Light is known to be massless in vacuum. However, when it enters a conductor, does it still have zero mass? In a transparent medium, the speed of light is less that c, which would imply that light acquired some mass. What is the mass of light when a conductor is accelerating?
1- If light behaves like a quantum particle inside a conductor, then its effective mass will be m=(1/2)*mu_0*hbar*sigma, sigma being the electric conductivity.
2- Under gravity the mass of light inside a conductor is mc^2=hbar*g/(2c), where g being the acceleration due to gravity.
The mass in 2 can be linked to that of Unruh and Hawking who relate it to thermal energy of the emitted radiation
(pi k_B T=hbar g/(2c)
Light may be "massless" in the strictest sense, but it still has energy and momentum, if E=hf, and E=mc^2, then the effective mass of a photon (even in a vacuum), is then m = hf/c^2.
Light may be "massless" in the strictest sense, but it still has energy and momentum, if E=hf, and E=mc^2, then the effective mass of a photon (even in a vacuum), is then m = hf/c^2.
Indeed, it has longitudinal inertia m = hf/c2 , but light deflection by the Sun shows that it has only transverse inertia m = hf/2c2.
The speed of light is decreased by a factor which is called refractive index.
Concept of mass comes from photons, and photons have zero rest mass.Thanks to the dual nature of light, this slowing of speed can be understood by the interference of incoming light with the newly generated light after its interaction with the medium or matter.
Light don't have types,its an EM field,which slows down in a medium not because it decelerate there but because its interaction(basically with E field) with the atoms and electrons of the medium which cause them to oscillate and thus produce a new wave front and all this process takes time thus make a propagating field to lag in its path.It is due to this process refractive index of any medium originates.I think concept that light has mass come from pressure exerted by a wave (radiation pressure).
Who are saying that the light do not have more forms, and the light its only EM field is negates the principle of matter-enegy’s conservation ! The photo electric phenomena is the best and spectacular demonstrations that the light is not only electric field!
Reg
Laszlo
Light travelling freely in flattish empty space at cBackground is usually considered not to have mass, because mass tends to be considered to be a property with known position(s), and if energy is moving at lightspeed, its location is difficult to pin down.
However, if the light is trapped inside a stationary mirrored box (effectively giving it an identifiable persistent location), then it contributes m=E/c^2 of equivalent inertial mass to the box:
- If you push against the box, the box shows an increased resistance to being accelerated, due to the transient increase in internal light-pressure pushing back against the side being pushed (inertial mass).
- If you place the box on a set of scales, the internal lightrays are always being deflected downwards by the background gravitational field, producing a greater internal light-pressure on the base of the box than the lid, causing the box to press more strongly against the scales (gravitational mass).
With a brief sufficiently-compact concentration of pure light-energy, we can also argue that the light should generate an enclosing gravitational horizon, creating a black hole made entirely out of self-trapped light (a "kugelblitz").
For intermediate cases where light is slowed but not 100% trapped (or where the reference scale is so large that lightspeed is proportionally quite slow, so that we can point to the location of the light-energy and say "There it is!"), the localised light-energy ought to be associated with an identifiable field. So if we point at a galaxy in the sky, we might expect the galaxy's internal radiant EM energy to contribute to its overall total gravitational field, and I suppose by analogy, if we have light moving through a block of glass (or slowed as it skims a star due to the Shapiro effect), we might expect some sort of increase in the gravitational field, due to the persistent localised increase in light energy-density over background.
Perhaps we can calculate the increase in light-density within a block of glass due to light-slowing, and then treat that excess (around ~50% over background?) as if it was permanently trapped within the block as extra localised energy.
For an accelerated glass block, I suppose we have to try to calculate the internal radiation-pressure difference.
.... PS, a potentially interesting gravitational nonlinearity ...
... If we have a known nominal mass, and a defined associated gravitational field, and we place them in a region of deep space, then the cosmological background radiation energy-density should be more concentrated in the region surrounding the mass, due to the region's slower speed of light causing a persistent concentration in EM energy over background. Alternatively, we can argue that the background energy-density should be uniform per unit volume, but that spactime curvature creates more spatial volume around the mass, giving the region more EM energy than flat background.
... this persistent excess should then contribute more massenergy to the region, making the field stronger than we'd originally calculated from just the initial known mass.
Light has a dual aspect. It is at the same time a corpuscle (the massless photon) and a wave (an electromagnetic wave). Some physical phenomena can be explained by considering only one aspect of light. The refraction is an example of that. Light refraction can be explained by considering only the wave nature of light. So, in my humble opinion the notion of mass is irrelevant in this case.
As Vinay and Vipul correctly pointed out, light inside a transparent material doe snot slow down per-se. It is only an apparent slowing due to internal interactions when the entering time and the exiting time are compared.
At the same time, as
Photon has mass at rest 10^-5 eV. See
I. A. Urusovskii. Multidimensional Treatment of the Observed Dependence of the Speed of Photon on Its Energy. / Journal of Modern Physics, 2015, 6, 1220-1226.
I think there are some misconceptions in this thread. In General Relativity the inertial and gravitational properties of electromagnetic radiation, or any other form of energy, are derived from the stress-energy tensor. Any kind of energy contributes to the gravitational field but that does not mean that the corresponding quantum (photon or whatever) is massive. Light follows the geodesic of space-time and, for that reason we have the phenomenon of light bending for rays grazing the Sun. You can partially deduce that from a massive photon with m=h v/ c^2 in a Newtonian theory but the result is incorrect (the correct result being the double). This is attributed to curvature (see Relativity- The Special, General and Cosmological by W. Rindler). From another point of view: radiation verifies the condition p=\rho/3 among pressure and energy density and this is true for a gas or ultrarelativistic particles as well. This can be derived from the stress-energy tensor both from the vacuum or a dielectric in which light travels slower than in vacuum. So, an interpretation of the reduction in light velocity in transparent dielectrics as a consequence of a massive photon is not in tune with standard Einstein-Maxwell theory. However, some people has pursued this approach in the past: See "Maxwell Equations, Nonzero Photon Mass, and Conformal Metric Fluctuation" by Kar, Shina and Roy, Int. J. Theor. Phys. Vol. 32 N. 4 (1993) in which they used De Broglie relation
m c^2=h f (1-vg2/c2)2 to attribute a mass to the photon. But in dispersive media, group velocity has been observed to be faster than the velocity c in vacuum (Nature 406(6793) by Wang et al.) and, then, the relation of Kar et al. may be misleading under certain circumstances.
I think that perhaps the more interesting question would be "what is the speed of light when interacting with a non-transparent body", and I am thinking specifically of a black hole.
Our current understanding is that the immense gravity of these object prevent light from inside the black hole from crossing the event horizon to the outside, but the theory fudges around what happens to the light that tries but fails to cross.
Is it held in a state of permanent stasis, just on the 'surface' of the black hole? This would mean that the speed of light is zero in this scenario and the photon density at the boundary would be infinite.
Does light / EM radiation not exist while it is inside the event horizon? Is it some kind of fifth state matter?
Does gravity whip light back into the bulk of the body at some superluminal speed?
Similar questions could be asked about light interacting with another non-transparent body - a perfect mirror.
I have never seen a speed of light measurement where it is not assumed that any mirror reflects the light infinitely fast. But is this really the case? More modern theories tend to suggest that mirrors reflect light by absorbing photons and then transmitting it. This is not a timeless activity. so how can we trust the speed of light measurements?
Some of Arbab's excellent extensions to Maxwell's EM equations may well provide an answer
Photon has mass at rest m = E/c^2, where the energy at rest is E = 10^-5 eV. See
I. A. Urusovskii. Multidimensional Treatment of the Observed Dependence of the Speed of Photon on Its Energy. / Journal of Modern Physics, 2015, 6, 1220-1226.
Interaction between elementary particles needs account of its motion in additional subspace. See
I. A. Urusovskii, RADIATION SPECTRUM OF THE HYDROGEN ATOM WITH THE ACCOUNT OF MOTION OF ITS ELECTRON IN THE EXTRA SPACE, Journal of Modern Physics Vol. 8, No. 12, 2017.
Best Regards
I. A. Urusovskii
Dear Igor - What you are describing is referred to as "virtual mass" or "mass equivalent". It is not mass per-se as the term is commonly defined at this time.
Meaning it has energy and one can use m = E/c^2 going "backwards" to calculate an mass equivalence.
For example, the relativistic momentum of a photon has to be calculated by p=h/lambda instead of p=mv/Sqrt(1-v squared/ c squared) whch would otherwise give a meaningless result of a division by zero.
Also, there is no such thing as a photon at rest. In fact a photos of actual [vacuum] speed of anything less then c would have zero energy, failing to meet any definition of existence.
Dear Arbab,
It is with lots of caution that one should talk of the properties of light.
Yes, if E = mc^2 , then your statement that m increases if c decreases seems reasonable, but since E = hf , this only results in an increase of frequency.
This seems to be acceptable: when one compresses the longitudinal scale of a fluctuating wave in a rope, one gets a denser number of cycles, and a slower velocity per (initial) unit of length.
Dear André,
You wrote: "Indeed, it has longitudinal inertia m = hf/c^2 , but light deflection by the Sun shows that it has only transverse inertia m = hf/2c^2."
That is indeed a possibility. However, I found another explanation for the double bending of light, as explained in the annexed excerpt of paper, paragraph 2.1.
The velocity-dependent, magnetic-like component of gravity causes it.
It follows that "m" for light is rather fictive, and certainly not Newtonian.
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