Almost a century ago, scientists discovered a cosmic mystery, the cosmic rays — charged particles zipping through space. But where do the most energetic cosmic rays come from?
Cosmic rays are charged particles that zip through space at high speeds. And one of the biggest mysteries in astrophysics today is where very high-energy cosmic rays come from. Stefan Westerhoff — a researcher at Columbia University in New York — told us that only about one hits a square kilometer of the atmosphere every year.
Stefan Westerhoff: And that’s pretty bad for a satellite — you can’t send up a satellite that has a square kilometer area. So you’re stuck on Earth. You have to build ground-based experiments.
What’s more, scientists can’t detect high-energy cosmic rays directly. That’s because they hit atoms and molecules high in the atmosphere — releasing a shower of other particles and light. It’s this light that Westerhoff detects in his current experiment.
Westerhoff is planning a new experiment — called GrandScan — that’ll search in the direction of the sky toward the possible black hole at the center of our Milky Way galaxy. His detector has to be far from city lights — where the only source of electricity will be solar energy. He’s now developing the electronics needed to run his experiment on stored solar power.
Cosmic Rays (a web site maintained by the HiRes collaboration)
“Eleven Physics Questions for the New Century” by Discover Magazine, February 2002 (See “Where do ultra-energy particles come from?”)
The lowest energy cosmic rays striking Earth come from the sun. But the sun can’t account for many of the cosmic rays out there. For one thing, there isn’t much difference in the amount of cosmic rays hitting our atmosphere in the daytime or the nighttime.
In the first half of the 20th Century, scientists didn’t have the large particle accelerators that they have today. But nature provided a laboratory of sorts to discover exotic new particles at high energies. Stefan Westerhoff says, “In the history of cosmic rays, a lot of particles that we know like the muon or the positron — the anti-electron — have first been discovered in cosmic rays at a time when it was not possible to build these huge accelerators like we have now at Fermilab or so on. So at that time, nature was our accelerator. We discovered all these particles in cosmic rays in the first half of the century. So it’s an interesting lab that nature provides.”
Westerhoff’s current project — based in Utah and called “HiRes” — looks at the entire sky — so it usually detects a couple of these showers each night.
Westerhoff plans to begin building GrandScan in a remote location in Australia in the summer of 2004. He expects it to take about two years to construct. Then, he expects to take data for five or more years.
Excerpts from an interview with Stefan Westerhoff:
Well these are the [challenges of doing this work] — the big collecting area and the fact that … future detectors like GrandScan have to be orders of magnitude bigger — so we will probably end up in an area far away from civilization. So in that case, the experimental challenge is that you have to build a detector where the camera will work on solar power alone. So you can — it has little environmental impact — so you can put it wherever you want without having to dig any electricity lines out there. So I think that’s the additional challenge for GrandScan. For HiRes that is not an issue because we are on a military base which already has electricity at the two sites.
Well, in the quest of where cosmic rays come from, it makes sense as the next experimental step to look at the sources that are probably the closest. So if you study stars, you would probably look at your own sun first. So we know that a large fraction of cosmic rays come from galactic sources — and the galactic center is the most energetic part of our Milky way — so it is very likely that the sources sit there. There is very weak experimental evidence for sources in the region of the galactic center, but there has been no dedicated instrument to look at high angular resolution at this part of the sky to actually detect sources and see if it is actually the black hole that we think is at the center of our galaxy or some source that is close by, so for that, you need to go to a location in the southern hemisphere with a detector that has the ability to resolve the region of the galactic center with very high precision. So that leads you to this effervescence technique I described which has a better angular resolution than building detectors on the ground.
So that would help to understand not only where galactic cosmic rays come from but where all cosmic rays come from because once we understand where within our own galaxy the closest — once we have identified one of the closest sources of cosmic rays, which we haven’t done yet, apart from our own sun, then we are able to look for this kind of source in other galaxies and understand where the highest energy cosmic rays come from. If you make a sky map right now of where all the cosmic rays we’ve measured in the last 10 years come from, the sky map is very isotopic and it does not seem to point back to any astronomical sources we know, so there’s no source we can point to at this point and say this is where cosmic rays come from.
We will apply this year and we’ll know then at the beginning of next year. There’s some initial funding already — I got a career grant form the National Science Foundation to build some electronics that work with solar power — so electronics for an effervescence camera that works on solar power, so the funding is already there and we are developing the electronics at NEVIS labs, which is the particle physics lab at Columbia — so there is initial funding for developing, for research and developing the electronics and since we now have a pretty good idea on building these electronics, we can go for a full proposal. And will be do that in a couple of weeks — and hopefully get fully funded next year then.
Whenever a new energy range is opened in astronomy, interesting things happen and new things are found. So this is currently the energy frontier in astronomy, these are the highest energies that you can go. Cosmic rays at the highest energies are the most energetic particles that we know in the universe — they are orders of magnitude higher than the particles that astronomers have dealt with so far.
And it’s also, for me personally, I have not studied astronomy, I am a particle physicist. This is exactly between the two fields — it’s a field called astro particle physics, which is where particle physics and astronomy are combined into one field, so it’s at the borderline between two established fields. But it’s not an established field yet because we have so little data. And to increase that data, to interpret that data is challenging. And so that’s what makes it interesting for us. Everything you do in this field has not been done before. So you’re not just measuring the mass or any property of particles that have long been known and now just have to be measured again with high accuracy. This is a field where whatever you do is something that no one has ever done before. And that’s exciting I think.
It’s a relatively small field. You want a number of people? Well, I’ll give you an example, modern physics collaborations — if you look at their papers — have something between two hundred to a thousand people as authors on their papers — and HiRes currently has about 30 people on it. It’s not a desktop experiment, but it’s a field where you can still make an impact and you’re visible and you don’t drown in the crowd of people working on it. These are small experiments so they don’t require a huge collaboration like modern particle physics experiments. A lot of people from particle physics join us these days and a lot of universities are starting programs in astro particle physics because it’s attractive for people and it’s also attractive for students to work not in a huge group but in a moderately sized group where your voice is still heard.