# Beam Of Light Slowed Down....

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Ok most people no that light travels at around 186.000 miles per second, and WOW thats some serious SPEED, and Here is somthing that i would love to see and i think is maybee? possible, so let me try to explain in my own words what id love to see.

By using a high powered hand held green laser preferably 400 mw and a high speed camera then turning the laser on and playing back the high speed recording of turning the laser on would it be possible to see the laser beam acctually moving, so there light has bin slowed down.

Sorry for my bad english but i hope you understand me, this is just sumthing i wana see so it is a proof that light travels, evan though i no light does travel but this would be proof.

I really wana see a video of somthing like this or somthing similar and i cant really find any videos on it anywere.

Thanks Chris......

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it would be theoretically possible but there are no cameras fast enough to capture this. you'll just have to stick to traditional methods of proving light takes time to travel(like bouncing it off the retroreflectors on the moon)

or looking at the conversations with astronauts in lunar orbit/on the moon. big gaps from the delay.

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Thanks for your reply, i was hoping there is a camera fast enough for this maybee one day there will be lol

Cheers...

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Well, let's look at this carefully.

I've heard of high-speed camera systems that can take a million frames per second, so let's assume that means one frame is a millionth of a second. Light travels about a foot in a nanosecond, and a millionth of a second is 1000 nanoseconds. So in the time it takes the camera to take one image, the light beam will travel 1000 feet. You'd have to set up the camera somewhere where it could see that distance.

High-speed camera systems like that would also require the beam to be very bright to pick up. It might be possible with a fast enough camera, a bright enough laser, and a clever enough setup, but I don't know if it's been done before. (I keep thinking I've seen a picture, but I can't find it. Maybe it was a picture of something else.)

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The beam would have to be in an atmosphere that would scatter the beam sufficiently.

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The beam would have to be in an atmosphere that would scatter the beam sufficiently.

Indeed you would need smoke and with that set up you wouldn't see it moving you would see the end of the laser before it hits the solid surface as you would only have one frame, one way you would be able to do it is to do it over a massive area with a fast extremely high resolution camera and you would get it moving in 3 pictures with the light moving 1000 feet per picture.

But even this would really work as for it to be visible in smoke or something similar it wouldn't go that far.

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The speed of light has always been known to slow down depending on its gravitational enviroment. The speed of light is slower on the earth, albeit it very very small rate of deceleration, than if it were moving through a deep void in space. Also think of a black hole event horizon, once anything including light crosses that god forsaken line nothing can escape and time itself is slowed down. So one can only speculate that light itself is slowed tremendously inside the black hole.

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The speed of light has always been known to slow down depending on its gravitational enviroment. The speed of light is slower on the earth, albeit it very very small rate of deceleration, than if it were moving through a deep void in space. Also think of a black hole event horizon, once anything including light crosses that god forsaken line nothing can escape and time itself is slowed down. So one can only speculate that light itself is slowed tremendously inside the black hole.

I thought the whole point of the speed of light was it was a universal constant.

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You r absolutely right in light being a universal constant, but just as gravity is a universal constant it deviates depending on its enviroment. Like for instant the gravity on earth is 9.80 meter per second sqr and gravity outside of earth is 6.673 times ten to the negative 11th. They are both constants in thier field of use. So the same principles apply to light. In the vacume of space its such and such speed and in the atmosphere of earth its slightly slower. Now the effect of the speed of light on earths atmosphere is so tiny tht it will not affect any experimentation or any mathematical calculations if you use the 186000 miles per sec number. It takes sohisticated instruments to calculate the difference and the difference really does not effect anything. Also the slight deceleration of light on earth is still faster than anything in the universe and nothing comes close. Also it being a constant means its not gonna fluctuate in its enviroment its gonna stay the same speed.

Edited by relativist

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I'm sorry, relativist... but that's just wrong. Light is constant in all frames. Sometimes it seems to slow as a result of absorption and re-emission, but the stuff you are saying is false.

If you wish to continue saying that the speed of light is not constant, then you need a reputable source supporting your claim. Until then, all readers should disregard your posts and recall that the speed of light is constant in all frames.

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Like for instant the gravity on earth is 9.80 meter per second sqr and gravity outside of earth is 6.673 times ten to the negative 11th. They are both constants in thier field of use.

$g_{\mbox{earth}} = 9.8 m/s^2$

$G = 6.67 \times 10^{-11} \frac{m^3}{kg \times s^2}$

They have decidedly different meanings. g is the acceleration due to gravity on earth. G is the universal gravitational constant. See http://en.wikipedia.org/wiki/Gravitational_constant

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Obviously u didnt read the whole section cause i argued for light being a constant and explained why. Now ask any physicist, light on earth travels a bit slower than light in space.

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Like for instant the gravity on earth is 9.80 meter per second sqr and gravity outside of earth is 6.673 times ten to the negative 11th.

In addition to what iNow said, this is also wrong. 6.673*10^-11 is the gravitational constant, as measured in (m^3)(kg^-1)(s^-2), not m/s^2. It's what appears in equations as G, and is not a measure of force or acceleration. The 9.8m/s^2 is the approximate acceleration at Earth's surface. Force of gravity between two objections is determined by the equation:

F = (Gm1m2)/r^2, where F is force, G is the aforementioned constant, m1 and m2 are the masses of the objections, and r is the distance between their centers of mass.

See:

http://en.wikipedia.org/wiki/Gravitational_constant

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In explanation of what seems to be my constant work error. G is the universal constant but g (gravitational acceleration on earth) is a constant tht never changes in earths atmosphere. So all the matter of fact individuals out there I was still right in my example. Guess I will have to be more literal

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The speed of light in Earth's atmosphere is less because the light is being absorbed and re-emitted by molecules of the various gases in the atmosphere, not because the light itself is going any slower.

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If its being absorbed and reemitted than it is going slower. Because of the nitrogen, hydrogen, and oxygen gases that make up 99 percent of earths atmosphere than the process of absorption and emmision is always. Those gases and process arent changing so light is consistenly slower on earth.

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In explanation of what seems to be my constant work error. G is the universal constant but g (gravitational acceleration on earth) is a constant tht never changes in earths atmosphere. So all the matter of fact individuals out there I was still right in my example. Guess I will have to be more literal

g(n) is standard gravity, defined as the acceleration due to Earth's mass at sea level, at 45.5 degrees latitude. g is local gravity. It changes depending on where you are. Neither is affected by the presence of atmosphere.

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http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html

It seems to be quite simple and too the point, and covers the historical thoughts on the speed of light.

My knowledge of general relatitiy isn't really good enough to give a good solid statement on this.

What I can say:

g (little g) is certainly not a constant in the earths atmosphere and varies depending where you are even on the surface.

The refractive index of the atmosphere is also not constant, as it varies with pressure, this can be shown quiet easily using a rayleigh refractometer...

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Since I am not anywhere near an expert at any of this, I could be wrong. If I am wrong, I would love to be corrected.

I would think that the absorption and re-emission of light would be more of a starting/stopping thing than an actual slowing down. When the light gets absorbed it gets absorbed at 300,000,000 meters per second. I would imagine that when it gets re-emitted it gets re-emitted at 300,000,000 meters per second as well. It doesn't get slowed down so much as it gets stopped altogether and started again, at the same speed at which it was absorbed. The light itself doesn't get slowed down, but the process of absorbing and re-emitting it just takes more time, causing an apparent "slowing" of the speed of light.

o.O?

I think this is basically what Cap'n Refsmmat said, only more wordy.

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Since I am not anywhere near an expert at any of this, I could be wrong. If I am wrong, I would love to be corrected.

I would think that the absorption and re-emission of light would be more of a starting/stopping thing than an actual slowing down. When the light gets absorbed it gets absorbed at 300,000,000 meters per second. I would imagine that when it gets re-emitted it gets re-emitted at 300,000,000 meters per second as well. It doesn't get slowed down so much as it gets stopped altogether and started again, at the same speed at which it was absorbed. The light itself doesn't get slowed down, but the process of absorbing and re-emitting it just takes more time, causing an apparent "slowing" of the speed of light.

o.O?

I think this is basically what Cap'n Refsmmat said, only more wordy.

Yep, that's pretty much the gist of it.

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I found this neat thing on google about Astronomical Photometry that addresses the absorption and re-emission of photons by atmospheric molecules. To quote them directly:

"Absorption of radiant energy by an atmospheric molecule is a destructive process: a photon is annihilated, and its energy is transferred to the absorbing molecule, which undergoes a transition from a lower to a higher state of energy. Absorption may lead to subsequent emission. It may also lead to destruction of the molecule."

From Astronomical photometry

By Christiaan Sterken, J. Manfroid

Sorry for the huge ugly link, but the preview looks pretty cool.

Edited by OliviaMcQ

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When I went to school, in the 1960's ,refraction of light through a prism, glass block or even water ("the bent pencil"), was always explained as being due to a change is the speed of light as it entered a more or less dense medium.

There also seems to be some confusion between the Universal Gravitational Constant 'G' and the acceleration due to Earth's gravity 'g'.

The gravitational force between two objects is given by:

m1 m2

F=G-------

2

d

where F is the force, m1 and m2 are the masses of the two bodies, and d is the distance between them.

Using the mass and radius of Earth:

G m(earth)

g=-------------- = 9.81 m/s approx

2

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The idea that refraction is due to a chance in the speed of light is partially true, but it is the speed of the propagation of the wavefront that has altered, not the speed of individual photons, photons always travel at c, but inside materials they spend some time absorbed before being re-emitted.

ignore sorry

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