OT: Sci-fi fantasy - Torchships! - a reaction

Seems you are talking about Robert L Forward's book "industinguishable from magic".

The best you can do in practice is around 42% efficiency by dropping stuff into a spinning black hole in the right direction.

A working number for astrophysics is more like 10% efficiency for a typical object although quasars seem to manage something closer to

30-35% - this compares with a puny 0.7% for nuclear fusion in stars.

Most efficient conversion of mass to energy known apart from matter antimatter annihilation which is obviously 100% efficient but requires a ready source of antimatter to be viable. The latter is rarer than hens teeth.

Regards, Martin Brown

What about Hawking radiation? If it really exists, then a small enough black hole should be converting it's mass to energy quite vigorously, never mind any infalling matter.

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http://www.wescottdesign.com

I tried once, too -- I assumed it was just me.

Presumably if you can arrange for the black hole, you could arrange for the nuclear fusion.

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http://www.wescottdesign.com

That is "only a theory" - although I think most physicists do think he is probably right about it there is no prospect of observational astrophysical evidence of it in the forseeable future. Most black holes we know about are far too big to have significant Hawking radiation and they will outlive all other matter in the universe. Cosmos has a nice popular article on the timeline of our universe from 0 to 10^100 years.

If the LHC ever manages to make a nano BH then we will know for sure!

Interestingly there may be other laboratory experiments that could demonstrate analogous behaviour in extremely non-linear optics.

If that experiment is confirmed then it would pave the way for Hawking to win a Nobel prize for Physics for his work on black holes.

Regards, Martin Brown

ISTR at the time it was thought that some small ones may have been formed in the big bang - so that they might be "expiring" right now.

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John Devereux

That's because I don't bother to reply to your stuff, because you're not only an idiot, but actively hostile toward me for noticing that you're an idiot.

I haven't "reinvented" squat, albeit my question is equally applicable to the Bussard ramjet - how many joules of energy are released in one

4H -> 1He event, and how does that translate to the velocity of that helium atom if all the kinetic energy went to shoving the helium atom out the exhaust, and how much thrust would it produce?

But, having no answer, you go into your old familiar ad hominem attack mode.

If you can be civil, and actually answer a simple question rather than go on a personal crusade, then I might respond in kind. Otherwise, you're more than welcome to go f*ck yourself.

Thanks, Rich

Indistinguishable.

Probably. Unfortunately, we are moving house, and my science fiction collection has been reduced by around 98%, so I can't easily check.

-- Bill Sloman, Nijmegen

Thanks for the correction. I was aware that my "almost 100%" was a very rough approximation, but detailed calculation of the efficiency of a fantasy machine isn't a sensible way of spending one's time.

Astronomers are presumably interested in the hypothetical proposition that at least some quasars, pulsars and X-ray bursters are more or less local alien space-craft travelling directly away fom us at some appreciable fraction (red-shift)of the speed of light, which would complicate the interpretation of observational astronomical results ...

Not quite that rare. There are physics experiments that make a few atoms of anti-hydrogen, while hens do seem to be uniformly tooothless.

-- Bill Sloman, Nijmegen

There is still some haggling about that - and it may well be that they do exist and are expiring now but manage to be out of reach of our present instrumentation (but possibly not by much). eg

(popular science version)

Observation of a primordial BH evaporating would be a huge coup!

(more technical)

Tricky thing is they will be very fast transients - so hard to pin down.

Regards, Martin Brown

;)

Jon

Ignoring the problem that you would never be able to get a 4 particle H nuclear reaction to go in the first place. D2 + H1 -> He3 would be more realistic and has the lowest activation energy with a net yield 5.5MeV. The numbers sound better in MeV and a rough guide in stars is 26MeV per Helium4 nucleus formed (accounting correctly for energy that escapes with fast neutrinos).

If you ignore energy lost to neutrinos then the total energy released by fusing four hydrogen atoms is 0.7% efficient at converting rest mass into energy. back of the envelope.

KE = (4mH - mHe)c^2/Na = (4x1.007825-4.002603)x9x10^16/(6x10^23) = 4.3 nJ

Where c is speed of light, Na Avagadro's constant.

Avoiding the conversion to Joules KE of Helium afterwards is from

4mHc^2 = gamma.mHe.c^2

Hence gamma = (4mH/mHe) ~ 1.007

Or ignoring relativistic effects entirely KE = 1/2 mHe. v^2

v^2 ~ 4.3x10^-9 x 2 x 6 x 10^23/4 ~ 12 x 10^14 ~ 3.4x10^7m/s

Or roughly 10% of the speed of light so relativistic effects are not quite negligible at around 0.5% but neither are they strong.

Regards, Martin Brown

The bad news is that you have to drag the black hole along for the ride. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Just balance it out with an equal negative mass.

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John Devereux

alli

I don't think that a "dead in the egg embryo chick" counts as a "hen", though.

--
http://www.wescottdesign.com

Well, if it's OK to be rude, how come you're not smart enough to look up the atomic weights of hydrogen and helium, apply E=mc^2, and go from there?

--
http://www.wescottdesign.com

Since we're fantasizing anyway, we can fantasize that we've figured out how to make it work with a CNO cycle.

--
http://www.wescottdesign.com

Since I consider you to be close to brain dead, I don't attach much significance to your opinions, and certainly wouldn't bother with active hostility. I do correct the nonsense you post from time to time, but that's for the benefit of lurkers - I couldn't care less about how you feel about being corrected.

Here is a link that gives the mass defect of the helium atom in units that you can translate into kilograms. Einstein tells us that e=3Dm.c^2, so you can get the energy in joules.

tm

You know the mass of the leium atom, so you can calculate it's velocity from e=3D0.5m.v^2, which gives you the momentum involvd in squirting the helium atom out of the exhaust.

It isn't thrust, but that's because your question was ill-posed - what you want to know is specific impusle.

I've got an answer, but since you couldn't even ask the right question, getting you into a state where you could appreciate the answer to the right question is a bigger job that it would be sensible to tackle in this forum.

You might consider posting your questions to a more suitable user- group - something like elementary.physics.basics if it exists. As for being civil, you might think about setting an example.

My post did point out the inadequacies of your post, but I was tolerably civil about it.

-- Bill Sloman, Nijmegen