Is emission-less propulsion possible? I believe it is...

[mikelewis]

32. Why this is actually inline with physics...

Explain why someone would at first think this violated the laws of physics and now why it doesn't

ChatGPT
Your friend's scenario with the curved barrel might initially seem to violate the laws of physics, but upon closer examination, it actually adheres to them. Here's why:

Why It Might Seem to Violate Physics:

Perceived Violation of Conservation of Momentum:

At first glance, it may seem that redirecting the bullet back towards the shooter, thereby seemingly reversing its momentum while still maintaining the shooter's forward momentum, violates the law of conservation of momentum. This law states that the total momentum of a closed system must remain constant if no external forces are acting on it. The idea of shooting, then catching or redirecting the bullet, and not being pushed back to the original position might appear as if momentum is not conserved.
Misinterpretation of Newton's Third Law:

Newton's third law states that for every action, there is an equal and opposite reaction. One might initially think that redirecting the bullet directly back towards the shooter would cancel out the initial recoil completely, which seems counterintuitive because it suggests a net zero movement, despite the obvious action of firing a bullet.
Why It Actually Doesn't Violate Physics:

Conservation of Momentum Still Applies:

In reality, the scenario still respects the conservation of momentum. When the astronaut fires the gun, the total momentum of the system (astronaut + gun + bullet) is conserved. The bullet moves forward, and the astronaut moves backward. When the bullet is redirected back and caught or hits the astronaut, its momentum is transferred to the astronaut. This transfer adds to the astronaut’s momentum in the same direction as the recoil, not canceling it out.
Newton's Third Law in Action:

Newton's third law is indeed at play here. When the bullet is redirected, it exerts a force on the barrel, and the barrel exerts an equal and opposite force on the bullet. However, this interaction involves complex forces due to the curvature of the barrel and does not directly translate into a simple forward or backward movement of the astronaut.
Energy Considerations:

Energy loss through heat, sound, and friction, especially in the curved barrel scenario, plays a significant role. These energy losses mean that not all the energy from the gunshot is efficiently used for propulsion. This aligns with the principles of physics, as energy conservation doesn't imply that all energy must be converted to useful work.
In conclusion, while the scenario might seem to challenge basic physics principles at first, a more detailed analysis shows that it actually adheres to these principles, including the conservation of momentum and Newton's third law. The apparent paradox arises from a simplified view of the situation and is resolved when considering the full complexity of the forces and energy transformations involved.