What's Wrong With E=MC^2?

http://www.scientificblogging.com/hammock_physicist/whats_wrong_emc2

by jawngee • 17 years ago

41 22 17 years ago

zandorg• 17 years ago

I like how it uses Google to test an opinion: I do this for things like spell checking and phrase verification.

dakellog• 17 years ago

As someone with a Ph.D. in physics, I found the whole article wrongheaded and partly wrong. Sure, Einstein's original paper (I did read it in its original German) did not have E=mc^2 in the short form, but don't worry, since it is still true. If you compress a spring, it weighs more. If you add a photon to a box, it weighs more. The net momentum of both does not matter here.

cool-RR• 17 years ago

Not to detract from what you're saying, but just a note: The author has a PhD in physics as well.

TweedHeads4• 17 years ago

I am no physicist and would appreciate if somebody can explain why the speed of light is used everywhere in physics, specially in that E=mc2 formula.

What is the meaning of using light when calculating energy or mass?

And what if light is not the fastest thing in the universe, would we need to rewrite all formulas?

Excuse my ignorance, but this topic really picks my interest.

Anon842• 17 years ago

I am no physicist and would appreciate if somebody can explain why the speed of light is used everywhere in physics, specially in that E=mc2 formula.

This is a consequence of the invariance of the speed of light. You can pretty much derive Lorentz transformations (and a large chunk of special relativity) by simply forcing the speed of light to always be the same regardless of how fast you are moving with respect to the light source. (For a sketch see http://astro1.panet.utoledo.edu/~ljc/voigt02.jpg and http://en.wikipedia.org/wiki/Lorentz_transformation#From_phy... for a more detailed example).

A direct consequence of the form of the transformation factor \beta=1/(sqrt(1-(v/c)^2) is that you can't go over the speed of light without getting all sorts of imaginary (as in non physical) quantities.

thristian• 17 years ago

I also am no physicist, but I've read a bunch of books!

Picking 'the speed of light in a vacuum' does seem to be fairly arbitrary, almost as if physicists had decided to write equations in terms of 'the top speed of a stock 1972 Dodge Viper', but as I understand it, light really does have a special place in the scheme of the universe. Newton's laws are an excellent description of the way the universe works at the scales humans work with, but with extremes of mass, energy or velocity Newton's laws break down.

Einstein's theories have a number of consequences that seem strange to our limited human experience; things like time running slower for things moving at an appreciable proportion of c, the mass of fast-moving objects approaching inifinity as their velocity approaches c, and time running lower in the vicinity of very large masses. Despite how strange these predictions sound, they seem to match our observations of the world around us - for example, one of Einstein's early triumphs was that he was able to accurately predict the orbit of Mercury (deep within the Sun's enormous gravitational field) when Newton's laws could never get it quite right.

As for 'why light specifically', well, it isn't exactly - it's the speed of all electromagnetic radiation. Of the four fundamental forces in the universe, half (strong and weak nuclear forces) only work on atomic scales, and gravity works by its own special rules, leaving electromagnetism as pretty fundamentally important.

TweedHeads4• 17 years ago

Thanks for your response, but many things don't compute.

"time running slower for things moving at an appreciable proportion of c" - Time doesn't slow down, perception of time changes from the observers point of view.

Mercury's orbit is easily predicted with newtonian laws, again, perception of it changes due to the strong gravitational effect of the sun on it and its light we use to measure it.

In other words, I really don't accept things like time travel, infinite mass, getting younger if traveling at the speed of light and bullshit like that.

I don't know if they use that stuff to make it interesting to the unwashed masses but it is an insult to our intellect.

Going back to the speed of light as a measure of energy, I take as you say, it is the speed of all electromagnetic radiation, but why related to energy and mass?

And what if there is something faster? say like hypothetical tachyons?

ars3• 17 years ago

It's actually the opposite. The universe has a speed limit. And light moves as fast as it is possible to go.

As opposed to light itself being the decider of the speed limit.

Light is not the only thing that moves at the speed limit. Gravity does too. Magnetic and electric forces do as well (which makes sense since light is made out of a combination of both, but the point is that the individual elements also move at the speed limit).

Now, why did the universe pick that specific speed as the limit? No one knows.

TweedHeads4• 17 years ago

"The universe has a speed limit. And light moves as fast as it is possible to go."

My question has always been, how sure are we? Any room for error?

chrischen3• 17 years ago

So basically to be uber geeky you should start espousing e=mc2 + mv2/2?

Anon842• 17 years ago

Or something like this, even http://www2.wolframalpha.com/Calculate/MSP/MSP20811972138108...

(From http://www.wolframalpha.com/input/?i=Series[%28m0*c^2%29%2FS...] )

chrischen3• 17 years ago

holy expletive! Wolfram alpha can solve equations, show steps, and graph it? Wow I knew it could tell me the population of the world and stuff but didn't know it could do my math homework too. Screw maxima.

chrischen3• 17 years ago

It's so smart! http://www.wolframalpha.com/input/?i=are+you+stupid%3F

DougBTX• 17 years ago

That's only when the speed is much smaller than the speed of light (v << c). The one you are after is:

  E^2 = m^2 c^4 + p^2 c^2

(Set momentum, p, to zero and divide everything by itself to get E = mc^2 again.)

http://en.wikipedia.org/wiki/Energy–momentum_relation

xenophanes• 17 years ago

It's one character longer than necessary. Try E=MCC

lutorm• 17 years ago

I don't quite know what he's arguing against. The E that's mc^2 has always been the rest-mass energy. Hence what Doug wrote for the total energy

E^2 = m^2 c^4 + p^2 c^2

I'm not saying the article is wrong, but he sure could have explained more clearly what wrongheaded thing he's trying to debunk because I didn't get it. It sounded kind of like railing that E=mgh is "wrong" because it should be E=mgh+mv^2.

What's Wrong With E=MC^2?

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