

Highly relativistic spaceflight would be fatal for passengers and instruments - arthuredelstein
http://www.scirp.org/journal/PaperInformation.aspx?paperID=23913

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noonespecial
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.

~~~
jessriedel
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_.

~~~
noonespecial
Ahh. Understood. Most enlightening. Thanks for that.

~~~
Evbn
Accelerating relativistically greatly increases mass, so enlightening is
indeed the safe way to avoid these problems.

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celerity
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
Alastair Reynolds describes ice shields in a lot of detail.

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

~~~
Sharlin
Also with the "lighthuggers" in the Revelation Space universe.

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waqf
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
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."

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linuxhansl
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)

~~~
juiceandjuice
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.

~~~
linuxhansl
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.

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arikrak
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
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
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.

~~~
gliese1337
> So it's not equivalent to shielding--it's designed to avoid shielding.

Not quite. Electromagnetic shielding is a potentially viable thing, and can be
much less massive than material shielding; a Bussard Ramjet doesn't avoid
shielding so much as repurposes the electromagnetic shielding you already
needed anyway.

Ionizing is not that big of a problem. If you're going fast enough, hydrogen
atoms will be ionized by the interaction with your electromagnetic field. If
you're not going fast enough, you can shine a maser ahead of you to ionize
things in your path (which cuts in to the efficiency of the ramjet).

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superkuh
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>

~~~
gliese1337
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).

~~~
lmm
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.

~~~
arethuza
I think intersteller travel at sensible speeds would require rather more
energy than that - the relevant Wikipedia page gives estimates (depending on
the approach used) of 30 million to 300 thousand 1MT bombs which is,
fortunately, a few orders of magnitude larger than any nuclear arsenal:

[http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propul...](http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29)

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BjoernKW
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
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
"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.

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salimmadjd
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.

~~~
schiffern
>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.

~~~
salimmadjd
>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.

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iuguy
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...](http://www.scirp.org/journal/PaperDownload.aspx?paperID=23913&returnUrl=http%3a%2f%2fwww.scirp.org%2fJournal%2fHome.aspx%3fJournalID%3d69)

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ekurutepe
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.

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metatronscube
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?

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

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mukaiji
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.

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mbq
Another point for using warp drive (=

~~~
gliese1337
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
Do you have any other info/links on the problems associated with the
Alcubierre?

~~~
gliese1337
Here is a paper on the results of interactions with external matter en-route:
<http://arxiv.org/abs/1202.5708>

With a couple of popularizations:

<http://www.theregister.co.uk/2012/03/04/killer_warp_drive/>

[http://www.universetoday.com/93882/warp-drives-may-come-
with...](http://www.universetoday.com/93882/warp-drives-may-come-with-a-
killer-downside/)

(Essentially, particles encountered en-route pile up near the boundary and are
released in a large burst when you turn the thing off, which is potentially
very bad for the local environment, of which you are a part.)

And here's a paper on the horizon-like properties of the warp bubble boundary:
[http://staff.science.nus.edu.sg/~scilooe/srp_2003/sci_paper/...](http://staff.science.nus.edu.sg/~scilooe/srp_2003/sci_paper/phy/research_paper/pua_yeong_heng.pdf)

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bjhoops1
Thanks a lot, Buzz Killington...

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

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rorrr
Future world problems.

