Supernovae, neutrinos and photons, energy conservation, a little about star structure and general relativity.
NO MATH NEEDED!
If you are in high school you no doubt know the answer to this easy question:
A and B travel from C to D.
A and B start out from C at the same time, at the same speed. .
Which of them will get to D first?
That's easy you say. Of course they'll get there at the same time, anybody knows that. Well, what if I told you about an A and a B starting out from a C at the same time and at the same speed, and that one of them got to D six hours earlier than B. Before you ask me whether I failed algebra, I'll tell you about a special A and B and a special C. D is simple. It's down a mine shaft somewhere.
C is in outer space
I guess you've figured out by now that we are not talking about garden variety A’s, B’s or C’s. So I'll have to tell you about the ones I have in mind, one at a time. I'll start with C.
A supernova called SN1987A
The special place, C, where A and B start from is a supernova (SN) which exploded in 1987, called not surprisingly SN1987A. As you probably have heard, a supernova is a star which has exploded in a major way. 1A Supernovas are very interesting to astrophysicists because we know just how bright they are. That's because they all blow up in the same way. For instance, if you took 100 sticks of dynamite and blew them up, and compared the explosion to another 100 sticks of dynamite someone else blew up, you'd expect the explosions to be the same. And that's the way it is with 1A supernovas. If one of them looks dimmer than another, we conclude that it must be further away. If we know the distance to a close one, we can figure out how far away the dim ones are. Cool? Well, you have to do things like that in astronomy. You can't exactly use a ruler. In fact it takes 160,000 years for light to get from SN1987A to the earth. That is so amazing because it means that what we saw in 1987 happened 160,000 years ago. If you want to time travel (unfortunately only backwards), just look up at the sky.
A and B - Particles with (just about) no mass
Mass is what gravity pulls on to give something weight. You, an iron ball, a feather, whatever. With the same mass you’d weigh more on Jupiter and less on Mercury.
When I went to high school atoms were very easy to understand. They were made of electrons, protons and neutrons. Since then life on the sub-atomic level has become more complicated. New particles have sprung up like mushrooms – well, a lot more have been discovered. These days, there is a branch of physics that just deals with particles. In fact particle physicists are even predicting particles which no one has ever seen.
Well we’re not going to go on The Great Particle Hunt. We’re going to concentrate on two particles, A and B, remember them? Which started out from supernova C and one of them got to D (earth), SIX HOURS EARLY!
A is a photon and B is a neutrino. Here’s what you need to know about them to understand to understand why there was a six hour difference in arrival time.
A neutrino was the first “I can’t see them but they’re there” particles. It was needed to conserve energy. Energy conservation is very important! Without it, the world would not be the world. Well not the world we are used to. Physicists get dewy-eyed when they think of how nice it is that energy is conserved. In addition, they get down right nervous if they think it might not be. So maybe it’s no big surprise that they feel obliged to do something about conserving energy, if at all possible. In 1930, Wolfgang Pauli, an Austrian physicist, predicted that there would have to be a neutrino to conserve energy in a reaction called beta decay. Whew, we saved energy conservation. But you have to go underground with lots of equipment to actually see a neutrino.
Neutrino detector, courtesy of the Cambridge (England) MINOS Group, located in Fermilab USA
Why they are so hard to see
For starters, neutrinos have no mass to speak of*, and no charge like the electron and the proton. Neutrons have no charge either, but they feel ‘atomic strong’ forces. Neutrinos don’t feel that strong force. So they don’t interact with basic atomic particles. And that’s what everything is made of! They could go straight through you, and the entire earth for that matter. We detect neutrinos underground because we want to be sure that we are not seeing any other particle. All other particles would be stopped by the vast amount of material covering the underground detector.
* The neutrino does have a mass but it is extremely small - 100 million times smaller than the mass of the electron.
In the 19-th century light was shown to be made up of waves, which were reflected from mirrors and could be focused through lenses. The waves carried energy. The shorter the waves, the higher the energy. X-rays and radio waves and spot lights in a disco are all light rays. X-rays are shorter than the visible. Radio waves are longer.
Then in the early 20-th century an amazing property of light was discovered. Sometimes light acts like a wave, and sometimes it acts like a particle! The name given to the light particle is the photon. Charged particles interact by exchanging photons! So unlike neutrinos, photons are particles which interact with other particles.
Now we are getting to the important difference between photons and neutrinos. One of them interacts with other particles and the other doesn’t.
Photons and Neutrinos and Star Structure
Let’s assume that neutrinos and photons move at the same speed, the speed of light. If we don’t assume this we can stop right here!
When a star explodes into a supernova, it gives off photons and neutrinos. Now we are back to the old problem, A and B leave C at the same time and the same speed and reach D, but not at the same time. How can this be explained? Well it has to do with two factors, one of which I haven’t gone into yet.
1) The structure of the star.
You can consider the star as having two parts, outer and inner. The inner part is called the core which is very dense, where all the star’s energy is produced. The outer part or envelope consists of less dense gas. In a supernova explosion, the neutrinos come from the envelope, while the photons come from the core. Ah, you say, A and B both come from C, but their C is not EXACTLY the same place. Now we are getting somewhere. Not only is the core different from the envelope, it emits photons perhaps not at the same time as the envelope emits the neutrinos. So being emitted from the envelope versus being emitted by the core (which happens only when the envelope is blown away) makes for a difference in starting time, even though to get to us, they both have to travel 160,000 years!
2) Particle Interaction
We have to take into account that the photons in the core are being delayed in their progress outwards towards the earth, by their interactions with charged particles which are also to be found in the core. So instead of going straight out, their path may be similar that that of a drunk walking in a street filled with debris, knocking in to this and that while going forward. Even in space there are a few charged particles with which our photon will interact, zig zagging it’s path some more.
The little neutrino has no such problems. It just goes! It goes through space and doesn’t interact with the particles there any more than it interacts with the atoms in a rock or an ice cream soda.
Prof. Michael Longo of Michigan University* has done the calculation of the difference between the velocity of light versus the velocity of the SN1987A neutrino using all the big guns of general relativity. And he finds that the difference in velocities (divided by the speed of light) is less than 0.000000002 which is as close to zero as you can get. So my conclusion is that photons and neutrinos go at the same speed. Only the difference lies in the physics - that is, what actually happens in a supernova explosion, and what happens when the two particles do or don’t interact with the matter around them. This accounts for one of them being SIX HOURS EARLY.
*Longo, Michael J, in 1988 Physical Review Letters, volume 60 pages 173-175
The effects of General Relativity on Photons and Neutrinos
You may be surprised to know that gravity bends empty space. Einstein proposed this in his General Relativity Theory in 1916. General relativity is really hard to explain and has super hard mathematics. Even Einstein had to beef up his math to do it.
Simply, we can understand from it that space is not a smooth road. Space has ‘pot holes’ or distortions that are produced by the gravitation pull of stars and galaxies. So the neutrinos and the photons having the same speed also means that they go through the same pot holes on their way to earth.
A couple of good presentations on Neutrinos for high school students from PARTICLE 2005 at the University of Rochester
2) Neutrino detection
3) The Cambridge MINOS GROUP home page – a well known group doing Neutrino Physics
Most textbooks including modern physics will have a section on photons.