The evolution of lasers...

Thread Starter

cmartinez

Joined Jan 17, 2007
7,015
I've just read this rather interesting article:

And found two outstanding facts:
...in the 1980s we paid $200,000 for a laser that I could buy today for less than $5

That's pretty impressive... for a guy whose teenage years where lived in the 80's, it brings the economics of technology in perspective.

But this other one, is simply mind boggling:
Well, a project called the Extreme Light Infrastructure is now being built in Romania. It will focus laser pulses lasting less than a trillionth of a second to intensities so high that they can ionise the vacuum, creating positrons and electrons. It takes black holes to do that in nature.

How is it possible to actually excite the vacuum to the point of ionization? Is it because the photon energy and density is so intense that virtual particles are not allowed to re-collapse on themselves? Would such phenomena make it possible to build an engine capable of propelling a spaceship without needing a propellant?
 

wayneh

Joined Sep 9, 2010
16,398
I'm extremely skeptical of that claim. Fusion reactions can convert energy to mass. This is the opposite of using fusion to release energy, but it's what happens for fusing atoms more massive than iron, if I recall correctly.

Anyway, I don't think you can shoot pure light energy into the void and get mass out of it.

Would love to be shown wrong.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,015
I'm extremely skeptical of that claim. Fusion reactions can convert energy to mass. This is the opposite of using fusion to release energy, but it's what happens for fusing atoms more massive than iron, if I recall correctly.

Anyway, I don't think you can shoot pure light energy into the void and get mass out of it.

Would love to be shown wrong.
Well, there really are virtual particles out there, and they're responsible for the already proved casimir effect.
Besides, the article's source is serious enough to at least give it some objective consideration.
 

nsaspook

Joined Aug 27, 2009
7,356
There is a point where it is theorized that EM becomes nonlinear. Schwinger limit

The field would be so strong it would split apart the vacuum virtual particle/anti-particle pairs before they recombine. The problem is no one really knows if these so called virtual-particles ( disturbance in a field) interactions will result in true matter (most likely not).
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,015
Wow!:

Maxwell's equations predict the impossibility of any but trivial elastic photon–photon scattering. In QED, however, non-elastic photon–photon scattering becomes possible when the combined energy is large enough to create virtual electron–positron pairs spontaneously, illustrated by theFeynman diagram in the adjacent figure.


The entire article is extremely interesting, especially the last paragraph:

Photon–photon scattering and other effects of nonlinear optics in vacuum is an active area of experimental research, with current or planned technology beginning to approach the Schwinger limit.[5] It has already been observed through inelastic channels in SLAC Experiment 144.[6][7] However, the direct effects in elastic scattering have not been observed. As of 2012, the best constraint on the elastic photon–photon scattering cross section belongs toPVLAS, which reports an upper limit far above the level predicted by the Standard Model.[8] Proposals have been made to measure elastic light-by-light scattering using the strong electromagnetic fields of the hadrons collided at the LHC.[9] Observation of a cross section larger than that predicted by the Standard Model could signify new physics such as axions, the search of which is the primary goal of PVLAS and several similar experiments. Even the planned, funded ELI–Ultra High Field Facility, which will study light at the intensity frontier, is likely to remain well below the Schwinger limit[10] although it may still be possible to observe some nonlinear optical effects.[11] Such an experiment, in which ultra-intense light causes pair production, has been described in the popular media as creating a "hernia" in spacetime.[12]

By "hernia" I assume the article means a spacetime rift?

Thanks for the excellent reference, nsaspook.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,015
This is astonishing:

http://www.foxnews.com/tech/2018/02/01/china-is-building-laser-10-trillion-times-more-intense-than-sun-that-could-tear-space-apart.html

The end goal is to create a laser so powerful it can produce 100-petawatt laser pulses –that's 100 million billion watts.
For context, that's 10,000 times the power of all the world's electrical grids combined.
These ludicrously powerful pulses could be targeted at incredible precise spots measuring just three micrometers across – that's 2000 times less than the thickness of a standard pencil.
...
According to the Science journal, this laser would be so powerful it "could rip apart empty space".
The idea is to achieve a phenomenon known as "breaking the vacuum", whereby electrons are torn away from positrons (their antimatter counterparts) in the empty vacuum of space.
 

nsaspook

Joined Aug 27, 2009
7,356
Sorry, more media hype. Pop-science virtual particle misconceptions. Its the science reporters. Complex ideas often get simplified down, warped and twisted between leaving a scientist's mouth and appearing in print.

Virtual particles are simply a mathematical artifact of the modeling process, they don't actually exist in modern QFT.
https://www.physicsforums.com/insights/vacuum-fluctuations-experimental-practice/

https://www.physicsforums.com/insights/vacuum-fluctuation-myth/
https://www.physicsforums.com/threads/so-quantum-fluctuations-dont-exist.925247/page-2#post-5843430

http://www.mat.univie.ac.at/~neum/physfaq/topics/virtcoul
 

nsaspook

Joined Aug 27, 2009
7,356
Unless I've misunderstood what you've said, then how do you explain the very real force exerted by the Casimir Effect? ...
I will let the Prof. answer that question: https://www.physicsforums.com/insights/author/a-neumaier/
http://www.mat.univie.ac.at/~neum/physfaq/topics/casimir

Most of the 'breaking the vacuum' articles are variations on experiments to approach the Schwinger_limit.
In QED, however, non-elastic photon–photon scattering becomes possible when the combined energy is large enough to create virtual electron–positron pairs spontaneously, illustrated by the Feynman diagram in the adjacent figure.
These virtual pairs are not actual particles but they do have real effects in fields.
https://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/
The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle. A particle is a nice, regular ripple in a field, one that can travel smoothly and effortlessly through space, like a clear tone of a bell moving through the air. A “virtual particle”, generally, is a disturbance in a field that will never be found on its own, but instead is something that is caused by the presence of other particles, often of other fields.
 

nsaspook

Joined Aug 27, 2009
7,356
So, there really is no reason to believe (and many reasons not to believe) that vacuum fluctuations or virtual particles are fundamentally needed for the Casimir Effect. Van der Waals quantum forces from uncertainty in the location/momentum of the electrons (real particles) can be used to explain it.

https://en.wikipedia.org/wiki/Casimir_effect#Relativistic_van_der_Waals_force
In fact, the description in terms of van der Waals forces is the only correct description from the fundamental microscopic perspective,[19][20] while other descriptions of Casimir force are merely effective macroscopic descriptions.

http://www.sciencemag.org/news/2018/01/physicists-are-planning-build-lasers-so-powerful-they-could-rip-apart-empty-space
Back to the laser, as the science article says correctly "This will be completely new physics". Where have we heard that term before?
 

nsaspook

Joined Aug 27, 2009
7,356
Virtual particles are always a calculation of an effect. What escapes from the macroscopic black hole's event horizon is a real photon generated from a process where virtual particles are used in diagrams to describe that process.

Hawking himself uses the analogy of virtual particles in his paper. He says:

One might picture this negative energy flux in the following way. Just outside the event horizon there will be virtual pairs of particles, one with negative energy and one with positive energy.

However he goes on to say:

It should be emphasized that these pictures of the mechanism responsible for the thermal emission and area decrease are heuristic only and should not be taken too literally.
https://physics.stackexchange.com/questions/251385/an-explanation-of-hawking-radiation
The dry facts are that two real particles (e.g., two photons, or an electron and a positron) are created from the energy in the very strong gravitational field near the horizon of the black hole - from a classical external gravitational field (if gravitation is treated classically), or possibly from two gravitons (in effective quantum gravity at lone loop), not from the vacuum. [Strong external fields with energies significantly above the pair creation energy threshold necessarily create the corresponding particle pairs. See the postscript below for more details.] The particle pair creation reduces the gravitational energy by the energy (including the rest mass energy equivalent) of the two particles. One particle escapes, the other is absorbed by the black hole. The net result (black hole energy - 2 particle energies + 1 particle energy) is a loss of mass corresponding to the energy of the escaping particle. A valid description is given on p.645 of the book B.W. Carroll and D.A. Ostlie, An Introduction to Modern Astrophysics, 2nd. ed., Addison Wesley 2007.
[added January 11, 2017] I recently discovered that already in his ground-breaking paper on the subject, Hawking says on p.2462 (left) that ''One can interpret such a happening as being the spontaneous creation in the gravitational field of the black hole of a pair of particles, one with negative and one with positive energy with respect to infinity. The particle with negative energy would fall into the black hole [...] The particles with positive energy can escape [...]''. (Note that only energy differences are meaningful, hence Hawking's reference to (zero energy in flat space at) infinity that gives meaning to the sign of the energy.) No "virtual pairs" of particles and antiparticles that pop in and out of existence, as in Carlip's fairy tale!
https://physics.stackexchange.com/a/252183
 
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nsaspook

Joined Aug 27, 2009
7,356
Hawking stated that virtual particles are converted into real particles in the vicinity of a black hole's event horizon.
This generates radiation from a black hole.
https://en.wikipedia.org/wiki/Hawking_radiation#Overview
As you can see he didn't really say that in his peer reviewed paper because he's a physicist who understands what virtual particles really are and didn't get his understanding of the true physical process from pop-science articles.
 

crutschow

Joined Mar 14, 2008
25,123
As you can see he didn't really say that in his peer reviewed paper because he's a physicist who understands what virtual particles really are and didn't get his understanding of the true physical process from pop-science articles.
I bow to your expert knowledge.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,015
I'm glad to learn that the Chinese are not the only ones after this sort of science:

http://www.iflscience.com/physics/scientists-are-creating-lasers-so-powerful-they-could-tear-matter-out-of-empty-space/

Three projects top the “one to watch list” of the laser world, prepared by the journal Science. They are China’s Station of Extreme Light (SEL), Russia’s Exawatt Center for Extreme Light Studies (XCELS), and the Department of Energy’s Optical Parametric Amplifier Line (OPAL).
 

nsaspook

Joined Aug 27, 2009
7,356
Cool, but I still cringe at “break the vacuum” because it's pop-science BS.

https://www.forbes.com/sites/startswithabang/2018/02/03/ask-ethan-can-a-laser-really-rip-apart-empty-space/#15bb1299f4b0
The story is real, verified, and a little bit exaggerated in terms of claims that it can break the vacuum, as though such a thing were possible. Let's dive into the real science to find out what's really happening.
...
According to the theory of quantum electrodynamics, the zero-point energy of empty space isn't zero, but some positive, finite value. Although we visualize it as particles and antiparticles popping in-and-out of existence, a better depiction is to recognize that, with enough energy, you can — through physics — use these electromagnetic properties of empty space to generate real particle/antiparticle pairs. This is based on the simple Einsteinian physics of E = mc2, but requires a strong enough electric field to build those particles: around 1016 volts per meter. Light, since it's an electromagnetic wave, carries with it both electric and magnetic fields, and will reach that critical threshold with a laser intensity of 1029 W/cm2.
...
In any case, the quantum vacuum never breaks, but rather does exactly what you expect of it: responds to matter and energy in accordance with the laws of physics. It might not be intuitive, but it's something even more powerful: it's predictable. The art of doing that prediction and doing the experiments to verify or refute them is what science is all about! We may not be there yet, but every leap upwards in power and intensity is another step closer to this "holy grail" in laser physics.
 
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