# Where does Light get its energy?

Traveling at 186K miles a second, that would seem to require quite a bit of energy to sustain. What powers Light to travel? Does it ever run out?

In very rough terms, it would require energy to sustain if light had mass that needed to be pushed (and/or if there was some force resisting its movement), but it doesn’t have mass, so a tiny little push makes it go a very long way very fast; also, it doesn’t operate on the same scale as macroscopic objects, so it doesn’t experience things like wind resistance.

You do need to rer-orient your world view. Light isn’t like a battery-powered toy that you fill with Duracells and let go across the gulf. (For one thing, there’s nothing for it to push against…) Light is energy. Each photon is an energy packet. It travels at the speed of light (c divided by the refractive index of the substance, if any) becauase it’s the only speed it can travel at – it doesn’t go at a slower rate or a faster. This is the only explanation consistent with observation , and it fits into theory based upon these and other observations.

One other thing. You appear to have the common misconception that motion requires energy to “sustain” it. Newton’s first law states that an object in motion travels at a constant velocity unless acted upon by a net external force.

For example, when the astronauts travelled to the Moon back in 1969, they spent the vast majority of the trip coasting. The same is true of the various probes we send to other planets. Indeed, one problem with sending a probe for a soft landing on another planet is how to slow it down when it gets there.

My point is that nothing (matter or light) requires energy to “sustain” its motion. That notion goes back to Aristotle and was debunked by Newton.

What about light emitted from a lighthouse?
There is a maximum distance from which it can be seen. Does the external forces on earth affect its travel?

[QUOTE=robby]
One other thing. You appear to have the common misconception that motion requires energy to “sustain” it. Newton’s first law states that an object in motion travels at a constant velocity unless acted upon by a net external force.QUOTE]

Eh, I don’t think Newton’s physics apply in this case. But please correct me if I’m wrong.

I’m no physicist, but I would guess that the scattering of the light means that after a certain point, the light is so scattered by bouncing off everything, that it can’t be detected by us anymore. Even lasers have a set limit for how far out you can see them.

[QUOTE=Squee]

I think you’re right, however, even if light was subject to Newtonian physics, it would still behave unlike, say, ping-pong balls, because it has no mass.

The big difference between earth and space is that the earth is covered with air. The oxygen, nitrogen and water vapor in the air all absorb and emit photons, which is why the sky turns different colors depending on the time of day. If there’s a lot of water vapor the sky looks white or gray, if there’s little water vapor the sky looks blue, if the light source (that is, the sun) has to travel through a lot of atmosphere the light looks orange or red. So yes, the air can block, bend or transform light.

Also the earth is curved, so a very distant lighthouse would be behind the horizon and its light would be blocked by the crust of the earth.

However, in space there’s very little to block the photons. So a photon emitted by (say) the star Sirius has a very good chance to travel 8 light years without hitting or being blocked or absorbed by anything. Of course, only those photons on the exact path to your eye can be detected by you, which is why Sirius appears so faint even though it’s many times brighter than the Sun.

Lasers have an intrinsic divergence angle based upon the size of the aperature and wavelength of light; you can treat this classically as a result of diffraction of the beam wavefront, or you can handle it with QED as a quantum mechanical effect; either way, light–even coherent light–scatters. Regular photons distributed across a spectrum will diffract with and be absorbed/reemitted by normal matter so as to prevent light from travelling to far in an atmosphere; even in “empty” space between stars the light gas of hydrogen compounds and dust will serve to block light, which is why the Milky Way is cut through the middle with a black band; even though you’re looking down the most dense concentration of stars nearby, most of the light is deflected or absorbed by non-illuminating dust.

As for the OP, as others have mentioned, light, free from any external influence, will continue to move indefinitely without requiring additional energy to be input; light is energy, and energy (in thermodynamic terms) is motion (or potential motion). So, just remember that next time you use your Maglight to flash an “SOS” to outer space. Somewhere out there, some bug-eyed alien on a Stellar Guard runabout is catching your distress message and responding with all available means to your mayday. Won’t you be embarrassed when he shows up?

Stranger

You can make a photon heavy by splitting it in half, creating an electron-positron pair, both of which have mass. Don’t try this at home.