About interference, momentum and thrust....

Click For Summary

Discussion Overview

The discussion revolves around the concept of electromagnetic wave interference within a closed box containing two antennas. Participants explore the implications of photon distribution and momentum transfer on the movement of the box, considering both theoretical and practical aspects of radiation pressure and reaction forces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes a scenario with two antennas emitting electromagnetic waves, questioning why the box would not move towards the side with more photon impacts.
  • Another participant argues that the diagram used does not account for the inverse square law of electromagnetic waves, suggesting that the difference in photon flux may not lead to a net force when considering the 3D nature of the waves.
  • Some participants assert that if there is asymmetric radiation, a net force should exist, while others emphasize the importance of reaction forces on the antennas during radiation emission.
  • One participant expresses skepticism about the conservation of momentum if the antennas are attached to the box, suggesting that any force on the box would be countered by forces on the antennas.
  • Another participant discusses the distinction between waves and photons, noting that while waves may be emitted symmetrically, the photon impact could be asymmetric, leading to different momentum transfers.
  • Some participants highlight the need for concrete equations to substantiate claims about impulse and momentum transfer, indicating a lack of mathematical backing for the assertions made.
  • One participant introduces the concept of quantum mechanics, questioning the need for wave-particle duality if the antennas could predict photon impacts, which raises further discussion about the nature of photons and their momentum transfer.
  • Another participant presents an animation showing how the interference pattern changes with antenna separation, suggesting that closer antennas may yield a more significant imbalance in photon impacts.
  • One participant points out that radiation pressure is a well-established theory, questioning the doubts expressed regarding its application in this specific arrangement.
  • Another participant discusses how the reaction forces on the antennas depend on the differing electromagnetic fields around them, indicating that this could lead to different forces acting on each side of the antennas.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the implications of photon impacts and radiation pressure on the movement of the box. The discussion remains unresolved, with no consensus on the effects of the antennas' configuration and the resulting forces.

Contextual Notes

Participants note the limitations of their arguments, including the need for mathematical equations to support claims and the complexity of the relationship between waves and photons in the context of momentum transfer.

  • #31
cala said:
Can you explain what would make the forces asymmetric in the sources if the fields are interfering on another region far from them?
The whole point of finding conservation laws is that the details are totally irrelevant. From the basic laws governing the system we know that momentum is conserved, regardless of the specific details.

If you don't have momentum leaving the system then both the magnitude and the direction of the momentum of the system is constant. That implies that, on average, any net force on one part of the system is balanced by an equal and opposite net force on another part of the system.

Do you understand that?
 
Science news on Phys.org
  • #32
Dale said:
The whole point of finding conservation laws is that the details are totally irrelevant
I think that cala is just not recognising this.
cala said:
If the sources are not inside the interference region,
For two coherent sources there is no "interference region". The interference is there over all space. If what you claim were true then directional antennae would not work over large distances (inter-galactic, even). By concentrating on the near field situation around your two sources, you are not grasping the whole problem and your conclusions are not valid. Actually, the same thing would apply if you consider tennis balls being fired from a serving machine. If you operate it in a box with just one hole in it, that hole will provide drive from the balls that happen to get out (possibly after bouncing around inside for some time).
 

Similar threads

  • · Replies 21 ·
Replies
21
Views
2K
  • · Replies 8 ·
Replies
8
Views
8K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
Replies
20
Views
5K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 39 ·
2
Replies
39
Views
7K
  • · Replies 4 ·
Replies
4
Views
5K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 4 ·
Replies
4
Views
5K