# How Does QED Explain the Electromagnetic Force

• Physicist97
In summary, Quantum Electrodynamics (QED) explains the interaction between two charged particles, such as two electrons, by describing the exchange of virtual photons. These virtual photons are not physical particles, but rather mathematical tools used to understand the calculations of the theory. The differential cross-section, which measures the probability of the interaction occurring, can be calculated using QED and results in the Rutherford formula for repulsive forces between like-charged particles and an attractive potential for opposite-charged particles. This can be understood intuitively as a plane wave reaching a cloud of free electrons and causing them to oscillate perpendicular to the direction of propagation.
Physicist97
From a conceptual point of view, how is it that Quantum Electrodynamics explains the interaction between two charged particles, (let's say two electrons)? I've heard people say that the electrons exchange photons, but how is it that an electron would know when to send off a photon and that it will be absorbed by another charged particle? And how is it that an exchange of photons would be different for two electrons (which repel) and an electron and, say, proton (which attract)? Also, by exchanging photons, would the photons not carry away a little bit of energy, thus causing the electron to lose energy? Thank you for any explanation :)

Physicist97 said:
I've heard people say that the electrons exchange photons
Virtual photons are an attempt to describe the theory in words, that attempt works well in some setups but it cannot answer questions like those you asked. The actual theory (the formulas) can, but then you have to understand the calculations.

Physicist97 and bhobba
Basically, if you calculate the differential cross-section for a process like you have described (an electron exchange a virtual photon with another electron) in QED, in the non relativistic limit you will find the Rutherford formula, which is calculated assuming the repulsive Coulomb force. If you do the same with opposite charged particle, you will find an attractive potential.

Physicist97
Is there a simple way to explain the following fact, at the level of a reader who has just made his way through "QED The Strange Theory..."

When a plane wave reaches a cloud of free electrons, they oscillate at right angles to the direction of propagation.

How does this emerge from QED, (intuitive picture if such is possible)?

## 1. What is QED and how does it explain the electromagnetic force?

QED stands for Quantum Electrodynamics, which is a branch of physics that describes the behavior of electromagnetic interactions at the quantum level. It explains the electromagnetic force as the result of the exchange of virtual particles called photons between charged particles.

## 2. How does QED differ from classical electromagnetism?

Classical electromagnetism is a theory that describes the behavior of electric and magnetic fields at the macroscopic level, while QED describes the behavior of these fields at the microscopic level. QED takes into account quantum effects and interactions between particles, while classical electromagnetism does not.

## 3. What is the role of virtual particles in QED?

Virtual particles are particles that exist for a very short amount of time and cannot be directly observed. In QED, they mediate the electromagnetic force by being exchanged between charged particles, causing the force to be either attractive or repulsive.

## 4. How does QED explain the properties of the electromagnetic force?

QED explains the properties of the electromagnetic force by using mathematical equations to describe the interactions between charged particles and virtual photons. These equations take into account factors such as the strength of the charge and the distance between particles.

## 5. Are there any limitations to QED's explanation of the electromagnetic force?

While QED is a very successful theory in explaining the electromagnetic force, it has some limitations. It does not take into account the effects of gravity or the behavior of particles at very high energies. Additionally, it cannot fully explain the phenomenon of quantum entanglement.

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