# Tag Info

27

You talk about light as if it were a person carrying a clip board writing down things on its way to you. It is a physical phenomenon that gets affected as it propagates. Depending on the various processes that it goes through before it reaches your eye, its amplitude,polarisation, frequency (or wavelength), pulse time etc. get affected from which we can ...

15

When a photon hit the retina, it only has two pieces of information: Its wave length and its position/direction. That is all. But it is not alone. We are bombarded with billions of photons every second and the pattern these photons make is where the information is hiding. And we have a brain that is pretty good at figuring out these patterns. Let's say a ...

7

Does light have a rest mass ? No, the rest mass of a photon is zero. Is light affected by a gravitational fields ? Yes, because it has energy. In particular, light moving against a gravitation field does work and loses energy - we can see this because the light is red-shifted.

7

For most parameters, it is often impossible to differentiate them being a certain value from being extremely close to that value. Even if you finally have an experiment precise enough to differentiate, you could always then ask "What if the difference is even smaller!" In the context of special relativity, a particle going at light speed is related ...

5

I think that trying to think in terms of light "getting encoded with information" is a confusing and excessively complicated way of thinking about things. Suppose I'm standing next to a window, and there's a lamp on the other side of the window. When light from the lamp encounters the glass, what happens? If you wanted, you could describe what ...

4

First, Bekenstein's bound has been found to hold in all physically-sensible relativistic quantum field theories for which it has been checked. See ref 1 for an example. It definitely is applicable to the quantum realm. Second, Bekenstein's bound is a bound on the number of mutually orthogonal quantum states that have volume $\leq \frac{4\pi}{3}R^3$ and ...

4

The atoms in the Sun's atmosphere that are excited by absorbing photons will indeed de-excite by emitting photons. But an emitted photon won't usually be emitted in the same direction that the absorbed photon was travelling. So if the absorbed photon were travelling towards the Earth, the emitted photon almost certainly won't be. Hence the dark absorption ...

4

A “photon” is a quantum entity while “gravitational wave” is an entirely classical (non-quantum) concept. Generally, a good classical description of a quantum physical system could only be achieved when the number of quanta is large. Consequently, question about a single photon generating a (classical) gravitational wave does not have a consistent answer. ...

4

Let me first deviate to radio&television: a radio wave with constant frequency does not carry information, since it is absolutely predictable on the basis of a few parameters: its amplitude, its frequency and its initial phase: $$X(t) = A\cos(\omega t +\varphi)$$ The information is encoded into the wave by modulating these parameters, i.e. by changing ...

4

Light may carry information defined by which frequencies of the light spectrum it has. For example, the colour of an object is information carried by light. White light from the sun is actually many different wavelengths combined to create "the colour white". These wavelengths can teach us about: What object it reflected off of What created this ...

3

One has to be careful in quantum electrodynamics to disntiguish between photons, real elementary particles on mass shell, and virtual photons , as in this simple feynman diagram of electron-electron scattering: The Casimir effect is more complicated, but the fields that give the effect are represented by virtual photons. When this field is instead studied ...

3

Imagine an alien drops a transparent artifact nearby. Not knowing what it is, or whether it's dangerous, you decide not to try to walk up to it and touch it. All you know is that you can see through it, and thus, you don't really know what its outline looks like. However, you have several children and lots of plastic balls. So you give each child a ...

3

Let me first comment a sentence from SuperCiocia’s answer. The photodetector clicks ... are caused by the photoelectric effect, that is bound electrons in the photodetector are in quantised orbits and are only capable of discrete energy jumps. (1) In addition to this statement, please recapitulate that any observation of the wave behaviour of light during ...

2

We must be careful with such a question, because both the notion of photon and that of temperature are not straightforward. Photon: Lamb (1995, of the Lamb shift) wrote: the author does not like the use of the word "photon", which dates from 1926. In his view, there is no such thing as a photon. Only a comedy of errors and historical accidents led ...

2

The easiest way would be to not just look with one eye, but with two. The image of an object is projected on the retina of each eye, but slightly displaced and the brain can from this compute the distance. It's basically a distance measurement by parallax, i.e. measuring the angle under which the source is seen from two points with a known distance (as is ...

2

By 'combined' with the holes in the valence band (not shell), you mean the recombination process. To answer your question, you must understand the concept of 'hole'. You probably already know 'hole' is not a separate particle rather it is a lack of electron. In overly simplified words, you can think a positively charged ion has a hole in it as it requires a ...

2

Mass is defined in relativity by $m^2=E^2-p^2$, where $E$ is the mass-energy and $c=1$. A ray of light or an EM plane wave has zero mass. However, mass is not additive, and a collection of light rays, or a more complicated wave pattern can have nonzero mass. It's not really correct to argue about whether light has mass based on gravitational effects as it ...

2

Final edit. If it is the first time you read this go to the bottom, where I have summarized the arguments about classical electromagnetism modeling light, and mass in special relativity, replying to the title. Here once again is an example of how light is an emergent phenomenon from single photons. This is a double slit experiment one photon at a time: ...

2

yes they do, and for the reasons you sketched out. In principle, it would be possible to construct a mirror "sail" which, when deployed near a star, could be used to propel a spacecraft via the photon reaction force. However, the reaction force is tiny and to generate useful accelerations, a sail many miles across would be required. Isaac Asimov ...

2

This lesson from Richard Fitzpatrick’s Quantum Mechanics course calculates the cross section for a hydrogen atom in its ground state to be ionized by an incoming photon. The result (eq. 8.259) is $$\sigma\approx\frac{256\pi}{3}\alpha\left(\frac{I}{\hbar\omega}\right)^{7/2}\left(1-\frac{I}{\hbar\omega}\right)^{3/2}a_0^2$$ where $a_0$ is the Bohr radius, $\... 2 One eye perceives the angular size of an object. With two eyes we can get an estimate of the distance to the object. In combination these can let us estimate the actual size. Our estimate of size will often be dependent on the context in which the object is observed. 2 Light does not behave like a wave some times and like a particle some other times. Light behaves as light. Trying to categorise some behaviour as "wave-like" or "particle-like" is just an attempt to build an intuitive understanding for quantum phenomena by relating them to simpler everyday things like water waves or marbles hitting a wall.... 2 The simple answer is no. For a photon to be observed, all its energy must be collected. You cannot observe half a photon, either you observe it or you do not. The observation or detection can only happen in one place. This is often referred to as "the collapse of the wave function". As an electromagnetics engineer I sometimes monitored very faint ... 2 Usually you deal with it classically, but the classical explanation includes the uncertainty principle in disguise. See Interesting relationship between diffraction and Heisenberg's Uncertainty Principle? Ray tracing is often used when designing lenses. The position of rays and the lens surfaces are perfectly known as they are designed. It is possible to ... 2 Photons do interfere, there are places where you can see the classical interference patterns like in the double slit experiment (or every interferometer) and some places you can see quantum interference (e.g. Hong Ou Mandel experiment). The "sorting" of photons is an outcome of the lens in our eye, sorting photons coming from different directions ... 2 You just raised a question about a very important topic, the distinction between interference and interaction. A lot of answers on this site mention interference in connection to the double slit experiment. And you see other phrases like "photons do not interact with each other". I think this needs a little clarification: Interference, you can see ... 1 As R.W. Bird said, our 2 eyes give us a perception of distance for not too far objects. Our brain uses that information and the apparent size to estimate the real size. But it doesn't work for distant objects. The moon and the sun have almost the same (apparent) size. When guided only by perception, we are completely unable to estimate their real sizes. 1 Somehow questions about inertia are related to these about a photon mass. The discussion about a photon mass can be conducted endlessly. In general: it is clear that a photon has no rest mass. Because it cannot be at rest. It can only exist after its emission until it is not absorbed. In between it moves at the speed of light. the emission of photons (... 1 If$\hbar\omega<W$, then no matter how intensely you shine light at the metal, no current will flow. In other words, no electrons are ejected. This was one of the key experimental observations of the photoelectric effect. 1 From statistical physics, if a system is in contact with a "reservoir" of heat and particles, then the energy of the system$E$and the particle number$N\$ are allowed to fluctuate. If the set of microstates of the system form a discrete set (which would be the case for a system of particles which can inhabit discrete energy levels), then the ...

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