Monday, April 13, 2009

Good Physics, Bad Reporting

This Wired article points out to an acceleration scheme for proton particle accelerator. This actually is new for protons, but not a new concept in general, because the electron-driven plasma wakefield acceleration has been demonstrated by several groups already a few years ago.

Still, popular media such as this should hire someone with at least a solid grasp of physics to write such articles. There are at least a couple of either strange statements, or outright errors in the article.

Building bigger proton accelerators, such as Fermilab's Tevatron in Illinois and the Large Hadron Collider in Europe, is still possible because protons can be accelerated in a circle. But electrons need linear tracks such as that of the SLAC National Accelerator Laboratory or the proposed International Linear Collider.

This, obviously, is wrong. Any intro physics student can see that. There's nothing inherently different between protons and electrons that allows protons to be in a circular accelerator while electrons ".. need linear tracks..". Synchrotron centers all over the world have circular storage ring for electrons, and LEP, the predecessor of the LHC at CERN, was a circular electron accelerator/collider. So the statement in that paragraph is utter nonsense.

But this strategy also has its limits. The maximum energy of the accelerated electrons depends on the energy of the particle bunches. SLAC currently produces the highest-energy electrons of any accelerator, at 50 gigaelectronvolts (GeV, or a thousand million volts).

This is nitpicking, but "a thousand million volts" is not a unit of energy.

Today's accelerators can bring protons to much higher energies than they can electrons. Protons at the Tevatron can hit 1 TeV (hence the name), and those at the LHC will be seven times as energetic.

This makes no sense either. It is so much easier to accelerator electrons than protons. So to claim that protons can be acccelerated to much higher energies than electrons is again, nonsense. What is often confused is the center-of-mass energy and also the energy per nucleon or parton.

While the intention is good that media outlets such as Wired are covering such issues, I wish they also pay attention to the accuracy of their article.



Anonymous said...

If "wired" or similar source asks, would you become their consultant?
on which terms? :)

ZapperZ said...

If they pay me enough money, sure!


notevenwrong said...

Your complaints about the Wired article are simply wrong.

1. In circular accelerators, energy loss due to synchrotron radiation goes up as the 4th power of the energy. For electrons, a circular accelerator of higher energy than LEP is impractical for this reason. Higher energy electron accelerator designs are all linear accelerators. Protons are much heavier, so this problem is not relevant until one gets to much higher energies.

2. In particle physicist's units, an electron volt (or a GeV) is a perfectly conventional unit of energy. And there's nothing wrong with telling people that a GeV is a thousand million electron volts.

ZapperZ said...

1. Er... read how the article was written. It implied that circular electron accelerator isn't done. THAT is what I was countering. I am certainly aware of why high energy electron colliders will need to be linear, since I work with advancing linear electron accelerator.

2. "In particle physicist's units", sure! I write "frequency" in eV as well. But this was NOT written for "particle physicists". This is an article for the general public, in which even simple terms can get very confusing. I have had to answer this exact question on many public forums where people got confused by the terminology and the units. So this is definitely a valid complaint.


Seth Zenz said...

I'm befuddled by your criticism of what actually appears to be an unusually well-informed article. The first two comments having been addressed above, let me address your last complaint.

The center-of-mass energy of the proton-antiproton system (or the energy of either particle in the lab frame) at the Tevatron is higher the equivalent values for any electron-positron collider ever built. It is universally understood to be a very great challenge to bring electrons and positrons to the same energy -- which is why the Tevatron has been built and the ILC and CLIC are still on the drawing board. The statement that "today's accelerators can bring protons to much higher energies than they can electrons" is entirely accurate.

ZapperZ said...

I think a lot of you are missing the point here. Read the article not from the perspective of someone who understands physics, but from someone who doesn't. For example, read this paragraph that I quote earlier:

"Building bigger proton accelerators, such as Fermilab's Tevatron in Illinois and the Large Hadron Collider in Europe, is still possible because protons can be accelerated in a circle. But electrons need linear tracks such as that of the SLAC National Accelerator Laboratory or the proposed International Linear Collider."

If I don't know any better, I'd think that electrons CANNOT be accelerated in a circular accelerator!

{actually, if we want to be TECHNICAL about it, the accelerating structures for electron, protons, and even heavy ions are all LINEAR. Those accelerating cavities (LINACs) containing the accelerating RF are all straight!}

And this applies to using "energy" in "volts". This is not a complaint if it was written in a physics paper. But the target audience here are not physicists, or are even familiar with physics.

Now, let's go back to that last complaint. Here's a fact: it takes A LOT more effort to accelerate protons than electrons. So electrons gain very high speeds very easily and very quickly. Still, if both electrons and protons are at 0.9c, a proton will have a larger energy, simply because it has way more mass than an electron. And that is a crucial point in which the article doesn't mention. It gives a misleading impression that it is difficult to accelerate electrons, which it isn't. Electrons move at higher speeds than protons given the same accelerating gradient. I had to explain that to a non-physicist after she read that article because she thought electrons move "slower" than protons!

To repeat, the article was not written for experts. It's written for the general public, and I read it under that light. How would the average Joe understand that article after reading it? I can see several places where it gives a misleading, if not outright errors, if someone doesn't know what is going on.


Seth Zenz said...

I'm afraid I think that you're the one considering the article at the wrong level. They are presenting accurate statements about the current challenges at the forefront of work on accelerators; those challenges are not intuitively related to basic physics because they don't arise from basic physics. Thus if a non-physicist makes up a simple model in their head about why the statements in the article are true, their model won't be right. But they'll still have learned some interesting stuff about what physicists work on and why!

Asking Wired to lead the reader all the way from non-relativistic electromagnetism to synchrotron radiation to current problems in accelerators is asking the impossible. They have to choose between presenting the world from the physicists perspective and giving a high school physics lecture, and I think they were wise to pick the former.

At the very least, you should be more careful to differentiate between these two things: (1) a person explaining different aspects of a complex problem than you would, and (2) that person being wrong.

ZapperZ said...

So you don't see anything wrong even with the phrase that I quoted being given to the general public?

Since I last typed my response, I gave the article to several people who are not physicists, and asked them in particular about that phrase. 4 out of 5 of them gave me EXACTLY the response I was anticipating - that electrons cannot be accelerated "circular" accelerator, only protons. And this comes from people (support staff) who work several yards away from a huge synchrotron center.

I have no idea what the intention of that phrase was, but it certainly gave the wrong information.


Anonymous said...

I'm really happy to see you encourage open dialogue. And since I'm not an expert in this field, and you are, I'll take your side.

Anonymous said...

nah, i think the problem areas hightlighted are properly critiqued.

electrons CAN be accelerated in circles. and a volt is NOT a unit of energy.

then again i ain't no high energy physicist. i am just a condensed matter physicist.

(we got inferiority complexes! dang high energy and astro dudes get all the attention.)

Tometheus said...

Fortunately they have corrected the major gaffe of "GeV, or a thousand million volts". eV has got to be one of the worst named units of measurement, considering how often people confuse it with Volts. ("Oh, it must just be another scale, like mV or kV" ... etc.) I don't blame Wired for that mistake, but us physicists. As for the linear vs circular issue, my first thought was "uh oh, I'd better flee the lab before the Advanced Photon Source finds out that it can't exist and destroys reality around it".