Can light slow down?

reply to post by intergalactic fire


That is what I am inclined to think. Thanks for the Wikipedia link.

reply to post by definity


In the experiment they just made a medium of cold sodium atoms with the highest refractive index.
refractive index of 17,621,000

www.jupiterscientific.org...

reply to post by jiggerj


The speed which light travels and the "speed of light" are not the same. You can change the speed which light travels, but nothing can ever go faster than whatever that speed is.

If anything you should reverse your thinking. They should speed light up and then they can go much faster without being FTL.

Light only appears to slow down but you have to keep in mind that time also slows down in a strong gravity field so if you where in the black hole with the beam of light your watch would be running so slow that you would still measure light at C. time and length are not constant and good way to think about light is it always slows with time and length to create a constrain measurement of C

reply to post by Tbrooks76


Yes, exactly, that is the thinking of Einstein. So, in theory, light never slow down.

reply to post by swan001


No, light is a constant in a vacuum. Einstein never said light is a constant no matter what, if you think he did show me the quote. Once you add variables such as different mediums you can affect the speed of light. This is completely in line with Einsteins theories.

Originally posted by swan001
reply to post by TeslaandLyne

 

I tought that Einstein's Relativity relied on the fact that the speed of light is always constant, no matter the emitter or observer's movements?

In outer space the medium (its usually called a vacuum) is constant for the Einstein
theories to take place. See through mediums like glass of different mixtures and
such the physics determined in order to analyze the observed properties of light
they decided to make light slow down. Since wave motion in the math model
depends on density the physics is undoubtedly correct.

The only problem I found was the EM wave of Maxwell using transverse waves which
up until then were only seen in solid objects under vibration. As you can see pressure
waves in a gaseous medium are not to be considered for antenna propagation but were
the sole area of Tesla's work. Using induction of one coil in a circuit into another with
one connection, basically an antenna, made gas filled tubes light up and one wire
filament bulbs light up. If the power input was greater than the illuminations needed
probably ended those experiments for everyone. To increase voltage without using
more power and give greater illumination would set back the need for more power
plant construction at least for household illumination.

Originally posted by subject1145

Originally posted by 11I11
A good thought provoking thread, S&F good sir.

I agree, if light is being sucked into a black hole, it must surely be slowed down by the gravitational pull?

I am not sure if it would be slowed down, or merely not able to escape the gravity of the singularity. Basically spinning at the speed of light but not able to achieve escape velocity and travel faster then the speed of light due to the extreme gravity. Again I have no idea what I am talking about so I'll defer to experts on these subjects.

LOL Hey, it sounded good to me.
Then again, I know even less than you.

reply to post by jiggerj


If "they" slowed it down any further, you could "trip over the light fantastic."

Scientists have sped light up as well as slowed it down. Here's an article talking about light going faster than light.

FTL light

reply to post by jiggerj

Hi again, jiggerj. Intergalactic fire's post has the correct answers. When you read about scientists slowing light down, what really happens is best visualized by imagining the photons being bounced back and forth between the atoms of a medium like pinballs before emerging. All this bouncing, though, is still happening at the (invariant) speed of light.

*

reply to post by Tbrooks76

Light only appears to slow down but you have to keep in mind that time also slows down in a strong gravity field so if you where in the black hole with the beam of light your watch would be running so slow that you would still measure light at C.

It doesn't matter how fast or slow your clock is; the speed of light is the same in all frames of reference. Gravity has no effect on it. Gravity alters the geometry of spacetime, which causes photons to change direction – to 'bend' – but does not slow them down.

The reason why light cannot emerge from a black hole is that the gravitational force around the singularity is so strong, the path of emerging photons is bent until it can't escape. The photon falls into orbit around the singularity somewhere below the event horizon and circles indefinitely or spirals back, depending on its energy. This is what makes black holes 'black'.

reply to post by Astyanax


More to the point, a medium slows the phase velocity of the light wave, but doesn't slow its group velocity.

The group velocity, as I always say, can be thought of as how fast the water in a stream is moving, while the phase velocity, on the other hand, can be thought of as how fast waves are moving through the water. The latter deals with the speed of the wave through the water, and the former deals with the speed of the water itself.

In every example given of slowing light down or speeding it up, it is merely the slowing down or speeding up of light's phase velocity -- how fast the photon's wave is travelling. In all cases, regardless of the medium or experiment conditions, the photons are always (I can't stress that enough: ALWAYS) travelling at a constant speed of 299,792,458 m/s.
Always.

Also, to the point of the thread: if the speed of light ever deviated at all from its exact value of 299,792,458 m/s, light would be unable to travel at all. It must propagate at that speed in order to exist (this according to Maxwell's equations -- in order for EM waves to self-propagate, they must always be measured, in any and all reference frames, to be travelling at a single constant velocity).

reply to post by CLPrime

More to the point, a medium slows the phase velocity of the light wave, but doesn't slow its group velocity.

You're right; I was just trying for an explanation that could be more easily visualized. All those pinballing photons are, of course, part of a wave that advances slowly through the medium. I have a question, though: does the actual absorption and re-emission of photons in the medium take place more slowly in these 'light-slowing' experiments, or is there just a lot more of it?

reply to post by Astyanax


Absorption and re-emission of photons, given a quantum understanding of such things (as I know you're well-familiar with), is instantaneous. Slowing phase velocity would be akin to giving a pinball more obstacles to bounce around between, in a classical sense.

The more rocks you put in the stream, the slower the waves in the water will go, as energy gets dissipated, but the water will keep going at the same speed, regardless.
Or, at least, pretend it does -- the example isn't perfect, but it's good enough. And if you build a dam... well, that's where the analogy fails altogether. I suppose, we could say it's a dam made of sponge, soaking up the water as it comes (as the medium absorbs photons without re-emitting them), rather than having the water/photons pile up at the face of the dam. Luckily, this sort of thing (a blackbody) doesn't happen all that often.

Can light slow down like this?







It makes logic to me now.

Actually, you can stop it dead in it's tracks. It's called black paint or paper. As soon as it hit's, it stops and transfers into heat energy. Or so I think anyway.

reply to post by swan001


Assuming I'm understanding that diagram right, then that's an alright way of looking at it. Regardless, the speed of every photon (group velocity) is always constant and equal to exactly 299,792,458 m/s.

reply to post by Fromabove


There's a difference, though, between the slowing/stopping of light and the absorption of light. You're taking about absorption.

reply to post by CLPrime


Exactly. The slowing down is due to interaction (in my example, mirrors) only, but the light itself never varies its speed.

reply to post by swan001


Since it's my thread am I allowed to take it in another direction? Let's find out? You have a square tube, with each side measuring 12 inches wide. You shine a light in the tube so that it will reflect on all sides (I have to make a diagram). Is it possible to trap that light inside the tube by capping off the ends with mirrors? Of course I'm thinking the tube would have to be LONG to allow for time to cap the ends.