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Christian95's avatar

Why E=mc2?

Asked by Christian95 (3260points) September 23rd, 2009

I don’t see what’s the connection between mass,speed of light and energy.I understand what it means but I don’t understand why is so.

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30 Answers

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gailcalled's avatar

You may have to channel Einstein from the great beyond.

You may not understand why earth’s gravity has apples falling off trees to ground rather than being launched like an ICBM. Make your peace with the laws of nature.

ragingloli's avatar

mass is energy in a different form. e=mc² is the formula to calculate how much energy you get when you losslessly convert matter into energy.

Christian95's avatar

@raqinqloli I understand what it does but i don’t understand why it’s E=mc2 and not E=me3(e=Euler number)

Darwin's avatar

Because that is how the Universe works. It just takes us a while to figure out the rules.

Or as some might say, because that is how God wants it to be.

gailcalled's avatar

Maybe because Einstein has better hair than Euler.

Christian95's avatar

I said Euler number just as an example

gailcalled's avatar

Not a good idea to use random constants in serious math equations, especially when they involve everything. Why the speed of light and not the speed of mice running away from cats?

Darwin's avatar

Because the speed of mice running away from cats is a variable, not a constant.

mattbrowne's avatar

Why are electrons unpredictable?

FrankHebusSmith's avatar

The reason you don’t understand it is the same reason it took one of the smartest men in the history of mankind to figure it out in the first place. It has a TON of calculations, proofs, assumptions, etc, behind it….. I would dare to say that less than 1% of the population today understands it, and that’s only because they have been taught by those who came before them. Most of us simply accept it as fact, much the way we have come to accept many other things as fact that would otherwise boggle our minds.

andrew's avatar

@Christian95 Have you looked at–energy_equivalence?

Sorry for all the crappy answers you’ve received so far.

Val123's avatar

I have a hard time understanding it too, because it’s not a literal equation. It can’t be! Nothing can go faster than the speed of light, much less light squared! But…it has to do with explaining the physical difference between mass, and energy, but at the same time showing how they are related. I think. I don’t know for sure. Now I have a headache, like I always get when discussing physics, and must lay down.
@mattbrowne Does that sound sorta right? (I mean, not the headache part….)

Val123's avatar

Sorry…I just realized that you understand what it means, just not why he used that particular formula. Well, using the speed of mice running away from cats is just silly. A better formula would be the speed at which a four year old eats 12.5 french fries. From Mickey D’s.

ragingloli's avatar
i read it and i still don’t understand it, as one would have to know einsteins entire theory of relativity to understand that equation. plus i suck at mathematics.

grumpyfish's avatar

In some ways, think of “the speed of light” as just another constant. Like absolute zero, or the planck constant, or something like that.

Notably, in @andrew ‘s link above:

Will explain the math behind it (quite simple, really)

finkelitis's avatar

Here’s the idea. Einstein’s observation was that matter and energy are two different forms of the same thing. So the next question is, if you transform matter into energy (i.e., via a nuclear reaction), how much energy do you actually get?

e=mc^2 or (energy) = (mass)(speed of light squared) is the equation that tells you the answer. The energy you get is proportional to the mass… you just have to multiply by the right constant. That constant turns out to be the speed of light squared. In other words, you get a lot of energy from a little bit of mass. Thus, the effectiveness of nuclear bombs (and the sun).

Does that help?

Strauss's avatar

@gailcalled “speed of mice running away from cats”

only if the cats belong to Schroedinger~!

mattbrowne's avatar

@Val123 – Yes. But if we’re looking for an ultimate answer, we would either have to ask the lawmaker of the universe or assume the universe/multiverse by itself simply had no other choice but to put the natural laws in place the way they are.

Of course the better our comprehensive understanding of all natural laws get the more we are able to explain particular parts as a result of more fundamental laws. A quantum theory of gravity might better explain why E=mc2. As explained by the Wikipedia article referred to by @andrew

“the concept of mass–energy equivalence unites the concepts of conservation of mass and conservation of energy, allowing rest mass to be converted to other forms of energy, like kinetic energy, heat, or light. Kinetic energy or light can also be converted to particles which have mass. The total amount of mass–energy in a closed system remains constant because energy cannot be created or destroyed and, in all of its forms, trapped energy has mass”

which means Einstein’s formula combines earlier know laws like the conservation of energy related to the first law of thermodynamics.

But we have to keep in mind that any theory describing the real world has to rely on a number of “axioms” i.e. stuff considered to be either self-evident. A given. Then there are “theorems” which can be deduced from “axioms”. Like for math, physicists want to come up with a minimal set of simple, beautiful “axioms” from which everything else can be deduced without contradictions. For the four elementary forces plenty of work has still to be done. Einstein’s general relativity and quantum mechanics contradict each other.

For the fundamental units (see SI, the international system of measurement) all we need are the following 7 units: meter, kilogram, second, ampere, kelvin, candela, mole. Everything else can be deduced for example volt, joule, watt, newton, pascal and so forth.

Val123's avatar

@mattbrowne Geez, Matt! Have you considered writing a book? :) The only thing I understood there was that “Kelvin, Candela, Joule, Watt, Newton and Pascal” would make good baby names. “Mole and Second” would not.

Darwin's avatar

@Val123 He does write books actually.

Val123's avatar

@darwin I know! I have it!

mattbrowne's avatar

@Val123 – A baby girl named Joule? Wow, that woman would be hot when she grows up. Pure energy!

gailcalled's avatar

Ohm y god, watt a bad idea.

ragingloli's avatar

forced pun is forced

gailcalled's avatar

And may the forced be with you, too.

Val123's avatar


grumpyfish's avatar

Watt a bad idea… Lord Kelvin would be a pretty cool name…

Lord Kelvin Grumpyfish has a nice ring to it.

dabbler's avatar

The squared thing has some parallels in kinetic energy where momentum is mass*velocity^2. It’s not that anything in the system is going the speed of light squared, but the embodied energy is correlated to velocity squared.

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