# Einstein's Mass-Energy Relation

1. Nov 27, 2013

### curious bishal

We know that the mass of any body turns to zero as it gains the velocity of light.
From Einstein's Mass-Energy relation,
E=mc2
so that,
m=E/c2
It is clear that mass is directly proportional to energy.
For a body to gain light's speed, we have to apply infinite amount of energy to the body to accelerate it. So, the mass of the body also increases in large amount till it attained the velocity of light. From this, it is clear that the body will have maximum mass just a moment ago when it gains the velocity of light.
As soon as it gains the velocity of light, how does all the mass change to zero? Doesn't it should have more mass?

2. Nov 27, 2013

### bapowell

Massive bodies simply cannot achieve the speed of light. Period.

3. Nov 27, 2013

### HallsofIvy

Staff Emeritus
Well, that's what comes of "knowing" something that is NOT true!

No, the mass of any body does NOT turn to 0 as it "gains the velocity of light". What is true is that no body with mass greater than 0 can "gain the velocity of light". It is only bodies that have mass 0 (photons, gravitons, etc.) that can travel at the speed of light to begin with (and they cannot travel at any speed lower than the speed of light).

4. Nov 27, 2013

### Naty1

Only massless objects, like photons, can travel at "c", as measured locally, as already posted. If you were to try to accelerate any particle with mass, no matter how much energy is used, the 'rest mass', better called 'invarient mass', remains constant and the particle will never attain speed 'c'.

A cornerstone of relativity is that all massive bodies [meaning observers, or a particle or object with mass] measure the local speed of light the same...."c". So no matter how fast, say your spaceship goes, light stills zips past you at the same old "c". So you can never 'catch up' with lightspeed!

example: You are on a rocket traveling at 0.8c relative to some observer: If you turn on a flashlight the lightspeed relative to you, the slight speed your would measure, [as well as the other observer] is incredibly, still the same old 'c'.

5. Nov 27, 2013

### nitsuj

That's a great point often not mentioned!

6. Nov 27, 2013

### PAllen

As I just said in another thread, the whole concept of mass increasing with speed of a body is obsolete and should be forgotten. Energy increases. Even more, the fraction of total energy that is kinetic energy approaches 100%. You can arrive at the photon limit by keeping total energy constant, and decreasing rest mass. Then speed approaches c, and when rest mass is zero, total energy = kinetic energy, which is true for a photon.