[ukmicky (OP)]

A particle of light can travel ftl as shown in experiments when it was sent through a chamber of caesium atoms.

My question is, what then happens to that particle if, after leaving the last atom of caesium, it enters a vacuum?

Would it slow down, releasing energy in some form?

Or would it keep going but change into some form of exotic matter and maybe losing energy as light? 

Or does the way that it transmits itself through the caesium atoms allow it to exit at the speed that light travels through a vacuum?

Or are my questions silly?

[daveshorts]

Do you have any links to the experiment as there are several possibilities of what is happening depending on what exactly the experiment was.

[ukmicky (OP)]

this is the first one from the list when i googled it http://archives.cnn.com/2000/TECH/space/07/20/speed.of.light.ap/
it was a pulse from a lazer

[gsmollin]

According to the researchers, the exit beam has the same properties as the entrance beam, with a reduced amplitude.

I would urge some caution here, this is new work, and has not been verified. There have been claims of this type in the past, that were found to have no merit. People have been passing light beams through cesium gas for many years, without seeing superluminal velocities. I am skeptical.

[qazibasit]

hmmm

[chris]

It looks like this poor question, posted 12 years ago, never got a decent treatment. Can anyone "shed any light" () on this, or at least provide an answer?

[jeffreyH]

Fooling the uncertainty principle with light, a vibrating membrane and caesium atoms.
http://www.nbi.ku.dk/english/news/news17/clever-atomic-cloud-solves-heisenbergs-observational-problem/
Not exactly an answer to the OP but fascinating in its own right.

[evan_au]

A particle of light can travel ftl as shown in experiments when it was sent through a chamber of caesium atoms.

Faster Than Light (ftl) seems rather unlikely - the speed of light in a medium is lower than the speed of light in a vacuum.

I haven't seen the report to which the OP was referring. But I do know that scientists use clouds of cesium vapor to slow down light below its usual speed by irradiating it with lasers of exactly the right wavelength.

This is used as a variable pulse delay, for example:
https://www.ncbi.nlm.nih.gov/pubmed/17501346

[Colin2B]

Faster Than Light (ftl) seems rather unlikely - the speed of light in a medium is lower than the speed of light in a vacuum.

I agree. There have been a number of experiments claiming light faster than accepted speed in a medium, but on closer examination have confused group and phase velocities.

I think the op may be referring to an experiment at Princeton in around 2000 which was hailed in the press as ftl. Quote from one of the researchers suggests not:

“The pulse may look like a straight beam but actually behaves like waves of light particles. The light can leave the chamber before it has finished entering because the cesium atoms change the properties of the light, allowing it to exit more quickly than in a vacuum.
The leading edge of the light pulse has all the information needed to produce the pulse on the other end of the chamber, so the entire pulse does not need to reach the chamber for it to exit the other side.
The experiment produces an almost identical light pulse that exits the chamber and travels about 60 feet before the main part of the laser pulse finishes entering the chamber, Wang said.
Wang said the effect is possible only because light has no mass; the same thing cannot be done with physical objects.”

Another example of popular press distorting the results by not explaining the full detail.

[jeffreyH]

Do the electric and magnetic constants of the vacuum limit light to a speed of c or are they forcing light to travel at that speed?

[evan_au]

Do the electric and magnetic constants of the vacuum limit light to a speed of c or are they forcing light to travel at that speed?

According to Maxwell's classical theory of electromagnetism, the permeability and permittivity of a vacuum determine the "stiffness" of space for electromagnetic waves, and hence the velocity of electromagnetic waves through a vacuum.

Other materials have greater permeability and/or permittivity than a vacuum, due to the presence of electrons and protons. These material are less "stiff", and light travels more slowly than in a vacuum.

Maxwell's equations define light very well at the macroscopic level, but when you are dealing with light at the level of photons or atoms, you need a quantum description of the propagation of light.

Even today's quantum description can't tell us why the vacuum has the permeability and permittivity that we observe.

See: https://en.wikipedia.org/wiki/History_of_Maxwell%27s_equations#Relationships_among_electricity.2C_magnetism.2C_and_the_speed_of_light

[Bill S]

“The pulse may look like a straight beam but actually behaves like waves of light particles

That's a thought provoking statement; especially the "of"..

[jeffreyH]

It is just like saying in the sea we get waves of water molecules. The ensemble produces the wave.

[evan_au]

"stiffness" of space for electromagnetic waves

Maybe I oversimplified there...
As I recall, there are two factors influencing the velocity of a wave in macroscopic objects:
- The restoring force: This can be due to tension in a string, the stiffness of a solid, or the strength of gravity for water waves
- The inertia: This can be due to the mass/length of a string, or the density of a solid, liquid or gas.

Could it be that the presence of electrons and protons responding to the electromagnetic field play the role of adding "inertia" to the vacuum, which slows down the electromagnetic waves? (...to use a semi-classical analogy)

[Bill S]

It is just like saying in the sea we get waves of water molecules. The ensemble produces the wave.

No quibble with that, in the macro-world.  At quantum level we have Waves and particles, waves or particles and neither waves nor particles; now we add waves of particles, and I find myself wondering where that fits with our observations of the wave/particle duality. 

Takes me a while to process these things.    Running them past people on TNS helps - usually.