S U N D A Y   F O R U M             October 23, 2005

 

The debate goes on over the October 9th Commentary article on Einstein’s famous equation.

 
Yes, that MP3 player does lose mass when it plays
 
Given the often made claim that the general public is not interested in science, it is heartening to see the lively discussion (in last Sunday’s and Tuesday’s letters to the editors) engendered by the article by Brian Greene on Einstein’s famous equation E = mc² (Commentary, Oct. 9).
 
But who is right in this debate — are Greene’s examples really germane to Einstein’s "great idea" or are they even fallacious?
 
Greene’s central point is that this equation, published by Einstein 100 years ago, is not only about nuclear energy, atomic bombs and the like. In fact, Einstein couldn’t know at the time how soon his equation would be confirmed by nuclear processes.
 
Instead, what the equation says is that every form of energy (nuclear, chemical, electrical) contributes to the mass (inertia) of the object that holds that energy. And vice versa, mass can be converted into energy given the right circumstances (unfortunately, not at will, or we all could power our cars and homes with a few extra pounds of weight we are carrying around).
 
The difference between nuclear and, say, chemical processes is simply that the energies involved in the former are a million times larger than in the latter, which leads to a noticeable change in mass if a nucleus undergoes a reaction. Because of the huge conversion factor c², even the sizeable energy involved in burning fuel in your car (much less the puny one needed to run a MP3 player or to think) leads to such a minuscule change in mass of the reaction partners that it would be exceedingly hard to demonstrate this change directly.
 
That’s why the examples in the sidebar to Brian Greene’s article may not have been chosen very wisely from a pedagogical point of view — but they are indeed correct. It’s important to understand that even in most nuclear reactions, matter is neither created nor destroyed. The fragments of a uranium nucleus after fission contain among them the same number of protons and neutrons as the original nucleus. What changes is just the total mass of all the parts taken together.
 
This is just one other revolutionary aspect of Einstein’s equation: Mass is not equivalent to "amount of stuff" but rather a property of matter that can (and will) change, just like speed. However, there are subatomic reactions where matter can be made to disappear and pure energy appears in its stead — for instance, the mutual annihilation of electrons and their antiparticles, the positrons.
 
And vice versa, new particles (matter) can be created from sheer energy in the most violent collisions inside particle accelerators ("atom smashers", like the one we have at Jefferson Lab in Newport News).
 
This is probably the most impressive, but by no means the only, confirmation of Einstein’s 'great idea," E = mc².
 
Sebastian Kuhn
Professor of Physics, Old Dominion University
Norfolk
 
Author of Einstein article really is an expert
 
I got one of my biggest laughs last Sunday — not from the comics, but from the letters questioning Brian Greene’s article stating that E = mc² is behind everything we do.
 
One of the letters said that "Prof. Greene needs to go back and do his homework."
 
In my frustration, I contacted Greene, a leading professor of physics and mathematics at Columbia University. I attended his talks last year when I lived in New York City and taught science education at Wagner College.
 
Greene’s critics claim that E = mc² only describes nuclear reactions, not the gasoline in your car being converted into energy or the food in your stomach.
 
But the point Greene makes in his article is that Einstein’s equation is universal — it is at work in any situation in which there is an energy exchange. For the case of gasoline, as Greene emphasized to me in a recent e-mail exchange:

"The point is that the conventional description, which people b e l i e v e d prior to 1905, claims that mass is conserved — the energy released comes from the rearrangement of chemical bonds, with no change in mass.
 
"After E = mc², however, scientists realized that the energy in those chemical bonds contributes to the mass of the gasoline. That is the burden of energy and mass being equivalent. Change the energy in the bonds and you thus change the gasoline’s mass.
 
"The pre-1905 description ignores this. That is a wonderfully accurate approximation since the mass contribution of the bonds is E = mc² (E = bond energy), which is typically very small. But it is an approximation nonetheless. If energy is released, mass necessarily is decreased."

I wonder if Greene’s critics have ever read one of his books, The Elegant Universe or The Fabric of the Cosmos, or if they’ve ever watched his PBS special, also titled The Elegant Universe, and if they realize that he is one of only a handful of physicists in the world who are working toward a unifiedfield theory of the universe. Even Einstein didn’t achieve that in his lifetime.
 
Greene is an expert of the "superstring theory" or "string theory", which may enable physicists to unify quantum physics and astrophysics, which at the moment seem to abide by two very different sets of physical laws. His critics are the ones who need to do their homework.
 
Clair Berube
Old Dominion University
Norfolk