Thursday, October 19, 2017

Virtual particles

Werner Heisenberg
Heisenberg's uncertainty principle, one of the consequences of quantum mechanics, makes possible the birth of virtual particles in the void, apparently transgressing the principle of energy conservation, the most holy in physics. The reason is that Heisenberg’s principle can be expressed in several ways, one of which relates the uncertainty about the energy to the uncertainty about time:
This expression can be interpreted in the sense that a pair of objects, each with energy E, can appear spontaneously in the vacuum, provided that they lasts at most a time Dt<ħ/(2E). These pairs of objects are called virtual particles. One of these particles is always matter, the other antimatter, and their duration, according to this principle, is ridiculously small. A virtual electron, for instance, would last 1.3×10-21 seconds (just above one sextillionth of a second). The higher the mass (energy) of the virtual particle, the less time it will last. After that time, the two particles will annihilate each other and disappear. Due to their short duration, the existence of virtual particles has not been experimentally verified.
Is it possible for these virtual particles to become real under certain circumstances? Yes it is, and it is believed that there are at least two situations (somewhat drastic, it is true) where this could happen.

Stephen Hawking
Hawking radiation, proposed in 1974 by Stephen Hawking. If a pair of virtual particles arises spontaneously, exactly at the event horizon of a black hole, it is possible that one of the two particles moves casually into the black hole while the other moves outwards. In that case, the two particles can no longer annihilate each other, and the one outside will automatically become a real particle. As the principle of conservation of energy then comes into effect, it is necessary for the mass of the black hole to decrease by an amount equal to the mass of that particle. This process will be repeated many times over billions of years, so in the long run a black hole would not last forever, but would eventually evaporate completely. The smaller the mass of the black hole, the faster it would evaporate.
     • The Unruh effect, also proposed in the 1970s by the Canadian William Unruh. It consists in the fact that an object (i.e. a spaceship) moving through space with increasing velocity (accelerated motion) will sweep away the pairs of virtual particles that arise spontaneously in front of it, separating them and converting them into real particles. Then the temperature of the void would increase, from the point of view of the moving body. Of course, the phenomenon is extreme and would only be perceptible for accelerations trillions of times greater than the acceleration of gravity on the surface of the Earth, so we should not fear that it will affect our spaceships in the foreseeable future.
It can be seen that both effects mentioned above are beyond our reach. Hawking radiation, because there are no near black holes to experiment with. The Unruh effect, because we cannot reach the accelerations that would make it perceptible. Does this mean that these things are empty imaginations, a waste of time? Perhaps not, for both effects have been simulated using devices as simple as a water bucket.
Unruh effect simulation

        A Canadian team has tried to simulate Hawking radiation by making waves on the surface of water that behave similarly (in a sense, of course) as a black hole. The idea, which was suggested also by William Unruh, is based on the fact that the equations governing the movement of fluids resemble those governing the gravitational fields around black holes. By manipulating these waves, Unruh and his team created the hydraulic equivalent of an event horizon, and by causing pairs of fluid waves on the horizon, generated something similar to Hawking radiation. The Science News magazine echoed this in December 2010.
         Recently, an Israeli team has simulated the Unruh effect using waves on the surface of the water and computing how they would be seen by an alleged observer moving with great acceleration. This experiment has also been discussed in Science News in September 2017.
Critics of these experiments argue that the waves at the surface of water are not the same as black holes or impossibly accelerated movements, and that even though the equations are similar, the situations under comparison don’t have to be the same. It has also been said that the Israeli experiment has not even simulated the acceleration, it has just calculated how the waves caused on the water would be seen if the observer moved in that way. Finally, both effects rely on quantum mechanics, while their simulation in water is purely classical.
What can we conclude? For the time being, neither the existence of particle pairs, nor Hawking radiation, nor the Unruh effect have been experimentally demonstrated, at best they have been simulated. It is better to wait and see what happens.

The same post in Spanish
Thematic Thread on Particle Physics: Previous Next
Manuel Alfonseca

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