Can light slow down?

reply to post by jiggerj


It's more than possible...it's a relatively recent achievement:

Physicists trap light in a bottle

Originally posted by intergalactic fire
Yes and no,
Speed of light is an absolute constant, it always travels the same speed. It just appears to go slower in a medium because the interaction with the matter, the photons are being absorbed by the atoms and reemits them at the same speed. It just takes more time to travel through the medium. The delay of absorption and emmission.
Depending on the refractive index of the medium. Like for the refractive index of water is around 1.3 so it's C/1.3.
In a vaccuum it's 1.
en.wikipedia.org...

So you could say light slows down when travelling through a medium but it just takes longer to travel through.

For light travelling near a black hole, photons are being bend or 'trapped' not slowed down.

edit on 24-4-2012 by intergalactic fire because: (no reason given)

Still here, Intergalactic? I checked the list of refractive indices and understood a good none of it. lol But it did stir up some questions, if you don't mind answering?

1. If we create a vacuum on earth, would the number for air, 1.000277, be reduced down to 1?

2. Is it possible to create an even stronger vacuum than what's in outerspace, thereby allow light to travel faster?

3. We have regular lightbulbs, halogen lights, Glow in the dark nightsticks. There are lightning bugs, and things that absorb light and then are illuminated in the dark. Different lights and different colors; are all these photons the same?

If you don't mind, since I'm here, I'd like to answer these:

1. If we create a vacuum on earth, would the number for air, 1.000277, be reduced down to 1?

The index for air is 1.000277 and the index for vacuum is 1.000000. If you create a vacuum here on Earth, you're creating a region with an index of 1.000000. It's not changing the index of air from 1.000277 to 1.000000, it's changing the medium, and the different medium has a different index.
But, yes, if you create a vacuum on Earth, the resultant index of that vacuum (as with any vacuum) will be 1.

2. Is it possible to create an even stronger vacuum than what's in outerspace, thereby allow light to travel faster?

Essentially, a vacuum is absolute. A quantum vacuum can spontaneously collapse to a lower state (or, in a sense, become a "stronger" vacuum), but we can't cause that to happen, and it obviously doesn't happen very often, if at all. It's just something that Quantum Mechanics says has a very slight probability of happening...like quantum tunnelling and the slight probability of you falling through the floor.

3. We have regular lightbulbs, halogen lights, Glow in the dark nightsticks. There are lightning bugs, and things that absorb light and then are illuminated in the dark. Different lights and different colors; are all these photons the same?

All photons are the same. Colour is caused by light of different wavelengths. In general, differing wavelength is responsible for the EM spectrum - from radio waves to gamma radiation, with visible light in the middle. It's all caused by light/photons travelling at different wavelengths/frequencies.

i was reading Jenny Randles' book about the rendlesham forest ufo encounter last night, and one airman, present during the encounter, related how his shadow seemed to move a moment or two later than it should have, as he approached the alleged ufo that night. it was as if the shadow (cast upon the craft a few metres from him) was in a different 'time moment' than he himself, as if somehow distorted by the ufo's energy field. so, if the account is accurate, this seemingly illustrates time as susceptible to certain forces, possibly able to be manipulated (given a sufficiently advanced technology).

reply to post by jiggerj



I was thinking about the same thing! I am glad you asked. I would say, The photon (light particle-wave) would die rapidly in that tube. It would stay trapped, but die very fast. Because each time the photon reflects, it loses energy, right? I mean, even if the mirrors are perfectly reflective, The photon can't survive that much reflections and would fastly merge with the surrounding atoms.
I hope someone could tell me if I'm wrong.

reply to post by CLPrime


Oh, I just saw your post. Thanks for your opinion!

reply to post by swan001


Ultimately, you're right...the light will eventually get absorbed/dispersed and, in effect, it would go away. It would probably heat up the container.

Originally posted by swan001
reply to post by jiggerj

 

I was thinking about the same thing! I am glad you asked. I would say, The photon (light particle-wave) would die rapidly in that tube. It would stay trapped, but die very fast. Because each time the photon reflects, it loses energy, right? I mean, even if the mirrors are perfectly reflective, The photon can't survive that much reflections and would fastly merge with the surrounding atoms.
I hope someone could tell me if I'm wrong.

Die? Never heard of this. How come light can travel billions of lightyears without any change? Its bent by gravity, so there are forces working against light that should cause some kind of change in the photons. Right?

And how come photons that are being emitted from two different sources don't collide with each other?

Originally posted by CLPrime
reply to post by swan001

 

Ultimately, you're right...the light will eventually get absorbed/dispersed and, in effect, it would go away. It would probably heat up the container.

Learning new stuff everyday! It just never dawnd on me that photons are the source of heat from the sun. So, just to be clear, when we stand out in the sun, the photons are hitting us and causing us to heat up and get a sunburn if we stay exposed for too long?

reply to post by jiggerj


Basically, yup! That's it. The photons carry energy and that energy is distributed to your atoms when the photon collide with you (sorry if that sounds a bit Greek-Mythology-dramatic :lol.

Originally posted by CLPrime
reply to post by jiggerj

 

It's more than possible...it's a relatively recent achievement:

Physicists trap light in a bottle

Glad you found the link -- I remember the story and was going to post a comment on it but it was great re-reading the actual story.

edit:

Seems like that article is a bit different -- the story I read was about was scientists using a mix of gases and extreme cold to effectively stop light from moving. It lasted a very very short time, nanoseconds I think.

Still -- its amazing that science is able to do it at all.

Originally posted by jiggerj

Die? Never heard of this. How come light can travel billions of lightyears without any change? Its bent by gravity, so there are forces working against light that should cause some kind of change in the photons. Right?

And how come photons that are being emitted from two different sources don't collide with each other?

"Die" is in reference to the absorption of photons. Photons never lose energy on their own...they must be absorbed by atoms.

Light is bent by gravity due to the fact that gravity is the bending of spacetime (through which light travels), but this has no affect on the photons, it just causes them to change direction.
Forces don't work against light. Moreover, light is a force (the electromagnetic force). The only other force that affects light is gravity, and, again, that only changes light's direction slightly.
As light travels through space, it can be absorbed and, possibly, re-emitted, which will diffuse and disperse it. A very good example of this is starlight passing through the atmosphere, which causes stars to twinkle, and sometimes in different colours.
Also, light is stretched (that is, the wavelength of the EM wave is stretched) by the expansion of the universe. That's how we know how much the universe has expanded in the past 14 billion years...the light from things 14 billion light-years away has been stretched proportionally. This is called redshift.

As for photons bumping into each other... photons are force particles, and force particles (unlike matter particles) can exist in the same location at the same time. For anyone interested in the technical reason, it's because photons are bosons. Unlike fermions, bosons don't obey Fermi-Dirac Statistics, the extension of the Pauli Exclusion Principle. Fermions are unable to occupy the same quantum state, but bosons can coexist at a single point in space.

Originally posted by jiggerj

Learning new stuff everyday! It just never dawnd on me that photons are the source of heat from the sun. So, just to be clear, when we stand out in the sun, the photons are hitting us and causing us to heat up and get a sunburn if we stay exposed for too long?

That's right. Specifically, sunburns are caused by ultraviolet light, which is energetic enough to get just below the surface of the skin and cause damage to it (the idea that UV light can cause cancer is arguable...though, it is interesting that sunscreen has a greater chance of causing a more lethal kind of skin cancer than UV light...so consider that the next time you put sunscreen on).

Photons are a great source of heat (solar energy anyone?) but you need a lot of them.

Photons are also responsible for radio signals, microwaving your food, and radiation poisoning following nuclear explosions. And then you have virtual photons, which are the source of magnetism.

Originally posted by PhoenixDown

... the story I read was about was scientists using a mix of gases and extreme cold to effectively stop light from moving. It lasted a very very short time, nanoseconds I think.

Anything of that nature always deals with the phase velocity of light (as I explained earlier). They aren't actually stopping the photons from moving, they're just stopping the light wave. The photons keep going at 299,792,458 m/s at all times. It's still impressive, but not nearly as impressive as actually getting photons to stand still, which would either collapse the laws of physics or completely rewrite our understanding of them.

Originally posted by CLPrime

Originally posted by jiggerj

Learning new stuff everyday! It just never dawnd on me that photons are the source of heat from the sun. So, just to be clear, when we stand out in the sun, the photons are hitting us and causing us to heat up and get a sunburn if we stay exposed for too long?

That's right. Specifically, sunburns are caused by ultraviolet light, which is energetic enough to get just below the surface of the skin and cause damage to it (the idea that UV light can cause cancer is arguable...though, it is interesting that sunscreen has a greater chance of causing a more lethal kind of skin cancer than UV light...so consider that the next time you put sunscreen on).

Photons are a great source of heat (solar energy anyone?) but you need a lot of them.

Photons are also responsible for radio signals, microwaving your food, and radiation poisoning following nuclear explosions. And then you have virtual photons, which are the source of magnetism.

Hate to press the matter, but I really want it clarified for me, and you seem like a bloody genius!
The heat from the sun is no more and no less photons. Right? I have forced hot water for heat; do photons play a role in that?

reply to post by jiggerj


The heat we feel is basically a result of vibrating molecules. Those molecules are vibrating because the atoms that make them up are energetic. The more energetic an atom is, the more it vibrates (or oscillates). In the case of a heating element (for example), the atoms in the metal are vibrating rapidly. When you touch it, it causes the atoms/molecules in your skin to vibrate as well, which you feel as heat (and can damage cells...giving you first, second, or third degree burns).

Colloquially, this is the "heat" we feel.
Technically (that is, scientifically), heat is thermal energy (the energy contained within the vibrating atoms... the energy that's causing them to vibrate). This energy is imparted by photons. The more photons an atom absorbs, the more energetic it gets and the more "hot" it feels.

In the case of hot water, the water is (or can be...in the case of a hot water heater, for example...or a stove) heated by an element. The element is heated by electricity. Electricity is one half of electromagnetism, which is the force propagated by photons. So, hot water isn't a direct result of photons, but they are responsible for the mechanism that is used to heat the thing that heats the water. And then the water will heat the air around it.

So it is safe to put the kettle on then
Seriously though great explanation given

reply to post by artistpoet


The photons emitted by a kettle, in terms of heat, are in the infrared range. IR photons are also emitted by stoves, people, dogs, sasquatch, engines, tables, and anything else that has any sort of noticeable temperature. Infrared is heat, and, luckily, it doesn't cause cancer (if it did, then visible light would cause cancer as well, because IR light is less energetic than visible light).

Originally posted by CLPrime
reply to post by artistpoet

 

The photons emitted by a kettle, in terms of heat, are in the infrared range. IR photons are also emitted by stoves, people, dogs, sasquatch, engines, tables, and anything else that has any sort of noticeable temperature. Infrared is heat, and, luckily, it doesn't cause cancer (if it did, then visible light would cause cancer as well, because IR light is less energetic than visible light).

So does this mean that the red shift or infra red wavelength is longer as seen in stars and does this mean these stars are cooler

reply to post by artistpoet


Redshift "cools" the light from stars by stretching its wavelength to the red end of the spectrum (that is, toward the visible red, infrared, microwave, and radio side). This shift is very much accounted for when we determine star temperatures. This can actually go both ways -- knowing the redshift can determine the star's temperature, and knowing a star's temperature can determine the redshift.
This is the same with the Cosmic Microwave Background. The CMB was blackbody radiation with an initial "temperature" of 3000 K, but the expansion of the universe over the past 13.7 billion years has stretched out the light and redshifted it to such an extent that it's current "temperature" is just 2.7 K.