Finding Quality Video Lectures: Feynman and Beyond

[arindamsinha]

There does not seem to be any real 'exact' proof of E = mc². This was a very good and logical approximation made by Einstein in his 1905 paper "Does the Inertia of a Body depend on its Energy-Content?"

No substantial experimental proof has yet appeared to disprove this, so we must assume this is close to correct (perhaps similar to accepting that Newtonian gravitational theory is close to correct, till we learned better). There is the possibility that there may be additional correction factors with small influence which have not yet been discovered.

I have seen some claims that this has been proved 'exactly' later on, using 4-vectors etc. in GR, but from what I can make out, those depend ultimately on the assumption of correctness of this equation itself.

I am no real expert in relativity theory, so the above is based on my limited knowledge and understanding.

 

[DrGreg]

Then we have an interpretation problem with special relativity.
Usually we say, 'one reason speed faster than light is not possible is mass of the
object will become so high it would require all energy of the universe to move'.
This statement then is not correct. The actual mass the energy would work on
is always the rest mass, increased mass is just its energy.

You just need to argue in terms of energy instead of mass. As I hinted in the tiny footnote to my last post, the correct relativistic formula for kinetic energy isn't \tfrac{1}{2}mv^2, it is really<br /> mc^2 \left( \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} - 1 \right)<br />where m is rest mass, which approximates to \tfrac{1}{2}mv^2 when (v/c) is very small. So an object with non-zero rest mass would have infinite kinetic energy if it could travel at the speed of light -- impossible.

 

[DrStupid]

For example, mass of the moon we use for gravitational pull may not be its rest mass, but rest mass + energy from its motion.

Gravitational mass of a moving body does not necessarily be equal to its inertial mass. The weak equivalence principle is limited to bodies at rest.

 

[krash661]

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

The cosmological constant

is equivalent to an energy density in otherwise empty space.

 

[shivaniits]

No, nuclear fission does not convert mass into energy.
It releases binding energy of a nucleus.

To me so far, E=mc² is a conversion factor.
Conversion of Kg into Joules.

ok so nuclear reactions don't produce something from nothing but they however converting the electron positron pairs from neutrons that is something that resides in nucleus and the relation e=mc^2 is just a conversion factor to judge the energy of a body...! can i say this by summing all..??

 

[shivaniits]

Here is a dumb thought.
Say, I have a ball in my hand of mass 1 Kg.
I throw it with speed 20m/sec.
Its kinetic energy is 200J. Mass equivalent of this energy is
2.22x10^-15 Kg.
Can we say mass of the ball is increased by 2.22x10^-15 Kg.
In another words, mass of the moving ball is 1+2.22x10^-15Kg, and it will act like a ball of mass 1+2.22x10^-15Kg.
Is this the mass increase we see in special relativity caused by speed?
In such case, it is not a physical increase of mass at all.
A.

oh yes that i was asking all along..! that i want to point..as if how one could explain such conversion...??