Phonons (kintetic waves) in a plasma?

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    Phonons Plasma Waves
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Discussion Overview

The discussion revolves around the concept of using phonons and explosions in a thruster design for space propulsion, particularly focusing on the mechanics of fuel ejection and momentum conservation. Participants explore the implications of phonons in plasma, the nature of explosions, and the dynamics of momentum in relation to spacecraft propulsion.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant proposes a thruster design that utilizes phonons to push spent fuel back into the spaceship for recycling.
  • Another participant points out that phonons cannot propagate through a vacuum, questioning the feasibility of the initial proposal.
  • Several participants clarify that an explosion is not a vacuum and discuss the mechanics of how explosions can propel a spacecraft by ejecting mass.
  • There is a discussion about conservation of momentum, with some participants arguing that if gases are sucked back into the spaceship, the overall speed would not increase due to momentum conservation.
  • One participant raises the idea that momentum is vectored, suggesting that the explosion could push the spaceship in one direction while the returning mass could affect its motion.
  • Another participant emphasizes that the force of an explosion is due to the ejection of mass, and without this ejection, there would be no acceleration.
  • There is a mention of electric cars and how their momentum changes, with a discussion on mass and energy transfer during acceleration.
  • Participants discuss the relationship between stars, mass loss, and the mechanics of solar sails, questioning the assumptions about stars gaining mass over time.
  • One participant expresses a desire to understand the effects of vibrations on fuel trajectories during explosions.

Areas of Agreement / Disagreement

Participants express differing views on the role of phonons in plasma and the mechanics of propulsion through explosions. There is no consensus on the implications of momentum conservation in the context of the proposed thruster design, and the discussion remains unresolved regarding the relationship between star mass and solar sails.

Contextual Notes

Participants highlight limitations in terminology and assumptions regarding the nature of phonons and mechanical waves. The discussion also reflects unresolved questions about the dynamics of momentum and the effects of vibrations on fuel trajectories.

treehouse
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What I am imagining is a thruster in space igniting a fuel to create an explosion to push a spaceship and phonons (mechanical waves) pushing the spent fuel to the sides of the thruster where vents vacuum it back into the spaceship for recycling. What do you think of this?
 
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slight problem there, phonons cannot propagate through vacuums
 
An explosion is not a vacuum.
 
treehouse said:
An explosion is not a vacuum.

Can you provide a citation where phonons exist in plasma? There is a reason why there is a "plasmon".

Zz.
 
Besides the problem of using proper terms (not any mechanical wave means phonons), it seems you'll end up with the same speed as before you ejected the gases.
Assuming no forces external to the spaceship, if the gases are sucked back inside you will end up with the same speed as before ejecting them (or less), just from conservation of momentum. Same final mass, same momentum...
 
nasu said:
not any mechanical wave means phonons
I'm not being sarcastic when I tell you "I want to be lectured to on this."

nasu said:
, it seems you'll end up with the same speed as before you ejected the gases.
Assuming no forces external to the spaceship, if the gases are sucked back inside you will end up with the same speed as before ejecting them (or less), just from conservation of momentum. Same final mass, same momentum...
But isn't momentum vectored? Can't the explosion push the spaceship one direction and the mass of the spent fuel going back into the spaceship push it in basically the same direction? Besides, when an electric car accelerates doesn't its momentum increase even though its resting mass doesn't change?
 
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The reason an explosion can push something in a direction is because it is ejecting mass away from the vehicle. If there was no mass ejected then there wouldn't be any acceleration.

Besides, when an electric car accelerates doesn't its momentum increase even though its resting mass doesn't change?

The electric motor is using energy from the batteries. The charged battery has more mass than the empty battery, with the vehicle losing mass through friction and heat and such. If you run the vehicle until the battery is dead and then stop, the vehicle will have less mass.
 
Drakkith said:
The reason an explosion can push something in a direction is because it is ejecting mass away from the vehicle. If there was no mass ejected then there wouldn't be any acceleration.

But isn't it the force of the explosion pushing all things within its range away from its center that pushes both the mass and the vehicle?
 
treehouse said:
But isn't it the force of the explosion pushing all things within its range away from its center that pushes both the mass and the vehicle?
Yes. I think you are saying the same thing we are, but in a different way. If I were to "shoot" high velocity particles at a rocket nozzle in space, the vehicle it was connected to would be accelerated. The same effect is happening when you use a rocket. Ideally the velocity of the exhaust would be zero,(I think) as that would mean that 100% of the energy of the combustion would be input into acceleration of the rocket, but that is not reachable.

I just want you to understand that the "eplosion" IS the fuel. If no fuel exhaust went backwards, there would be no explosion in that direction.
 
  • #10
treehouse said:
I'm not being sarcastic when I tell you "I want to be lectured to on this."

But isn't momentum vectored? Can't the explosion push the spaceship one direction and the mass of the spent fuel going back into the spaceship push it in basically the same direction? Besides, when an electric car accelerates doesn't its momentum increase even though its resting mass doesn't change?

Yes, the momentum is a vector.
In order for the spaceship to move forward, the gases ejected must have a velocity with at least some component in the opposite direction.
If you want to somehow bring the gas back in the spaceship you need to reverse the direction of the momentum of these gases. The effect of this will be a deceleration of the spaceship.
The components of the momentum of the gas particles transverse to the direction of the motion are irrelevant. They cancel out all the time. Once the gas is ejected, no internal process can change its overall momentum.

A car of any kind moves due to the interaction with the road and the Earth. If you consider the system car-road, the motion of the car forward results in a change in the momentum of the Earth (or maybe just part of it) in the opposite direction.
Imagine you are in the middle of long boat. You can move towards one end of the boat and in the process the boat will move in the opposite direction because you are pushing it with your feet. You gain some forward momentum (relative to the water) and the boat an opposite momentum. If you replace the boat with the Earth, the velocity gained by the Earth is so much times smaller that we usually neglect it. But it is there.
Going back to the boat, your idea would be similar to a scenario in which once you reach one end of the boat, you try to pull the boat back (under your feet) so you are again in the middle and start all over.

Regarding phonons, you can read any book on introductory solid state or read a little on Wikipedia: http://en.wikipedia.org/wiki/Phonon
 
  • #11
nasu said:
A car of any kind moves due to the interaction with the road and the Earth. If you consider the system car-road, the motion of the car forward results in a change in the momentum of the Earth (or maybe just part of it) in the opposite direction.
Imagine you are in the middle of long boat. You can move towards one end of the boat and in the process the boat will move in the opposite direction because you are pushing it with your feet. You gain some forward momentum (relative to the water) and the boat an opposite momentum. If you replace the boat with the Earth, the velocity gained by the Earth is so much times smaller that we usually neglect it. But it is there.
Going back to the boat, your idea would be similar to a scenario in which once you reach one end of the boat, you try to pull the boat back (under your feet) so you are again in the middle and start all over.
What about solar sails? Don't stars get more massive over time?
 
  • #12
treehouse said:
What about solar sails? Don't stars get more massive over time?

What about them? I am not sure what point are you trying to get to.
And from what I know, the stars, if anything, will loose mater and energy over time. Why would you think they get more massive? And how is related to the topic (whatever that is)?
 
  • #13
nasu said:
What about them? I am not sure what point are you trying to get to.
And from what I know, the stars, if anything, will loose mater and energy over time. Why would you think they get more massive? And how is related to the topic (whatever that is)?
Some stars get less massive; but some stars which emit photons that can push solar sails turn into black holes - and when stars get less massive it is because as part of a nuclear reaction generating an immense amount of energy they eject matter which was previously relatively stationary inside the star.
 
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  • #14
  • #15
treehouse said:
Some stars get less massive; but some stars which emit photons that can push solar sails turn into black holes -
This statement has quite a few problems.
Most of the stars emit photons and any photons can be used to push solar sails, in principle (there are no special photons for this). And how is the star becoming a black-hole related to photons pushing the sails??
And by the way, when a star becomes a black-hole does NOT became more massive.

In rest, I still don't understand your goal. Do you aim to find a case when the conservation of momentum does not hold?
 

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