Photons have mass!!

GCNRevenger · 2004-03-19 09:15:46

Sure its easy with a small solar pannel that weighs a few hundred kilos, but you are talking about a mirror thats bigger around than a small town and will weigh quite a bit. And when you are in space, with somthing really heavy, are you pushing it or is it pushing you? ...it will make it extremely difficult to maneuver.

And when you are in blackout with your VASIMR engine, how do you propose to keep it from melting? Those superconductor magnets won't keep without active cooling for long, at which point it will fail and explode. Thats why you need continuous power, which you can't get from storage with this scale (multimegawatt).

And yes photons will impart momentum to atoms, provided the electrons remain bound to the atoms. In a solar cell, the electrons don't remain bound up and infact this is why they work; the electrons come off the surface of the junction and move through the wires, becoming electricty, not adding their momentum to the atom. So no, a solar cell will make a lousy solar sail.

And again, photons do not have rest mass. They are free energy packed into a neat little bundle, they are not matter as you traditionally think. They do not cause objects to accelerate because of collision, because photons cannot collide with objects without being destroyed.

ERRORIST · 2004-03-19 09:22:48

So yes a solar cell will make a great solar sail because the photon does two things.It will impart momentum to atoms, provided the electrons remain bound to the atoms. It will also impart momentem at the same time. It is like killing two birds with one stone.

GCNRevenger · 2004-03-19 09:25:03

No, because the electrons DONT stay bound to the atoms, they move, so they don't push the atoms of the parent material. It cannot impart any substantial momentum unless the electron stays asscociated with the atoms of its parent material, which does not happen in solar cells, otherwise you would get no current.

ERRORIST · 2004-03-19 09:31:03

The moment the electron becomes reassociated with another atom is when the momentum exchange occurs.Also, what happens to the photon?

GCNRevenger · 2004-03-19 09:37:51

No, because the momentum of the electron, that is electricty, is used to power whatever you are doing and will STAY unassociated until the excess energy is used up, stopping the flow.

The photon ceases to exsist. Being that it IS energy, and its energy is transferred to the electron, its GONE.

ERRORIST · 2004-03-19 09:54:48

But doesn't that electron make another photon? Also, at the time of impact the kinetic energy from the photon is transferred to the electron before the energy absorbtion phase occurs in the electron. As you can see you killing two birds with one stone. What force holds the electron to the proton?

SBird · 2004-03-19 10:43:30

Zubrin's salt water reactor has higher thrust, no argument. However, the photonic drive has a much better ISP. The energy extraction fron the fissible material approaches 100% since you are basically using all of the emitted visible and IR waste from the reactor. Also, with the SWNR, you are carrying around fuel that is at least 50% water which provides no thrust.
If you were using the SWNR for interplanetary travel, I'd agree that it's better but really, I doubt that a plume of highly radioactive exhaust, even if it is at sun escape velocity will win you many friends. Where these drives are the best is interstellar probes. In that regime, the photonic drive is inherently more efficient and better since you are dealing acceleration periods of years and travel times of centuries.

Having just re-read the SWNR proposal, it appears that the proposed Titan mission SWNR would have an energy flux of 427 GW. That's with a 90% enriched uranium salt at about 3% concentration. Getting a 1 GW energy flux from a solid core reactor is easy - keeping it from exploding or vaporizing is probably the hard part.

I just don't know what the practical limit for thermal transfer is in materials. Obviously, the outer tungsten jacket can easily handle 1MW/m^2 of thermal flux but at what point does the interior structure of the reactor just turn to plasma? I imagine that a 1MW core would be fine. With a 1GW core, though, I have trouble envisioning the heat being able to get out to the radiator fast enough to prevent the reactor from exploding.

Some naive calculations seem to show that if you've got a molten core reactor jacketed with a high surface area, C12 isotopically enriched, 2cm thick diamond jacket with a tungsten thin film coating, a 1GW reactor wouldn't melt. Of course, the chemical reactions and pressures generated would probably render such a reactor useless in a fraction of a second.

Oh well, it was a good idea while it lasted.

GCNRevenger · 2004-03-19 15:26:08

A new photon is not always generated when an electron gains energy, this is only ONE way that it can disapate the excess. Mirrors do this >90% of the time generally speaking, some mirrors being very close to 100%, but most materials convert the excess energy into other forms.

And no, there is no "two birds - one stone" because the process of the photon being conveted into the motion of the electron is instantainious by nature of the photon's energy character.

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Yeah even if the fancy photonic engine didn't melt down from its own heat, you still have to worry about the kenetics of the fragile carbon-carbon bonds in the diamond coming lose and reverting to carbon black graphite. Diamond is actually a higher energy form of carbon than graphite or fullerines are, and will simply fall apart at high temperatures. Diamonds can burn even.

The NSWR that Zubrin cooked up would be a viable political alternative to the Orion engine, but worse environmentally. I wonder if the Uranium fuel could be shipped up as the oxide free metal, combined with whichever substance you want to make it soluble, and then put that in water mined from asteroids/Moon or brought up on another ship.

SBird · 2004-03-19 16:24:42

You're right about the diamond-graphite conversion. Although graphite is a great heat conductor in-plane, it's a terrible heat conductor out of plane. Of course, by Murphy's law, the planes would all probably be normal to the heat source.

I looked at the numbers a bit more and the nuclear photonic come out on top if you run the engines at a lower temperature. However, your thrust goes down to tiny levels (tens of millinewtons) and you definately don't end up going anywhere fast. It's a better drive system if you're going 100 LY or so and don't mind a travel time of a few thousand years.

A NSWR as a first stage to get a decent starting speed and then a photonic second stage to derive thrust for several hundred years would be an ideal system to go to, say 18 Scorpii.

Earthfirst · 2004-03-22 13:48:11

I think think that 70 virgens is a great star to go too.
Is that were the terrist go after they blow them selfs up?

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#76 2004-03-19 09:15:46

Re: Photons have mass!!

#77 2004-03-19 09:22:48

Re: Photons have mass!!

#78 2004-03-19 09:25:03

Re: Photons have mass!!

#79 2004-03-19 09:31:03

Re: Photons have mass!!

#80 2004-03-19 09:37:51

Re: Photons have mass!!

#81 2004-03-19 09:54:48

Re: Photons have mass!!

#82 2004-03-19 10:43:30

Re: Photons have mass!!

#83 2004-03-19 15:26:08

Re: Photons have mass!!

#84 2004-03-19 16:24:42

Re: Photons have mass!!

#85 2004-03-22 13:48:11

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