Highly relativistic spaceflight would be fatal for passengers and instruments

http://www.scirp.org/journal/PaperInformation.aspx?paperID=23913

112 69 14 years ago

arikrak• 14 years ago

Why not use the speeding hydrogen in a nuclear reaction that fuels the ship, thereby killing 2 atoms with one stone? See http://en.wikipedia.org/wiki/Bussard_ramjet

jonathanyc• 14 years ago

Because, if I am reading the article correctly, some hydrogen (which has become radiation) will go through you anyway. This kills you. Somehow gathering up all of the radiation and trying to use it to fuel a ramjet is the same thing as trying to make some sort of shield, which the article says would be very difficult.

wging• 14 years ago

I think the idea is that it doesn't ever touch your ship--you just funnel it into your reactor directly. (You have to ionize it somehow...) So it's not equivalent to shielding--it's designed to avoid shielding.

maaku• 14 years ago

The article points out some reasons why it might be difficult (not impossible!) to construct the electromagnetic field required for such a ramjet, but yes this point seems to be completely lost on the author. A Bussard ramjet specifically exploits the presence of interstellar hydrogen as a valuable energy resource.

adrianhoward• 14 years ago

Unfortunately those who have worked out the physics for Bussard ramjets in detail find that they don't really work as an energy source or a propulsion mechanism while cruising. Indeed - more useful as a brake.

See http://www.centauri-dreams.org/?p=22115 for some pointers.

gliese13377• 14 years ago

Because nobody knows how to make a Bussard Ramjet actually work. From the abstract: "Stopping or diverting this flux, either with material or electromagnetic shields, is a daunting problem."

And assuming that you can divert the protons, ironically enough, you'd have a serious problem cooling them down sufficiently to achieve fusion (because if you don't, the plasma will not be sufficiently dense to cause a high probability of fusion events, despite the high temperature); and slowing their passage through the ship so they stick around long enough to fuse would create significant drag. And after you have solved the problems of collecting, matching velocity with, and cooling/compressing the plasma, a Bussard Ramjet is only worthwhile if you have an extremely efficient system for capturing all of the waste heat from those processes and injecting it back into the exhaust. Otherwise, at any speed above about .12c, you lose more energy to drag while collecting protons than you gain back from fusing them.

Now, there's no particular physical reason why all of those problems couldn't be solved. But if you've already got some way of accelerating to >.5c, and you just need to worry about radiation shielding, building a whole Bussard Ramjet rather than just trying to push protons out of the way with minimal effort or absorb them in some very massive physical shield, is not the way to go.

salimmadjd3• 14 years ago

I think once we are advanced enough to figure out how to propel anything that fast or able to pack the needed energy, this part might be easy to solve.

Remember per special relativity mass increases as we approach C. Also I think there will be other physiological problems while accelerating to C. It would take like 34 days at 10g acceleration to get to C. I wonder how our bodies would handle 34 days at that kind of g limits. At 2g it will take like 173 days to reach C.

Lastly, we don't really have a good physical understanding of matter at near C speed limits. With mass increasing as we approach C, I don't know we can assume the same physical properties of any material in classical models.

gliese13377• 14 years ago

> Lastly, we don't really have a good physical understanding of matter at near C speed limits. With mass increasing as we approach C, I don't know we can assume the same physical properties of any material in classical models.

Yes, we do, and yes, we can. Matter travelling at near c relative to us behaves exactly the same as any other matter. That's what frame invariance means. From our perspective, yes, the ship would seem to have more mass, which is a direct result of the equivalence of mass and energy and the fact that it has a crap-tonne of kinetic energy. But from the ship's own perspective, its mass does not change at all- rather, the entire surrounding universe seems to become more massive.

schiffern• 14 years ago

>It would take like 34 days at 10g acceleration to get to C.

So why not take 340 days at 1g?

There are many problems with high relativistic travel. This is not one of them.

salimmadjd3• 14 years ago

>So why not take 340 days at 1g?

Sure! My reasoning was why add 2 years just to accelerate and decelerate. And as I said, "once we are advanced enough to figure out how to propel anything that fast or able to pack the needed energy" there were other challenges to overcome.

Strilanc2• 14 years ago

You need to learn more about relativity.

First, people on the ship will not feel as if they "have more mass". Their physical properties will be exactly identical, because they are at rest in their frame of reference. Flying through a galaxy at 0.999c is identical to "sitting still" while a galaxy flies by at 0.999c.

Second, you're ignoring relativistic effects in your calculation of the amount of time it takes to "reach c". You can't just divide light speed by the acceleration and convert to days. You need to include the effects of time and space dilation, in which case you'll find you have to pick a target less than c because no matter how long you accelerate you only approach c.

salimmadjd3• 14 years ago

>>Flying through a galaxy at 0.999c is identical to "sitting still" while a galaxy flies by at 0.999c.spaceship.

You're right if you were sitting in a train and watched the station go by, but not at .999c. You're ignoring relativistic mass.

Unlike General Relativity where we were able to measure the effects of sun's gravity against the position of emitted light from the stars, The truth is we have no empirical evidence of the effects on solid objects (not single accelerated particles) approaching C.

mbq• 14 years ago

Another point for using warp drive (=

altrego99• 14 years ago

Meh. We just need radiation shields.

fleitz• 14 years ago

Which in this case would just be an incredibly strong magnetic field...

gliese13377• 14 years ago

Not necessarily. There are some interesting potential radiation issues with "warp drives" as well. For example, at the boundary of a warp bubble of the generally-Alcubierre-like variety, space is shearing / expanding such that virtual particle pairs are incapable of recombining, just like at an event horizon. That could potentially create a great deal of radiation draining energy from the warp field. Additionally, what happens to particles that are intercepted by the travelling bubble? They don't just disappear!

serfdomroad• 14 years ago

Do you have any other info/links on the problems associated with the Alcubierre?

Ygg2• 14 years ago

Wouldn't a jump drive work slightly better in this case?

rorrr• 14 years ago

Future world problems.

noonespecial3• 14 years ago

There's one thing I never quite got about these speculations. When you consider a ship leaving a solar system and accelerating, and the speed gained turning the "ambient hydrogen" into deadly radiation, why does it always seem to assume that hydrogen atoms are just kind of hanging out at low velocities relative to the home solar system and only turn into a problem when the bald captain says "engage"?

Its a big universe, might the hydrogen atoms already be traveling at these deadly speeds relative to the ship once it leaves Sol? Might there be a "galactic current" so that the ship can go with the flow?

I know its just for fun but its still silly to think that anyone who could propel a canned ape at .8c wouldn't be able fend off a few rouge hydrogen atoms.

gliese13377• 14 years ago

Because the hydrogen atoms we care about are bound in galactic orbit, and that means they have a relatively low maximum relative velocity to us. If they weren't, they would all fly out into the intergalactic void in geologically short timescales, star formation would cease, and we'd have no such radiation problems after all.

smoyer• 14 years ago

That might require us to change the name of the intergalactic void though.

jessriedel• 14 years ago

In short, because the Milky Way disk sets a preferred rest frame. Other matter in the galaxy all rotates with approximately the same speed around the galactic center. Here are some numbers to give you intuition.

Tangential speed of the galactic disk, obtained by averaging over nearby stars: ~ 250 km/s

Speed of sun (and other nearby stars, in random directions) with respect to the galactic disk: ~ 15 km/s

Speed of Earth with respect to the Sun: 30 km/s

Speed of satellites in low-Earth orbit: 7.8 km/s

Speed of light: 300,000 km/s.

If you want more, this article discusses these speeds in the context of estimating the dark matter velocity distribution.

http://pa.brown.edu/articles/Lewin_Smith_DM_Review.pdf

It's fairly accessible as far as physics articles go.

Even if you get outside the Milky Way, the cosmic microwave background sets a preferred rest frame. I can't find the exact speed of the Milky Way in that frame, but the speed of the Earth is ~360 km/s. (This means the Milky Way's speed is somewhere between 100 and 600 km/s.)

The take-home message is that the although the laws of physics have no preferred rest frame, the actual matter out there in the universe most certainly does.

noonespecial3• 14 years ago

Ahh. Understood. Most enlightening. Thanks for that.

etherealG• 14 years ago

thanks very much, this makes a whole lot more sense now

monochromatic• 14 years ago

The other take-home message is that all of the speeds in question are small relative to c. So trying to go with the flow won't get you anywhere very fast.

Nursie2• 14 years ago

'a few rouge hydrogen atoms'

It's rogue. Sorry, pet hate, can't help myself. That and people typing 'tounge' when they mean tongue.

mdpye• 14 years ago

Unless the atoms were moving away from the observer, then they might appear a little rouged.

noonespecial3• 14 years ago

Yes, sorry. I'm severely dyslexic. There's a limit to mechanical spelling help.

I meant the particles that are more like Han Solo and less like Darth Maul.

superkuh• 14 years ago

That's okay, there's time to work on the shielding. 0.03 c is the most that can be feasibly achieved using top-end near future technology like dusty plasma fission fragment rockets.

https://en.wikipedia.org/wiki/Fission-fragment_rocket

gliese13377• 14 years ago

Thermonuclear pulse propulsion (a la Project Orion) can do better than that- up to 0.1c. And that requires no unproven technology; just a lot of money and a lot of politics (and a lot of fission fuel to buy with that money; I dunno if it's more than you could actually get on the market or not, but it would definitely cost A Lot).

philhippus• 14 years ago

>a lot of black politics

FTFY

lmm• 14 years ago

IIRC you're talking on the order of several hundred nuclear bombs for a single interstellar trip, i.e. a significant fraction of the current US warhead supply, but by no means the whole lot.

rweir• 14 years ago

> requires no unproven technology

what will you make the pusher plate out of?

celerity2• 14 years ago

You know what else is an obstacle to relativistic spaceflight? Being able to do relativistic space flight.

Fun fact: people realized this a long time ago, and proposed different "shields" in front of the ship (it turns out that the most effective shape is a smoothed-out cone [of ice, say] -- see Asimov's Song of a Distant Earth).

lucian1900• 14 years ago

Alastair Reynolds describes ice shields in a lot of detail.

andrewingram• 14 years ago

In Pushing Ice if I recall correctly. Fantastic novel, also discusses the threat of the EM spectrum being blue-shifted to harmful wavelengths.

Sharlin2• 14 years ago

Nitpick: The Songs of a Distant Earth is actually by Arthur C. Clarke.

celerity2• 14 years ago

Thank you! I always mix them up.

squires• 14 years ago

This also reminded me of the "Archangel" class ships described in Endymion by Dan Simmons. FTL travel on these ships instantly kills all passengers but then conveniently resurrects them once they reach the destination.

linuxhansl2• 14 years ago

There's also the risk of hitting small dust particles, etc.

A quick back of the envelope calculation reveals that hitting a spec of dust that weighs just 1 ug (a millions of a gram) at 90% c produces ~36 mega joule. Not a huge amount of energy (about the energy of burning 1kg of coal), but still.

Hitting a grain of sand (10mg) at 90% c produces ~360 giga joule (energy of a small lightning)

steve19• 14 years ago

That's the energy of about 20,000 rounds of 5.56mm NATO. The real problem is that it would be concentrated on such a tiny part of the spaceships hull.

DanBC• 14 years ago

The NASA orbital debris programme has a few pictures of impacts on returned space craft. Obviously these craft are travelling very much slower than 0.9 c.

(http://orbitaldebris.jsc.nasa.gov/protect/impacts.html)

(http://orbitaldebris.jsc.nasa.gov/photogallery/photogallery....)

And some of the debris is quite large - (http://orbitaldebris.jsc.nasa.gov/photogallery/gallarypage/s...) (Is that tape inches?? So that lump is about 1.5 to 2 inches?)

There are existing projects researching near earth impacts from high speed objects (http://ares.jsc.nasa.gov/ares/hvit/problem.cfm) so maybe there's some trickle through to research between very fast craft and small objects.

quattrofan• 14 years ago

So you need a very lage ablation shield

juiceandjuice• 14 years ago

You are making a fundamental error by assuming classical kinetics. At .9c, your first interactions would be electronic (ionization), then nuclear and radiative. The complete and initial ionization of all the atoms of 10mg of, say carbon, would be a lot less than 360 GJ. Then you'd have different interactions depending on your hull material, magnetic shielding/Bremsstrahlung losses, etc...

With some complicated magnetic fields, you could probably do something smart to take advantage the different charge/mass ratio of an ionized atom and positron and minimize interaction with a steady/confined positronic cloud. You might be able to annihilate enough electrons of the dust particle to ionize it enough to subsequently deflect it from the ship. Then you'd still have to worry about gamma ray radiation and back pressure and maybe some other interactions.

I don't really know about all of this for sure, just a thought.

linuxhansl2• 14 years ago

That's quite possible.

The mass would not go away, though, and since energy is preserved it would need to dissipate somehow.

If you can avoid the collision (as you suggest) by deflecting the particle and thus not change its momentum there would indeed be no (or less) energy to dissipate.

mukaiji• 14 years ago

That abstract is epic. It would be funny if all scientists made a mention of the impact of their research on interstellar highly-relativistic flight.

_b8r0• 14 years ago

I love this quote from the abstract:

  Diffuse interstellar H atoms are the ultimate cosmic space mines and
  represent a formidable obstacle to interstellar travel.

The paper[1] is an interesting read too, but of course there are other obstacles to relativistic spaceflight, such as reaching velocities capable of relativistic spaceflight.

[1] http://www.scirp.org/journal/PaperDownload.aspx?paperID=2391...

waqf• 14 years ago

Is this a reputable journal? It seems to be from the same publisher (https://en.wikipedia.org/wiki/SCIRP) as Advances in Pure Mathematics, which recently accepted a paper written by a random text generator.

dalke• 14 years ago

No, it is not.

Beall's List of Predatory, Open-Access Publishers includes Scientific Research Publishing on the list, saying "Do not do business with the above publishers, including submitting article manuscripts, serving on editorial boards, buying advertising, etc. There are numerous traditional, legitimate journals that will publish your quality work for free, including many legitimate, open-access publishers."

BjoernKW• 14 years ago

I can't help thinking that statements like these are merely meant to bog us down. I've heard and read statements like 'Interstellar travel isn't possible.' or my favourite 'We've enough problems down here on Earth already so let's forget about the more visionary ones altogether.' since the eighties.

I mean, why does one become a scientist if one doesn't at least have some greater vision?

This kind of statement seems like conceding failure before even having tried.

ksmiley• 14 years ago

I agree that both of the statements you gave are not productive. "Interstellar travel isn't possible" rejects all theories past and future without considering their individual merits, which is awfully arrogant. "We have enough problems down here on Earth" is short-sighted, in terms of a geological timescale. In a billion years, our most pressing problem on Earth will be that the expanding Sun will kill all terrestrial life. Hopefully some forward-thinking individuals will have worked out space travel by then.

I only read the abstract, but I don't think the article is nearly as prohibitive as your statements. It says that diverting the radioactive hydrogen is a "daunting problem" i.e. hard but not impossible. Even if Near-C space travel _is_ invariably fatal, that's not so broad as "interstellar travel isn't possible". There's still wormholes and hyperspace and whatever exotic ideas we might come up with in the future.

talmand• 14 years ago

"X isn't possible" statements have always held a special interest for me. To me it is that person stating that they, or possibly they think we, know everything there is to know about whatever the subject is they are describing. Many of the scientific advances from the past 100 years that we take for granted today were "impossible" 200 years ago.

As for the "enough problems here on Earth" statements, I always say that most likely the solutions to many of those problems will be solved directly, or indirectly, by visionaries chasing the dream of traveling to the stars. More than likely anyone who says they don't want to spend money on space exploration because of "problems" here is just saying they want to spend the money on their own preferred project, whether there's a benefit or not.

ekurutepe• 14 years ago

Reminds me of how the experts argued that the passengers would die of suffocation, if trains were to go faster than 50mph, back in the 19th century.

If we could figure out how to approach light speed, I'm sure we can also figure out how to shield people and electronics from harm.

nraynaud• 14 years ago

they already said that about speeds over 100km/h in the 1900 :)

metatronscube• 14 years ago

I have always wondered what the probability of hitting a near-c object/dust particle/micrometeorite is between solar systems is within the galaxy. No doubt a near-c object (of any size) would end up trashing a spaceship, but whats the chances of it actually happening? Is it significant?

stox• 14 years ago

This has been speculated for some time, and one solution would be a Fusion Ramjet: http://en.wikipedia.org/wiki/Bussard_ramjet

bjhoops1• 14 years ago

Thanks a lot, Buzz Killington...

Highly relativistic spaceflight would be fatal for passengers and instruments

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