Forgot your password?
typodupeerror
Science

Lasers Approach Their Ultimate Intensity Limit 384

Posted by samzenpus
from the biggest-pointer dept.
Flash Modin writes "Death Star style superlasers? Don't bet on it. High-power lasers currently in development appear to be nearing the theoretical laser intensity limit, according to new research set to be published in the journal Physical Review Letters. Ultra-high-energy laser fields can actually convert their light into matter as shown in the late '90s at the Stanford Linear Accelerator (SLAC). This process creates an 'avalanche-like electromagnetic cascade' (also known as sparking the vacuum) capable of destroying a laser field. Physicists thought it might be a problem for lasers eventually, but this work indicates the technology is much closer to its limit than researchers believed. A preprint is available here."
This discussion has been archived. No new comments can be posted.

Lasers Approach Their Ultimate Intensity Limit

Comments Filter:
  • Re:Maybe, maybe not (Score:4, Informative)

    by NeutronCowboy (896098) on Wednesday August 11, 2010 @03:34PM (#33219482)

    Or even running out of lighter fluid.

    If you could track every atom of the lighter fluid, you'd see that there are as many atoms from the lighter fluid around after the combustion as before. In a nuclear explosion, there are fewer atoms around.

    Also, they're not talking about a single laser, they're talking about colliding two laser beams.

    They're aiming an electron beam at a laser - not quite the same thing as aiming two lasers at each other. Furthermore, the key part is not the e-beam, but the gamma-rays that come from the electron-photon collision, which then interact with the laser. The issue is that once you create one electron-positron pair from photons, you can get a cascade reaction where there are so many electrons/positrons floating around that you don't have a coherent laser field anymore.

    It'll be a fascinating sight to see, surely.

  • by ceejayoz (567949) <cj@ceejayoz.com> on Wednesday August 11, 2010 @03:35PM (#33219486) Homepage Journal

    Well gee, if only there were a link to an article about it.

    In a report published this month by the journal Physical Review Letters, 20 physicists from four research institutions disclosed that they had created two tiny specks of matter -- an electron and its antimatter counterpart, a positron -- by colliding two ultrapowerful beams of radiation.

    As for this being new...

    The possibility of doing something like this was suggested in 1934 by two American physicists, Dr. Gregory Breit and Dr. John A. Wheeler.

  • by bunratty (545641) on Wednesday August 11, 2010 @03:37PM (#33219538)
    Energy converts to matter, and matter to energy, all the time. Check out Feynman diagrams [wikipedia.org] for many examples. Particle colliders are machines built for the purpose of converting energy into matter. When particles collide, some of their energy converts to various forms of matter.
  • Re:Maybe, maybe not (Score:0, Informative)

    by Anonymous Coward on Wednesday August 11, 2010 @03:44PM (#33219634)

    Burning lighter fluid is a chemical reaction, the same amount of matter exists before and after, it just exists in new compounds. Nuclear explosions actually destroy matter.

    Well, no. Some of the matter was converted into energy, and dissipated as heat and light.

  • Re:lighter fluid. (Score:5, Informative)

    by NeutronCowboy (896098) on Wednesday August 11, 2010 @03:49PM (#33219712)

    But it is energy that was stored in a either a chemical bond, or an electron state. Matter does not disappear, it is just electrons rearranging their orbits. If you count all the protons, neutrons and electrons before and after the chemical reaction, they're all still there.

  • Re:lighter fluid. (Score:5, Informative)

    by blueg3 (192743) on Wednesday August 11, 2010 @04:05PM (#33219898)

    Both nuclear and chemical reactions destroy matter, if you can call that destroying matter.

    In a chemical reaction, electrons change states. In an exothermal chemical reaction, the energy of those electron states is lower than the energy of the electron states before the reaction, and energy is released in another form (photons, kinetic energy, etc.). If you count the neutrons, protons, and electrons, they're all still there. But mass has been lost, because the binding energy of the electrons counts in the mass of the molecule. (In the reaction, binding energy was lost and converted to another form. Energy is mass.) However, chemical binding energy is tiny compared to the energy in the rest mass of protons, neutrons, and electrons.

    In a nuclear reaction (fission and fusion), the states of nucleons (neutrons and protons) also change. Again, if you count the neutrons, protons, and electrons, the same ones present before are present after. (Sometimes they change form, like n p + e.) But mass has been lost, because the binding energy between the nucleons counts in the mass of the atom. (In the reaction, binding energy was lost and converted to another form. Energy is mass.) Nuclear binding energy is still small compared to energy in rest mass, but it's a lot bigger than chemical binding energy.

  • Re:Maybe, maybe not (Score:3, Informative)

    by sconeu (64226) on Wednesday August 11, 2010 @04:06PM (#33219920) Homepage Journal

    That comes from the binding energy of the nucleus. The number of nucleons remains the same.

  • by Anonymous Coward on Wednesday August 11, 2010 @04:12PM (#33220046)

    "When particles collide, some of their energy converts to various forms of matter."

    No. When particles collide, energy is used to break internal bonds, decomposing them into various smaller bits of matter we can observe. It was all already there.

  • by bunratty (545641) on Wednesday August 11, 2010 @04:34PM (#33220478)

    This is a common misconception. No, the particles that result from collisions were not already there. The top quark was created from a collisions of particles that did not contain a top quark. The same is true of bottom quarks, strange quarks, and charm quarks. The particles come from the energy of the colliding particles. That's why the energy of the collisions determines the maximum amount of mass of the particles the collider can create.

    Just think about it for a few seconds. If new particles could not result, how can we make new types of quarks and antimatter? When we collide electrons and positrons, how could other types of particles possibly result?

  • by bunratty (545641) on Wednesday August 11, 2010 @04:52PM (#33220826)
    Einstein disagrees with you. He says that energy and matter and interconvert, according to E=mc^2. This is exactly how particle colliders work. The LHC pumps lots of energy into hadrons and smashes them together, converting some of the energy into many different particles. We hope to be able to see Higgs bosons that are created in this way. Because they are proposed to be massive particles, lots of energy is needed to create them.
  • by aepervius (535155) on Wednesday August 11, 2010 @05:21PM (#33221388)
    Chemical energy is energy and is matter too. If you measure 8 tons of oxygen and 2 tons of hydrogen (hopefully I got my stochiometry right), and let them react, and cool off, and measure the total weright afterward you will find it changed.
    Mass energy equiavelence, scroll to "Binding energy and the "mass defect". [wikipedia.org]
  • Re:lighter fluid. (Score:3, Informative)

    by FrangoAssado (561740) on Wednesday August 11, 2010 @05:43PM (#33221742)

    Mod parent up, please (I'd do it if I had mod points).

    People talk about "transforming mass into energy" in nuclear reactions, but they almost never say that it's actually much more mundane than that. You don't need nuclear reactions (or even chemical reactions): a sinning top, for example, has more mass than one that's standing still. Here [discovermagazine.com] is a somewhat known physicist talking about that, if you don't want to believe a random person on Slashdot.

  • by Chris Burke (6130) on Wednesday August 11, 2010 @08:30PM (#33223484) Homepage

    What E=mc^2 actually means is that energy includes a term involving mass. If you wanted to count up all the energy in something, you have to include some that is due to mass-energy. So suppose you want to get a bunch of kinetic energy to blow something up. One way to do that is to convert some chemical potential energy into kinetic energy; that's how dynamite works. Einstein is saying that there's another way: by converting some mass energy into kinetic energy; that's how a nuclear bomb works.

    No, what it means are that mass and energy are literally the same, and "mass-energy" is redundant. An object with kinetic energy has more mass than an object with no kinetic energy. A compound with stored chemical energy has more mass than the elements that comprise the compound on their own. An atomic nucleus containing many protons and neutrons has more mass than the sum of those protons and neutrons individually.

    Some things have a property called rest mass, which is different than mass. The "term involving mass" in energy calculations that you're thinking of is related to the rest mass (which is represented by m0, not just m). But in every dynamic system, if you use the rest mass instead of the mass when mass is called for, you will get the wrong answer.

    Nuclear bombs don't actually convert mass into energy in any way different than that of chemical reactions. In both cases the mass that was lost was the mass of the energy that was released.

  • Re:Maybe, maybe not (Score:3, Informative)

    by Artifakt (700173) on Wednesday August 11, 2010 @09:22PM (#33223858)

    Nuclear reactions don't conserve the number of subatomic particles. They conserve matter/energy, Baryon number (if there are any Baryons involved), and charge. Some also conserve 'spin'. As simple a reaction as neutron decay shows this. A neutron (1 particle) splits into a proton, an electron, and a neutrino (3 particles). That's beta decay. When the initial neutron is in a nucleus, the resulting proton stays there. The total mass of the three particles, plus the kinetic energy added to the electron and neutrino (which both fly out of the nucleus) is the same as the total mass and kinetic energy of the original neutron. If you're talking massive particles, the electron certainly at least has a bit of mass, and the neutrino very probably does by current theories. You do start with one Baryon and end with one, as the other particles aren't Baryons - that's probably what you are thinking of. That's all classical Nucleonics.
            You can also model it in Quantum Chromodynamics as a reaction involving three quarks. The quarks are two downs and an up in the neutron, and you end up with two ups and a down in the proton, so the number of quarks is technically conserved, but you also end up with the non-quark based particles (the electron and neutrino), so looked at that way, you're going from three particles to five. The more detailed version of the Quark conversion postulates a W- boson briefly exists, and that's a rather massive*, if short lived particle, so maybe the whole thing goes 3->4->5. Oh, and just because two quarks appear unchanged, doesn't mean that what's actually happened. It's not really true to assume one down flipped to an up and the others didn't do anything.

    * 80.4 GeV/c^2, roughly 100 times as massive as the initial neutron, and actually heavier than a whole iron atom. That's certainly massive particles being changed, which is a darned good reason Baryon conservation should not be translated to mean massive particle conservation. Baryon conservation is also considered an empirical law - it's assumed to be true so as to explain why protons don't decay, but maybe protons do decay over very long timespans and the law may not be absolute.

  • by drerwk (695572) on Wednesday August 11, 2010 @09:55PM (#33224072) Homepage
    I have not seen anyone answer you. Twenty years ago when I studies such things, 1% for something like a HeNe laser was good. I hear the National Ignition Facility lasers are good to maybe 4%. The quantum efficiency of a laser diode might be as high as 60%. You have a couple of considerations. In an optically pumped laser, you have the efficiency of creating the pump photons. These put the lasing medium in an excited state which happens as some fraction less than 100%. Lastly, the excited state medium can either spontaneously emit, or emit via stimulation. You need the one to start the laser, but any spontaneous emission after that counts towards inefficacy.

    http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=307495 [ieee.org]

    Suggests NIF lasers might be 10% efficent http://en.wikipedia.org/wiki/Inertial_confinement_fusion [wikipedia.org]
  • Re:Maybe, maybe not (Score:5, Informative)

    by ridgecritter (934252) on Wednesday August 11, 2010 @10:02PM (#33224122)

    Well, let's see. Suppose we decide to accelerate an asteroid 100km in diameter using whatever long-term propulsion we can (nuke-powered VASIMIR, big solar sails, whatever) and use the well-known gravity assist that the planets can provide. If the asteroid has an average density of 4 g/cc, how fast would we have to get it going when it impacted earth to give enough energy to blow the planet apart?

    Blow the planet apart = move all of its mass to escape velocity. Earth escape velocity is about 11.2 km/sec. 1kg moving at 11.2 km/sec has about 6.27e7 Joules of kinetic energy. Earth's mass is about 5.97e24 kg. (No, I didn't weigh it, but Google is my friend). So, to move all of the earth's mass away at a speed of 11.2 Km/sec would require (6.27e7 J/kg)*(5.97e24kg) = 3.75e32 Joules.

    OK, this doesn't count the energy needed to break the rock up, but cut me some slack, this exercise is tuned to the accuracy standards of physicists, i.e., we're happy if we get it within a few orders of magnitude.

    Back to our 100Km diameter billiard ball. It's mass is about 2.09e18kg. So, to get about 10^32 Joules of kinetic energy on target, it will have to be moving at about 10,000,000 m/sec. This is about 3% the speed of light.

    This is surely overkill in that it's the energy needed to push all the earth's mass to escape velocity. Probably less than 1% of this energy would suffice to crack the planet into pieces. Would this count as blowing the earth up?

  • Re:lighter fluid. (Score:3, Informative)

    by nmos (25822) on Wednesday August 11, 2010 @10:31PM (#33224298)

    GP is correct, the chemical and nuclear reactions are completely analogous, in each there is a change in mass due to a change in bond energies, but this change is much smaller in magnitude in chemical reactions.

    No, in chemical reactions there is NO change in mass.

  • Re:lighter fluid. (Score:2, Informative)

    by waives (1257650) on Thursday August 12, 2010 @12:43AM (#33224992)
    No, you are wrong. There is a change in mass due to the change in bond energies, which is completely analogous to the energy released in nuclear reactions, only the magnitude is of course far smaller.

    Of course this mass difference is far too small to be observed in everyday situations, but the rule you are quoting is a high-school chemistry approximation, not the full reality.
  • Re:Maybe, maybe not (Score:3, Informative)

    by dakameleon (1126377) on Thursday August 12, 2010 @12:53AM (#33225024)

    Seriously, you're going to NPoV Star Wars?

    "Although the instant destruction and death of millions of residents and visitors to Alderaan was considered a major turning point in the Rebel movements popularity, Darth Vader was considered to have been acting within his remit by the wider Imperial corps. Also, they didn't want to get force-choked."

  • Re:lighter fluid. (Score:1, Informative)

    by Anonymous Coward on Thursday August 12, 2010 @02:42AM (#33225522)

    I don't mind you not knowing.
    Being wrong when asked is worse, but not so bad.
    Going out of your way to post something wrong is unacceptable.

    Know what you know and stay silent about the rest.

"Marriage is low down, but you spend the rest of your life paying for it." -- Baskins

Working...