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Can we call matter without mass as energy? just a simple defintion to the word energy; massless matter= energy? and I'm not asking about photons.

Sensebe
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Farah Bh
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    Energy is the ability to do work. That definition won't take you far wrong. – AngusTheMan Aug 01 '15 at 13:18
  • Definition...Oh my friend, it is a difficult topic, going deep into philosophy (for example in math) with Russell, Wittingstein, Godel, etc. I can't make any statement as I am still thinking whether we can define anything at all! what I can suggest you is to read about the people I said above. – Sensebe Aug 01 '15 at 13:29
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    In physics, energy is a property of objects which can be transferred to other objects or converted into different forms, but cannot be created or destroyed.The "ability of a system to perform work" is a common description, but it is difficult to give one single comprehensive definition because it has many forms. – axelonet Aug 01 '15 at 13:58
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    To eloborate on what I am saying, read this real Feynman's statement: "We can't define anything precisely. If we attempt to, we get into the paralysis of thought that comes to philosophers... one saying to the other: you don't know what you are talking about! The second one says: what do you mean by talking? What do you mean by you? What do you mean by know?" – Sensebe Aug 01 '15 at 14:10
  • The "ability to do work" version is typical of an intro mechanics text and allows you to bootstrap your way through kinetic and various potential energies to understand the more general, but less articulated version that @AdityaBlaze is alluding to. – dmckee --- ex-moderator kitten Aug 01 '15 at 16:32
  • Check the answer to this question out. It will help you. – Omar Nagib Aug 01 '15 at 17:17
  • Related: http://physics.stackexchange.com/q/9731/2451 – Qmechanic Aug 01 '15 at 21:14

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Both in classical as in quantum mechanics, energy is the conserved charge (in the sense of the Noether's theorem) of a system (Hamiltonian) which is invariant up to time translations.

This is the most general definition of energy. It does not depend on the nature of the system (being either matter or radiation). Also note that at least in special relativity, mass and energy are basically the same thing (in the rest frame).

Noether's theorem and conservation of energy

sintetico
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  • still you are missing the key of my question; massless matter is it equal energy ? – Farah Bh Aug 01 '15 at 17:22
  • As I said in special relativity, mass and energy are basically the same thing. If you have energy, you have mass. Also, matter is everything which is not radiation. Matter essentially is what is build up of fermions (electrons, protons, neutrons, etc...). Everything else is radiation. What do you mean exactly with massless matter? – sintetico Aug 01 '15 at 17:27
  • Mass and energy are not the same thing in relativity, since we have the relation $E^2 = m^2 + p^2$. – Robin Ekman Aug 01 '15 at 17:34
  • @RobinEkman you are right, I was thinking about mass in the rest reference frame. – sintetico Aug 01 '15 at 17:39
  • @sintetico (If you have energy, you have mass) I don't think that's right since photons have energy but are massless – Farah Bh Aug 02 '15 at 13:52