Never mind in a vacuum, why and how does light even move at all? What propels it?
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6$\begingroup$ I could be wrong, but your question suggests that you may hold an Aristotelian view of the nature of motion - it's still the most popular view, despite being supplanted by Newtonian mechanics. Ask yourself the question "what propels a ball rolling for a long time across a flat surface?" If the answer that comes to mind is "the force that a person gave to the ball when he rolled it", then I recommend that you familiarize yourself with Newton's laws of motion as a prerequisite for this question. $\endgroup$– BrioniusJan 15, 2015 at 3:02
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9$\begingroup$ "Why" isn't a question physicists really answer. $\endgroup$– Kyle KanosJan 15, 2015 at 3:11
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2$\begingroup$ What propels a sound wave? What propels a water wave from a splash? Light is a "splash" in the electromagnetic field. $\endgroup$– Brandon EnrightJan 15, 2015 at 5:38
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2 Answers
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Light is not an object but a phenomenon: propagation of a disturbance of the electric field. So it's not the light that is moving. Light is the motion of the perturbation. Your question is like asking "why jogging is moving?". Don't get confused by the light description in terms of photons. Photons are nothing like little balls moving on a table or through vacuum.
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Photons are massless objects, and hence are always moving at the speed of light. Hence, the natural tendency of light is to move, so it is not propelled to move. Looking at a wave perspective, light is an electromagnetic wave, so light is the propagation of a disturbance in the electromagnetic field, similar to how water waves spread after the water in a lake is disturbed. The energy from the disturbance is spread through the waves.