
Does the Inertia of a Body Depend upon its Energy-Content? (1905) [pdf] - utkarshs12
https://www.fourmilab.ch/etexts/einstein/E_mc2/e_mc2.pdf
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plopilop
An interesting point to note is that Einstein derived m = E / c^2 rather than
E = mc^2. Even though both formulae are obviously equivalent, the
interpretations may vary.

For instance, for nuclear fission we use E = mc^2. Uranium breaks into smaller
atoms, but some mass has disappeared: it's because it has been radiated in the
form of immaterial energy, hence nuclear plants and A-bombs.

On the other hand, m = E/c^2 gives an interesting interpretation to what
inertial mass is. According to this equation, mass is actually "energy at
rest". If you want to move your object, you have to give it some energy, so
that the resulting energy of the body will result into the desired motion.
It's kind of similar to how hot air (the energy you give) and cold air (system
at rest) mix to form "medium-hot" air (system in motion).

And just because I like writing, note that we have no idea whether inertial
mass (the m in m = E / c^2 and Newton's Third Law, F = ma) is equivalent to
gravitational mass (the m in F_gravitation = G * m_earth * m / d^2), but
experiments dismiss any difference bigger than 1 in 10^12. The principle of
equivalency between these two masses is the fundamental postulate of general
relativity.

In a nutshell, m = E/c^2 defines the inertial mass, and general relativity
assumes it's equal to the gravitational mass.

~~~
nonbel
I'm not really seeing the difference between m = E / c^2 rather than E = mc^2.
Isn't c just a proportionality constant that we can set to 1 by choosing
different units?[1] Then you are saying that E = m suggests something
different than m = E.

[1]
[https://en.wikipedia.org/wiki/Natural_units](https://en.wikipedia.org/wiki/Natural_units)

~~~
plopilop
You can think of it as a dual approach. We can consider "E is defined as mc^2,
so basically we can extract energy from mass". Alternatively, we can consider
"m is E/c^2, so inertial mass is actually energy at rest".

Both explanations are valid, which doesn't mean they are contradictory.

It's kind of like one person in the train sees the person on the ground
moving, even though from the person on the ground's point of view, it's the
person on the train who is moving. Who's moving? Well, the interpretation
depends on the referential you choose. So here, you can say there is a
referential "mass" and a referential "energy". They will give seemingly
different interpretations, but which are actually equivalent.

(On a side note, there are indeed no differences between E=mc^2 and m=E/c^2,
it's just that it's easier to conceptualize inertial mass as being energy at
rest with the latter.)

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CamperBob2

       It is not impossible that with bodies whose energy-content 
       is variable to a high degree (e.g. with radium salts) the 
       theory may be successfully put to the test.
    

One of the more understated assertions in history, there.

~~~
robotresearcher
Here's my favourite understated claim in a paper:

"It has not escaped our notice that the specific pairing we have postulated
immediately suggests a possible copying mechanism for the genetic material".

Watson & Crick, "Molecular Structure of Nucleic Acids: A Structure for
Deoxyribose Nucleic Acid" (DNA), having almost certainly discovered the
mechanism of heredity in complex life forms: one of the most significant
discoveries in the history of biology. Very dry.

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monochromatic
Is this a translation from German, or was it originally published in English?
I ask, because this is so awkwardly worded as to be nearly unparseable:

> The laws by which the states of physical systems alter are independent of
> the alternative, to which of two systems of coordinates, in uniform motion
> of parallel translation relatively to each other, these alterations of state
> are referred (principle of relativity)

~~~
Luc
This 1920's translation by Perrett and Jeffery is so bad you might as well
read the original (search 'Ist die Trägheit eines Körpers von seinem
Energieinhalt abhängig?')

It makes a lot more sense in German, even though I'm not that fluent in it. I
searched but didn't find a better translation. It's been over a hundred years
now, surely someone made the effort...

~~~
detaro
working link:
[https://de.wikibooks.org/wiki/A._Einstein,_Ist_die_Tr%C3%A4g...](https://de.wikibooks.org/wiki/A._Einstein,_Ist_die_Tr%C3%A4gheit_eines_K%C3%B6rpers_von_seinem_Energieinhalt_abh%C3%A4ngig%3F_-
_Kommentiert_und_erl%C3%A4utert%2E)

~~~
Luc
Thanks, the full stop at the end of that URL was causing trouble.

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PhantomGremlin
For those who read comments before reading the article, and to riff off of the
recent HN discussion of _Classic Papers: Articles That Have Stood The Test of
Time_

It doesn't get much more "classic" (or maybe that should say "relativistic")
than A. Einstein demonstrating E = mc²

See the "About the Document" at the end for the details.

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aisofteng
>Neglecting magnitudes of fourth and higher orders...

What if we don't?

~~~
idlewords
You get essentially the same answer.

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ahh
Einstein discovered a counterexample to Betteridge's law?

~~~
JshWright
Many academic articles are titled with yes/no questions. That how the whole
"hypothesis" thing often works.

