# Does light actually travel

hi is there any proof that light does actually travel?

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Nugatory
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hi is there any proof that light does actually travel?
You mean, experimental proof that light travels at some speed as opposed to be being everywhere all at once?

Yes. The first reasonably convincing measurement of light travel was made in 1675 by an astronomer named Roemer observing the differences between the apparent position of one of Jupiter's moons and the shadow of that moon on the surface of Jupiter.

There have plenty of measurements since then; check out http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html for more history.

the easiest way i can explain what i mean is, if you imagine a mexican wave in a stadium, nobody is actually moving but it appears like there is a wave moving if this makes any sense?

Nugatory
Mentor
the easiest way i can explain what i mean is, if you imagine a mexican wave in a stadium, nobody is actually moving but it appears like there is a wave moving if this makes any sense?
Ah, OK, I think I understand what you're getting at. Two ways you can think about it:

1) Light is the wave, not the medium that's waving. So yes, the light is moving (and this is especially true when the light is traveling through the vacuum of empty space; in empty space there's nothing analogous the stadium audience which is always there even when a wave isn't moving through it).
2) You can think of light as a beam of particles called photons. These move, at the speed of light.

You mean, experimental proof that light travels at some speed as opposed to be being everywhere all at once?

Yes. The first reasonably convincing measurement of light travel was made in 1675 by an astronomer named Roemer observing the differences between the apparent position of one of Jupiter's moons and the shadow of that moon on the surface of Jupiter.

There have plenty of measurements since then; check out http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html for more history.
Minor correction- he was measuring the times on earth when the moon popped out from behind Jupiter's shadow. When the earth swung around its solar orbit by six months, the event was 92x2 million miles further away resulting in a 15 minute delay to the observed event.

russ_watters
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the easiest way i can explain what i mean is, if you imagine a mexican wave in a stadium, nobody is actually moving but it appears like there is a wave moving if this makes any sense?
The fact that the particles in a mechanical wave end back up where they started after a wave passes, does not mean the wave isn't moving.

The fact that the particles in a mechanical wave end back up where they started after a wave passes, does not mean the wave isn't moving.
yeah but in the end doesn't it just depend on what you define as moving... you could also say, the wave is not a thing moving but a series of the compression and stretching of the electric field. I mean trying to define it with human words that are relative seems kind of fuzzy, people can easily disagree with such things

yes i think you more understand what i am trying to say that space is not empty but full of something that is every where and it is not the light that is traveling.man im even starting to confuse myself.
Space is full of force fields, electromagnetism etc.

Spacetime can also bend and contract, in the presence of mass, so I'd say it is a medium in that way.