Information Theory and Intelligent Design.

The correct way to state it is that photons have zero rest mass. Also, by definition in relativity, photons are never at rest. As a result they always have a mass (which is why light sails are possible, and partially why the tails of comets always point away from the sun).The mass of a photon is given by the equation m = h/(cw), where h is planck's constant, c is the velocity of light, and w is the wavelength of the photon.A more telling counter example to the claim that because the unit of information is massless, therefore information is not physical is to point to the 25+ dimensionless (ergo massless) constants in current physical theories, including the fine structure constant:
Dimensionless physical constant - WikipediaIf the most fundamental constants of physics are not physical, we are using a very strange definition of physical.

I learnt my relativity from popular books and articles, so I have been surprised to find that you are right as regards the technical jargon. In particular, I found these articles to eloquently expound your view:What is the mass of a photon?
Relativistic Mass
If you go too fast, do you become a black hole?However, I wish to raise three quibbles. First, both formulations "mass equals rest mass" and "mass equals relativistic mass" are formally equivalent. The decision as to which to use is a matter of convenience rather than of fundamental disagreement.Second, having said that, I notice this:

If we now return to the question "Does light have mass?" this can be taken to mean different things if the light is moving freely or trapped in a container. The definition of the invariant mass of an object is m = sqrt{E2/c4 - p2/c2}. By this definition a beam of light, is massless like the photons it is composed of. However, if light is trapped in a box with perfect mirrors so the photons are continually reflected back and forth in the box, then the total momentum is zero in the box's frame of reference but the energy is not. Therefore the light adds a small contribution to the mass of the box. This could be measured - in principle at least - either by an increase in inertia when the box is slowly accelerated or by an increase in its gravitational pull. You might say that the light in the box has mass but it would be more correct to say that the light contributes to the total mass of the box of light. You should not use this to justify the statement that light has mass in general.

http://math.ucr.edu/...physics/Relativity/SR/light_mass.htmlI don't see why we should not use this example to justify the claim that light has mass in general. It clearly shows that while "mass equals rest mass" is convenient for some purposes, "mass equals relativistic mass" is convenient for others. The description of this case using the later convention is intuitive and informative, while that using the former convention (no matter how convenient in other contexts) is obtuse, indeed baroque.Which leads into the third quibble. The reason discussion of the light box is intuitive using the relativistic mass convention is because we are discussing an emperical case. In fact, SFAIK, only relativistic mass can ever be measured. Rest mass must always be calculated, based on the measured value of relativistic mass and other factors such as relative velocity, accelerations, etc. The significance of this is that, if we were to use a mass criterion of physicality, the only such criterion which could make sense is one based on a measurable mass, ie, relativistic mass. In other words, you are correct about the definitional conventions of physicists regarding mass. None-the-less, you are wrong about the definitional convention on mass used by Meyer in his physicality implies mass criterion. Using the convention Meyer must be using if his criterion is to be even coherent, photons do have mass.

In this context it is worth while reading Gordon Davisson's post of the month for October, 2000, at talk.origin. The most relevant conclusions:

First: the information-theoretic entropy (or Shannon-entropy) of a physical system contributes to its thermodynamic entropy, at the rate of 1 bit of Shannon-entropy => k (Boltzmann's constant) * ln 2 = 9.57e-24 Joule/Kelvin of thermodynamic entropy.

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Second: my first conclusion above doesn't really matter because under realistic conditions, the information contribution to thermodynamic entropy is so small that it can be safely ignored. For example, one terabyte (243 bits) of information corresponds to only 8.42e-11 J/K of thermo-entropy, which is the same as the entropy difference between 1 cc of water (about a thimblefull) at a temperature of 300 Kelvin (about room temperature) and the same amount of water at a temperature of 300.000000060 Kelvin (also pretty dang close to room temperature). This is a huge amount of information, but a negligible amount of thermo- entropy.

The Talk.Origins Archive Post of the Month: October 2000I leave as an excercise the determination of the relativistic mass increase due to increase the temperature of a gram of water by 6 hundred millionths of a degree kelvin.I believe that physical entropy only has units of joules/degrees kelvin for convenience. Expressed in units of kilograms/meters/seconds, it is dimensionless, because temperature is a measure of energy density. That means though a mass increase is involved in this example, however, small, it need not be involved in all examples of entropy increase.If I am correct about this, that means entropy is yet another "spiritual property" (according to Meyer) which is central to the concerns of physicists.