Ask Ethan: Can a laser really break empty space?

https://www.forbes.com/sites/startswithabang/2018/02/03/ask-ethan-can-a-laser-really-rip-apart-empty-space/
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In experiments with desktop lasers, it is possible that the energies are not the greatest, but in terms of power, they can even compete with lasers that trigger fusion reactions. Can a quantum vacuum be affected by such a laser?

Empty space, as it turns out, is not so empty. Fluctuations in vacuum mean that even if we eliminate all matter and radiation from a portion of space, there will still remain a finite amount of energy inherent in the space itself. If you shoot a powerful enough laser at it, is it possible, as written in the journal Science Magazine, to “break the vacuum and empty space”? This is what our reader is asking:
Science Magazine recently published an article that Chinese physicists are going to make a 100 PW laser this year (!!!) Can you explain how they plan to do this, and what unique phenomena this can help to investigate? And what does “breaking the vacuum” mean?

This story is real , it is confirmed, and a bit exaggerated in terms of the “vacuum gap” - one might think that this is possible in principle. Let's dive into real science and find out what is actually happening.


A set of laser pointers Q-line demonstrates a variety of colors and compactness - the phenomena now common for lasers. Lasers operating continuously and with very low power are shown here, only a fraction of watts - and record lasers operate with power up to petawatts.

The very idea of ​​a laser is still relatively new, despite their wide distribution. Initially, it was an acronym for light amplification by stimulated emission of radiation, and in principle, the name for lasers was chosen a little incorrectly. In fact, no gain occurs. In normal matter, there are atomic nuclei and various energy levels of electrons; in molecules, crystals, and other connected structures, the separation of electron energy levels determines the available transitions. In a laser, electrons oscillate between two available states, and emit photons of quite definite energy when they are transferred from a state with a higher energy to a state with a smaller one. These vibrations produce light, but for some reason,


Накачивая электроны до возбуждённого состояния и стимулируя их фотоном нужной длины волны, можно вызвать испускание другого фотона точно такой же энергии и длины волны. Именно так впервые был получен лазерный свет.

If you can bring the molecules or atoms into the same excited state and stimulate their spontaneous transition to the ground state, they will emit photons of the same energy. These transitions occur extremely quickly (but not instantaneously); therefore, there is a theoretical limit on the transition rate of an atom or molecule to an excited state with the subsequent emission of a photon. Usually, to create a laser inside a resonating or reflective cavity, there is a gas, a molecular substance or a crystal, but it can also be made from free electrons, semiconductors, optical fibers and, in theory, even from positronium .


ALICE free electron laser is an example of an exotic laser that does not rely on ordinary atomic or molecular transitions, but still produces narrowly focused coherent light

. time to emit a pulse. You could hear about petawatts, 10 15 W, and you think that this is a huge amount of energy. But this is not energy, but power - energy per unit of time. The petawatt laser power can be either a laser emitting 10 15J of energy (so much energy is contained in 200 kT of TNT) every second, or just a laser emitting one joule of energy (so much energy is contained in 60 μg of sugar) every femtosecond ( 10-15 s). In terms of energy, these options are very different, although they have the same power.


OMEGA-EP amplifiers at the University of Rochester, illuminated by flash tubes, could fuel American high-power laser

A 100 PW laser has not yet been built, but this is another incredible threshold that researchers plan to overcome in the 2020s. The hypothetical project is known as the Extreme Light Station, SEL, and is being built as part of the Shanghai ultrafast ultra-fast laser machine in China. An external pumping device, which is usually light of different wavelengths, excites electrons in the generating material, causing characteristic transitions that produce laser light. Photons appear in a tightly packed beam, or impulse, with a very small scatter of wavelengths. For many, it will be surprising to know that the threshold of 1 PW was overcome as far back as 1996; it took almost two decades to overcome the next threshold of 10 PW.


The preamplifiers of the National Ignition Complex are the first step towards increasing the energy of laser beams traveling towards the target chamber. In 2012, the NKZ reached the level of 0.5 PW, a thousand times at its peak, surpassing the power consumption of all the United States.

A national ignition complex in the United States may first come to mind when discussing high-energy lasers, but this is nothing more than a diversion. This array of 192 lasers focusing on a single point, compressing a hydrogen ball and triggering nuclear fusion, dangles around the 1 PW mark, but is not the most powerful complex. His energy is very high, over a million joules, but his impulses are relatively long. To set a record for power, you need to apply more energy in less time.

The current record holder uses a sapphire crystal with titanium impurities, pumps hundreds of joules into it, causes the light to reflect back and forth in destructive interference, destroying almost the entire pulse duration, and then squeezes the output light into a single pulse only tens of femtoseconds long. This is how it is possible to achieve output capacities of about 10 PW.


Part of a titanium sapphire laser; the bright red light on the left is a sapphire crystal with titanium; bright green light - pumping light scattered on the mirror.

To get even higher, crossing the threshold of the next order, it is necessary either to increase the energy pumped into the laser from hundreds to thousands of joules, or to reduce the pulse duration. The first is hard in terms of materials used. Small titanium-sapphire crystals will not withstand such energies, and large ones are prone to emitting light in unnecessary directions — at right angles to the path of the beam. Currently, researchers are considering three approaches to this problem:

  1. Take the initial impulse of 10 PW, stretch it using a diffraction grating, combine it into an artificial crystal, where it can be pumped again, increasing energy.
  2. Combining several pulses from a set of different lasers, creating the desired level of overlap is a difficult task for pulses lasting only a few dozen femtoseconds and moving at the speed of light.
  3. Add another stage of pulse compression, compressing it to a femtosecond pair.


Bend the light and focus it at a point regardless of the wavelength or the place of his fall to the surface - one of the key stages of maximizing the light intensity at one point in space

then you need to clearly focus the pulses, increasing not only energy, but also the intensity - that is, to concentrate power at one point. As written in the article :
If a pulse of 100 PW turns out to be focused on an area of ​​3 μm in size, [...], the intensity of the beam in this area will reach an incredible 10 24 per cm 2 - this is 25 orders of magnitude, or 10 trillion trillion times more than sunlight Of earth
This will open the way to the long-awaited possibility of creating particle-antiparticle pairs from nothing — but this is unlikely to be a “quantum vacuum break”.


Visualization of quantum field theory calculations shows virtual particles in quantum vacuum. Even in empty space, the vacuum energy is not zero.

According to the theory of quantum electrodynamics, the zero energy of empty space is not zero, and has a positive, finite value. Although we imagine it in the form of particles and antiparticles, appearing and disappearing again, it is best to describe it in such a way that with enough energy we can use the electromagnetic properties of empty space to create real particle / antiparticle pairs . This is based on the simple Einstein physics E = mc 2but requires fairly strong electric fields: about 10 16 V per meter. Light, being an electromagnetic wave, transfers electric and magnetic fields, and will reach this critical threshold at a laser intensity of about 10 29 W / cm 2 .


Zettawatt lasers, reaching an intensity of 10 29 W / cm 2 , should be enough to create real electron-positron pairs from a quantum vacuum. This will require even more energy, shorter pulses and / or an increase in focus compared to what we can imagine in the near future.

You may have noticed that even an ideal version from a scientific article gives an intensity that is still 100,000 times less than this threshold, and before it reaches your ability to create particle / antiparticle pairs is exponentially suppressed. The real mechanism is very different from the simple reversal of the process of creating pairs, in which, instead of two photons appearing during the electron and positron annihilation, two photons interact and produce an electron / positron pair. (This process was first experimentally demonstrated.in 1997. ) In a laser, individual photons will not have enough energy to produce new particles - instead, their combined effect on the vacuum of space will cause particle / antiparticle pairs to occur with a certain probability. But unless this intensity approaches the threshold value of 10 29 W / cm 2 , this probability will be zero.


The laser from Shanghai set records for power, but it fits on the table. The most powerful lasers are not necessarily the highest-energy ones, but more often they are simply lasers with the shortest pulses.

The ability to create matter / antimatter particle pairs from empty space will be an important test of quantum electrodynamics, and a remarkable demonstration of the capabilities of lasers and our ability to control them. It is possible that the first particle / antiparticle pair can be obtained without reaching the critical threshold, but for this you will either need to go very close to it, or be very lucky, or come up with a mechanism to increase the probability of producing pairs of particles in relation to naive calculations. In any case, the quantum vacuum is not broken, but it does exactly what is expected of it: it reacts to matter and energy in accordance with the laws of physics. This may not be intuitive, but predictably, and this is sometimes even more useful. All science is in the art of making a prediction and conducting experiments, confirming or refuting it. We have not yet reached the threshold, but each jump up in power and intensity is another step that brings us closer to the holy Grail of laser physics.

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