Antimatter
Practice
practice problem 1
| pair | energy (MeV) |
frequency (ZHz) |
wavelength (fm) |
|---|---|---|---|
| electron- positron |
|||
| proton- antiproton |
|||
| neutron- antineutron |
solution
The mass of the proton and neutron are slightly different, so for best results use values that have at least 4 significant digits.
Find the voltage half of the energy in electronvolts using the masses of the particles in kilograms and the value of the elementary charge in coulombs. We're trying to make a pair of particles here, so double the masses in Einstein's most famous equation. Remember that the M in MeV stands for mega or 106.
| E = | 2mc2 | |
| qV = | 2mc2 | |
| V = | 2mc2 | |
| e |
Alternatively, you could look up the masses of the particles already given in electronvolts (or keV, or MeV, or GeV) then double those after getting them all into MeV if they aren't already.
| pair | energy (MeV) |
frequency (ZHz) |
wavelength (fm) |
|---|---|---|---|
| electron- positron |
1.022 | ||
| proton- antiproton |
1877 | ||
| neutron- antineutron |
1880 |
Use Planck's equation for the energy of a photon to compute its wavelength. Since were dealing with multiples of the electronvolt use the version of Planck's constant in eV s (or it's equivalent, eV/Hz).
|
⇐ | E = hf |
Remember that the Z in ZHz stands for zetta or 1018.
| pair | energy (MeV) |
frequency (ZHz) |
wavelength (fm) |
|---|---|---|---|
| electron- positron |
1.022 | 0.2471 | |
| proton- antiproton |
1877 | 435.9 | |
| neutron- antineutron |
1880 | 454.4 |
There are two ways to find the wavelength: using the frequency with the speed of light…
|
⇐ | c = fλ |
…or using the energy with the combined constant hc in eV nm.
|
⇐ |
|
In either case, watch those powers of ten and remember that the f in fm stands for femto or 10−15.
| pair | energy (MeV) |
frequency (ZHz) |
wavelength (fm) |
|---|---|---|---|
| electron- positron |
1.022 | 0.2471 | 1213 |
| proton- antiproton |
1877 | 435.9 | 0.6605 |
| neutron- antineutron |
1880 | 454.4 | 0.6597 |
Your table is now complete.
practice problem 2
Physicists working in Europe announced yesterday that they had passed through nature's looking glass and had created atoms made of antimatter, or antiatoms, opening up the possibility of experiments in a realm once reserved for science fiction writers. Such experiments, theorists say, could test some of the basic tenets of modern physics and light the way to a deeper understanding of nature….
By corralling clouds of antimatter particles in a cylindrical chamber laced with detectors and electric and magnetic fields, the physicists assembled antihydrogen atoms, the looking glass equivalent of hydrogen, the most simple atom in nature. Whereas hydrogen consists of a positively charged proton circled by a negatively charged electron, in antihydrogen the proton's counterpart, a positively charged antiproton, is circled by an antielectron, otherwise known as a positron….
According to the standard theories of physics, the antimatter universe should look identical to our own. Antihydrogen and hydrogen atoms should have the same properties, emitting the exact same frequencies of light, for example….
Antimatter has been part of physics since 1927 when its existence was predicted by the British physicist Paul Dirac. The antielectron, or positron, was discovered in 1932. According to the theory, matter can only be created in particle-antiparticle pairs. It is still a mystery, cosmologists say, why the universe seems to be overwhelmingly composed of normal matter.
- Which particle mentioned in the article has the charge incorrectly identified?
- How should the emission spectrum of antihydrogen compare to the emission spectrum of hydrogen?
- Identify one characteristic that antimatter particles must possess if clouds of them can be corralled by electric and magnetic fields.
- According to the article, why is it a mystery that "the universe seems to be overwhelmingly composed of normal matter"?
solution
- The author mistakenly stated the charge for the antiproton. It's negative, not positive.
- The emission spectrum of antihydrogen and hydrogen should be the same. It would be very interesting if they were different.
- Charge is the characteristic that makes it possible to corral antimatter particles with electric and magnetic fields.
- When matter is created, it is always created with antimatter. Given what we know now, the universe should be half matter and half antimatter — but it isn't.