Mass, masslessness and the speed of light

[pero2912]

As far as I understood the relativity:
1. Massless particles travel at the speed of light.
2. As the speed increases, so does the mass of the moving body. It goes to infinite as the speed gets close to that of light.

My questions: Why would particles with infinite mass (theoretically) move at the same speed as the massless particles? What's the connection between these two kinds?

You may say, it is impossible to reach infinite mass, but anyway, why does mass of let's say electron moving at 90% speed of light gets very big, while the mass of the photon which travels at 100% of the speed of light is zero?

 

[micromass]

It becomes infinite as the speed reaches that of light.

It will never reach the speed of light.

My questions: Why do particles with infinite mass move at the same speed as the massless particles?

They don't because there is no such thing as infinite mass and particles with mass will never reach the speed of light.

 

[pero2912]

ok, it was wrong choice of words on my side. edited now.

 

[jtbell]

As far as I understood the relativity:
1. Massless particles travel at the speed of light.
2. As the speed increases, so does the mass of the moving body. It goes to infinite as the speed gets close to that of light.

You're talking about two different kinds of mass here.

1. refers to what is often called "rest mass" (which sounds silly for a particle that can only travel at the speed of light, but that's the way it is, for historical reasons) or "invariant mass". Most physicists call it simply "mass."

2. refers to what is often called "relativistic mass." Most physicists don't use it, except for a while during the early history of relativity. Nevertheless, it still appears in many books for laymen, and in some low-level textbooks.

 

[sardar]

I can provide an explanation for the relationship between mass, masslessness, and the speed of light based on the principles of relativity.

Firstly, it is important to understand that mass and energy are two sides of the same coin, as stated by Einstein's famous equation E=mc^2. This means that as an object's speed increases, its energy also increases, and this energy can manifest itself as mass. Therefore, the faster an object moves, the more energy it has and thus the more mass it has.

Now, according to the theory of relativity, the speed of light is the maximum speed at which anything can travel. This means that no matter how much energy an object has, it cannot travel faster than the speed of light. So, when an object with mass approaches the speed of light, its energy and thus its mass continue to increase, but its speed remains constant at the speed of light. This is because the object's energy is now contributing to its mass rather than its speed.

On the other hand, massless particles such as photons do not have a rest mass. This means that they always travel at the speed of light since they do not have any mass to increase their speed. They also do not experience the effects of increasing mass as they already have no mass to begin with.

To address your question about the connection between massless and infinitely massive particles, it is important to remember that these are theoretical concepts and cannot be observed in the real world. The concept of infinite mass is used to explain the behavior of objects approaching the speed of light, but it is not a physical reality. The connection between these two kinds of particles lies in the fact that both have a constant speed of light, but their mass and energy behave differently due to their different nature.

In summary, the relationship between mass, masslessness, and the speed of light is governed by the principles of relativity and the equivalence of mass and energy. As an object's speed approaches the speed of light, its energy and mass increase, but its speed remains constant. Massless particles, on the other hand, always travel at the speed of light as they do not have a rest mass.