After watching the video on Einstein’s Theory of Relativity,
many people have commented that the mass of an object does not actually increase
as the object moves faster, and that the equation E=MC^2
is therefore not actually valid for objects in motion.
The confusion surrounding this topic is due to what we mean by the word “mass”.
It is true that the gravitational mass of an object does not change.
That is, the gravitational attraction of an object does not increase
due to the fact that the object is moving close to the speed of light.
What does change is that the more an object’s speed approaches the speed of light,
the greater the force necessary to increase the object’s speed even further.
If a spaceship’s speed is equal to the speed of light, then an infinite force
would be required to increase the spaceship’s speed any further.
There are two ways to describe this phenomena.
One way is to say that as an object approaches the speed of light, it changes
the way in which it reacts to an applied force, but that its mass never changes.
Another way to describe this phenomena is to introduce the concept of
relativistic mass, and to say that the relativistic mass of the object increases.
Both approaches give all the exact same predictions,
so they are mathematically equivalent to each other.
However, the relativistic mass can never be used to
calculate the gravitational attraction of an object.
The mass that an object has when it is at rest is what we
call the object’s “rest mass”, and it is the rest mass that we
use to determine gravity, even when the object is in motion.
This is one of the reasons why many people feel that the
concept of relativistic mass should never be used.
Einstein initially used the concept of relativistic mass, but later regretted it,
and insisted that only rest mass should be used.
Advanced text books on Einstein’s Theory of relativity always only use rest mass.
However, many introductory text books and videos
still use the concept of relativistic mass.
If it is the relativistic mass that is used in the equation E=MC^2
then this equation is still valid for objects with mass that are in motion.
More information about Einstein’s Theory of Relativity
is available in the other videos on this channel.