April 30, 2010

R. wins 357-303

No bingos tonight, but Renae cleaned up with the Z, J, K, X, S (3 of them) and a blank.

I was at a gathering of physics teachers earlier this week and the teaching of modern physics at the high school level came up a few times during the meeting.  It was really cool to see that there are high school physics teachers who are able to cover topics like the photoelectric effect, Compton scattering, and of course, the meaning of E=mc2.

In conversation with a few teachers the topic of antimatter came up.  I've always been fascinated with the existence of antimatter (thank you, Star Trek) and the means of producing antimatter.

One of the ways of producing antimatter is through a process called pair production.  The pair production process is the transformation of a photon into a positron (the antimatter complement to the electron) and an electron.  The positron has the mass of an electron, but has a positive charge. The production of a positron-electron pair from a photon requires a minimum energy of 1.022 MeV be involved in the reaction.  The mass of the electron and positron are each 0.511 MeV/c2. So if all the of the energy carried by a photon is converted into the mass of the two particles, the required energy is just 2mc2, where m is the mass of the electron (remember the positron has the same mass).  If the energy involved in the pair production process is just at that threshold energy, then the electron and positron would be produced at rest.

Scientists (and especially physicists) love conservation relationships.  The pair production process conserves charge, since the photon is not charged and the electron-positron pair is oppositely charged. Energy is conserved as I was discussing above.  The real issue with pair production is the conservation of momentum.

I don't want to get into the math of the momentum conservation right now. But, if you think about our hypothetic case where the pair are produced at rest, there is a problem which should be obvious: before the production the photon had momentum, but if the pair is produced at rest, then they have no momentum.  This is BAD, since momentum conservation is a fundamental principle of physics.

The teachers and I were standing around talking about pair production and the conservation of momentum when one of the teachers said that he explained to his class that two photons were required to  produces an electron-positron pair.  The collision of two photons with opposing momenta satisfies the momentum conservation law.  Then another teach piped up and said that he had read a paper saying that in face FIVE photons were required.  I tried to explain that I was under the impression that the typical observation of pair production was done in the presence of a heavy nucleus which would account for the momentum conservation.

None of the teachers really seemed to believe my explanation and they probably didn't think that I was going to be swayed by their reasonings, either.  The next day I flipped through some textbooks to see if I could make sense of what we were talking about.   I only found the type of pair production that I was familiar with, and the question of photon-photon (or multi-photon) interactions was never brought up in any of the books I had with me.

So I went to google and did some digging.

The first thing I found was an astrophysics book which discussed photon-photon interaction leading to pair production. The relevant part of the text is on pages 127-8 of that book.  It turns out that photon-photon collisions leading to pair production provide a means of screening high energy gamma rays in some cosmic environments. (Gould & Schreder PRL 1966)

But I had to figure out if the five photon interaction claimed by the other teacher had any basis in reality. What I found was a paper by Burke et. al. in PRL from 1997. (If you aren't reading this from a place where you can see the full text of the article, I apologize.)  I haven't fully digested this article, but the relevant quote says:

"...the multiphoton Breit-Wheeler reaction  

becomes accessible for n ≥ 4 laser photons of wavelength 527 nm colliding with a photon of energy 29 GeV."
Whoa!  That's cool!  The point of that article was to show that while in cosmic sources, two photon collisions can produce electron-positron pairs, it has not been observed in a lab.  But, using one high energy photon and four (or more) photons from a laser, the pair production can be done in the lab.

What I learned was that all three of us standing around were right in what we each understood, but that our individual understanding of the topic was incomplete.  I'm so glad that I went and had that discussion with the teachers!

(The prediction of the existence of antimatter was a surprising result of the unification of Einstein's relativity theory with the burgeoning field of quantum mechanics in the earth 20th century.  This prediction was made by Dirac, a brilliant and eccentric theoretical physicist.  There was a book recently published on Dirac which I have not had a chance to read, but I'm linking to below.) 

April 02, 2010

Roundup of iPad review roundups

So there is a new device from Apple called an iPad.  If you want an opinion on what the iPad could do for you, you could either try to read Apple's marketing material on it, or you could look for reviews by people who have actually tried the device out.

Most of the people who have actually tried the device out and written about it are journalists (or really lucky geeks), but there are some journalists out there who have not had the opportunity to try out the iPad and write a review on it.

The next best thing to actually writing a review is to go out and review other people's reviews.  But, you don't actually critique the reviews.  You make a "roundup" which is a fancy way of saying a list of reviews.

But there are SO MANY roundups of iPad reviews!  Where to start?!  Never fear, I have a list roundup of iPad review roundups for you:

First is Macworld:
With the iPad a few days away, now begins one of the great rituals of the release of a new Apple product: the posting of the reviews from the lucky few journalists who were given a week to play with the product.

Here are the reviews we’ve found so far:
They then go on to quote 8 reviews. The infamous Stephen Fry gets a link, but no quote.

Then there is Buzzfeed:
Howdy, pardners, it's iPad review day! And all the tech cowboys out there are a hootin' and a hollerin'.
They got Fry's quote, but only a total of 5 reviews.  Photos from the reviews does not make up for the low numbers.  Slackers.

The Wall Street Journal's Digits blog has a roundup:
The reviews are in on Apple’s new iPad, and the consensus is no surprise:
I'm not going to ruin the surprise for you.  You can click the link and read their roundup.  It's a little excessive for the WSJ to have a roundup, since they were one of the news outlets to get a review unit.

Zatznotfunny has a roundup:
Yeah, there will be a disproportionate amount of iPad coverage this week. However, we promise not to run a single April Fools Day post. Fair enough?
Zatz not going to hide the fact that you only linked to five reviews as well.  Even a youtube clip at the top won't make up for your laziness.

Looking across the pond, the Telegraph has a roundup:
Here is what the critics had to say about the latest offering from Apple:
What a perfunctorily and typically British way to introduce a round up.  Where's the zazz!?!  No surprise, the Limeys link to Fry's review first.  Again, they could only dig up five reviews to include in their round up.  Lame.

So anyway, there's a bunch more roundups out there, but these are the ones that were at the top of my google search, so they are the only ones I actually skimmed.

Happy roundup reading!