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.
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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.
Neuneck
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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.
Blackbody Blacklight
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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 .
anna v
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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.
jdj081
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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.
wendy.krieger
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