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127 votes

Why isn't my calculation that we should be able to see the sun well beyond the observable universe valid?

The problem with your derivation is that you distributed the photons over a 360° circle, so the photons only spread out in a two-dimensional circle. This means that the intensity of light drops off at ...
Mark H's user avatar
  • 23.9k
118 votes

What exactly is a photon?

The word photon is one of the most confusing and misused words in physics. Probably much more than other words in physics, it is being used with several different meanings and one can only try to find ...
Ján Lalinský's user avatar
107 votes

Can photons be detected without being absorbed?

It is indeed possible, as demonstrated by the group of Serge Haroche in 1999 using so-called quantum non-demolition Ramsey interferometry. The idea was to observe the presence or absence of a photon ...
ZeroTheHero's user avatar
96 votes

Why can't the light from a candle light the whole of a dark room?

It might well be that the photons reach the corners of the room but that doesn't mean that you can see the corners. Why? For you to see something, photons have to be emitted or reflected by the object ...
Virft's user avatar
  • 954
94 votes

What exactly is a photon?

The photon is a construct that was introduced to explain the experimental observations that showed that the electromagnetic field is absorbed and radiated in quanta. Many physicists take this ...
flippiefanus's user avatar
  • 14.2k
87 votes

Why is the $S_{z} =0$ state forbidden for photons?

Massless particles with spin do not have a "$S_z = 0$" state because they actually do not have spin like massive particles do. They have helicity, which is the value of the projection of the spin ...
ACuriousMind's user avatar
  • 122k
82 votes

Force of photons from the Sun hitting a football field = weight of 1 dime?

Photons are massless so their weight is 0. However, photons do have momentum so they can exert force. This force is due to their momentum and would occur even in the absence of gravity, so it is not a ...
Dale's user avatar
  • 94.4k
80 votes

Do photons truly exist in a physical sense or are they just a useful concept like $i = \sqrt{-1}$?

There is lots of experimental evidence that the electromagnetic field exchanges energy with atoms in discrete chunks, and if we call these chunks photons then photons exist. Which is all very well, ...
John Rennie's user avatar
80 votes

What is the reason that Quantum Mechanics is random?

If it helps, it's not that the nature of the universe is random, it's that we model it as random in Quantum Mechanics. There are many cases in science where we cannot model the actual behavior of a ...
Cort Ammon's user avatar
  • 45.6k
76 votes

Why have our eyes not evolved to see "gluons"?

In short, the answer is: because gluons behave in a way that makes them useless for this purpose. To understand why, let's back up a little and look at how photons are useful, and then see how gluons ...
Mike's user avatar
  • 15.2k
72 votes

Do photons bend spacetime or not?

Classical electromagnetic fields carry energy and momentum and therefore cause spacetime curvature. For example, the EM field around a charged black hole is taken into account when finding the ...
G. Smith's user avatar
  • 51.3k
70 votes

What exactly is a photon?

This is the elementary particle table used in the standard model of particle physics, you know, the one that is continuously validated at LHC despite hopeful searches for extensions. The Standard ...
anna v's user avatar
  • 232k
70 votes

How can a red light photon be different from a blue light photon?

Some areas of physics are counter-intuitive. For them, your everyday experience is a poor guide to how the universe really works. This is one of those areas. Photons have no mass. They all have the ...
mmesser314's user avatar
  • 36.3k
66 votes

How do photons get into the eyes?

Yes - we are surrounded by a "sea of photons". An individual object that reflects light (let's assume a Lambertian reflector - something that reflects incident photons in all directions) sends some ...
Floris's user avatar
  • 118k
65 votes

How does light re-accelerate after slowing down?

When light travels through a physical medium the photons don't actually slow down. They still travel at the speed of light. What makes it look like it slows down is the interactions between the ...
macco's user avatar
  • 1,977
64 votes

If a photon truly goes through both slits (at the same time), then why can't we detect it at both slits (at the same time)?

Think of it this way: A photon is the detection event. When there is only one photon, there is only one detection event. The probability distribution of detection events is associated with the ...
S. McGrew's user avatar
  • 24.6k
58 votes

Why do electrons absorb and re-emit photons?

And electrons in an atom absorb and re-emit it. But why do electrons bother to absorb and re-emit light and not just let it pass all the time? There is a basic misunderstanding in your question. An ...
anna v's user avatar
  • 232k
56 votes

How can we see a beam of light?

You're not seeing the photons in the beam that are traveling from A to B (beam starting point to beam destination), you are seeing photons that are scattering off of dust particles that are in the ...
Time4Tea's user avatar
  • 4,042
55 votes

Why does paint mix to produce black, but light mix to produce white?

Mixing light does result in white, but the black mixture of paint happens due to how paint works. Paint has color not because it's emitting light, but because it's absorbing colors other than the one ...
Gabriel Golfetti's user avatar
55 votes

How can photons have an electric field without having a charge?

I believe electric fields can only be created by charged particles There are two things that can produce a (disturbance of the) electric field: A charged particle A changing magnetic field Since ...
The Photon's user avatar
  • 26.5k
53 votes

Do massless particles really exist?

Here is a quick & simple answer until the professionals arrive. In the Standard Model, it is zero. This $< 1\cdot 10^{-18} \frac{\mathrm{eV}}{c^2}$ is an experimental upper limit (i.e. if it ...
peterh's user avatar
  • 8,184
53 votes

What exactly is a photon?

The starting point to explain photons from a theoretical point of view should be the Maxwell equations. In covariant form, the equations in vacuum without sources are \begin{align} \partial_\mu F^{\mu\...
Bosoneando's user avatar
  • 5,614
53 votes

How to identify if a photon comes from the sun?

Photons are identical particles characterized by energy and a direction of propagation. If you see just a photon, without any other information, from these two properties, you cannot distinguish a ...
Davide Dal Bosco's user avatar
51 votes

Does the speed of light in vacuum define the universal speed limit?

It's the second one: the reason the speed $299792458\ \mathrm{m/s} = c$ is special is because it's the universal speed limit. Light always travels at the speed $c$, whatever that limit may be. The ...
David Z's user avatar
  • 75.8k
49 votes

Can a wire having a $610$-$670$ THz (frequency of blue light) AC frequency supply, generate blue light?

It would be hard to generate such a current and harder still to get it to produce any blue light - though this is theoretically possible. The main problem is that you are probably thinking of a metal ...
Paul Young's user avatar
  • 3,504
47 votes

Do nuclei emit photons?

Nuclei emit gamma rays, which are high energy photons. The photons emitted when electron in an atom changes its energy state are usually in optical spectrum, which are more frequently encountered in ...
Roger V.'s user avatar
  • 57.4k
47 votes

If light propagates like waves, why can't I see around corners?

The bending of waves around corners is known as “diffraction,” and its natural length scale is the wavelength of the diffracted wave. So if you want to block the sound from a speaker playing a middle ...
rob's user avatar
  • 86.2k
46 votes

How come waves and particles can have energy without having mass?

Equation $E=mc^2$ is incomplete. The proper form is (in units with $c=1$) $E=\sqrt{m^2+p^2}$. When an object is at rest then $E=m$ is recovered. But for massless objects $E=p$. So this means that even ...
Veritas's user avatar
  • 1,253
42 votes

Free electron can't absorb a photon

It is because energy and momentum cannot be simultaneously conserved if a free electron were to absorb a photon. If the electron is bound to an atom then the atom itself is able to act as a third body ...
ProfRob's user avatar
  • 128k
42 votes

If the Higgs field gives particles mass, and is present everywhere, then why are there massless particles?

The usual pop-sci explanation of "the Higgs field exerting a drag force on particles that move through it, sapping their kinetic energy" is unfortunately not very accurate. In technical ...
Nihar Karve's user avatar
  • 8,307

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