WEBVTT
00:00:00.000 --> 00:00:03.240
This is College Physics Answers
with Shaun Dychko.
00:00:03.460 --> 00:00:06.760
Suppose a proton and an anti-proton
annihilate,
00:00:06.820 --> 00:00:08.340
the question is what
00:00:08.940 --> 00:00:13.680
energy of gamma rays would we expect
to see from that annihilation?
00:00:14.080 --> 00:00:16.780
So there would be two gamma rays produced
00:00:16.780 --> 00:00:19.940
so 2 times the energy of
a single gamma ray.
00:00:20.060 --> 00:00:22.940
It's going to be 2 times the energy of
a single proton—
00:00:22.940 --> 00:00:25.560
the proton and positron energies
are the same—
00:00:25.980 --> 00:00:28.460
and so we can just divide both sides by 2
00:00:28.520 --> 00:00:31.920
and so the energy of the gamma ray
we expect to see
00:00:31.920 --> 00:00:33.920
is the energy of a proton.
00:00:34.220 --> 00:00:36.480
So the mass of the proton times
c squared
00:00:36.520 --> 00:00:41.500
is 938.27 megaelectron volts per
c squared times c squared
00:00:41.560 --> 00:00:46.120
gives an energy of 938.27
megaelectron volts.
00:00:47.280 --> 00:00:50.260
And this energy is not observed
00:00:50.300 --> 00:00:54.220
by telescopes very often
00:00:54.240 --> 00:00:58.860
which suggests that proton, anti-proton
annihilation is not
00:00:59.120 --> 00:01:01.180
common in the universe,
00:01:01.520 --> 00:01:04.380
which suggests that there's not
very much anti-matter.
00:01:05.500 --> 00:01:07.700
We are asked to compare this
with the energy
00:01:07.700 --> 00:01:09.620
you would expect in a gamma ray from
00:01:09.620 --> 00:01:14.180
electron-positron annihilation which is
0.511 megaelectron volts
00:01:14.340 --> 00:01:17.960
so we divide the 2 and this energy due to
00:01:17.960 --> 00:01:20.640
the proton, anti-proton annihilation is
00:01:20.700 --> 00:01:23.980
greater than the energy from
electron-positron annihilation
00:01:23.980 --> 00:01:27.700
by a factor of 1840.