Are Relativistic Mass Calculations Still Relevant in Modern Physics?

[elemis]

Rest mass ? What on Earth ?!

I have a data table given to me by my examining body and it lists the REST mass of a proton and the REST mass of an electron along with their values.

My question is why do they specify the REST mass ? Does the mass of either sub atomic particles change when they start accelerating ?

 

[K^2]

Anything with non-zero mass becomes more massive when it moves. It's the special relativity in action. Relativistic mass is given by m_rel=γm, where gamma is the Lorentz boost factor.

\gamma = \frac{c}{\sqrt{c^2 - v^2}}

This is one of the ways to show that it is impossible to reach the speed of light. Force required to accelerate the particle diverges to infinity as v goes to c.

You can also use a more general equation to find the mass. m[rel]=p/v. This equation also works for objects whose rest mass is zero, such as photons. These always travel at the speed of light, and their relativistic mass is simply p/c.

 

[elemis]

Relativity... something that has yet to be taught to me in my course, but I've skimmed through it in my textbook.

Nonetheless good information to know :D

Is there a reputation system on this site ?

 

[K^2]

The reputation system on this site is reputation-based. :)

Special Relativity is very straight forward. If you have interest in particle physics, you really should catch up on it. Nearly everything in particle physics is relativistic.

 

[elemis]

The reputation system on this site is reputation-based. :)

No, you misunderstand. On other forums you can 'add rep' to a person to increase their 'rep points'. You get what I mean ? Is there something like that on this forum ?

Its General Relativity that's slightly more complex with some scary math, right ?

 

[Fredrik]

No, you misunderstand. On other forums you can 'add rep' to a person to increase their 'rep points'. You get what I mean ? Is there something like that on this forum ?

Its General Relativity that's slightly more complex with some scary math, right ?

The admins here haven't implemented reputation points. General relativity is much more complex, and uses lots of scary math. Special relativity can be understood without any scary math at all.

You need to know that there are two different conventions. Let E be the energy of a particle with speed v.Convention 1: Define the "mass" m, and the "rest mass" m₀ by

m=\frac{E}{c^2},\quad m_0=\frac{E}{\gamma c^2}Convention 2: Define the "mass" m by

m=\frac{E}{\gamma c^2}So the word "mass" can refer to two different things. Because of that, people sometimes choose not to use that word, and say "rest mass" when they mean E/(\gamma c^2), and "relativistic mass" when they mean E/c^2.

I think almost all physicists prefer convention 2. They also like to use units such that c=1, and in those units, E/c²=E, so there's no reason to call that quantity "relativistic mass". It already has a name, "energy".

 

[harrylin]

I have a data table given to me by my examining body and it lists the REST mass of a proton and the REST mass of an electron along with their values.

My question is why do they specify the REST mass ? Does the mass of either sub atomic particles change when they start accelerating ?

It is rare to see "rest mass" mentioned in tables of particles, as such tables always refer to properties of particles in rest. But the inertia of accelerated matter is increased: accelerated particles certainly behave more "heavy".

Now, it depends on what definition of mass one uses if it changes or not with acceleration. There is a battle between people who prefer "mass" to mean "relativistic mass" (popular with many teachers) and others who want "mass" to mean "rest mass" (popular in particle physics and in general relativity). Not all physicists agree with one camp. See for a discussion the physics faq:
http://www.xs4all.nl/~johanw/PhysFAQ/Relativity/SR/mass.html

 

[bcrowell]

Now, it depends on what definition of mass one uses if it changes or not with acceleration. There is a battle between people who prefer "mass" to mean "relativistic mass" (popular with many teachers) and others who want "mass" to mean "rest mass" (popular in particle physics and in general relativity). Not all physicists agree with one camp.

I think it would be more accurate to say that relativistic mass is old-fashioned. Working physicists today never use relativistic mass.

 

[harrylin]

I think it would be more accurate to say that relativistic mass is old-fashioned. Working physicists today never use relativistic mass.

You just called all physicists who use that concept "non-working"