### Re: VRD: Neutrinos and relativity

Date: Wed Jun 14 16:30:58 2000
Posted By: Meghan Gray, Grad student, Astronomy, Cambridge University
Area of science: Astronomy
ID: 960839276.As
Message:
```
Hi Julius,

When physicists talk about mass they almost always mean the "invariant
mass".  The invariant mass of a particle will have the same mass measured
in all inertial reference frames, and is sometimes also referred to as
the rest mass.  This won't change no matter how fast the particle is
moving.

When we bring relativity into it, we can write

p =       m v
---------------
sqrt(1-(v/c)^2)

where p is momentum, m is the invariant mass of the particle, v is the
velocity of the particle, and c is the speed of light.  You can see that
as v approaches c, the relativistic momentum of the particle approaches
infinity.

Now you may also see reference to the "relativistic mass" (which is what I
think you're referring to by "effective mass"):  m' = m/sqrt(1-(v/c)^2).
You get this by simply separating out the right hand side of the above
equation, and by that reasoning m' does indeed become infinite as v
approaches c.  However, this notation is not popular and is actually
actively discouraged in my relativity textbooks as being misleading.

Quite a lot more discussion of this matter can be found at
http://www.weburbia.com/physics/mass.html

Cheers!
Meghan

[Moderator's note: the neutrinos from supernovae can have a lot of energy - in
fact they take a lot of the supernova's energy with them when released. But
because neutrinos are only "weakly interacting" with baryons (the stuff we're
made of), we don't notice them - and it's actually very difficult to detect
them!]

```

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