Light speed

[drawkcab]

Been thinking about relativity. I read that the reason A. E. came up with the theory was to explain the seeming paradox of a constant light speed independent of the speed of the light's source. I've no scientific training or background whatever, so I'm sorry if this is stupid, but...

 

What if light emanates outward from a source at a range of speeds and the light we can see and measure is simply travelling at the right speed for our senses to pick up? It would mean that light would always appear to travel at a constant velocity.

 

If there's anything to this idea, it might explain why all of the formulas predict that nothing can go faster than light. As an object approaches the speed at which we see light, the light emanating from it will be travelling at a speed too great for us to see. So the object would appear to have disappeared.

 

I know the theory of relativity does an amazingly good job of predicting things and is therefore probably not too far from the mark, so just take this as it is: a layman's afternoon thought experiment.

 

I guess if there were anything to it there would be some form of weak radiation accompanying light wherever it went, and I don't know of any such thing, so the answer is probably no. Still interesting though...

[timo]

Many measurement are made by some kind of measurement device, not by human senses. I do not know if [save for production of neutrinos on the subatomic scale] there are any objects that just disappear. People would start to wonder where they went if that happened often.

[swansont]

The constant speed of light is in Maxwell's equations, though it took a while for people to realize this (that light was an electromagnetic wave). So e.g. a radio picking up a signal when it's moving relative to the source is another example of this phenomenon.

[GDG]

Light actually does travel at different speeds, depending on the material it is passing through. It is the speed of light in a vacuum that is constant (and the absolute limit).

 

So when you open your eyes underwater, you're seeing light traveling at a different velocity than it does through air (or through vacuum, although most of us don't open our eyes in vacuum ).

 

We can rule out the presence of other forms of light using Conservation of Energy, e.g., accounting for the mass balance in a nuclear reaction.

 

Make sense?

 

Grant

[iNow]

Just to clarify, IIRC light still travels at the same speed, it just appears slower in water do to the index of refraction (it bounces off more stuff before reaching the retina, so has travelled farther... it still goes at the same speed, though).

[GDG]

Just to clarify, IIRC light still travels at the same speed, it just appears slower in water do to the index of refraction (it bounces off more stuff before reaching the retina, so has travelled farther... it still goes at the same speed, though).

 

Nope: the index of refraction is due to the fact that light travels at different speeds in air vs. water, glass, etc. Look at the electromagnetic wave equation, which depends on the electric and magnetic constants for the medium.

 

It is in fact possible to slow light down to a stop.

 

Enjoy,

 

Grant

[iNow]

Aren't you mixing reference frames? I may be wrong, I don't deny that, but I am relatively certain that C is invariant.

[Klaynos]

Photons ALWAYS move at c.

 

It appears that light moves slower through materials due to the absorption and re-emission of photons.

[iNow]

It appears that light moves slower through materials due to the absorption and re-emission of photons.

 

That's the piece I was missing. Thanks, Klaynos.

[swansont]

It is in fact possible to slow light down to a stop.

 

The only way light stops is if it's been absorbed. "Stopped light" is IMO a horrible pop-sci journalism/press-release term.

[Klaynos]

The only way light stops is if it's been absorbed. "Stopped light" is IMO a horrible pop-sci journalism/press-release term.

 

I could not agree more.

[moth]

The only way light stops is if it's been absorbed. "Stopped light" is IMO a horrible pop-sci journalism/press-release term.

 

like this?

http://www.hno.harvard.edu/gazette/1999/02.18/light.html

 

the line about no lower speed limit on light makes me wonder what they're smokin at harvard

the laser cooling technique she uses is a pretty amazing tool though.

Edited February 17, 2009 by moth
speling

[drawkcab]

Thanks for all the input guys. At least I can put the thought to rest now.

[GDG]

The only way light stops is if it's been absorbed. "Stopped light" is IMO a horrible pop-sci journalism/press-release term.

 

See C. Liu et al., Nature (2001) 409:490-93 ("Observation of coherent optical information storage in an atomic medium using halted light pulses"):

"Electromagnetically induced transparency [fns. omitted] is a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium; a 'coupling' laser is used to create the interference necessary to allow the transmission of resonant pulses from a 'probe' laser. This technique has been used [fns. omitted] to slow and spatially compress light pulses by seven orders of magnitude, resulting in their complete localization and containment within an atomic cloud [fn. omitted]. Here we use electromagnetically induced transparency to bring laser pulses to a

complete stop

in a magnetically trapped, cold cloud of sodium atoms. Within the spatially localized pulse region, the atoms are in a superposition state determined by the amplitudes and phases of the coupling and probe laser fields. Upon sudden turn-off of the coupling laser, the compressed probe pulse is effectively

stopped

; coherent information initially contained in the laser fields is

'frozen'

in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated: the stored coherence is read out and transferred back into the radiation field. We present a theoretical model that reveals that the system is self-adjusting to minimize dissipative loss during the 'read' and 'write' operations. We anticipate applications of this phenomenon for quantum information processing."

 

Seems to me that this is the language coined by the researchers themselves, not the press. Or are you saying that Nature is "pop-sci journalism"?

[moth]

"frozen in the atomic medium" sounds like absorbed to me. the light pulse is being stored in the electrons around supercooled sodium atoms. light always travels at c.

[Klaynos]

The difference is the paper uses phrases like:

 

effectively stopped

 

Other less precise media state that the light HAS stopped...

 

I saw a lecture last week where a researcher had tracked his research through various publications and watched as slowly the meaning of the research was skewed to mean something very similar to this. He simply did it by using the titles of each article. The last was quite similar to the first (more so than some in between) but had dropped some very important quote marks...

[swansont]

"coherent information initially contained in the laser fields is 'frozen' in the atomic medium for up to 1 ms." Means absorption, albeit in a specially prepared system.

 

Seems to me that this is the language coined by the researchers themselves, not the press. Or are you saying that Nature is "pop-sci journalism"?

 

Sometimes it is made up by the researchers. I don't know what they're thinking, because it's picked up by the popular press, who reshape and retell it without understanding. (Just like quantum teleportation is so often written about with a Star Trek reference. It was a milestone, IMO, that in the most recent teleportation stories (involving ions separated by a meter) most reports didn't talk about Scotty.)

 

 

Anyway, I've already ranted about this at length

http://blogs.scienceforums.net/swansont/archives/412

[NowThatWeKnow]

Photons ALWAYS move at c.

 

It appears that light moves slower through materials due to the absorption and re-emission of photons.

 

Most probably consider this thread over but could someone please explain how absorption and re-emission of photons is not slowing them down? I visualize a drop of water falling into a sponge and eventually making it out the other side, effectively being slowed down. I also do not see how refraction of something moving 186,000 miles per second can make it look like it is standing still. You can't shine a light into two opposing mirrors and expect it to continue when the source is turned off. It would seem the time between two points determines the speed.

Edited February 19, 2009 by NowThatWeKnow

[swansont]

Most probably consider this thread over but could someone please explain how absorption and re-emission of photons is not slowing them down? I visualize a drop of water falling into a sponge and eventually making it out the other side, effectively being slowed down. I also do not see how refraction of something moving 186,000 miles per second can make it look like it is standing still. You can't shine a light into two opposing mirrors and expect it to continue when the source is turned off. It would seem the time between two points determines the speed.

 

"Photons move at c" and "light moves at c" are not saying the same thing. The time spent being absorbed is what makes the light propagation speed decrease. Meanwhile, the photons were traveling at c.

[moth]

Most probably consider this thread over but could someone please explain how absorption and re-emission of photons is not slowing them down?

 

it is slowing them down. thats why you usually hear the speed of light is constant in a vacuum, but it's not constant when moving through matter. it still travels at c between interactions with electrons around the atoms but takes longer to cross a distance through matter than a vacuum because of these interactions.

the press paints a picture of photons just hanging in space when they are actually stored in the supercooled atoms.

 

you got me with the new avatar i took a couple swings at that bug on my screen before i realized it wasn't.

Edited February 19, 2009 by moth
expanding

[Klaynos]

During the time they are absorbed they don't exist. When they are re-emitted some time (very very very short) has passed and so it appears they've taken more time than constantly moving at c.

[NowThatWeKnow]

you got me with the new avatar i took a couple swings at that bug on my screen before i realized it wasn't.

 

It does make you want to slap at it.

 

As far as light speed being constant. I can see why many would be confused. The explanations given and the physics definition of speed are not harmonious.

[swansont]

As far as light speed being constant. I can see why many would be confused. The explanations given and the physics definition of speed are not harmonious.

 

How so?

 

You drive in your car for 100 km, and it takes you 2 hours. But you stopped along the way, for a total of 1 hour. What was your average speed? How fast was your car traveling while it was in motion?

[NowThatWeKnow]

How so?

 

You drive in your car for 100 km, and it takes you 2 hours. But you stopped along the way, for a total of 1 hour. What was your average speed? How fast was your car traveling while it was in motion?

 

Does stopped or moving sound like a constant speed? If the photon can only maintain its speed the distance between molecules, then would it be harmonious with "Distance traveled divided by the time of travel"? It would be relative to a particular distance and your point of view.

[ydoaPs]

Does stopped or moving sound like a constant speed? If the photon can only maintain its speed the distance between molecules, then would it be harmonious with "Distance traveled divided by the time of travel"? It would be relative to a particular distance and your point of view.

 

It's not [math]\frac{d}{t}[/math], it's [math]\frac{dd}{dt}[/math]. What you described is AVERAGE speed. It's the average speed that slows down due to the absorption. The photon's speed, however, is always c.