Is That a Particle Accelerator in Your Pocket, Or Are You Just Happy to See Me?

By Sean Carroll | October 4, 2006 5:02 pm

The Large Hadron Collider accelerates protons to an energy of 7000 GeV, which is pretty impressive. (A GeV is a billion electron volts; the energy in a single proton at rest, using E=mc2, is about 1 GeV.) But it requires a 27-kilometer ring, and the cost is measured in billions of dollars. The next planned accelerator is the International Linear Collider (ILC), which will be similarly grand in size and cost. People have worried, not without reason, that the end is in sight for experimental particle physics at the energy frontier, as it becomes prohibitively expensive to build new machines.

That why it’s great news that scientists from Lawrence Berkeley Labs and Oxford have managed to accelerate electrons to 1 GeV (via Entropy Bound). What’s that you say? 1 GeV seems tiny compared to 7000 GeV? Yes, but these electrons were accelerated over a distance of just 3.3 centimeters, using laser wakefield technology. You can do the math: if you could simply scale things up (in reality it’s not so easy, of course), you could reach 10,000 GeV in a distance of about a hundred meters.

The LHC and the ILC won’t be the end of particle physics. Even the Planck scale, 1018 GeV, isn’t all that big. In terms of mass-energy, it’s only one millionth of a gram. The kinetic energy of a fast car is of order 1016 GeV, close to the traditional grand-unification scale. (Why? Kinetic energy is mv2/2, but let’s ignore factors of order unity. The speed of light is c = 200,000 miles/sec = 7*108 miles/hour. So a car going 70 miles/hour is moving at 10-7 the speed of light. The mass of a car is about one metric ton, which is 1000 kg, which is 106 grams, and one gram is 1024 GeV. So a car is 1030 GeV. [Or you could just happen to know how many nucleons/car.] So the kinetic energy is that mass times the velocity squared, which is 1030*(10-7)2 GeV = 1016 GeV.)

The trick, of course, is getting all this energy into a single particle, but that’s a technology problem. We’ll get there.

  • nc

    ‘… 10^16 GeV, close to the traditional grand-unification scale…’

    Unification is often made to sound like something that only occurs at a fraction of a second of the BB:

    Problem is, unification also has another meaning: that of closest approach when two electrons (or whatever) are collided. Unification of force strengths occurs not merely at high energies, but close to the core of a fundamental particle.

    The kinetic energy is converted into electrostatic potential energy as the particles are slowed by the electric field. Eventually, the particles stop approaching (just before they rebound) and at that instant the entire kinetic energy has been converted into electrostatic potential energy of E = (charge^2)/(4*Pi*Permittivity*X), where R is the distance of closest approach.

    This concept enables you to relate the energy of the particle collisions to the distance they are approaching. For E = 1 MeV, R = 1.44 x 10^-15 m (this assumes one moving electron of 1 MeV hits a non-moving electron, or that two 0.5 MeV electrons collide head-on). OK, I do know that there are other types of scattering than the simple Coulomb scattering, so it gets far more complex, particularly at higher energies.

    But just thinking in terms of distance from a particle, you see unification very differently to the usual picture. For example, experiments in 1997 (published by Levine et al. in PRL v.78, 1997, no.3, p.424) showed that the observable electric charge is 7% higher at 92 GeV than at low energies like 0.5 MeV. Allowing for the increased charge due to reduced polarization caused shielding, the 92 GeV electrons approach within 1.8 x 10^-20 m. (Assuming purely Coulomb scatter.)

    Extending this to the assumed unification energy of 10^16 GeV, the distance of approach is down to 1.6 x 10^-34 m, and the Planck scale is ten times smaller.

    If you replot graphs like (or Fig 66 of Lisa Randall’s Warped Passages) as force strength versus distance form particle core, you have to treat leptons and quarks differently.

    You know that vacuum polarization is shielding the core particle’s electric charge, so that electromagnetic interaction strength rises as you approach unification energy, while strong nuclear forces fall.

    Considering what happens to the electromagnetic field energy that is shielded by vacuum polarization, is it simply converted into the short ranged weak and strong nuclear forces? Problem: leptons don’t undergo strong nuclear interactions, whereas quarks do. The answer to this is that quarks are so close together in hadrons that they share the same vacuum polarization shield, which is therefore stronger than in leptons, creating vacuum energies that allow QCD. If you consider 3 electron charges very close together so that they all share the same polarized vacuum zone, the polarized vacuum will be 3 times stronger, so the shielded charge of each seen from a great distance may be 1/3 of the electron’s charge (a downquark). Just an idea.

  • Steuard

    The weird thing about this result to me is that you apparently can’t string two of these things together one after the other. Assuming I understood a recent colloquium speaker correctly, t sounds like there’s some strict theoretical maximum in this class of methods of doubling the input energy. You can’t just double it again: somehow, the system would catch on and not let you do it. (I have no clue what the mechanism for that is, and the speaker’s answers didn’t manage to enlighten me.) So it sounds like we’ll still need existing high energy accelerators to get halfway to our goals for a long time yet.

  • Mary

    As a friend of mine once said, “don’t put all your physics eggs in one large hadron collider in Switzerland”.

    I think there’s still quite a ways to go either way you look at it, and I’m excited to see what pops up.

  • Plato

    The situation in experimental basis was helped by reductionism to see events in relation to cosmological particle collisions?

    You put your eggs in one basket when you limit yourself in perspective.

    The story “is,” then at what point does “such a exchange of energies” allow you to look at the manifestations states of particle with regard to energy considerations?

    So out of that energy came? :)

    Creation of “microstate blackholes” serve what purpose?

    Mein Gott!

  • Thomas Dent

    Reminiscent of Jim Rosenzweig’s talk at SUSY – lasers and plasma are apparently the way to go in the decades after ILC, which means they should be being developed just about now.

    There are tradeoffs in the current setup:

    ‘After propagating for a distance known as the “dephasing length” the electrons outrun the wake. This limits how far they can be accelerated and thus limits their energy. To increase the dephasing length requires lowering the plasma density, but at the same time the collimation of the laser beam must be maintained over the longer distance.

    ‘ One way to do this is to increase the spot size of the laser beam, since this reduces diffraction. “The trouble with this approach is that if you double the size of the spot, you have to quadruple the laser power just to maintain the same intensity over the area of the spot,” Leemans says. Increasing spot size enough to achieve 1 GeV beams would require petawatt lasers (10 to the 15th watts). “The more powerful the laser, the more expensive and cumbersome,” he says. “Plus it takes the laser a lot longer to charge up, which limits its pulse repetition rate.” ‘

    … not sure how this prevents you from putting one on the end of another, unless the leftover plasma drifting about screws you up somehow.

  • Blake Stacey

    Sounds like all we have to do is slam a Mack truck into a proton in an elastic collision.

  • Mark

    ” … that’s a technology problem.”

    I had to laugh. I don’t know whether you intended it as such, but that sounds like a theoretician’s attitude. I pretty much think along those lines myself at times, although I am not a theoretician. I have on a few occasions (fairly facetiously) said that once I have stated the problem or, in some cases, laid out the equations to solve it, that I have the answer. It also reminds me of some other quotes, like this from a newspaper bio of some person of would-be note, ” … completed all the requirements for a phd except the dissertation.”

  • citrine

    Airport customs officer : Ma’am, what is that?
    JoAnne : It’s a portable particle accelerator.

    Officer : Why are you carrying it around?

    JoAnne : I’m a Physicist. I work with these.

    Officer : What is the square root of .0000098?

  • Aaron

    Sweeeet! I like the idea of wakefield acceleration, and it’s nice to hear more about it! :)

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