B Propellantless Rocket: Unleashing the Power of Breit-Wheeler

[dansmith170]

TL;DR Summary

A rocket that builds its own propellant.

Hi,

I've posted recently about multiphoton Breit-Wheeler matter production. The goal I had in mind for producing a large quantity of positrons and electrons was so that a rocket could make its own propellant without having to carry any propellant along with it. Ultimately, the goal being to send a spaceprobe to the Alpha Centauri star system in less than 500 years.

https://en.wikipedia.org/wiki/Breit–Wheeler_process

I don't think the rocket equation exactly applies since there is no onboard propellant to speak of.

The bad news is we would need to find a way to produce a total of 1e24 electrons in 1 second (assuming a 1000 kilogram spaceship) in order to produce 1000 N of force per second on the spaceship. (I can show my reasoning on that if you're interested).

I guess it's kind of unrealistic, although perhaps not impossible! What do you think? Any comments, suggestions, or objections are welcome.

 

[Ibix]

You'd have to carry trapped photons or an energy source to produce them. Either of those has mass (mass does not add linearly, so a system of two photons can have mass even if single photons don't).

 

[berkeman]

Maybe compare the efficiency of using your battery (or solar) power to make electrons and positrons to eject, versus using a high-efficiency light production process to just shoot photons out the back of the rocket...?

 

[dansmith170]

You'd have to carry trapped photons or an energy source to produce them. Either of those has mass (mass does not add linearly, so a system of two photons can have mass even if single photons don't).

Yes, thanks for pointing that out. The energy source could maybe be an array of lasers across the diameter of the solar system that would aim for the rocket to supply energy to it. That, I think, allows one to get around the problem of onboard propellant.

 

[Ibix]

Yes, thanks for pointing that out. The energy source could maybe be an array of lasers across the diameter of the solar system that would aim for the rocket to supply energy to it. That, I think, allows one to get around the problem of onboard propellant.

In that case, compare efficiency to a laser driven solar sail.

 

[dansmith170]

Maybe compare the efficiency of using your battery (or solar) power to make electrons and positrons to eject, versus using a high-efficiency light production process to just shoot photons out the back of the rocket...?

I'm personally kind of skeptical that light momentum can be used to accelerate a spaceship (I understand that it can impart momentum to electrons, I'm just skeptical about an entire spaceship gaining momentum through light).

 

[dansmith170]

In that case, compare efficiency to a laser driven solar sail.

I don't think light can push physical objects (except electrons I guess). Is there experimental evidence for this?

 

[Bandersnatch]

Physical objects contain electrons. That radiation exerts pressure follows from Maxwell's equations, and first experimental evidence was published over a hundred years ago (see Nichols radiometer). There are even spacecraft demonstrating the physics in practice: the IKAROS solar sail, the involuntary 'experiment' of the Pioneer anomaly, and any spacecraft that needed to take radiation pressure into account in trajectory calculations.

 

[berkeman]

I'm personally kind of skeptical that light momentum can be used to accelerate a spaceship (I understand that it can impart momentum to electrons, I'm just skeptical about an entire spaceship gaining momentum through light).

https://en.wikipedia.org/wiki/Photon_rocket

 

[Astronuc]

I'm personally kind of skeptical that light momentum can be used to accelerate a spaceship (I understand that it can impart momentum to electrons, I'm just skeptical about an entire spaceship gaining momentum through light).

One would need a lot of light (photons) for an appreciable thrust.

See my comment here
https://www.physicsforums.com/threads/breit-wheeler-matter-production.1010438/post-6581023

There have been many considerations of propulsion systems using as little matter as possible.

I have seen some wacky ideas proposed, e.g., a systems storing positrons to be combined with electrons for propulsion. A very nutty idea.

Others have proposed lasers beamed from Earth or Earth orbit.

Still another study proposed using the 'solar wind' by capturing it and accelerating it backward.

Most systems require some massive structure, so one needs to consider the energy/mass for the energy production and propulsion system, and the thurst/mass ratio, which determines who fast one can accelerate.https://www.nasa.gov/feature/deep-in-directed-energy-propulsion-for-interstellar-exploration/
https://dsiac.org/articles/nasa-dir...-in-project-investigates-photonic-propulsion/
https://www.forbes.com/sites/starts...ing-you-to-mars-anytime-soon/?sh=50d83bf642cf

https://www.nasa.gov/directorates/spacetech/niac/index.html
https://www.nasa.gov/directorates/spacetech/niac/NIAC_funded_studies.html

 

[mfb]

Just radiating the light backwards produces more thrust than doing pair production and ejecting the produced particles, even if the production would be 100% efficient. It's not - not even close.

I'm personally kind of skeptical that light momentum can be used to accelerate a spaceship (I understand that it can impart momentum to electrons, I'm just skeptical about an entire spaceship gaining momentum through light).

Spacecraft have electrons, and light sails have been tested successfully in space.

 

[dansmith170]

Physical objects contain electrons. That radiation exerts pressure follows from Maxwell's equations, and first experimental evidence was published over a hundred years ago (see Nichols radiometer). There are even spacecraft demonstrating the physics in practice: the IKAROS solar sail, the involuntary 'experiment' of the Pioneer anomaly, and any spacecraft that needed to take radiation pressure into account in trajectory calculations.

I did not know about the IKAROS solar sail. I stand corrected, and I learned something new today. Seems that light can make things move. Thanks!

 

[dansmith170]

One would need a lot of light (photons) for an appreciable thrust.

See my comment here
https://www.physicsforums.com/threads/breit-wheeler-matter-production.1010438/post-6581023

There have been many considerations of propulsion systems using as little matter as possible.

I have seen some wacky ideas proposed, e.g., a systems storing positrons to be combined with electrons for propulsion. A very nutty idea.

Others have proposed lasers beamed from Earth or Earth orbit.

Still another study proposed using the 'solar wind' by capturing it and accelerating it backward.

Most systems require some massive structure, so one needs to consider the energy/mass for the energy production and propulsion system, and the thurst/mass ratio, which determines who fast one can accelerate.https://www.nasa.gov/feature/deep-in-directed-energy-propulsion-for-interstellar-exploration/
https://dsiac.org/articles/nasa-dir...-in-project-investigates-photonic-propulsion/
https://www.forbes.com/sites/starts...ing-you-to-mars-anytime-soon/?sh=50d83bf642cf

https://www.nasa.gov/directorates/spacetech/niac/index.html
https://www.nasa.gov/directorates/spacetech/niac/NIAC_funded_studies.html

Ok, thanks for the links too.

 

[dansmith170]

https://en.wikipedia.org/wiki/Photon_rocket

Ok, thanks for the link.

 

[Nugatory]

I don't think light can push physical objects (except electrons I guess). Is there experimental evidence for this?

Yes.

Comet tails are one of the most cited examples, but we’ve also measured satellites drifting under the influence of radiation pressure. Less obviously, much of the force preventing stars from collapsing under their own weight is due to radiation pressure, and lasers are routinely used to compress gas in fusion experiments. Radiation pressure from the primary is also used to compress the secondary in a multistage nuclear weapon.

And a Google search will find plenty of papers describing lab experiments directly measuring the radiation pressure.

 

[mfb]

and lasers are routinely used to compress gas in fusion experiments. Radiation pressure from the primary is also used to compress the secondary in a multistage nuclear weapon

As far as I understand these are largely the result of the heated outer material, not directly radiation pressure.
Wikipedia has a comparison.

 

[Nugatory]

As far as I understand these are largely the result of the heated outer material, not directly radiation pressure.
Wikipedia has a comparison.

Ah - interesting. I may not have looked at this stuff since the Morland article.

 

[Klystron]

A basic radiometer -- a common toy in the 1950's -- demonstrates the principle on one's tabletop, even providing comparisons of different light sources at various intensity.

My first radiometer consisted of a four-vane rotor balanced on a needle point within an evacuated glass globe; each vane painted black on one side, reflective white on the other.

One can count revolutions at low light (EMR) intensities; tracking time on a stopwatch. Eventually, I measured rapid rotation at higher intensities by recording audio of the spinning vanes in a quiet room, then comparing audio signals from various sources using different filters such as UV, visible colors, and IR.

Crooke's radiometer reacts across a swath of the electromagnetic spectrum depending on several factors including the size and shape of the vanes and quality of the apparatus.

 

[mfb]

A basic radiometer works by heating the gas next to the darker side. If photon pressure would be driving the rotation then the lighter side would feel a stronger force (reflection is more efficient than absorption).

 

[Bandersnatch]

You want the Nichols radiometer, not the Crookes one that's in the picture.