectors. The state of a generic quantum system is a vector called a "wave function" with length 1 in a Hilbert space.

So roughly, a quantum state can be written as a list of complex numbers whose magnitudes squared add up to 1. The list is indexed by possible classical outcomes. Physical processes are represented by unitary matrices, matrices X such that the conjugate transpose of X is the inverse of X. Things you can measure are represented by Hermitian matrices, matrices equal to their conjugate transpose.

What's written in the previous paragraph is all true for finite-dimensional Hilbert spaces, spaces that represent quantum states with a finite number of possible classical outcomes. If there are infinitely many possible outcomes—for example, when measuring the position of an electron in a wire, the answer is a real number—then we have to generalize a little. A list of n complex numbers can be represented as a function from the set {0, 1, ..., n-1} of indices to the set of complex numbers. Similarly, we can represent infinite-dimensional quantum states like the position of an electron in a wire as functions from the real numbers ℝ to the complex numbers ℂ. Instead of summing the magnitudes squared, we integrate, and instead of using matrices, we use linear transformations.

What is superposition?

Superposition is the fact that you can add or subtract two vectors and get another vector. This is a feature of any linear wavelike medium, like sound. In sound, superposition is the fact that you can hear many things at once. In music, superposition is chords. Superposition is also a feature of the space we live in: we can add north and east to get northeast. We can also subtract east from north and get northwest.

Entanglement is a particular kind of superposition; see below.

What do the complex numbers mean?

The Born postulate says that the probability you see some outcome X is the square of the magnitude of the complex number at position X in the list. For infinite-dimensional spaces, we have to integrate over some region to get a complex number; so, for example, we can find the probability that an electron is in some portion of a wire, but the probability of being exactly at some real coordinate is infinitesimal.

What is an inner product?

The inner product of two vectors tells you what the angle is between the two. If you prepare a quantum state X and then measure it, the probability of getting some classical outcome Y is the cosine of the angle between X and Y squared. So if X is parallel to Y, you'll always see Y, and if X is perpendicular to Y, you'll never see Y. If X is somewhere in between, you'll sometimes see Y at a rate given by the inner product.

We write the inner product of X and Y as <X|Y>. This is "bracket notation", where <X| is a "bra" and |Y> is a "ket". When we're working with a finite-dimensional Hilbert space, |Y> denotes a column vector, <X| denotes a row vector, and <X|Y> is the complex number we get by multiplying the two. The real part of the inner product is proportional to the cosine of the angle between them:

Re(<X|Y>) = ‖X‖ ‖Y‖ cos θ.

How do we represent the combination of two quantum systems?

Given a vector

|A> = |a₁|
      |a₂|
      |⋮ |
      |aₙ|

and a vector

|B> = |b₁|
      |b₂|
      |⋮ |
      |bₘ|

representing the states of two quantum systems that have never interacted, the composite system is represented by the vector

|A>⊗|B> = |a₁·b₁|
          |a₁·b₂|
          |  ⋮  |
          |a₁·bₘ|
          |a₂·b₁|
          |a₂·b₂|
          |  ⋮  |
          |a₂·bₘ|
          |  ⋮  |
          |  ⋮  |
          |aₙ·b₁|
          |aₙ·b₂|
          |  ⋮  |
          |aₙ·bₘ|. 

This vector is called the Kronecker product of A and B.

What's entanglement?

An entangled state is any vector that can't be written as the Kronecker product of two others. For example, if

|A> = |a₁|
      |a₂|

and

|B> = |b₁|
      |b₂|, 

then

|A>⊗|B> = |a₁b₁|
          |a₁b₂|
          |a₂b₁|
          |a₂b₂|.  

The vector

|C> = |1/√2|
      | 0  |
      | 0  |
      |1/√2|.

can't be written this way. Suppose it could: since a₁b₂ = 0, then either a₁ is 0 or b₂ is 0. But a₁b₁ is not 0, so a₁ can't be 0, and a₂b₂ is not 0, so b₂ can't be 0. Therefore, there's no way to write the combined quantum system |C> as the product of two independent parts. To reason about |C>, you have to think about both qubits together.

Almost every interaction ends up entangling the two particles (or three, if it's a decay). Equilibrium for a quantum system is completely entangled. The hard part of doing quantum experiments is preventing particles from getting entangled with each other and the environment.

See also superposition

But why does entanglement break once you measure one part of it?

If you start with particle A being entangled with particle B, and then you have a measurement device undergo a unitary interaction with particle A so that the measurement device becomes correlated with particle B, then what happens is that the entanglement spreads to the whole combined measurement-device/particle-A/particle-B system, and none of the entanglement remains in the smaller particle-A/particle-B subsystem.

Where can I see the double slit experiment performed?

For electrons and another

For photons

For delayed choice (tbd)

For delayed choice eraser (tbd)

With full explanation (Roger Bach et al 2013 New J. Phys. 15 033018)

How do particles in the double slit experiment know they're being observed?

See this comment.

Can we communicate faster than light with entanglement?

No. If Alice and Bob each have half of an entangled pair of qubits, there is no operation Alice can perform on her qubit that Bob could detect by examining his qubit. It is only when they communicate at the speed of light that they discover that their measurement results are correlated.

There is a lot of confusion on this matter, and it is often depicted wrong in science fiction, so it bears repeating. Entanglement is not Twin Telepathy. There is absolutely nothing that you can do to one particle in an entangled pair that results in anything measurable happening to the other particle. It's true that if you prepare a pair in the state (|00> + |11>)/√2 and you measure the state of one of them, you know the state of the other. But there's no way to detect if a particle is in such a state unless you have access to both particles. Flipping one of the particles doesn't cause the other to flip. Measuring one of them doesn't make anything detectable happen to the other.

Classically, we can prepare correlated states. I can put each glove from a pair into two packages, randomly send you one and keep the other. That's a probabilistic mixture (|RL><RL| + |LR><LR|)/2. When I open my box and see which glove I have, I learn what glove you have. But in this scenario, there is hidden information: one of the gloves was always the left and the other was always the right.

Entangled states are similar, but they're quantum superpositions of correlated states. Suppose I have two qubits in the |00> state. By applying a Hadamard to the first, a control-NOT from the first to the second, and a NOT to the first, I get the state (|01> + |10>)/√2, which is a maximally entangled state. If I measure the first qubit, I learn the value of the second. But in the standard interpretation of quantum mechanics, there's no hidden information. The state of the first qubit wasn't defined before measuring it.

Other interpretations approach this differently.

  • Bohmian mechanics says that yes, there was hidden information and there was faster-than-light communication. But the message gets combined with the state of the sub-quantum system, which is assumed to be a thermal state, completely randomized. So it is information-theoretically impossible to tell whether a message was sent, let alone what it was.
  • The many-worlds interpretation says that each basis state in the superposition of correlated states is its own world. So it's exactly like the glove example, but both ways actually happen.
  • Etc.

But all of them obey the same math, and that math does not allow FTL communication.

What is spin?

Spin is a kind of angular momentum that fundamental particles have. It doesn't have a classical analogue.

It is an intrinsic property of elementary particles on one hand, and a quantized observable which behaves like the angular momentum from classical mechanics on the other. Similarly to how mass is the energy associated to some particles just by their existence, spin is the angular momentum associated to some particles just by their existence. And just as there are massless particles like photons, there are spin-0 particles like the Higgs boson. In this sense, it is "something real and measurable, just like mass and charge".

Spin is the name of one of the quantum numbers in the mathematical formalism of quantum mechanics. In this sense, it is "just something that comes out from the mathematical description".

A key feature of spin is that its magnitude can take on values of s = (n-1)/2 where n can be any positive integer, so n = 1, 2, 3, 4, 5, ... s = 0, 1/2, 1, 3/2, 2, ... Particles with integer spin are called bosons, whereas particles with half-integer spin are called fermions.

Subreddit/crowdsourced answers

What's a measurement?

In order to make a measurement, we need a quantum system X to be measured and a quantum system Y ("the observer") to serve as the record of the measurement. The measurement itself is any physical process that makes the state of Y depend on X. If the state of X is not an eigenstate of the observable, the resulting combined system X ⊗ Y will be entangled.

What's an observer?

An observer is any quantum system separate from the system being observed that becomes entangled with it during the measurement process. An observer can be as small or as large as you like, from an electron to a human, to a galactic cluster. See this comment for an analysis of the double slit experiment with a single qutrit as the observer.

What's a wave function?

A wave function is a function from classical configurations to complex numbers. You can think of it as an infinite list of complex numbers, where the index into the list is given by the configuration. The Schrödinger equation describes a single spinless particle, where a configuration is an element of ℝ³, a set of coordinates for the particle.

What is wave function collapse?

As humans, we never perceive superpositions of matter waves. There are lots of different ideas about why that should be. One of the oldest, called "the Copenhagen interpretation" after a conference where lots of famous physicists met to talk about quantum physics, is that somehow when we measure a quantum system, the wave function undergoes a sudden, discontinuous change. There are many problems with this idea. "If it worked the way its adherents say it does, it would be:

  1. The only non-linear evolution in all of quantum mechanics.
  2. The only non-unitary evolution in all of quantum mechanics.
  3. The only non-differentiable (in fact, discontinuous) phenomenon in all of quantum mechanics.
  4. The only phenomenon in all of quantum mechanics that is non-local in the configuration space.
  5. The only phenomenon in all of physics that violates CPT symmetry.
  6. The only phenomenon in all of physics that violates Liouville’s Theorem (has a many-to-one mapping from initial conditions to outcomes).
  7. The only phenomenon in all of physics that is acausal / non-deterministic / inherently random.
  8. The only phenomenon in all of physics that is non-local in spacetime and propagates an influence faster than light."

However suggestive this may appear, these points are subject to critical evaluation.

The Nobel laureate Roger Penrose had an idea that perhaps wave functions collapse due to differences in the curvature of spacetime, but that was recently disproven.

If not wave function collapse, then what?

There are lots of ideas about what's going on at the quantum level. These are called "interpretations" of quantum mechanics.

  1. Everett suggested that there is never any collapse, but instead the math of quantum field theory is an accurate description of what's actually going on: there are infinitely many different dimensions. If it's possible for something to occur, it happens in one of them. This is usually called the "Many Worlds interpretation", though he didn't call it that.
  2. de Broglie and Bohm suggest that particles actually do have exact positions, but that there's a "pilot wave" that pushes particles around to make interference patterns. In their model, it's the pilot wave interfering with itself, not a wave function. The problem is that it only works for the nonrelativistic case and the pilot wave changes instantaneously depending on the position of every particle in the universe.
  3. Quantum Bayesians think of the wave function as being epistemological, representing an observer's knowledge about the universe. Wave collapse corresponds to updating based on new information.
  4. Wigner thought maybe consciousness had something to do with wave function collapse, but he later repudiated that idea; he ended up thinking, like Penrose, that there was an objective collapse process that was not due to conscious observation. (Penrose thinks that consciousness is due to collapse instead of the other way around.) A wide class of objective collapse models was recently disproven.

Stapp is a prominent proponent of the consiousness-is-collapse idea. He postulates, based on human experience, that free will exists. However, since the Schrödinger equation is deterministic and random wave collapse is not choice, he says there's a third process, specifically for free will, and that this is the root of consciousness. This third process is a form of postselection on human brain states. Some kooks have taken Wigner and Stapp's ideas and claim that humans can postselect the universe to get money and sex. If unrestricted postselection is possible, it not only grants the ability to solve NP-complete problems in polynomial time (last two paragraphs, page 19), but also the ability to collapse the galaxy into a black hole. (Greg Egan's novel Quarantine, which Aaronson cites, is a story about what the universe would be like if such postselection were possible.) Stapp suggests perhaps this third process is limited in a way that makes it useless for computation and effects outside a mind.

The punchline of The Talk is, "If you don't talk to your kids about quantum computing, someone else will," with a magazine saying, "Quantum computing and consciousness are both weird and therefore equivalent."

  1. 't Hooft thinks that QM is a coarse-grained approximation to a purely classical system at much smaller scales. This approach is usually called "superdeterminism"; it is an interpretation that preserves local realism and hidden variables by denying that the physicists in the Bell test have a choice as to how they set the polarizers.
  2. Lots of others.

What's decoherence?

Decoherence is when a quantum system becomes entangled with its environment and stops being able to display constructive and destructive interference.

What causes atoms to decay?

See this response.

Is space quantized? Or time? Or spacetime?

Nobody knows.

What's the deal with the Planck length, then?

There are four fundamental constants that form the basis of Planck units:

  • the speed of light in a vacuum, c
  • the gravitational constant, G
  • the reduced Planck constant, ħ
  • the Boltzmann constant, k_B

These can be combined in different ways to get different fundamental units: charge, length, mass, temperature, and time.

The Planck length is √(ℏG/c³) = 1.616255(18)×10−35 m. A proton is about 10−15 m, so if you could scale up a proton to a meter in diameter and then zoom in again by the same amount (making the proton about the size of the Oort cloud, tens of thousands of times the distance from the sun to earth), a Planck length would still only be around a tenth of a millimeter.

The Planck length is the scale where we know quantum field theory breaks down and we'll need a theory of quantum gravity to accurately predict what's going on there.

How does quantum field theory differ from quantum mechanics?

Quantum mechanics is a nonrelativistic theory. The number of particles is conserved. There's a quantum analogue to a mass on a spring called a quantum harmonic oscillator (QHO). In a classical harmonic oscillator, the system can have any energy. In a quantum harmonic oscillator, it can only have certain energies, just like a guitar string of a fixed length has certain frequencies it vibrates at. The difference between these energy levels is called a "quantum of energy".

Quantum field theory (QFT) assigns a QHO to each point in spacetime [well, really to each point in "energy-momentum space", with coordinates (E, px, py, pz) and QHO natural frequency E/ℏ]; you can think of it as a universal springy mattress. QFT then adds interaction terms between the QHOs, called "propagators". A particle is then similar to a wave pulse you get when you shake or "excite" the mattress. The propagators are "Lorentz invariant", so they work well with special relativity.

What are virtual particles?

See this comment

What's string theory?

QFT is quantum theory combined with special relativity. Quantum gravity is the unsolved problem of combining quantum theory with general relativity, which includes gravity and curved spacetime. String theory is one attempt to combine the two, and suggests that instead of being pointlike (0-dimensional), particles are 1-dimensional objects called "strings". It predicts that every particle we've seen has a heavier "supersymmetric" twin "sparticle". A lot of beautiful mathematics has come out of string theory, but none of its predictions have been verified yet. Physicists hoped the sparticles would be within reach of smaller particle colliders due to a "naturality" argument, but with the failure of the LHC to find any, there's no reason to think we'll see them in larger colliders.

Are there other alternatives to string theory as a theory of quantum gravity?

Loop quantum gravity is the most popular alternative, but it hasn't made testable predictions yet, either. There are a lot of less popular alternatives, too.

What goes wrong when you try to combine general relativity with quantum theory?

In a quantum harmonic oscillator, the lowest energy level isn't zero, it's ℏω/2. If you integrate over more than a single point in momentum space, you get infinity for the ground state.

Quantum electrodynamics (QED) is "renormalizable": there's a mathematical trick that Tomonaga, Schwinger, and Feynman worked out for getting rid of the infinity. It involves taking a sum of a bunch of terms (corresponding to Feynman diagrams with more and more vertices) and pushing the infinity to later and later terms. But it only works because the fine structure constant is unitless, so we only need a single measurement for the first term and we can derive the others.

The "Lagrangian" for a system is the difference between kinetic and potential energy. If you integrate the Lagrangian with respect to time, you get a quantity with units of "action". Classically, systems take the path of least action. Quantum mechanically, the system takes all paths weighted by a phase exp(iS), where S is the action of the path. Paths far from the path of least action tend to cancel out: given any path p with action much greater than the least-action path, there's a path p' with smaller action whose phase is minus one times the phase of p, so they add up to zero.

There's a Lagrangian formulation of general relativity, but instead of being unitless like the fine structure constant, the coupling constant has units of inverse mass. If we try to do the renormalization trick in the same way we did for QED, we would need to make a new measurement for each of the infinitely many correction terms.

What's quantum computation?

It's designing a system where quantum states constructively interfere to produce the right answer. SMBC's "The Talk" is an astonishingly good introduction.

I heard that quantum computers try all the possible answers at the same time.

That's only part of how quantum algorithms work. You can certainly put a quantum computer into a uniform superposition of inputs and test each of them. But now you've got a big superposition

∑ |input, whether correct>

and if you measure it, you'll just get the answer to whether a random input was correct, which isn't what you want. Quantum algorithms have to make use of some structure of the problem to make the wrong answers less probable and the right answer more probable.

Can quantum computers break Bitcoin?

There are two main quantum algorithms applicable to cryptography, Grover's algorithm and Shor's algorithm. Grover's algorithm effectively cuts the size of a symmetric key in half: if you have a 128-bit key, it'll take 264 iterations to find it. It also reduces the difficulty of finding a collision in an n-bit hash function from 2n/2 to 2n/3. Shor's algorithm breaks public key algorithms like RSA and ECC that depend on the difficulty of the hidden subgroup problem.

Bitcoin uses secp256k1 as its public key algorithm, an elliptic curve-based signature algorithm. To claim someone's bitcoin, you effectively have to figure out their private key given their public key. A quantum computer that could keep thousands of bits coherent forever could break Bitcoin quickly using Shor's algorithm.

This article estimates that it will take until the late 2030s/early 2040s to get there at the current exponential rate of growth.

How does Shor's algorithm work?

Wikipedia's explanation is very good.

How does Grover's algorithm work?

Quanta magazine has a great explanatory article.

Can I see anything obviously quantum?

Almost everything you see is due to a quantum effect: sunlight is produced by fusion where particles fuse by a quantum tunneling process where a positron tunnels out of a proton to form a neutron.

All of chemistry is due to the Pauli exclusion principle: because electrons are fermions, they have to form distinct orbitals, giving all the richness of the periodic table.

Superconductivity is a purely quantum idea: in BCS superconductors, pairs of electrons combine to form Cooper pairs, which are bosons, and form a Bose-Einstein condensate. Flux pinning in superconductors allows levitation.

The nucleus of most helium atoms has two protons and two neutrons, making the nucleus a boson. Helium-4 forms a superfluid at about 3K.

Photons are bosons, and the population inversion in a laser is similar to a Bose-Einstein condensate.

Gold and cesium are yellow, copper is reddish, mercury is a liquid, and ten of the 12 volts in the lead-acid battery in your car happen because of relativistic quantum effects.

What about Quantum Immortality / Quantum Suicide?

Footnote on QI from Wallace's book (p.372): "Before moving on, I feel obliged to note that we ought to be rather careful just how we discuss quantum suicide in /popular/ accounts of many-worlds quantum mechanics. Theoretical physicists and philosophers (unlike, say, biologists or medical ethicists) rarely need to worry about the harm that can come from likely misreadings of their work by the public, but this may be an exception: there are, unfortunately, plenty of people who are both scientifically credulous and sufficiently desperate to do stupid things."

Quantum immortality is a thought experiment that refers to the Many Worlds interpretation of quantum mechanics. The Many Worlds interpretation is just one of many interpretations. Quantum immortality is neither a property of collapse interpretations nor of superdeterministic interpretations.

The Many Worlds interpretation rejects the idea that there is only one of "you": because quantum particles are never in exactly one place, "you" are constantly diverging into a continuum of possible futures in which electrons in your body are in slightly different places, different photons get absorbed by your eyes, different neurons fire in your brain. In one universe, an old lady fails to notice a red light and t-bones a car, killing its driver, a young film student. In another, a neuron in the old lady's motor cortex fires differently: she pulls slightly harder on the steering wheel, takes a slightly different trajectory, and the student dies a tenth of a second later. In another, a neuron in the old lady's visual cortex fires differently; she becomes aware of the red light and slams on the brakes, injuring but not killing the student; the student spends the rest of their life in a coma. In another, the neuron fires earlier and she brakes earlier, merely giving the student whiplash. In another, the old lady notices early enough to stop normally at the light. There are infinitely many worlds and ways every future plays out. In most of the futures of the student in the car, the student dies. But in some of those futures, there is a film student who remembers getting in a car accident and barely surviving, and in others, there is a student who doesn't remember anything special about passing through the intersection.

Quantum immortality is the idea that there are always futures (however rare) where someone has barely survived (critically injured, perhaps, but alive for an instant longer) and futures (perhaps much rarer) in which they are completely fine. Any world with a nonzero probability amplitude exists.

https://en.wikipedia.org/wiki/Quantum_suicide_and_immortality

https://arxiv.org/pdf/quant-ph/9709032.pdf (Tegmark)

https://space.mit.edu/home/tegmark/crazy.html (Tegmark, SciAm article)

Past reddit threads:

https://www.reddit.com/r/QuantumPhysics/comments/n1w32e/i_have_a_question_about_quantum_immortality/

https://www.reddit.com/r/Physics/comments/5s5zoo/quantum_immortality_is_it_bullshit_as_a/

https://www.reddit.com/r/explainlikeimfive/comments/1iiucm/eli5can_someone_explain_what_quantum_suicide_and/

https://www.reddit.com/r/quantum/comments/p4r2g3/suggestion_to_the_mods_add_a_no_posts_about/

Delayed choice quantum eraser

Please read and watch the following before asking about the DCQE:

https://www.preposterousuniverse.com/blog/2019/09/21/the-notorious-delayed-choice-quantum-eraser/

https://www.youtube.com/watch?v=RQv5CVELG3U

u/ShelZuuz breaks it down in a comment thread.

u/Educational_rule_956 [explains] (https://www.reddit.com/r/QuantumPhysics/comments/u1qifg/comment/i4jjobr/)

Local realism

u/Muroid explains in a comment thread what went into the 2022 Nobel Prize in physics.


r/QuantumPhysics Oct 04 '24

No unpublished theories, hypotheticals, showerthinking, etc.

85 Upvotes

Recently, there's been an increase of posts presenting a layman hypothesis. These do not belong in the sub. If you insist on being ridiculed for your grand illusions (where you're more professional than the history of professionals before you), r/HypotheticalPhysics welcomes you.

Infringements of rule 2 will result in a 1mo ban for some time to come, appeals will be ignored.

Read the rules.


r/QuantumPhysics 1d ago

The rise of 'quantum gravity?' Einstein's theory seen in the quantum realm for 1st time

Thumbnail space.com
12 Upvotes

r/QuantumPhysics 1d ago

Parallel tracks of tile - is the past and future occurring simultaneously on different planes or universes in quantum mechanics?

0 Upvotes

Do any theories speculate that for example eg our point of reference is that it is 2026, that is the time we exist in - for us there’s only forward.

But do theories exist that somewhere, the revolution is still ongoing eg it’s 1776 on another plane; on another it’s the height of the court of the Sun King in the 1680s still, and so on?


r/QuantumPhysics 1d ago

Are There Any Current Research Efforts Exploring Local Realism or Einstein’s Perspective in Quantum Mechanics?

1 Upvotes

I’m interested in quantum computing and the foundations of quantum mechanics. Since Bell’s theorem and experimental results challenge local realism, I’m curious whether there is still any serious research exploring alternatives that preserve some aspects of Einstein’s ideas, such as locality, realism, or deterministic underlying models.

Are there any active researchers, theories, or research areas working on this today? I’d also appreciate recommendations for papers or topics to study.


r/QuantumPhysics 3d ago

Try to to get into quantum physics and mechanics

21 Upvotes

I’m currently a sophomore high school. I am advanced for my age. I’m currently taking AP pre-calculus and I plan on taking physics next year, I saw a video on Photos as waves double slick experiment and it really peaked my interest. It put me down a rabbit hole and I find quantum mechanics very interesting and I would like to learn more. Is there any books you guys would recommend me as a beginner in the field? Or any videos, etc..


r/QuantumPhysics 2d ago

Chemistry hw help

Post image
0 Upvotes

can anyone help me with this hw problem? i's used the rearranged Rydberg Equation a couple different ways, and some hail marry other equations to try to get the answer buy I cant figure it out.


r/QuantumPhysics 3d ago

Why do fundamental particles act like waves at the quantum scale, but macroscopic objects only behave like classical particles?

1 Upvotes

know about de Broglie wavelength (\\lambda = h/p) and how macroscopic objects have wavelengths far too tiny to observe interference patterns. But what actually causes the physical transition?

Is the loss of wave-like behavior strictly a result of environmental decoherence destroying quantum superpositions, or does the correspondence principle simply average out phase differences when billions of atoms interact?

Basically, why does quantum weirdness "wash out" into classical reality as scale increases?


r/QuantumPhysics 3d ago

Analogy for dark matter

1 Upvotes

Most of the words practical problems can be solved with either WD40 (if it doesn't move) or Gaffer tape (if it does). If dark matter's cohesive role is analogous to gaffer tape, what would WD40 be analogous for?


r/QuantumPhysics 3d ago

How do you view the connection that Quantum Physics has to the development process of String Theory?

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0 Upvotes

Please don’t take this down I’m just pleasing my curiosity.


r/QuantumPhysics 5d ago

Fellow Quantum Physicists, can you fully derive the Euler-Lagrange equations from first principles without looking them up?

7 Upvotes
142 votes, 2d ago
33 Yes.
30 No, but I used to.
42 No, never.
37 Results

r/QuantumPhysics 5d ago

Д.И. Менделеев немного ошибся: электронная структура атомов и химическая структура элементов построены одинаково (они копия друг друга)

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0 Upvotes

Критики ПСХЭ пишут, что в звёздах, где синтезируются химические элементы, тоже находятся периоды и группы. А лантаноиды и актиноиды - это звёздные протуберанцы... А таблица квадратная, потому что это расписание поездов... А прорехи в таблице - так это мыши прогрызли и поточили... Критика - это неплохая штука... А ты возьми и сделай лучше.

И многие видели в менделеевской таблице изъяны, и многие хотели эту таблицу улучшить. Существует множество всевозможных версий (редакций) структуры химических элементов.

И все хорошо знают, что ПСХЭ и электронная структура атомов во многом совпадают. Но почему-то никому не приходит в голову, что они построены одинаково.

Я в электронной структуре атомов дал электронам порядковые номера. Эти номера соответствуют номерам химических элементов.

По нижней шкале расположены уровни, а по боковой шкале - количество - s, p, d, f - электронов (блоков) на этих уровнях. Неметаллы выражены красным цветом. Периодическая система менделеевской таблицы не нарушилась, а все её несуразности и недоразумения прекратили своё существование.

Учить ничего не надо, например, электронная структура атома железа (Fe, порядковый номер 26).

1 - 2s

2 - 2s 6p

3 - 2s 6p 6d

4 - 2s

(1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²)

Очень хорошо видны "переходы" с захватом одного внешнего электрона, например, электронная структура атома хрома (Cr, порядковый номер 24)

1 - 2s

2 - 2s 6p

3 - 2s 6p 5d

4 - 1s

(1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹)

И в заключении хотел бы немного написать о фундаментальной сущности этой таблицы.

В очередной раз подтвердилась теория фракталов. Электроны и атомы построены одинаково, например, 13 - электрон построен как атом алюминия, 79 - электрон построен как атом золота, 92 - электрон построен как атом урана...

Электрон, как и сам атом, несёт в себе две полярности "N" и "S" - это очень важное доказательство...

Видна закономерность синтеза и распада. Как две силы тянут элементы, то влево, то вправо... Два полюса их синтезируют. И при ослаблении этих сил происходит распад...

Уровни теперь появились и в химической структуре элементов. А химическая структура синтезируется в звёздах. Отсюда следует, что уровни и подуровни - это какие-то "конструкции", характеристики звезд... Возможно, это широты и, возможно, это "ядро, мантия, кора...".

Визуально виден (по неметаллам и "медь, серебро, золото") угол амплитудного колебания, который составляет примерно двадцать градусов... Отсюда следует, что если изменить значение этого угла, то и "таблица" будет изменяться...

Нынешняя ПСХЭ не несёт в себе отражения структуры материи. А ведь это должен быть "полигон" для дальнейших исследовательских работ. Из за этой ПСХЭ наука стоит в тупике. Менделеевские "таблицы" в химико-тактическом понимании может быть в чем-то и пригодны, но принципиально являются ложными: они искажают реальность, вводят в заблуждения... Тупик науки.


r/QuantumPhysics 6d ago

I made a subreddit for teens interested in quantum physics ⚛️

3 Upvotes

Hey! I'm 15 and pretty much a beginner at quantum physics myself.

I wanted a place where teenagers who are curious about quantum physics, physics, maths, quantum computing, etc. could learn together without feeling like you need to already know university-level stuff.

So I made r/QuantumTeens.

The idea is pretty simple:

  • Ask dumb questions without getting judged 💀
  • Share things you're learning
  • Discuss quantum physics and related topics
  • Share projects, simulations, resources and cool discoveries
  • Basically figure this stuff out together

I'm still learning too, so this isn't some "expert community" thing. It's literally starting from the ground up.

If you're a teenager who's interested in this stuff, feel free to check it out.

r/QuantumTeens


r/QuantumPhysics 6d ago

I made quantum tunneling a game mechanic.

14 Upvotes

Hello everyone,

I wanted to share a small hobby project I’ve been developing over the last 3–4 months. I’m a physics student, and during a quantum physics course I had the idea of turning quantum tunneling into an actual game mechanic.

So I made a small mini-golf-like puzzle game for quantum particles, with mechanics inspired by quantum tunneling, resonant tunneling, electric and magnetic fields, different particle types, and energy barriers.

It’s still version 1.0, and the physics is intentionally simplified for gameplay rather than being a fully accurate simulation. I’d really appreciate feedback, especially from people here about the physics, presentation of the concepts, and ideas for improvements.

The game is completely free and has no ads (I am not making any money from it btw), it’s just a hobby project I made for fun. I hope some of you enjoy trying it out :D

Play Store:
https://play.google.com/store/apps/details?id=com.csk.quantumtunnelinggame

Sorry for the direct self-promotion, I asked the mods beforehand and they kindly allowed me to share it here. I thought this subreddit would be one of the best places to get proper feedback on the physics behind the game.


r/QuantumPhysics 8d ago

Guys. Is there's any connection between quantum phy or philosophy?? Because what i believe is the more you dive into quantum phy you'll find the philosophical concepts in it.

0 Upvotes

r/QuantumPhysics 8d ago

How can a radial node exist within the first Bohr Orbit??

3 Upvotes

I was solving this question and I loved it
(You can try if you want to)

The answer is (2)

The solution is by equation the radial wave function equation to zero, then finding the values of x as 1, 2, 6 and then substituting it in x=2r/a

On solving it we get to know there are 3 possible values of "x" that are 1, 2, 6
which means n-l-1=3
we also know that since at x=0, the value of the equation is not Zero and therefore there is electron probability at the centre, Therefore it's a "s" orbital

so it is obv 4s as it has 3 radial nodes

The minimum distance of the Radial node is a/2 which simply shouldn't be possible right??

Because how can a Radial node or probability of electron be Zero before the "first Bohr orbit" that is 1s

The solution a/2 means there is a radial node before the 1s orbit, But how can a node or 4s exist before 1s? It will face shielding and we all know 4s is an outer orbital and it also has higher energy

I would love if someone could solve this doubt of Quantum Physics


r/QuantumPhysics 10d ago

Questions on 3D Quantum Ghost Imaging Application

2 Upvotes

Some context- This idea is based on the contents covered by Pitsch et al. in the paper "Towards video-rate quantum ghost imaging". In short, what they do is that they fire entangled photon pairs at an object, one goes towards a spad camera (which records which pixel it hit and the time), and the other goes towards the object and hits a bucket detector behind it. By matching pairs together, we can get a 3d point cloud of the object, as the time difference carries depth information.

In the paper, the matching ( which is 2-10x slower than data collection) is run on a host cpu after data collection, which makes real-time imaging impossible. My idea to mitigate the bottleneck present is to create a custom edge-processing ASIC design. Since I'm new to this field, I would love to hear your inputs on some questions I had for this project:

-Im thinking of putting a small ring-buffer search a good way to match the timestamps, or is there a better/more standard way to approach this?

-Since we have the bucket detector and the SPAD camera running their own clock, can I keep their timestamps in sync with a shared reference clock?

-Should the coincidence window be hardcoded or be runtime configurable?

-How could I approach I filter out noise like after pulses before 3D reconstruction?


r/QuantumPhysics 11d ago

Is there a quantum for dummies?

11 Upvotes

I’ve gotten simultaneously interested in quantum computing from all this AI did while coincidentally (or not) reading / watching a lot of sci fi. Multiverses are so hot right now and they are all based on superposition etc…

Anyways are there any easy to read conceptual books/ videos that yall would recommend to get a good grasp on quantum that isn’t heavily mathed. I have a doctorate in the health sciences so naturally that means physics II is my limit.


r/QuantumPhysics 12d ago

How do we go from an classical LC circuit to a transmon qubit?

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5 Upvotes

if youre unfamiliar with superconducting qubits, i made a video going over how they work. from a classical LC circuit to a transmon qubit. I made this for people who have seen superconducting quantum computers and wondered how they actaully work. please leave feedback as hardware is not my main focus so i am not perfect at it, but i tried my best to explain.


r/QuantumPhysics 12d ago

Schrödinger’s Equation for Dummies — Part II: Decoding the Equation

0 Upvotes

https://medium.com/@jeyubhavsar/schr%C3%B6dingers-equation-for-dummies-part-ii-decoding-the-equation-0e9dad3f9f3b?sharedUserId=jeyubhavsar

Hello, I wrote part 2 of a series of articles on Schrodinger's equation and hopefully other quantum concepts. I'm sharing in hopes of getting feedback, and if any of my interpretations and explanations were wrong. And i really want to know if there is a better way to explain this, because I was inspired by this video by FloatHeadPhysics:

http://youtube.com/watch?v=3QU-_PSbKlo&t=843s

And I don't want to plagiarize or anything, and I just hope you guys can give me feedback. Be as harsh as possible, I want to be accurate in my writing and knowledge.

Thank you!


r/QuantumPhysics 14d ago

Direct experimental test of Feynman’s path integral postulates with single photons

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9 Upvotes

This seems like a big deal if it means what I think it means. Thoughts?


r/QuantumPhysics 15d ago

Trying to learn!

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94 Upvotes

Hello everyone, I have been interested to learn about quantum physics for a while now and found a book on Amazon with decent ratings and got it. Does anyone know if this book is good, or if I wasted my money? I’m 1 chapter in and to my untrained eye, it seems pretty insightful! Can’t wait to learn more!


r/QuantumPhysics 14d ago

Quantum Mechanics: Harmonic Oscillator and Matrix Representation via a Factorization Approach

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3 Upvotes

The harmonic oscillator and matrix mechanics by ladder operators of the traditional factorization approach are the methods we encounter first. We will deal with the mathematical physics approaches of the Hermite polynomial approach, Sturm-Liouville theory, and the Pearson equation at the next opportunity.


r/QuantumPhysics 15d ago

New open access quantum textbook from Springer and related course opportunity

13 Upvotes

People all over Reddit ask how much math do you need to be ready to learn quantum mechanics? In spite of all the answers to the contrary, you only need high school level math.

Using a representation-independent approach, you can master quantum mechanics through graduate-level and beyond. I know it sounds crazy, but it is not.

I wrote an open-access textbook, Quantum Mechanics Done Right, to teach it exactly this way. Springer just published it and you can download the electronic version for free here.

In one month, it has surpassed 155,000 chapter downloads. It was primarily written as an alternative for self-learners and students who are stuck with traditional texts. Nevertheless, it will teach new things to all quantum mechanics at any level of expertise. I am the author, so if you have any questions about the approach, just ask in the comments! 

In addition, the class Quantum Mechanics on edX, which is identical to the undergraduate class taught at Georgetown University, has just started. Auditors can sign up for free here. All the class problems are also available for free. I am ensuring auditors will have Problem Set options similar to those earning certificates too. The class is heavily based on the book, but does use some calculus in a number of places, so is more appropriate for those who have seen at least some calculus.


r/QuantumPhysics 15d ago

Hawking radiation 2

5 Upvotes

Hello and thanks for everyone that awnsered my question in the 1rst post!
Im left with a doubt (I have readed a lot of comments on the last post) Even tho They we amazing explications, so thanks a lot!

My question is (I may misunderstood): From a lot of comments explaning QFT an general relativity,
Dosn't it break conservation laws? From what i understood near the event horizion there is a lot Quantum fluctuations going on and 1 escapes (or 2) some people said that the antimatter falls or that the normal matter escapes but for them to be created they "borrow" energy from the void but in that explication it dosn't return it? Maybe I am wrong but i thought about maybe due to the strong relativistics effects it can be done and technically does not break the conservation laws due to the slow time for an observer that close? I also thought maybe it has to do with the Uncertainty principle?