Cracking the Continuum

Cracking the Continuum

My third year as a graduate student at Trantor University (not to be confused with the University of Trantor), Go Raptors, I was promoted to being the senior intern-in-residence of the Physics Department Dean. I was informed that among my privileges as senior intern is that I could use any parts in lab inventory for personal projects so long as I did not unduly break them or wear them out. The only word I probably heard was 'any' and something about 'unduly'.

Inside my cramped laboratory, me and my fellow guerrilla interns created what I later dubbed (for the 8 hours before being ordered to disassemble it) The Superintense Ultrafast Laser Facility (SULF). At the heart of the SULF was a wafer thin single cylinder of titanium-doped sapphire. After kindling light in the crystal and shunting it through a system of lenses and mirrors, the SULF distilled it into pulses of mind-boggling power achieving an unprecedented 5.3 million billion watts (5.3 petawatts). Things went well, until we tried to extend the duration of the pulses beyond a trillionth of a second. The brownout got me a few demerits and I was ordered to disassemble the SULF, but not before a few brilliant 10-petawatt pulses.

Undeterred by a handful of demerits, and the administration naively not rescinding my access to lab inventory, my merry band of guerrilla lab tech interns helped me to construct a 100-PW laser known as the Station of Extreme Light (SEL), flinging pulses into a chamber 20 meters underground, subjecting targets to extremes of temperature and pressure not normally found planetside, a boon to astrophysicists and materials scientists alike. That is, if they allowed it to stay assembled long enough for me to conduct more experiments. The SEL could power demonstrations of a new way to accelerate particles for use high-energy physics. Most alluring, is that the light could tear electrons and their antimatter counterparts, positrons, from empty space—a phenomenon known as cracking the continuum, a striking illustration that matter and energy are interchangeable. Nuclear weapons attest to the conversion of matter into immense amounts of heat and light, doing the reverse is not as easy to demonstrate. This was not just epic destruction for my personal aggrandizement and amusement, this was also educational. That’s my story and I am sticking to it.

By the time they finally took away my access to physics lab inventory and the main power grid, we had completed a 180-PW laser known as the Exawatt Center for Extreme Light (XCEL).

Conceptually simple, even these powerful lasers use an external "pump," such as a flash lamp, to excite electrons within the atoms of a lasing material—usually a gas, crystal, or semiconductor. When one of these excited electrons falls back to its original state it emits a photon, which in turn stimulates another electron to emit a photon, and so on. Unlike the spreading beams of a flashlight, the photons in a laser emerge in a tightly packed stream at specific wavelengths.

Power equals energy divided by time. There are basically two ways to maximize power: Either boost the energy of your laser, or shorten the duration of its pulses. Our primary method was boosting laser energy by routing beams through additional lasing crystals made of glass doped with neodymium. Beams above a certain intensity damaged the amplifier crystals. To avoid this, we used larger amplifiers, many tens of centimeters in diameter. We figured out that a short laser pulse could be stretched in time—thereby making it less intense—by a diffraction grating that spreads the pulse into its component colors. After being safely amplified to higher energies, the light could be recompressed with a second grating. The end result: a more powerful pulse and an intact amplifier.

Focusing 192 high-energy pulses on a tritium/deuterium target to induce fusion.

Chirped-Pulse Amplification (CPA) Intense pulses can damage amplifiers. CPA avoids that by stretching a laser pulse with diffraction gratings. After safe amplification, the pulse is compressed.

Optical ParaMetricAmplification/ A high-energy pump beam can amplify a stretched seed pulse within a nonlinear crystal that can be made large to withstand intense inputs.

Chirped-Pulse Amplification (CPA) is a staple of high-power lasers.

To get to higher powers, we turned to the time domain: packing the energy of a pulse into ever-shorter durations. One approach is to amplify the light in titanium-doped sapphire crystals, which produce light with a large spread of frequencies. In a mirrored laser chamber, those pulses bounce back and forth, and the individual frequency components can be made to cancel each other out over most of their pulse length, while reinforcing each other in a fleeting pulse just a few tens of femtoseconds long. Pump those pulses with a few hundred joules of energy and you get 10 PW of peak power. Higher than that, some components melt and some vaporize. That is how the SULF and other sapphire-based lasers can break power records with equipment that fits in a large room and costs just tens of millions of thalers.

Raising pulse power from 10 PW to 100 PW, will require more wizardry. One approach is to boost the energy of the pulse from hundreds to thousands of joules. But titanium-sapphire lasers struggle to achieve those energies because the big crystals needed for damage-free amplification tend to lase at right angles to the beam—thereby sapping energy from the pulses. After a few regrettable and expensive failed tests, we pinned our hopes on what are known as optical parametric amplifiers. These take a pulse stretched out by an optical grating and send it into an artificial "nonlinear" crystal, in which the energy of a second, "pump" beam can be channeled into the pulse. Recompressing the resulting high-energy pulse raises its power.

Finally, to approach 100 PW, we combined several such pulses—four 30-PW pulses for the SULF in our first test and a dozen 15-PW pulses at the XCELS. But precisely overlapping pulses just tens of femtoseconds long is very difficult, they can be thrown off course by even the smallest vibration or change in temperature. We couldn’t build a design to generate 75 PW using a single beam using off the shelf components from the lab’s inventory.

We later took a different route to 100 PW: adding a second round of pulse compression, using thin plastic films to broaden the spectrum of 10-PW laser pulses, then squeezing the pulses to as little as a couple of femtoseconds to boost their power to about 100 PW.

The secondary challenge is bringing the beams to a singularly tight focus. For the purposes we envision, we care more about intensity—the power per unit area—than the total number of petawatts. Achieve a sharper focus, and the intensity goes up. If a 100-PW pulse can be focused to a spot measuring just 3 micrometers across, as we are planning, the intensity in that tiny area will be an astonishing 10^24 watts per square centimeter (W/cm2)—some 25 orders of magnitude, or 10 trillion trillion times, more intense than the sunlight striking Araxes.

Yottawatt irradiance radiant flux opens the ability to crack the vacuum. According to Quantum Electro-Dynamics (QED), which describes how electromagnetic fields interact with matter, vacuum is not as empty as classical physics would have us believe. Over extremely short time scales, pairs of electrons and positrons, their antimatter counterparts, flicker into existence, born of quantum mechanical uncertainty. Because of their mutual attraction, they annihilate each another almost as soon as they form.

A very intense laser could separate the particles before they collide and annihilate. Like any electromagnetic wave, a laser beam contains an electric field that whips back and forth. As the beam's intensity rises, so, too, does the strength of its electric field. At radiant flux intensities around 10^24 W/cm2, the field would be strong enough to break the mutual attraction between the electron-positron virtual particle pairs. The laser field then shakes the particles, causing them to emit electromagnetic waves—in this case, gamma rays. The gamma rays in turn, generate new electron-positron pairs, and so on, resulting in an avalanche of particles and radiation that could be detected. Gamma ray photons would be energetic enough to push atomic nuclei into excited states, known as "nuclear photonics"—the use of intense light to control nuclear processes.

One way to crack the vacuum is focus a single laser beam onto an empty spot inside a vacuum chamber, but colliding two beams makes it easier, because this jacks up the momentum needed to generate the mass for electrons and positrons. The SEL would collide photons indirectly. First, the pulses would eject electrons from a helium gas target. Other photons from the laser beam would ricochet off the electrons and be boosted into high-energy gamma rays. Some of these in turn would collide with optical photons from the beam.

Whereas classical physics insists that two light beams will pass right through each other untouched, Quantum Electro-Dynamics (QED) stipulate that converging photons will occasionally scatter off one another. This is the good part about the quantum uncertainty principle.

An engineering matter is that the flash lamps that pump the initial energy into many lasers must be cooled for minutes or hours between shots, making it difficult to hard to carry out research that relies on plenty of data. For example, occasionally photons transform into particles of the mysterious dark matter thought to make up much of the universe's mass. We need a lot of shots to see that.

A higher repetition rate is also key to using a high-power laser to drive beams of particles. In one scheme, an intense beam would transform a metal target into a plasma, liberating electrons that, in turn, would eject protons from nuclei on the metal's surface.

Physicists, for their part, dream of particle accelerators powered by rapid-fire laser pulses. When an intense laser pulse strikes a plasma of electrons and positive ions, it shoves the lighter electrons forward, separating the charges and creating a secondary electric field that pulls the ions along behind the light like water in the wake of a speedboat. This "laser wakefield acceleration" can accelerate charged particles to high energies in the space of a millimeter or two, compared with many kilometres for conventional accelerators. Electrons thus accelerated could be wiggled by magnets to create a so-called free-electron laser (FEL), which generates exceptionally bright and brief flashes of x-rays. A laser-powered FEL could be far more compact and cheaper than those powered by conventional accelerators.

Electrons accelerated by high-repetition PW pulses could slash the cost of particle physicists' dream machine: a 30-kilometer-long electron-positron collider that would be a successor to the traditional design for Large Hadron Colliders. A particle accelerator device based on a 100-PW laser could be at least 10 times shorter and at least that much cheaper.

Both the linear collider and rapid-fire FELs would need thousands, if not millions, of shots per second, well beyond off the shelf parts. One possibility is to combine the output of thousands of quick-firing fiber amplifiers, which don't need to be pumped with flash tubes. Another option is to replace the flash tubes with diode lasers, which are expensive, but worth the money.

What would be ideal for our envisioned wormhole factory would be a series of 12 such electron-positron collider channels with 16 highly durable beam splitters to focus 192 near simultaneous pulses on a 3 micrometre target or Rydberg state ultra-condensed hypermatter. By near simultaneous, I mean specifically within a quadrillionth (10^-15) of a second.

Although technically, I only need to cross the beams, the time frame becomes much smaller without a target. The timing becomes the time it takes for light to cross a photon wavelength rather than the time it takes to cross a micrometer. Although that can be spread to its wavelength, that is still 10^-32 metres instead of 10^-6 metres. A billion billion billion times faster with much more reliance upon the uncertainty principle. Fortunately I have the hypermatter to spare at the moment.