Classical and quantum fields

In summary: Doesn't the scalar mode always cancel out the longitudinal mode?Yes, the scalar and longitudinal modes always cancel out.
  • #1
karlzr
131
2
In QFT, all particles can be interpreted as excitations of some fundamental quantum fields in the vacuum. This is the quantum picture. But in classical world, only photons and gravitons have classical counterparts. How to explain this? The common feature of these two is that they are intermediate particles of long-range interactions. But This is far from satisfactory to me. So I was wondering whether there is a deeper and better explanation of this problem.
 
Physics news on Phys.org
  • #2
Roughly speaking, quantum fields look like classical waves when there are many particles in the same state. So classical electromagnetic waves are states of many photons and classical gravitational waves are states of many gravitons.

Fermion fields don't have any states that look classical, because you cannot put more than one fermion in the same state. So there is no classical analog of the electron field.

The W, Z, and Higgs fields are bosonic but they never look classical because these bosons all decay long before you could hope to construct a state containing lots of them. (This is pretty much the same thing as saying that they don't mediate long-range interactions.)

The gluon field undergoes confinement and so there aren't actually any states that look like gluons, let alone states with many coherent gluons. There are only hadrons and glueballs.
 
  • #3
karlzr said:
But in classical world, only photons and gravitons have classical counterparts.
What do you mean by this? What is "classical" about photons and gravitons?
 
  • #4
The_Duck said:
Roughly speaking, quantum fields look like classical waves when there are many particles in the same state. So classical electromagnetic waves are states of many photons and classical gravitational waves are states of many gravitons.

For the static electric field around an electric charge, how to understand it from the perspective of quantum fields? In the absence of test particle, I guess there is no photons except vacuum bubble. So can we say there is no electric field unless a test particle is placed nearby to have virtual photon exchange?

Meir Achuz said:
What do you mean by this? What is "classical" about photons and gravitons?
Of course we don't have quanta in classical world. I mean the classical wave or field: EM field or radiation, gravitational field or gravitational wave.
 
  • #5
karlzr said:
For the static electric field around an electric charge, how to understand it from the perspective of quantum fields?

In covariant QED the static Coulomb field is mediated by the scalar and longitudinal photons which exist even in vacuum (where no transverse photons are present).
 
  • #6
WannabeNewton said:
In covariant QED the static Coulomb field is mediated by the scalar and longitudinal photons which exist even in vacuum (where no transverse photons are present).

Doesn't the scalar mode always cancel out the longitudinal mode?
If there is no test particle, where do the scalar and longitudinal modes end?
 

1. What is the difference between classical and quantum fields?

Classical fields are described by classical mechanics and obey classical laws of motion, while quantum fields are described by quantum mechanics and follow the principles of quantum mechanics.

2. How are classical and quantum fields related?

Quantum fields can be seen as a more general and fundamental description of classical fields. In certain cases, classical fields can be derived from quantum fields through a process called "taking the classical limit".

3. What is the significance of fields in physics?

Fields are used to describe physical phenomena that vary in space and time. They are fundamental to our understanding of the universe and are used to explain a wide range of phenomena, from the motion of particles to the behavior of light.

4. What are some examples of classical fields?

Some examples of classical fields include the gravitational field, the electric field, and the magnetic field. These fields are described by classical theories such as Newton's laws of motion and Maxwell's equations.

5. How are quantum fields used in particle physics?

In particle physics, quantum fields are used to describe the behavior and interactions of subatomic particles. They are essential for understanding phenomena such as the creation and annihilation of particles, and are used in theoretical models to make predictions about experimental results.

Similar threads

  • Quantum Physics
Replies
1
Views
706
Replies
6
Views
747
Replies
6
Views
613
  • High Energy, Nuclear, Particle Physics
Replies
3
Views
2K
  • High Energy, Nuclear, Particle Physics
Replies
6
Views
2K
  • High Energy, Nuclear, Particle Physics
Replies
6
Views
2K
  • Quantum Physics
Replies
13
Views
697
  • Quantum Interpretations and Foundations
Replies
13
Views
498
  • Quantum Physics
Replies
3
Views
709
  • High Energy, Nuclear, Particle Physics
Replies
3
Views
16K
Back
Top