# Light always particle in vacuum because it has no medium

My physics teacher told me that light in a vacuum is always in the particle form. So what happens if you perform the double slit experiment in vacuum? Will the light spread like a wave or will it have a single dot on the other side of the slit(s)?

Also, please explain why either happens if I missed something. Heisenberg's uncertainty principle?

• Sadly for your teacher, they are wrong. The double slit experiment works just fine in vacuum. Sep 30 '15 at 0:58
• What happens in an unobserved situation is not observable. This is a tautology but it describes, that we cant speak about something what is not in observation. Your teachers statement is ok in the meaning that photons are traveling in straight lines when they are not influenced by interactions with fields. Sep 30 '15 at 4:29

Light is made up of photons that are really neither waves nor particles. Sometimes they appear to behave as particles (see photo-electric effect), sometimes as waves (see e.g. diffraction).

You have either remembered poorly or your teacher has taught you badly: electromagnetic radiation (photons) doesn't require a medium and does not behave as a particle just because a medium is absent.

The double slit experiment produces no different outcomes in air or vacuum.

Light always particle in vacuum because it has no medium

That's wrong I'm afraid. If you take a look at LIGO you can read them talking about "ripples in space-time (the fabled “fabric” of the Universe)". These gravitational waves are not particles. Instead, space waves. Because the vacuum is a medium. And whilst electromagnetic light waves aren't the same as gravitational waves, they're still waves.

My physics teacher told me that light in a vacuum is always in the particle form.

Have a look at the Wikipedia photon article: "The energy and momentum of a photon depend only on its frequency (ν) or inversely, its wavelength (λ)". It has a wavelength. The photon isn't a point-particle, it's a wave.

So what happens if you perform the double slit experiment in vacuum? Will the light spread like a wave?

The light spreads like a wave and goes through both slits. See a depiction of this on Aephraim Steinberg's website:

or will it have a single dot on the other side of the slit(s)?

It will also leave a single dot on the screen. People tend to see this as some kind fo fundamental contradiction that demonstrates quantum weirdness, and some start talking about the many-worlds multiverse. But see Steven Lehar's website and check out the optical Fourier transform:

There's nothing mysterious about that. A waves is a wave, but sometimes it gets converted into something pointlike. The dual slit experiment is not magic, IMHO something akin to the optical Fourier transform is occurring.

Also, please explain why either happens if I missed something. Heisenberg's uncertainty principle?

See Wikipedia and note this sentence: "It has since become clear, however, that the uncertainty principle is inherent in the properties of all wave-like systems". It's to do with waves, not to do with the possibility of finding a point-particle. There are no point-particles. It's quantum field theory, not quantum point-particle theory.