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I suggest getting used to Newtonian physics before starting to mix in relativistic and quantum chromodynamic physics.

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If make a barrel with mirror walls, fill it with light on the Earth, and attach to the ship, the ship will need no reaction to move, it may just open a tiny hole in one end of the barrel, letting the photons escape.
But it would be a lot of photonic mass stored inside, and the barrel walls would be enough strong to withstand the radiation pressure.

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9 hours ago, kerbiloid said:

If make a barrel with mirror walls, fill it with light on the Earth, and attach to the ship, the ship will need no reaction to move, it may just open a tiny hole in one end of the barrel, letting the photons escape.
But it would be a lot of photonic mass stored inside, and the barrel walls would be enough strong to withstand the radiation pressure.

Every time a photon hits one of the mirrors inside your bucket there is a reaction, it just gets cancelled out by the other reflections on the other mirrors(unless you open a hole, then there will be some uncanceled reactions when the light escapes instead of bouncing off the mirror where there is now a hole).

 

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12 minutes ago, Terwin said:

Every time a photon hits one of the mirrors inside your bucket there is a reaction, it just gets cancelled out by the other reflections on the other mirrors(unless you open a hole, then there will be some uncanceled reactions when the light escapes instead of bouncing off the mirror where there is now a hole).

Depends

https://www.quora.com/How-do-photons-get-trapped

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19 hours ago, Cheif Operations Director said:
19 hours ago, Snark said:

Not correct.

It's trading the momentum of the fuel (its mass times its velocity) for the momentum of the ship (its mass times its velocity).

Ok thanks

This also leads to an intuitive understanding of why engines with high exhaust velocity are more fuel-efficient than engines with low exhaust velocity.

On Earth, we talk about fuel efficiency in the context of distance, because an automobile must continually expend energy in order to keep moving against the drag of the air and the ground. With a rocket, however, we talk about efficiency in the terms of fuel consumption rate. The technical term is "thrust-specific fuel consumption", i.e., the amount of fuel/propellant being expended every second in order to produce a specific amount of thrust. Since thrust is just force divided by time, you can cancel out the "per second" on both sides, which gives you the amount of fuel required to produce a specific amount of impulse, or change in momentum. 

Suppose you have a spaceship with a mass of 1 kg and a prop tank containing 10 kg of propellant. The initial mass of the system is 11 kg and the initial momentum of the system is zero. If you throw 1 kg of props out the back at -100 m/s, it has a momentum of -100 kg*m/s. By the conservation of momentum, the spaceship must have an equal and opposite momentum of +100 kg*m/s. The spaceship now masses 10 kg, so dividing its +100 kg*m/s by 10 kg gives you a positive velocity of 10 m/s.

But if your propellant is more volatile (or, perhaps, is being accelerated by an extreme heat source like a nuclear reactor), it can come out the back end of your spaceship with much greater speed. For example, if you can throw propellant out the back of your spaceship at -1,090 m/s, then you only need to use 0.1 kg for the same effect. 0.1 kg * -1,090 m/s = -109 kg*m/s, and so the spaceship has a resulting momentum of +109 kg*m/s, and dividing by the remaining spaceship mass of 10.9 kg gives you a positive velocity of 10 m/s.

9 hours ago, kerbiloid said:

If make a barrel with mirror walls, fill it with light on the Earth, and attach to the ship, the ship will need no reaction to move, it may just open a tiny hole in one end of the barrel, letting the photons escape.
But it would be a lot of photonic mass stored inside, and the barrel walls would be enough strong to withstand the radiation pressure.

You'd also need a way to collimate the photons so that their thrust pushed you in the correct direction rather than dispersing out at angles.

And, as @kerbiloid's link indicates, photons do not actually remain trapped in resonant cavities indefinitely. Even with a perfectly-reflective mirror (which is itself impossible), there is a small but nonzero probability that the wavefunction of the photon will tunnel through, either being absorbed by the mirror substrate and turning into heat, or passing through entirely and escaping. 

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Posted (edited)
11 minutes ago, sevenperforce said:

You'd also need a way to collimate the photons so that their thrust pushed you in the correct direction rather than dispersing out at angles.

And, as @kerbiloid's link indicates, photons do not actually remain trapped in resonant cavities indefinitely. Even with a perfectly-reflective mirror (which is itself impossible), there is a small but nonzero probability that the wavefunction of the photon will tunnel through, either being absorbed by the mirror substrate and turning into heat, or passing through entirely and escaping. 

Of course. They are not storable propellant.

P.S.
This is a joke idea, just in case.

Upd.
Though, who knows what will they use as mirrors in future.

Edited by kerbiloid

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38 minutes ago, kerbiloid said:

Of course. They are not storable propellant.

P.S.
This is a joke idea, just in case.

Upd.
Though, who knows what will they use as mirrors in future.

There is work with photons in quantum systems where the wavefunctions can be trapped in closed timelike curves, which should remove the capacity for quantum tunneling. Imagine if we had a quantum photon rocket powered by entangled particles in time-traveling curves!

Or of course we could capture a black hole inside a resonating cavity and feed it mass whenever we wanted to fire our photon rocket.

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3 minutes ago, sevenperforce said:

There is work with photons in quantum systems where the wavefunctions can be trapped in closed timelike curves, which should remove the capacity for quantum tunneling. Imagine if we had a quantum photon rocket powered by entangled particles in time-traveling curves!

If @FreeThinker could be so kind to add it.

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5 hours ago, sevenperforce said:

we could capture a black hole inside a resonating cavity and feed it mass whenever we wanted to fire our photon rocket.

Just don't feed it after midnight

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This in turn makes to imagine the midnight, the midday, the midmorning sunrise holerise, and the midevening sunset holeset on a Gargantua close planet.

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On 8/12/2019 at 12:23 AM, kerbiloid said:

If make a barrel with mirror walls, fill it with light on the Earth, and attach to the ship, the ship will need no reaction to move, it may just open a tiny hole in one end of the barrel, letting the photons escape.
But it would be a lot of photonic mass stored inside, and the barrel walls would be enough strong to withstand the radiation pressure.

I feel like your "fuel tank" would have rather poor mass ratio..........

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8 hours ago, sevenperforce said:

I feel like your "fuel tank" would have rather poor mass ratio.

If instead of material mirrors use some immaterial physical effect (for strength), it can be as dense as photons of annihilation in the very beginning, i.e. as dense as solid materials and even denser.
You can treat this propellant as "post-antimatter" or "post-annihilation photons".
Like you don't annihilate antimatter while flying, but doing this in advance, and taking the annihilation results as a fuel.
This engine doesn't need a huge nozzle, just manage its transparency in desired spots on the tank surface.

Edited by kerbiloid

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I have to say thank you to Cheif Operations Director for continuing to argue for what he thinks is right when everyone tells him he is wrong, and for remaining positive when abuse comes his way.  That is the stuff I like to see on this forum, and in other areas of life.  

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On 8/14/2019 at 12:28 AM, kerbiloid said:

If instead of material mirrors use some immaterial physical effect (for strength), it can be as dense as photons of annihilation in the very beginning, i.e. as dense as solid materials and even denser.
You can treat this propellant as "post-antimatter" or "post-annihilation photons".
Like you don't annihilate antimatter while flying, but doing this in advance, and taking the annihilation results as a fuel.
This engine doesn't need a huge nozzle, just manage its transparency in desired spots on the tank surface.

At some point you create a kugelblitz, at which point, why not just go with a black hole starship?

Collimation of the thrust beam is always going to be the challenge.

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