Violation of Newtons third Law and conservation of momentum

In summary, QED describes electric fields in terms of the field theory of electrons and photons and their interactions as perturbations, rather than viewing them as separate objects with forces exerted on each other. Additionally, the Feynman diagram and thought experiment presented are not physically possible due to the violation of momentum conservation. A minimum of two photons is needed to produce a free electron-positron pair and a virtual electron is required to mediate the interaction.
  • #1
Danyon
83
1
Three High energy Photons spontaneously convert into electron positron pairs, The electron on the left marked A is created and and destroyed within a short interval, sending an electric field burst towards electron B shortly after it's created, just enough so that Electrons B's electric field does not have enough time to reach and interact with electron A's, meaning that A exerts a force on B without B exerting a force on A

https://scontent-sjc.xx.fbcdn.net/hphotos-xpf1/v/t1.0-9/q85/s720x720/10945668_869527739755713_6251030958447359498_n.jpg?oh=5479ca362edb62068d30b9c6cf9d2704&oe=5587A156
 
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  • #2
Newton's third law does not hold even in classical electrodynamics. You are right that momentum conservation is a consequence of the third law in Newtonian physics. The correct generalization is to discard the third law and keep momentum conservation.

 
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Likes DrClaude
  • #3
Danyon said:
sending an electric field burst towards electron B shortly after it's created

In QED electric fields are not viewed that way - rather its described in terms of the field theory of electrons, the field theory of photons as the quanta of the em field, and their interaction which, if i remember correctly, is treated as a perturbation.

Beyond that I will have to leave it to those more conversant in QED - but what you wrote above is not a correct analysis.

Thanks
Bill
 
  • #4
Your Feynman diagram (and, by extension, your thought experiment) is not physically possible, even though the basic observation about Newton's third law is accurate. A photon converting into an electron-positron pair is fine as an internal vertex, but such a process cannot conserve 4-momentum and so as a complete interaction has amplitude zero. You need a minimum of two photons to produce a free electron-positron pair. Similarly, an electron-positron pair cannot annihilate into a single photon. In both cases, you need a virtual electron to mediate the interaction.

Also, BTW, your arrows are not drawn correctly. One arrow flows into the vertex, the other flows out.
 
  • #5
VantagePoint72 said:
Your Feynman diagram (and, by extension, your thought experiment) is not physically possible, even though the basic observation about Newton's third law is accurate. A photon converting into an electron-positron pair is fine as an internal vertex, but such a process cannot conserve 4-momentum and so as a complete interaction has amplitude zero. You need a minimum of two photons to produce a free electron-positron pair. Similarly, an electron-positron pair cannot annihilate into a single photon. In both cases, you need a virtual electron to mediate the interaction.

Also, BTW, your arrows are not drawn correctly. One arrow flows into the vertex, the other flows out.

I see, thankyou
 

1. What is Newton's third law of motion?

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object exerts a force back on the first object that is equal in magnitude and opposite in direction.

2. How does the violation of Newton's third law affect conservation of momentum?

The violation of Newton's third law can lead to a violation of conservation of momentum. This is because if one object exerts a force on another object without experiencing an equal and opposite force, then the total momentum of the system will not remain constant as it should according to the law of conservation of momentum.

3. What are some examples of violations of Newton's third law?

Some examples of violations of Newton's third law include objects moving through a fluid such as air or water, where the fluid exerts a force on the object but the object does not exert an equal and opposite force on the fluid. Another example is a rocket thrust, where the exhaust gases exert a force on the rocket but the rocket does not exert a force back on the gases.

4. How can the violation of Newton's third law be explained in terms of energy?

The violation of Newton's third law can be explained in terms of energy by considering the transfer of energy between objects. When an object exerts a force on another object without experiencing an equal and opposite force, the energy of the system is not conserved. Some of the energy is transferred to the second object, leading to a violation of the law of conservation of energy.

5. How can the violation of Newton's third law be avoided?

The violation of Newton's third law can be avoided by ensuring that all forces in a system are balanced and equal in magnitude and opposite in direction. This means that for every action, there must be an equal and opposite reaction. In cases where it is not possible to completely avoid a violation, the effects can be minimized by reducing the difference in forces between objects.

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