Does a photon have a shadow
Interaction of light
According to the laws of classical physics, light does not interact with one another. However, under extremely rare circumstances, light particles can collide with one another. In the World of Physics podcast, Matthias Schott from the University of Mainz spoke about the first observation of such light-to-light collisions.
Particles of matter can collide, deflect one another, or even destroy one another. In short: matter interacts with one another. On the other hand, if light collides, nothing happens. This was already recognized by the Scottish physicist James Clerk Maxwell, who more than 150 years ago described the behavior of light as electromagnetic radiation with the equations named after him. What happens when two rays of light meet can be illustrated with a calm lake into which a stone is thrown - a ring-shaped wave pattern is created.
Matthias Schott from the University of Mainz
Matthias Schott: “If you throw another stone into the water, it also creates a wave pattern. If both wave patterns meet, they interfere. But the interesting thing is that usually both waves go through each other without interaction. This means that this will not create any new waves. Exactly the same thing happens with light. "
Light exhibits both wave and particle properties. Even if two light particles, or photons, collide, nothing happens.
“Photons don't interact with each other, at least not primarily. Roughly speaking, this means that it behaves like the stones in the water. Accordingly, photons simply fly through each other. "
The reason why light particles do not interact with one another can be explained by the electrical charge on particles.
“Photons are emitted by charged particles. Two charged particles interact with each other by emitting light particles. According to our theory, only charged particles can in principle emit photons. However, photons themselves are not charged and can therefore not send out any further photons. "
According to the laws of classical physics, interaction between light particles is therefore impossible. In the world of quantum physics, however, everything is a little different, including the behavior of light. Werner Heisenberg and his doctoral student Hans Heinrich Euler recognized this more than seventy years ago. The two worked on the fundamentals of what is known as quantum electrodynamics: They applied the laws of quantum physics to Maxwell's equations for electromagnetic radiation to describe how light propagates in a vacuum.
“Quantum electrodynamics describes that photons in a vacuum, that is, in a vacuum, can split into two particles for a very short time, namely into an electron and its antiparticle, the positron. We know from many experiments that this happens all the time. "
In addition to protons, the LHC also accelerates lead ions
At the quantum level, light becomes matter, which in turn immediately transforms itself back into light. And this matter can in principle interact with one another - and thus indirectly enable the interaction of light with light.
“We can imagine that we have two photons. One comes from the left, one comes from the right. And both split into an electron and a positron. If the photons are close enough and this splitting happens at the same time, the two electrons and positrons from both sides can annihilate in this very small time window and in the smallest of spaces and thus produce two new light particles. "
However, this is an extremely unlikely event.
“You have to let an extremely large number of light particles collide to be able to see this effect at all. One hopes that one day two new light particles will arise that will fly in a different direction. "
Matthias Schott and his colleagues are carrying out experiments on the ATLAS detector of the LHC particle accelerator at the CERN research center. There it is not light but particles of matter that are accelerated, either protons or charged lead atoms. These particles are brought to extremely high speeds and enable scientists to study light as well.
“The nice thing about relativistic charged particles is that they are always surrounded by an electromagnetic field. This field can be described by photons, which means that the lead ions are actually accompanied by a very, very large number of photons. In principle, this is exactly the starting point of our experiment. "
Usually, the researchers at the LHC want the lead ions to collide.
Proof of light-to-light scattering
"In our experiment, however, we are explicitly looking for events in which two lead ions fly very close to each other so that the photons that accompany them can interact with one another."
If two photons actually interact with each other, the ATLAS detector would not register anything apart from the two newly created light particles that fly in a different direction after the collision. In fact, Matthias Schott and his colleagues succeeded in tracing this characteristic signal in the detector's data - a total of 13 times in over four billion events.
“It must be said, however, that we made this discovery with a probability of 4.4 sigma. This means that it is very likely a real discovery and not a coincidence. In particle physics, however, one normally requires five sigma in order to actually be able to speak of a discovery: the probability that the observed signature of the 13 events came about by chance is then one in a million. "
In order to finally rule out chance, the scientists want to collect further events as soon as lead ions are accelerated again at the LHC in the coming year. But Matthias Schott is already pretty sure that with the measurements he has confirmed the prediction of Heisenberg and Euler after more than seventy years - and that when light collides, nothing always happens.
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