## The Energy – Mass Relationship

• Imagine you are sipping tea or coffee while discussing various issues with a broad and diverse network of students, colleagues, and friends brought together by the common bond of physics, graduate school, and the physics GRE.

htam9876
Posts: 12
Joined: Mon Mar 04, 2019 8:16 pm

### The Energy – Mass Relationship

One is Einstein’s energy – mass equation;
Another one is such representation as “for a massless particle, its energy E = pc” accoding to the energy – momentum equation.
But there should be no two kinds of energy – mass relationship in nature.

Liqiang Chen
Aug 26, 2020

phygre123
Posts: 5
Joined: Tue Jul 18, 2017 12:13 pm

### Re: The Energy – Mass Relationship

it is better to say hf or hc/lamda because p is also equal to mv so it is confusing.

htam9876
Posts: 12
Joined: Mon Mar 04, 2019 8:16 pm

### Re: The Energy – Mass Relationship

@ physicsgre123:
“...hf or hc/lamda...”
You are talking the special situation of photon. Yes, for a photon, E = hf or hc/lamda (in some articles, the frequency is denoted with the symbol “γ”). In quantum theory, the energy of photon is calculated that way.

“...p is also equal to mv...”
You meant that photon has mass. But some people insist that photon is a “massless” particle. Its momentum p is just p, not equal to mv, because if it’s mv, then p = 0, and perhaps E = 0 too. Then, no games can be played on.
Actually, piggy agrees that p = mv applicable to anything in cosmos: an elephant, a pig, a rabbit, an electron, a neutrino, a photon…There shouldn’t be two different conceptions for one thing in physics. If there is, that might be due to the flaw in theories.
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(Note: Piggy is talking the basic natural relationship between E and m, not limited to photon. Also, my thread of “how to understand the energy – momentum equation in a natural way” as well as the thread of “math vs physics” hint this topic.)