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Scientists Create New "Lightsaber-Like" Form of Matter
First time accepted submitter loftarasa writes "A group of scientists led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic have developed a form of matter by binding massless photons together in a special kind of medium to create 'photonic molecules', effectively bringing us a bit closer to a world with lightsabers. 'The discovery, Lukin said, runs contrary to decades of accepted wisdom about the nature of light. Photons have long been described as massless particles which don't interact with each other – shine two laser beams at each other, he said, and they simply pass through one another. "Photonic molecules," however, behave less like traditional lasers and more like something you might find in science fiction – the light saber.' The work is described in Nature (paywalled)."
As the photons enter the cloud of cold atoms, Lukin said, its energy excites atoms along its path, causing the photon to slow dramatically. As the photon moves through the cloud, that energy is handed off from atom to atom, and eventually exits the cloud with the photon.
These are not photons in free space being described. These are photons which have excited electron orbitals in some material.
Because it turns out you don't need mass to be affected by gravity.
A smart guy called Einstein did a lot of explaining about this.
Everyone knows photons were meant to be shot out of phasers, not for dueling with lasers.
Photons have no _rest mass_ or they couldn't go the speed of light. Their mass is their energy which is a function of their frequency.
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Physicists - please cover your ears, I'm trying to simplify.
When particle move near the speed of light their mass increases. At the speed of light it becomes infinite. Imagine a very light particle, moving very fast. By making it move near C I can get any mass I want. So now imagine i make the original particle lighter, an keep moving it faster in such a way that its moving mass stays the same. In the limit a particle with zero mass moving at the speed of light can have some moving mass. That is how a photon works.
Gravity will bend light, but the effect is very weak because light is moving very quickly. Gravity around a black hole is so strong that it will stop even light.
Real relativity and general relativity changes this a little, but the basic idea is the same. Photons are very light -> massless. They move very fast -> speed of light, so they have mass from their motion. Gravity doesn't bend light much - but black holes have very strong gravity so they do bend light.
Slightly OT question but TFA mentions that photons are massless particles. I've read that elsewhere, too.
I've also heard that black holes are so massive that the force of gravity does not let anything escape including light.
So, if photons have no mass, how do black holes keep the photons from escaping?
Gravity bends the fabric of space-time itself, which the photons are travelling through.
The BH does not stop light. It changes the path light takes so it never leaves the black hole's event horizon.
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Physicists - please cover your ears, I'm trying to simplify.
Sorry, but I can't let that go, especially since it's been modded up. Just no. Light does not have mass. There is no such thing as "moving mass."
General relativity: How does a large mass bend light? Because a large mass bends space around it. Light ends up bending around the object, kind of like a banked roadway. A black hole bends space so tightly that if light gets too close (the event horizon), it gets sucked in. That's why we refer to it as a hole.
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From what I can tell, they are simply creating a system of quantum-mechanically entangled photons, not a "molecule" of photons...
You can stop reading the summary after "A group of scientists led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic..." Everything after that is complete fabrication.
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A photon has mass energy. If I take an empty box made of perfect reflectors and add a photon, it will weigh more (by a tiny bit). It will have more inertia since inertial and gravitational mass are as far as we can tell exactly equivalent (as required if you use a curved space model of gravity).
In any case, words are a bit fuzzy. A photon has 4-momentum and the mass like term (or time like term if you wish) is non-zero.
btw- curved spacetime is a perfect model for all existing measurements involving gravity, but is incompatible with quantum mechanics at very (unreachable) high energies.
Imagine a gigantic sheet and stretch it out flat and taut. This is space. Take a marble and set it rolling across the sheet. This is your light particle. It reaches the other side.
Now attach a weight under the sheet so that it dips in the middle. This is the gravity of a star. Take the marble again and roll it across the sheet so it goes through the dip and keeps going. The curved path of the marble describes the influence of gravity upon the light particle.
Now imagine the weight is so heavy that the dip is effectively vertical at its heart, a hole in the sheet. This is the gravity of a black hole. Take the marble again and roll it across the sheet. If it gets too close, it rolls into the hole and doesn't come out.
http://xkcd.com/895/
No, photons do NOT have mass. You can only go at the speed of light (photon is light) if you do not have mass.
Photons have only energy associated with their speed. But NO energy associated with any mass.
And you do not need mass to be affected by gravity. Gravity is a distortion of space, and therefore everything in space (with mass and without) is affected by gravity.
This blog post explains everything in detail in a simple way:
http://profmattstrassler.com/articles-and-posts/particle-physics-basics/mass-energy-matter-etc/more-on-mass/the-two-definitions-of-mass-and-why-i-use-only-one/]
This is happening in a non-linear medium where photons interact: it can't happen in free air. Photons hardly interact on most transparent media, but there are materials with non-linear electric properties that can be used to generate harmonics ( see for example http://en.wikipedia.org/wiki/Second-harmonic_generation ). This is used to convert red light into green in some green laser light pointers. At high power levels, the refractive index increases in more normal materials, which is a nuisance in high-power lasers as light in NdYg glass laser elements can self-focus and damaage the apparatus if the power gets too great.
Hardly "contrary to decades of accepted wisdom about the nature of light" if you can find it in a green laser pointer. Meh.
What a terrible article summary.
Light is not self interacting at the tree level. This means there is no scattering of light in the classical sense, only through virtual particles (quantum loop corrections). Did this group of experimenters prove otherwise? No.
This is the most clear statement from TFA
"It's a photonic interaction that's mediated by the atomic interaction," Lukin said. "That makes these two photons behave like a molecule, and when they exit the medium they're much more likely to do so together than as single photons."
http://phys.org/news/2013-09-scientists-never-before-seen.html
What the experiment did do was place another particle in the theory, the electron, with which the light may interact. So the light continuously scattering off of electrons keeps the two light beams spatially near each other. This is not new physics. The sun does the same thing. How long does it take a photon to escape from the sun? Typically a few million years.
What is significant about this experiment is the level of control the experimenters have over the scattering proximity of the light. Unlike the sun, whereby a very large volume is effectively trapping the light, this is a tabletop experiment.
A proper summary would be that a breakthrough in experimental techniques allowed for controlling photon scattering to a higher degree than before achieved.