Relativistic energy/momentum, massless particles

• zippideedoda
In summary, a pion at rest decays into a muon and an antineutrino. Using conservation of energy and momentum, we can find the energies and momenta of the muon and antineutrino. The muon has an energy of 109.78 MeV and a momentum of 29.79 MeV/c, while the antineutrino has an energy of 29.79 MeV and a momentum of 29.79 MeV/c. These values satisfy the conservation of energy and momentum, resulting in a total energy of 139.57 MeV for the decaying pion.

Homework Statement

A pion at rest decays into a muon and an antineutrino. The mass of the antineutrino is zero, find the energies and momenta of the muon and antineutrino. Mass of the pion is 139.57 MeV/c^2 and the mass of the muon is 105.66 MeV/c^2

Homework Equations

pion -> muon + antineutrino

(1) E=mc^2
(2) E=pc
(3) E^2 = (pc)^2 + (mc^2)^2

The Attempt at a Solution

Conservation of energy: E(pion) = E(muon) + E(antineutrino)
Using equation 1 and the given masses:
E(pion) = 139.57 MeV
E(muon) = 105.66 MeV (?)
so E(antineutrino) = E(pion) - E(muon) = 33.91 MeV
Use equation 2 for the massless antineutrino and get p = 33.91 MeV/c

since momentum is conserved and p(pion) = 0 (at rest),
p(muon) = -p(antineutrino) = -33.91 MeV/c

I don't think I'm right because if the muon has momentum it is moving and thus I can't find its energy by simply plugging its mass into equation 1.

Thanks for any help.

Last edited:
You are correct in that you can't use Equation 1 for the energy of the muon. You have to use Equation 3. Otherwise you are on target.

p(muon) = p(antineutrino) = p
E(antineutrino) = pc
E(muon) = sqrt[(pc)^2+(mc^2)^2]

plug these into conservation of E and solve for p
after some math I get

p = 29.79 MeV/c
so E(antineutrino) = 29.79 MeV
E(muon) = 109.78 MeV

The two E's add up to equal E(pion), 139.57

Looks good to me.

Your approach is correct, but there is a slight error in your calculation. The energy of the muon should actually be 33.91 MeV, not 105.66 MeV. This means that the momentum of the muon is also 33.91 MeV/c, and the momentum of the antineutrino is -33.91 MeV/c, as you correctly calculated.

This result may seem counterintuitive, but it is a consequence of the relativistic equations for energy and momentum. When a massive particle (such as the pion) decays into two particles, one of which is massless (such as the antineutrino), the energy is not divided equally between the two particles. Instead, the energy is distributed in such a way that the total energy and momentum are conserved.

In this case, the antineutrino, being massless, receives all of the energy (33.91 MeV) and momentum (33.91 MeV/c) from the pion, while the muon, being massive, receives a smaller portion of the energy (also 33.91 MeV) and momentum (33.91 MeV/c). This is why the energy and momentum values for the muon are lower than its mass would suggest.

Overall, your approach was correct, but just be careful with the calculations and make sure to double check your units. Keep up the good work!

1. What is the equation for relativistic energy and momentum?

The equation for relativistic energy is E = γmc^2, where γ is the Lorentz factor and m is the rest mass of the particle. The equation for relativistic momentum is p = γmv, where v is the velocity of the particle.

2. How does the energy and momentum of a massless particle differ from a massive particle?

A massless particle, such as a photon, has zero rest mass and therefore its relativistic energy and momentum equations become E = pc and p = E/c. This means that for a massless particle, its energy and momentum are directly proportional to its frequency and wavelength, respectively.

3. Can a massive particle ever reach the speed of light?

According to Einstein's theory of relativity, it is impossible for a massive particle to reach the speed of light. As a massive particle approaches the speed of light, its mass and energy increase, making it more and more difficult for it to accelerate further.

4. How does the concept of mass-energy equivalence apply to massless particles?

Mass-energy equivalence, as described by Einstein's famous equation E = mc^2, applies to all particles, including massless ones. This means that even though massless particles have no rest mass, they still possess energy and therefore have mass due to their motion.

5. How do massless particles interact with matter?

Massless particles, such as photons, interact with matter through the electromagnetic force. This means that they can be absorbed, scattered, or emitted by charged particles in matter. The energy and momentum of the massless particle will then be transferred to the charged particle, causing it to move or vibrate.