|MadSci Network: Physics|
Alfred, Thanks for the question. The definitions of energy and mass can be tricky when we look closely at them. What I state here will certainly be considered incomplete or inaccurate by some, but at least it will give you an idea of what some of the issues are. 1. Energy is the motion or frequency of matter. Energy does not necessarily require the presence of matter or mass to exist. One example is a static electric or magnetic field. Such a field can store energy and we can talk about the energy density of the field. 2. Photons have zero rest mass. Another example of energy without matter or mass is electromagnetic radiation (photons.) It is confusing to talk about rest mass for a photon however, since photons only travel at the speed of light c. It is better to say that the invariant mass of the photon is zero. Also, to add to the confusion, a SYSTEM of photons with zero net momemtum could be considered to have a nonzero invariant mass. An illustration of this is the system composed of the two oppositely directed photons created when an electron and positron annihilate. From the perspective of general relativity it is possible for two massless photons to combine to form something that has a mass. Take a look at this link for a better explanation of what is going on: http://math.ucr.edu/home/baez/physics/light_mass.html 3. Neutrinoes have recently been found to have mass. Actually, the jury is still out on this one. It appears probable that neutrinoes have a small mass, but the results really need to be confirmed by other experiments. There are other probable massless particles however. An example would be the graviton, the carrier of the gravitational force. 4. Is there any form of energy which has no mass? From the examples mentioned I would say that energy does not have to be associated with mass. I think that what confuses many people learning about physics and relativity is the famous relation E = m c^2. The fact that an equivalence between mass and energy can be defined does not mean that a given amount of energy can be considered as a related amount of mass, at least not in all circumstances. This is particularly true with regards to the photon. Please see this related link where I have done most of my research on this topic: http://math.ucr.edu/home/baez/physics/mass.html The relationship between mass and energy seems to be fairly complicated once we enter the realms of special and general relativity. I would caution you to take explanations provided by any popularization of physics with a grain of salt. There are always many more details that the authors don't have time to discuss. Regards, Everett Rubel
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