# Tag Info

133

In fact, light is not really a wave or a particle. It is what it is; it's this strange thing that we model as a wave or a particle in order to make sense of its behaviour, depending on the scenario of interest. At the end of the day, it's the same story with all theories in physics. Planets don't "choose" to follow Newtonian mechanics or general relativity. ...

65

You are seeing particles. However there's more to this than meets the eye so I need to explain exactly what I mean by this. Light is neither a particle nor a wave. Instead it is a quantum field. As a general rule while light is travelling it appears as a wave, but when the light quantum field is exchanging energy with anything it does so in quanta that ...

60

"Is it because they are just simplified illustrations?" you ask. The answer is simply: yes it is because they are simplified illustrations. Furthermore, not only can the particle hit the barrier outside or between the slits, typically most of the particles do that. Only a small fraction make it through. I say 'typically' because in such experiments we don't ...

52

Asher's comment written above is simply wrong, and the reason is rather fundamental in quantum mechanics. "The car slightly hits you" isn't how it works in quantum mechanics. The reason is that weak effects – such as very small but nonzero values of the wave function of an electron very far from the nucleus – do not cause tiny but observable effects like ...

49

What is a wave? From sound and water waves we come to an association with sine and cosine variational behavior. Wave equations are differential equations whose elementary solutions are sinusoidal . In water waves and sound waves and even electromagnetic waves what is "waving", i.e. has a sinusoidal variation with time and space, is the energy of the wave, ...

44

I don't really like the whole wave-particle duality business because it obscures the more startling truth about particles: they aren't sometimes waves and sometimes particles, and they also don't transform into waves sometimes before reforming as particles, they are something completely different. It's like the story of the blind men and the elephant: a ...

42

Duality is the relationship between two entities that are claimed to be fundamentally equally important or legitimate as features of the underlying object. The precise definition of a "duality" depends on the context. For example, in string theory, a duality relates two seemingly inequivalent descriptions of a physical system whose physical consequences, ...

42

However, they must somehow occupy space, as I've read that light waves can collide with one another. That's not true. Yes, light waves can "collide" and interact with each other (rarely), but that itself doesn't imply that they need to occupy space. It's not even entirely clear what it means for a subatomic particle to occupy space. A particle like a ...

41

They were looking at the scattering of electrons from samples of nickel out of general interest, and because their nickel target was polycrystalline the electrons came off pretty much in all directions with a smooth angular distribution, which they carefully measured. Under the electron bombardment the sample of nickel got hot. At this point in their ...

33

I think there's a bit of confusion here. The double-slit experiment was not performed with "single photons" - it's very hard to even consider what that would mean. At its heart, it is a thought experiment, and it's not really possible to make a real-life device that tests it. The first low-intensity experiment (Taylor 1909) was challenging the EM field ...

33

From the comments, you seem to want the minimum possible math. There are 4 things you have to know first: First, what you have to know is that a basic quantum wavefunction can be imagined as exactly just a sine wave: Second, you should know that the amplitude of the wave across an interval is related to the probability of measuring your particle's position ...

32

Although a single photon can only be absorbed and emitted by a single electron, it leaves that electron in exactly its original state. There is no record, and no way of knowing, which electron absorbed and emitted the photon. According to quantum theory, to calculate the result when any electron could have absorbed and emitted the photon, we must form a ...

30

I read that phonons are (the quantum mechanical analog of) normal modes of vibration in a crystalline system of atoms or molecules, so I guess a superposition, i.e. a general vibration should also be a phonon. Is that so? Why would they then be described as normal modes? The reason that phonons are described in terms of normal modes is because the phonon ...

30

Consider a photon that strikes the center and is reflected straight back on itself. That photon gives the sphere twice its momentum. Consider a photon that strike the edge at a glancing angle and is only slightly deflected. It hardly affects the sphere at all. The momentum change is about $0$. If you integrate over the sphere, you get an average momentum ...

28

I) One must distinguish between the group velocity $$v_g~=~\frac{\partial E}{\partial p}~=~v,$$ and the phase velocity $$v_p~=~\frac{E}{p}~=~\left\{ \begin{array}{cl} \frac{v}{2} & \text{in non-rel. QM (Schr. eq.) where}~ E~=~ \frac{p^2}{2m}, \cr \frac{c^2}{v}& \text{in rel. QM, QFT (Dirac eq., KG. eq.) where}~ E~=~ \sqrt{(pc)^2+(m_0 c^2)^2}.... 27 Effectively, as the CERN website emphasizes The theories and discoveries of thousands of physicists over the past century have resulted in a remarkable insight into the fundamental structure of matter: everything in the universe is found to be made from twelve basic building blocks called fundamental particles, governed by four fundamental forces. It ... 25 The uncertainty principle should be understood as follows: The position and momentum of a particle are not well-defined at the same time. Quantum mechanically, this is expressed through the fact that the position and momentum operators don't commute: [x,p]=i\hbar. The most intuitive explanation, for me, is to think about it in terms of wave-particle ... 21 What you have there isn't actually de Broglie's equation for wavelength. The equation you should be using is$$\lambda = \frac{h}{p}$$And although photons have zero mass, they do have nonzero momentum p = E/c. So the wavelength relation works for photons too, you just have to use their momentum. As a side effect you can derive that \lambda = hc/E for ... 20 In non-relativistic Quantum Mechanics (NRQM), the dynamics of a particle is described by the time-evolution of its associated wave-function \psi(t, \vec{x}) with respect to the non-relativistic Schrödinger equation (SE)$$ $$i \hbar \frac{\partial}{\partial t} \psi(t, \vec{x})=H \psi(t, \vec{x})$$  with the Hamilitonian given ...

19

Let me first say that I'm a fan of this theory, so whilst I'm giving what I believe to be a neutral response, bare in-mind that I'm pro-Bohmian which is in many fields an atypical viewpoint. To respond to the core question, 'why would a pilot-wave theory be wrong?': There are plenty of reasons why 'a' pilot-wave theory could be wrong, but in terms of ...

18

How does light 'choose' where to be a wave and where to be a particle? It doesn't, YOU do. That's really the entire "weirdness" of quantum right there. It's not entirely crazy. If you measure a car with a scale it will tell you it's 1200 kg, and if you measure it with a spectrometer it will tell you its red. This is perfectly natural. The thing that makes ...

17

In the following answer I am going to refer to the unitary evolution of a quantum state vector (basically Schrodinger's Equation which provide the rate of change with respect to time of the quantum state or wave function) as $\mathbf{U}$. I am going to refer to the state vector reduction (collapse of the wave function) as $\mathbf{R}$. It is important to ...

17

The problem is that light is not a wave and it's not a particle. Light is a quantum field, or at least that's our current best description of it. Quantum fields can behave in ways that appear to be wave like, and they can also behave in ways that appear to be particle like, and this the origin of the claim that light is both a wave and a particle. It's more ...

17

Yes. No! Both! Neither? The electron is an excitation of the QED quantum field, which is not quite compatible with the classical notion of either fields or particles. All you can do is draw analogies to either of these. Both analogies are sometimes just wrong, as in, they suggest different behaviour from what electrons actually do in experiments. However, ...

17

The answer to this question is largely philosophical and depends on your definition of the word "exists." Physics is all about building models to describe real-world behavior, and this always involves making some approximation that only captures the "essential" physics. Phenomena like temperature, pressure, viscosity, etc. all break down at the single-atom ...

15

The waves of quantum mechanics are probability waves. The solutions of quantum mechanical equations are the wave functions and the square of the wave function gives the probability of finding the particle at $(x,y,z,t)$. That is why the solutions for the electrons in the field of a nucleus are not orbits, but orbitals, i.e. probability distributions. The ...

15

A light beam is not like a swarm of photons flying through space - the relationship between light beams and photons is rather more complicated than that. This is discussed in What is the relation between electromagnetic wave and photon? though a proper discussion requires going deeper into the subject of quantum optics that many of us consider sane. Anyhow, ...

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