# What causes light to move through a vacuum

I was looking at rockets and stuff and thought about how they move through a vacuum using newtons 3rd law, and then I started thinking of any other ways you could move through a vacuum without using this and then I thought about the photon. I then thought how does the photon move through a vacuum? So I searched it online and couldn't find an answer. So my question is simply how does a photon move through a vacuum? Is it because its massless? Is it because it is thrown from an electron or whatever like a rock from a slingshot? Or is it something completely different? I'm very curious!

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Any thing can move through vacuum, not only photon but also electron and any other particle. – Anixx Dec 25 '11 at 12:12

Firstly, it should be noted that a rocket does not need to use Newton's Third Law in order to move through a vacuum, only in order to accelerate: that is, to speed up, slow down, or change direction. This is Newton's First Law: if the net force on an object is zero, it moves at a constant velocity.

Photons always travel at the speed of light. When in a vacuum (and in the absence of gravity) they don't change direction either. So Newton's Third Law is not involved.

When a photon hits a mirror (in, for the sake of argument, a vacuum) it is reflected, i.e., it changes direction. In this case Newton's Third Law is involved: there is (in effect) a force which accelerated the photon, so an equal and opposite force acts on the mirror, causing it to accelerate in the other direction. (Of course, because a photon is very small compared to a mirror, the mirror won't accelerate as much as the photon did.)

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Actually Mirror is not actually effected in that sense, as only some of the energy is actually absorbed so not complete newtons laws are in affect not all look at this equation $E_2 = \frac {m_1}{1- cos(\theta) + \frac {m_1}{E_1}}$. This shows as the mass of the mirror increases it actually absorbs less energy thus as $m_1 -> \infty$ the mirror basically absorbs less energy thus not following newtons laws and not moving as much. But strictly you are still correct. – Rohan Vijjhalwar Aug 28 '14 at 11:51
@RohanVijjhalwar: the important thing is that the momentum is conserved. The amount of energy transferred isn't really relevant to the question. – Harry Johnston Sep 1 '14 at 21:14
I agree that momentum is conserved although momentum affect mirror much less because as $m_1$ approaches higher and higher masses $E_2 = E_1$ therefore mirror will not move as much (as momentum of photon is directly propotional to energy by $\vec p = \frac{E}{c}$ and once at infinite mass the mirror (impossible but intutive) simply reflects ALL energy so it does not move. Besides its just a point to show that change in light path could result in energy gain or loss. NOTHING BAD ABOUT YOUR POST. IT WAS JUST TO HELP THE OP UNDERSTAND MORE – Rohan Vijjhalwar Sep 2 '14 at 22:12

Electric and magnetic fields can exist in a vacuum. When an electric field changes, it creates a changing magnetic field, and vice versa. Oscillations between those fields travel at the speed of light through the vacuum. That's the classical view, which does not involve photons. In the quantum view, a photon is emitted at one location and absorbed at another location, and what happens in between is undetermined. However the equations used to work out the probabilities of various things being observed follow the the pattern of the classical view.

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