Speed of light

[Aki]

I'm curious, I have a few questions: can the speed of light be ever altered? Can we slow it down? And how much does a photon weight?

thanks

[Tim_Lou]

photon is massless, it has no weight....hmm, i wonder whats the volume of it...
when light travels throw some substances, it slows down, like water.
normally, it travels at a constant speed.

[deamonstar]

light only travels at "light speed" in a vaccuum. its realative velocity slows when travelling through a medium such as our atmosphere or water or glass etc... etc...

here is a link you may find interesting:
stop light

[Aki]

well how can something be completey massless? so a photon doesn't even weight 10000000000000 times less than (I don't know) the lighting thing on earth?

[deamonstar]

when doing basic calculations of theories where the mass of a photon isn't a critical variable, it would be allright to assume a value of zero for the mass of a photon. however, when we delve into calculations of theories based on special realativity or quantum mechanics, where the mass of a photon becomes more critical, we must give a non-zero value to avoid anomolies such as infinities or absolute zero's. but, what is this non-zero value, and how can it be determined?

an ingenius experiment carried out by Jun Luo and his colleageus at Huazhong University of Science and Technology in Wuhan, China using a very sensitive torsion balance (a device with rotational torque) have been able to determine the mass of a photon. they aimed a beam of light at the apparatus, for which the value of the torque (or rotational energy) of it is known. when the photons hit the device, there is a measurable increase in the overall torque. this method is a 20-fold increase in the accuracy to determining the value of the mass of a photon over previous methods for determining the photon's mass. the newly determined estimate for thenon-zero valueof the mass ofa photonis 10<SUP>-51</SUP> grams or 7 x 10<SUP>-19</SUP> electron volts.

for more info, go to the following link:

photon mass

[halogenesis]

if you ever visit www.madsci.org, and search their archives, you'll find a large amount of discussion to your questions about the whole massless photon debate (of course, there are MANY other topics). There was a recent discussion by a post-doc named Phil Marsden where he imparts his views on photon interactions. The most interesting thing I found was this new field of study on QUANTUM HOLOGRAPHY... here are some interesting links

http://unisci.com/stories/20014/ 1126013.htm

http://www.aip.org/enews/physnews/2001/split/566-1.html

I got these sites from Marsden's article.... enjoy them! They are interesting!

[Tim_Lou]

i found something in some website:

Q: Do photons have mass? If not, why does the gravitational field of a star bend passing light?

A: No, photons do not have mass according the present definition of mass. The modern definition assigns every object just one mass, an invariant quantity that does not depend on velocity, says Dr. Matt Austern a computer scientist at AT&T Labs Research. Under this definition, mass is proportional to the total energy, Eo, of the object at rest.

"A particle like a photon is never at rest and always moves at the speed of light; thus it is massless," says Dr. Michael S. Turner, chair of the Department of Astrophysics at the University of Chicago.

What about experimental evidence? Experiments don't determine exact quantities because of small errors inherent in making measurements. We have, however, put an upper limit on the photon rest mass. In 1994, the Charge Composition Explorer spacecraft measured the Earth's magnetic field and physicists used this data to define an upper limit of 0.0000000000000006 electron volts for the mass of photons, with a high certainty in the results.

This number is close to zero; it is equivalent to 0.00000000000000000000039 times the mass of an electron (the lightest particle), says Turner.

[Tim_Lou]

hmmm, if photons have mass, and it travels at the speed of light.....
how is it possible??
how can "massive" particals travel at the speed of light???

probably, the "mass" of photons is due to its energy.

mass and energy....probably the same thing...

[Aki]

Okay, photons are not "massless". They have a mass. So if we find something lighter than a photon, it'll be able to travel faster than light! But it is said that NOTHING can travel than the speed of light. So are photons the lightest thing we can ever find, or is there something else ligher, which can travel faster?

[wavelength?]

If E=MC2 and the speed of light slows down when passing through a medium such as water or the earths atmosphere then the photon should gradually gain mass as it slows it's velocity. and if a stars atmosphere such as the gas it's giving off or even its magnetic field could be considered a medium, then the photon should slow down and gain mass and gravity would begin to pull on it, therefor bending light.~