Faster Than The Speed Of Light?

[mistermack]

And Why(or more correctly How ) does light "slow down" in a medium like water or glass ? I've Not heard of a good explanation. I've had a lot of people angrily repeating the lies they tell students but no good explanation .

The clue is in the fact that the light doesn't travel in straight lines, in glass or water.
That's why you see it being refracted. Not travelling in a straight line means that it covers a greater distance. So, as it can't speed up, it takes longer to cover the greater distance.

If you have an experiment set up to measure the speed of light, it has to travel in a straight line, with no reflection. Imagine the same experiment, with a couple of mirrors in line, so that the light travels a million miles more. The EFFECTIVE speed of light, through the mirrors, just taking the time from point a to point b, is slower. But if you find out the real distance travelled it's still the speed of light.

So in short, the medium, like water or glass, adds distance through internal reflection.

[Feck]

NO, nice try......... but no .
That begs the question 'how is light reflected' which avoids my original question AND adds another one .

[JimC]

It's bloody weird stuff that's for sure. A photon takes c.8 mins to travel from the Sun to the Earth, yet for the photo itself, travelling at lightspeed, it takes no time at all.

And Why(or more correctly How ) does light "slow down" in a medium like water or glass ? I've Not heard of a good explanation. I've had a lot of people angrily repeating the lies they tell students but no good explanation .

This from Wikipedia:

In a medium, light usually does not propagate at a speed equal to c; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity vp. An actual physical signal with a finite extent (a pulse of light) travels at a different speed. The largest part of the pulse travels at the group velocity vg, and its earliest part travels at the front velocity vf.

The phase velocity is important in determining how a light wave travels through a material or from one material to another. It is often represented in terms of a refractive index. The refractive index of a material is defined as the ratio of c to the phase velocity vp in the material: larger indices of refraction indicate lower speeds. The refractive index of a material may depend on the light's frequency, intensity, polarization, or direction of propagation; in many cases, though, it can be treated as a material-dependent constant. The refractive index of air is approximately 1.0003.[58] Denser media, such as water,[59] glass,[60] and diamond,[61] have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than-c speeds in material substances. As an extreme example of light "slowing" in matter, two independent teams of physicists claimed to bring light to a "complete standstill" by passing it through a Bose–Einstein condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard–Smithsonian Center for Astrophysics, also in Cambridge. However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped," it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light.[62]

In transparent materials, the refractive index generally is greater than 1, meaning that the phase velocity is less than c. In other materials, it is possible for the refractive index to become smaller than 1 for some frequencies; in some exotic materials it is even possible for the index of refraction to become negative.[63] The requirement that causality is not violated implies that the real and imaginary parts of the dielectric constant of any material, corresponding respectively to the index of refraction and to the attenuation coefficient, are linked by the Kramers–Kronig relations.[64] In practical terms, this means that in a material with refractive index less than 1, the absorption of the wave is so quick that no signal can be sent faster than c.

A pulse with different group and phase velocities (which occurs if the phase velocity is not the same for all the frequencies of the pulse) smears out over time, a process known as dispersion. Certain materials have an exceptionally low (or even zero) group velocity for light waves, a phenomenon called slow light, which has been confirmed in various experiments.[65][66][67][68] The opposite, group velocities exceeding c, has also been shown in experiment.[69] It should even be possible for the group velocity to become infinite or negative, with pulses travelling instantaneously or backwards in time.[70]

None of these options, however, allow information to be transmitted faster than c. It is impossible to transmit information with a light pulse any faster than the speed of the earliest part of the pulse (the front velocity). It can be shown that this is (under certain assumptions) always equal to c.[70]

It is possible for a particle to travel through a medium faster than the phase velocity of light in that medium (but still slower than c). When a charged particle does that in a dielectric material, the electromagnetic equivalent of a shock wave, known as Cherenkov radiation, is emitted.[71]

[Feck]

Ok I found this.......https://en.wikipedia.org/wiki/Ewald%E2% ... on_theorem
which helps (A bit ;}

[mistermack]

This from Wikipedia:

In a medium, light usually does not propagate at a speed equal to c; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity vp. An actual physical signal with a finite extent (a pulse of light) travels at a different speed. The largest part of the pulse travels at the group velocity vg, and its earliest part travels at the front velocity vf.

The phase velocity is important in determining how a light wave travels through a material or from one material to another. It is often represented in terms of a refractive index. The refractive index of a material is defined as the ratio of c to the phase velocity vp in the material: larger indices of refraction indicate lower speeds. The refractive index of a material may depend on the light's frequency, intensity, polarization, or direction of propagation; in many cases, though, it can be treated as a material-dependent constant. The refractive index of air is approximately 1.0003.[58] Denser media, such as water,[59] glass,[60] and diamond,[61] have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than-c speeds in material substances. As an extreme example of light "slowing" in matter, two independent teams of physicists claimed to bring light to a "complete standstill" by passing it through a Bose–Einstein condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard–Smithsonian Center for Astrophysics, also in Cambridge. However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped," it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light.[62]

In transparent materials, the refractive index generally is greater than 1, meaning that the phase velocity is less than c. In other materials, it is possible for the refractive index to become smaller than 1 for some frequencies; in some exotic materials it is even possible for the index of refraction to become negative.[63] The requirement that causality is not violated implies that the real and imaginary parts of the dielectric constant of any material, corresponding respectively to the index of refraction and to the attenuation coefficient, are linked by the Kramers–Kronig relations.[64] In practical terms, this means that in a material with refractive index less than 1, the absorption of the wave is so quick that no signal can be sent faster than c.

A pulse with different group and phase velocities (which occurs if the phase velocity is not the same for all the frequencies of the pulse) smears out over time, a process known as dispersion. Certain materials have an exceptionally low (or even zero) group velocity for light waves, a phenomenon called slow light, which has been confirmed in various experiments.[65][66][67][68] The opposite, group velocities exceeding c, has also been shown in experiment.[69] It should even be possible for the group velocity to become infinite or negative, with pulses travelling instantaneously or backwards in time.[70]

None of these options, however, allow information to be transmitted faster than c. It is impossible to transmit information with a light pulse any faster than the speed of the earliest part of the pulse (the front velocity). It can be shown that this is (under certain assumptions) always equal to c.[70]

It is possible for a particle to travel through a medium faster than the phase velocity of light in that medium (but still slower than c). When a charged particle does that in a dielectric material, the electromagnetic equivalent of a shock wave, known as Cherenkov radiation, is emitted.[71]

Thass wot I sed innit

[Feck]

Ok if we agree that photons can't slow down from C and that the actual effect is (apparently ) that photons are absorbed by atoms which then release other photons why are these photons not scattered ? The deeper you question refraction the more tentative are the answers you get .Conversations about refractive index don't actually help me understand how a mass- les uncharged particle can be slowed .

[cronus]

At C precisely a change of state occurs. Before then it is business as usual albeit in ever more extremes. At C the equations are in full on singularity mode. Hence nothing can go faster and that's that, at the limit as you approach C. If things do go faster then time must go backwards because of the dilation equations.

[Brian Peacock]

But of course, as you speed approaches C you mass also approaches infinity. I guess this is why I feel fatter when I'm jogging.

[JimC]

Ok if we agree that photons can't slow down from C and that the actual effect is (apparently ) that photons are absorbed by atoms which then release other photons why are these photons not scattered ? The deeper you question refraction the more tentative are the answers you get .Conversations about refractive index don't actually help me understand how a mass- les uncharged particle can be slowed .

Interesting point about the scattering. If, in transparent materials, a photon which is absorbed and then quickly re-emitted always is re-emitted in precisely the same direction as it arrived, we would have an answer.

[mistermack]

One interesting thing about massless photons, is that if you "effectively" stop them, by trapping them between two mirrors, so that they are constantly reflecting back and forth, then the system gains a fixed unit of mass. So even though the photon has no mass, it can act as if it has.

When you talk about a photon being absorbed and re-emitted, then that's the same thing as reflection, right? When light is absorbed, it doesn't just sit there in the molecule. It can be still travelling at c, whizzing about within the matter. You would expect the molecule to gain in mass, like the mirrors, until the light is re-emitted.

And because a photon has momentum, even though it has no mass, the momentum has to be conserved. That's how asteroids can be pushed off course by light. The light is absorbed and not re-emitted, so the asteroid gains momentum in the direction to the incoming light.
Even if some is reflected, it can't be reflected in it's original direction, so there's still a push in the direction of the light.

[Brian Peacock]

I've often wondered about the quantum state of the photon as involved in the mechanics of vision or photosynthesis. I've read somewhere that the human eye can detect a single photon. It seems to me that as far as the mechanics of vision is concerned this is as good as any system can get. However, the human eye is an organ of the brain and so any explanation of vision must involve the role of quantum physics in neuroscience, just as quantum physics must play a role in plant biology when it comes to photosynthesis. As I say, I've wondered about this before, but after some fruitless googling I generally give up - until I wonder about it the next time.

[JimC]

One interesting thing about massless photons, is that if you "effectively" stop them, by trapping them between two mirrors, so that they are constantly reflecting back and forth, then the system gains a fixed unit of mass. So even though the photon has no mass, it can act as if it has.

When you talk about a photon being absorbed and re-emitted, then that's the same thing as reflection, right? When light is absorbed, it doesn't just sit there in the molecule. It can be still travelling at c, whizzing about within the matter. You would expect the molecule to gain in mass, like the mirrors, until the light is re-emitted.

And because a photon has momentum, even though it has no mass, the momentum has to be conserved. That's how asteroids can be pushed off course by light. The light is absorbed and not re-emitted, so the asteroid gains momentum in the direction to the incoming light.
Even if some is reflected, it can't be reflected in it's original direction, so there's still a push in the direction of the light.

When photons are reflected from an object, the added momentum of the object is double that if they were absorbed.

[mistermack]

When photons are reflected from an object, the added momentum of the object is double that if they were absorbed.

Using that principle, in theory, you could accelerate spacecraft for nothing.
You shine a lazer at it, from the Earth. And if it's fitted with a mirror, it's getting double the acceleration that it would, if it just absorbed the light.
And then you collect the reflected lazer light and use it to boil your kettle. Or reflect it back again for more acceleration.

In space, you could accelerate two craft in opposite directions, using the same light over and over again, just with two mirrors.

[Brian Peacock]

I've often wondered about the quantum state of the photon as involved in the mechanics of vision or photosynthesis. I've read somewhere that the human eye can detect a single photon. It seems to me that as far as the mechanics of vision is concerned this is as good as any system can get. However, the human eye is an organ of the brain and so any explanation of vision must involve the role of quantum physics in neuroscience, just as quantum physics must play a role in plant biology when it comes to photosynthesis. As I say, I've wondered about this before, but after some fruitless googling I generally give up - until I wonder about it the next time.

Quantum Mechanical Basis of Vision. (PDF)

[mistermack]

In space, you could accelerate two craft in opposite directions, using the same light over and over again, just with two mirrors.

I'm not so sure about that now. If the two craft are accelerating due to the light, where is the energy coming from? While there is no increase in momentum in the system, there is an increase in kinetic energy in the two crafts, and yet there is no energy input if you are using the same light over and over.
Must be a flaw somewhere. In fact, you're not just gaining kinetic energy, you're gaining potential energy, due to the increasing distance, working against their gravitational pull on each other.

Edit :
This is actually quite interesting. It is actually being done in space with satellites, and is a very efficient way of accelerating bodies in space. The two mirror technique increases the efficiency hugely, as I speculated. But it does require energy input. The photons lose a tiny bit of energy on reflection, so it's not actually creating energy out of nothing. Otherwise, you could easily make a perpetual motion machine, by bouncing light off mirrors, mounted on axles.

https://en.wikipedia.org/wiki/Photonic_laser_thruster