# How to create charge from light?

• per.sundqvist
In summary, the conversation discusses the creation of an electron-positron pair from a gamma-ray with sufficient energy and the role of QFT in explaining this phenomenon. The creation requires an external force, usually the Coulomb force of a nucleus, and QED provides a relatively simple explanation. However, there is no exact solution to QFT, only perturbative expansions, so the details of the creation process are still unknown.
per.sundqvist
We know that we can create an electron-positron pair out of a gamma-ray, if it has an energy of $$E=h\nu>2m_ec^2$$.

However a photon is described as an EM-wave that obeys the divergence criteria
$$\nabla\cdot\vec{E}=0$$

My question is simply if there is a "simple" explanation out there (QFT?) which describes microscopically what happens when the photon breaks up and 2 charged packages are created? I also wonder what the distance between the particles is in the creation moment.

My guess is that the EM-wave/photon must be disturbed in some way (by gravity?) such that it bends in some way, and that it is this bending which gives rise to the charge creation: $$\nabla\cdot\vec{E}=\rho/\epsilon$$, but where the total charge is neutral, like in the point-particle classical situation:
$$\rho=e(\delta(r-a)-\delta(r+a))$$ (some kind of dipole, but perhaps rater described with waves)

Any idea?

Pair production requires an external force, which is usually the Coulomb force of a nucleus.
QED gives a relatively simple explanation.

Ok thanks! I'm a little confused still since the four-potential $$A_{\mu}$$ in the QED-Lagrangian is the self-field. Do you use superposition in the case of a Coulomb potential, like $$A_{\mu}=A_{0,ext}(Coulomb)+A_{\mu}(self)$$?

Maybe it is just a personal shortcoming, but I am not aware of any QFT description at the level of realistic detail that you are describing.

Just as a particle in QM travels on all possible paths, so does a field in QFT take on all possible configurations. If we treat both the electrons and the vector potential as quantum field excitations (respectively, as a dirac field and a gauge field) then we are to think of the electron-positron pair creation/annihilations as happening super-frequently all over space. But there are no exact solutions to QFT, only perturbative expansions, meaning that we usually only take into account the simplest, and in QED most dominant, modes of creation/anihilation, when calculating the amplitude for some process.

Perhaps with an exact solution to QED it would be possible to give a more meaningful anwer, but at our current level of description these details are unknown to us.

## 1. How does light create charge?

Light creates charge through a process called the photoelectric effect. When light, which is made up of particles called photons, interacts with certain materials, it can transfer its energy to electrons in those materials, causing them to become "excited" and jump out of their atoms. This creates a flow of electrons, which is what we call electric charge.

## 2. What materials can be used to create charge from light?

Some common materials that can generate charge from light are silicon, gallium arsenide, and copper indium gallium selenide. These are often used in solar cells and other devices that convert light energy into electrical energy.

## 3. How can the amount of charge created from light be increased?

The amount of charge created from light can be increased by using materials that are more sensitive to light, increasing the intensity or frequency of the light, and optimizing the design of the device to maximize the surface area exposed to light.

## 4. Can charge be created from all types of light?

No, not all types of light can create charge. Only certain wavelengths of light, typically in the visible or near-infrared range, have enough energy to cause the photoelectric effect and generate charge.

## 5. What are some practical applications of creating charge from light?

The most well-known application is in solar panels, where light energy is converted into electricity. Other applications include photodetectors, which use light to detect and measure the intensity of light, and photovoltaic cells used in electronic devices like calculators and watches.

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