E=mc^2

[DyslexicHobo]

I don't really have much knowledge of the subject, but I'm reading "The Elegant Universe". Blah blah blah, massless particles. Blah blah blah, E=mc^2. If m = 0, then E = 0?

This obviously isn't true (at least according to the string theory), if these massless particles are made of strings... unless that the strings had no "vibrations" at all, but then all massless particles would behave the same, which is again, obviously false.

What am I missing here?

 

[Gannon]

The truth is they don't have the equipment to measure it. E=MC^2 only applies to certain domains right now, its not a universal. Many quantum theorists are talking about superluminal light/photon/electron quantum models. The truth is matter is "really wound up" vaccum energy and "light", in certain patterns. This appears the most likely answer, this is why when you have matter - anti matter collisions all the energy is released (light) the nature of light is probably the biggest questions of modern physics and its very hard to answer without the proper equipment.

Using boolean logic, either energy exists or it doesn't, so if you have a "massless particle" what you have is something not well understood because you're approaching the question from the wrong direction... the question should be.... if energy is massless, why do particles have mass?

 

[f95toli]

No, it isn't true. The reason is that you are not using the full equation.

The full relation is

E^2=(pc)^2+(mc^2)^2

which simplifies to E=mc^2 if p (the momentum) is zero. A massless particle has an energy E=pc=hf.
(h being planck's constant, f the frequency).

Note that his also means that massless particles can transfer momentum (since p=E/c=hf/c), which is how solar sails work.

 

[CSMR]

There is a relitivistic equation E sqr(1-v^2/c^2)=m c^2. Here if you have zero mass you have zero energy, unless your speed is c, in which case the equation doesn't say anything.

 

[triacontahedron]

E=mc^2 does not mean that the only way to "store" energy is mass. It just implies that if you get somewhere an energy E you can create a mass m=E/c^2 or if you somehow converted a mass m into something else this new thing would have energy mc^2. There is not need to drag string theory into this, STR (special theory of relativity) is enough to explain this. At the times E=mc^2 was written down there were no string theory or even GTR.

On a separate subject:
STR is based on some postulates that can not be explained by STR itself. They can be probably explained by say string theory but string theories are based on some postulates themselves. At the end of a day, you have to start from something, so postulates of STR are not that bad, they seem reasonable at least. Strictly speaking "string theory" does not exist what people refer to as "string theory" is a bunch of hypothesis that are pretty much useless at the moment. There theories dont agree with each other on a number of important things , they dont really explain all particles well (it is impossible to derive properties of all particles from these theories due to non-linearity), they dont have any predictions that can be verified now. At the present state of things, "string theory" is a mind exercise, more math then physics.

 

[DyslexicHobo]

Ok, so I guess I really don't understand a lot of this even a little bit.

Where should I start out learning if I'm interested in this sort of physics? Should my path of learning be STR -> GTR -> String (?). How accepted is string theory in the world of physics? I've really only read a few books, and all of the books I've read enforced string theory as correct... then again, half of what I hear is in the words of Brian Green, and even though I don't know what I'm reading, I can tell that I need to take it with a pinch of salt.

 

[f95toli]

STR isn't very complicated and the math is quite simple, it is usually covered in the first courses at university.
However, GR requires some very complicated math if you want to understand it properly. GR is, in fact, NOT part of the usual curriculum when you study physics; only a small minority of all physisicts have a working knowledge of GR; mainly because it is simply not very useful unless you are working in astrophysics and/or cosmology (STR is much more usefull).

String theory is even worse than GR, there are probably only a a few hundred people in the whole world who fully understands string theory; it is very "exotic".

 

[silverpig]

Most universities don't even offer courses in string theory. You'd have to get on as a grad student with a string theorist and do research with them in most cases.

I took a GR class and you spend the first third of the class learning some new math and non-euclidian geometry without even touching any physics.