# How is a vacuum able to propagate light?

We say that sound waves need a medium to propagate and we know that light doesn't need such a thing. But is that really how that works? There's no such thing as "nothing" according to Quantum Mechanics, so I'm wondering if light can travel through "perfect vacuum"? A vacuum that there's no field, no lowest energy thing, nothing. Or does it really need something like "aether" to propagate? I'm not talking about "Luminiferous aether" by the way. A kind of aether Einstein thought existed.

• Light's medium of travel is the background EM fields. A photon is the propagation of a change in the background EM fields. Without any fields present (if that were even possible), photons couldn't propagate. – Jim Apr 25 '14 at 14:01
• If your idea of a "perfect vacuum" really does not (and cannot) exist, then how does it make sense to ask whether light can propagate through it? – Brian Bi Apr 25 '14 at 14:23
• @Brian Bi, doesn't make much sense to me. We also know that no massless particle can travel at the speed of light, but we can clearly say that if it WERE to travel, its mass would be infinite. Jim, does the light create its EM field as it goes through space? – user34284 Apr 25 '14 at 14:33
• Furthermore, while we can ask what happens for masses travelling closer and closer to the speed of light, no similar limit can be taken here, except maybe the limit $\hbar \to 0$, which would give you classical electrodynamics. This is probably not what you want. – Brian Bi Apr 25 '14 at 15:03
• This is like asking: does a unicorn run faster than a horse? – garyp Apr 25 '14 at 15:22

Even in a "perfect vacuum", i.e. barring quantum fluctuations, all fields are present. Their field values just are zero, corresponding to no particles or electromagnetic fields present.

Still the fields are there! Therefore, just as a matter wave (which is nothing but a particle), light waves, i.e. waves in the electromagnetic field, can propagate through such a "perfect vacuum".

Vacuum fluctuations or quantum mechanics has nothing to do with it. This is true, even in purely classical electromagnetism.

• So do the "fields" give an absolute frame of reference like an aether? If not then what exactly are they? – Jus12 Jun 25 '17 at 14:28
• @Jus12 The fields do not give any reference frame and are therefore not comparable to any 'aether' ideal as were popular at the beginning of the 20th century. They are physical entities that are well-behaved under Lorentz transformations, thereby being in perfect agreement with special and general relativity. In fact, one of the key points of GR is that it promotes the spacetime metric to such an entitiy, a dynamic tensor field. – Neuneck Jun 28 '17 at 8:14

Sound waves need a medium because they are density waves, and without matter there is no density. Water waves cannot propagate without water. Who'd say that light cannot propagate without water?

Light waves are not density waves. They reflect a change in the electric field emanating from something. The change is not observed everywhere immediately, but instead the information takes time to arrive.

A wave is simply a periodic function. Any pattern may be interpreted as overlapping sine waves by the Fourier transform and its cousins. Light waves are a convenient way to think of a phenomenon causing the electromagnetic field to carry energy from one place to another, but they are only an approximation of that. The field is the "real thing."

Photons and other particles are probability waves, where the value determines the likelihood of observing a particle at a given location. This probability field happens to coincide with the electromagnetic field because the quantum (Compton) wavelength of a photon is the same as the electromagnetic wavelength. But either way, such waves don't exist in any way except as an approximation of a field which has a value at every point in space.

• Seems I misread that statement. Deleted comment. – Jus12 Mar 23 '17 at 11:23

We say that sound waves need a medium to propagate and we know that light doesn't need such a thing. But is that really how that works? There's no such thing as "nothing" according to Quantum Mechanics, so I'm wondering if light can travel through "perfect vacuum"?

Right, but light is not a quantum mechanical concept. It is a well fitted macroscopic observation, well fitted by the solutions of Maxwell's equations that posit motion of a wave in vacuum.

What does quantum mechanics say? All matter is made up of elementary particles, described in the Standard Model, and all matter is described by the solutions of quantum mechanical equations, and these solutions are wave functions which give the probability of finding an electron, or a photon, or a neutrino... at (x,y,z,t) in "vacuum".

So how does light as described by the wave function in (x,y,z,t) of the solutions of Maxwell's equations come out of photons? Photons have a wave function that is the solution of a quantized version of Maxwell's equations, the one expressed with the A potential, where the differentials are turned into quantum mechanical operators operating on the wave function of the photon. This is a complex function with amplitudes and phases, and the classical light can be shown to emerge smoothly from a confluence of innumerable photons of energy h*nu where nu is the frequency of the emergent light. One needs to study quantum field theory to appreciate how this works.

In some handwaving sense, light rides on the back of innumerable quantum mechanical "particles" called photons. Particles do propagate in vacuum after all .

Without getting into the wild discussion on the intricacies of what may be meant by a perfect vacuum, all waves may be thought of as requiring a medium. Light may seem to be an exception, leading many to say that light is a wave that can travel through a vacuum with no medium.

Light doesn't use EM fields as its medium; light IS an EM (electromagnetic) wave. Maxwell's equations tell us that a magnetic field changing in time causes an electric field to change in space, and an electric field changing in time causes a magnetic field to change in space, etc. This leads to 2 wave equations. As I like to visualize it, the electric wave travels along its medium, which is a magnetic wave. Likewise, the magnetic wave travels along its medium, which is the electric wave.

In conclusion, you can look at light as an EM wave that is its own medium. Or, you can separate it into an electric wave and a magnetic wave that serve as each other's medium.

Light travels through a vacuum like rocks do.

Unlike something like 'waves in water', light is to be thought of as something between particle and wave. Pretty much all stuff is (de broglie wavelength etc), so even though light has a wave-length, it is also made of 'particles' (eg photoelectric effect), and therefore a photon has some 'substance', which by quantum nature, does not fall apart like a rock.

So a photon simply wanders off from the source that projects it, rather like rocks do, and they fall on the ground etc, rather like rocks do.

• While this is true, it neglects the wave side of things, and that light travels almost the same through things besides a vacuum, such as glass. This train of thought tends to forget the big picture. (Not the downvoter.) – Blackbody Blacklight May 4 '14 at 12:59
• On the other hand, light does not travel throug wood. The big picture is that some things are porous to water or Heeven when light does not pass through them. The point of my answer is that light is a particle made of wavy material, and does not need something to wriggle. The photoelectric effect showsthe particle nature of light. @BlackbodyBlacklight – wendy.krieger May 4 '14 at 23:38
• @wendy.krieger - Certain frequencies on the EM spectrum can propagate through wood and most other materials... – honeste_vivere Feb 8 '16 at 17:40
• Parts of the EM spectrum pass through lead, too, but we suppose that these are not light in the usual sense, since they do not interact with the eye, The general proposition of my answer is still true. – wendy.krieger Feb 9 '16 at 2:49