Explain E = MC2

Originally posted by twhitehead
After some thought I think my issue is with natural units, not with the equation itself. I realize that the idea of natural units is to make time and distance essentially equivalent, but there is of course a conversion factor c. Although I realize that it is constant (or appears to be) for our universe, I am still uncomfortable with simply setting it to 1 and eliminating it.

That's the initial reaction for everybody. Believe me, but once it sinks in that what we are doing with natural units is indeed more fundamental than using SI units that feeling of discomfort passes. It takes a while but it does happen.

Originally posted by adam warlock
That's the initial reaction for everybody. Believe me, but once it sinks in that what we are doing with natural units is indeed more fundamental than using SI units that feeling of discomfort passes. It takes a while but it does happen.

can you explain 'natural units'? Not sure what that means.

Originally posted by sonhouse
can you explain 'natural units'? Not sure what that means.

When distances are divided by c so that they are measured in seconds, just like time. Distances are measured in light-years, light-months, light-days, light-minutes, light-seconds. Time units.

Or, in a more general sense,

In physics, natural units are physical units of measurement defined in such a way that certain selected universal physical constants are normalized to unity; that is, their numerical value becomes exactly 1.

http://en.wikipedia.org/wiki/Natural_units

Originally posted by zeeblebot
how did einstein happen to toss in c when he could have set it to 1 and left it out?

I suspect because c is the only constant he knew of that actually has units.

Originally posted by adam warlock
That's the initial reaction for everybody. Believe me, but once it sinks in that what we are doing with natural units is indeed more fundamental than using SI units that feeling of discomfort passes. It takes a while but it does happen.

Not me. The first time I was introduced to natural units was the first day of my gr class. Soon after that point, we were knee-deep in calculations, so I did not bother to worry whether or not I was uncomfortable with natural units. In fact, for a system I had never really used before, I was comfortable with it, and rather glad that these were the units we were using, otherwise a lot of our calculations would have been unnecessarily complicated. After a little bit of practice, the units felt natural to me, and the implications of using such a system sunk in.

Originally posted by AThousandYoung
I suspect because c is the only constant he knew of that actually has units.

Or maybe not. Maybe because it was a new constant with units that he hadn't played with before.

Originally posted by amolv06
After a little bit of practice, the units felt natural to me, and the implications of using such a system sunk in.

Just like me. Kinda strange at first, but I could use it still; then I really got what those units were for.

Originally posted by zeeblebot
lucky guy, knows quantum physics AND a GF with 1500+!

but has no computer...

Originally posted by AThousandYoung
I suspect because c is the only constant he knew of that actually has units.

Well, it only makes sense to set c = 1 AFTER you realize that it's a constant. If you are describing a theory that first postulates that it is, it's perhaps a bit pretentious.

Originally posted by twhitehead
After some thought I think my issue is with natural units, not with the equation itself. I realize that the idea of natural units is to make time and distance essentially equivalent, but there is of course a conversion factor c. Although I realize that it is constant (or appears to be) for our universe, I am still uncomfortable with simply setting it to 1 and eliminating it.

It's not "set to one". It's divided by the speed of light - a constant.

I think.

@ the OP and twhitehead:

I have derived Einstein's formula, and stated my case for why c^2 should be considered a conversion factor on my newly started blog. Its a bit of a long read, but hopefully it clarifies my position, and answers the OPs question, perhaps in more detail than he wanted. I did this on a blog and not here as here I can not add any mathematical symbolism.

http://diaryofaphysicsstudent.blogspot.com/

Originally posted by amolv06
@ the OP and twhitehead:

I have derived Einstein's formula, and stated my case for why c^2 should be considered a conversion factor on my newly started blog. Its a bit of a long read, but hopefully it clarifies my position, and answers the OPs question, perhaps in more detail than he wanted. I did this on a blog and not here as here I can not add any mathematical symbolism.

http://diaryofaphysicsstudent.blogspot.com/

Using relativistic mass is a bit of an outdated practise. But it is required to arrive at E = mc². You could also derive the formula for relativistic mass from the principle of relativity together with the assumption that c is constant in all intertial frames of reference.

Originally posted by amolv06
@ the OP and twhitehead:

I have derived Einstein's formula, and stated my case for why c^2 should be considered a conversion factor on my newly started blog. Its a bit of a long read, but hopefully it clarifies my position, and answers the OPs question, perhaps in more detail than he wanted. I did this on a blog and not here as here I can not add any mathematical symbolism.

http://diaryofaphysicsstudent.blogspot.com/

Very nice blog. It's on my favorites.

Originally posted by adam warlock
Very nice blog. It's on my favorites.

Thank you very much 🙂

Originally posted by KazetNagorra
You could also derive the formula for relativistic mass from the principle of relativity together with the assumption that c is constant in all intertial frames of reference.[/b]

This is probably something I would have to work out. I've never actually seen the derivation for relativistic mass -- it was only presented as an experimental fact. I've heard it can be done, though. I'll look into it, and perhaps make a post about it some time.