Probably the clearest example is antimatter.  Antimatter is very real; take for instance the case of pair production/annihilation.  If you have a photon of light that flies past the nucleus of an atom, it can suddenly turn its energy into matter, creating both an electron and a positron (an anti-electron).  Likewise, when a positron collides with an electron, both of their masses are converted into energy, releasing two photons of light.  There you have it: mass converting to energy via annihilation, energy converting to mass via pair production.

A neutrino is an extremely small (way smaller than the electron) uncharged particle.  There are many varieties of the neutrino.  An antineutrino is its antimatter counterpart, which also comes in many varieties.  So, in theory, a neutrino colliding with an antineutrino of the same variety would convert into energy just like the electron/positron pair.  But, the characteristic property of neutrinos is that they do basically nothing - the probability that they will interact is incredibly small.  So basically, for most calculations, it doesn't matter.

And as for matter/antimatter, that's just the way the world works.  Things are made  out of either matter or antimatter; both exist and make up what we call reality.

PaulovesPhysics wrote:

I know this is probably a basic question, but could someone out there explain the fundamentals behind string theory?  Try to explain it to someone who has completed the entry level physics courses in college, (kinematics, light, electromagnetism, waves etc...)

I suggest that you re-post this as a new topic.

xiaolong wrote:

M@Man wrote:

A neutrino is an extremely small (way smaller than the electron) uncharged particle.  There are many varieties of the neutrino.

But , in expriments, neutrino and electron are both dot particles. i mean ,if we say something is big or small,  it must have structure. but in expriments,we have not find neutrinos or electron has structure.

What Matt means by small is that it has an extremely small mass.

Six is more than one or a couple. Anything more than that is "many".

N i think our teacher told they are interconvertible...hence the relation

As far as this interconvertibility is concerned there is no consensus. Most physicists, including Einstein, claim that mass and energy are one and the same thing, i.e., not two different things that can convert into one another. Baierlein in "Newton to Einstein (Cambridge University Press, Cambridge, 1992), p. 324; or: Phys. Teach. 29, 174 (Mar. 1991)" even speaks of "the great heresy" when teachers speak of an interconvertibility of E and m. In his opinion, the source of this misconception is that the convertibility of matter and radiation is erroneously taken to imply a convertibility of mass and energy.
Here is another recent reference that is the best and all-inclusive one I know about the social and instructional aspects of the issue of "rest mass vs. relativistic mass; derivations of E=mc2; convertibility vs. equivalence of E and m":
M. Jammer, Concepts of Mass in Contemporary Physics and Philosophy (Princeton University Press, Princeton, New Jersey, 2000), pp. 51-89.

Last edited by fortaq (2006-04-06 15:40:44)

For those truly interested in the subject, you may want to check out Wheeler's Exploring Black Holes