For a sphere situated in deep interstellar space, what is the wattage
of incident radiation falling upon its surface from [1]- cosmic
microwave background, [2]- starlight, [3]- cosmic ray particles?

We have not sent a probe a billionth of the way to deep space so I can
only guess. If the sphere has a diameter of a thousand KM = 600 miles;
one watt each of Cosmic microwave background radiation, star light,
and cosmic ray particles. the last two would be much higher if there
were any stars within ten light-years or any galaxies within a million
light-years. Perhaps someone has made an educated guess.  Neil

Thanks for your thoughts. What I'm after, really, is the wattage of
radiant energy available in space far from the Sun that can (in
principle) be turned into useful work, regardless that it may be
miniscule. What I'm addressing is a claim I've read that interstellar
spacecraft might freeload on this available energy for sustained
high-g propulsion. What I thoroughly expect is that the wattage of the
sky is so small as to be of no use for any such thing, but I don't
have any firm numbers for the total from all likely sources. Once
again, thanks for your input.

One interesting fact I remember reading about in Robert Zubrin's book
"Islands in the Sky", is that the MOMENTUM (per unit area) of
radiation coming from a star, is in fact independent of the distance
to the star! This theoretically makes possible the building of "solar
sail" -based starships which will use this radiation to gain thrust.

As for cosmic radiation, neilzero, aren't there measurements of the
power of this radiation? This shouldn't vary between here and deep
space as it is presumably almost constant throughout the (known...)
universe.

dannidin

Hmmm? I'm not sure I can see why that would be so, since the momentum
would be carried by the radiant particles, which themselves would
become rarified by the inverse-square law. I'll see if I can get my
hands on Zubrin's book and check out his argument. Thanks.

The momentum can not be constant per unit area...
If the photons were absorbed, the momentum transfer would be (Energy
Absorbed)/(speed of light)
If they were reflected, it would be twice that.  Energy Absorbed is
inversely propotional to the square of the distance from the sun...

If you are going to construct a solar sail for interstellar travel,
you would probably want to rely on a laser of some kind where you can
continue to accelerate even when you are far away from the laser
source.

In theory, providing momentum for a depp-space vehicle would not
require a signifigant amount of energy to maintain.

ie. In deep space, there is little effect from gravity fields, and in
a near vacuum such as space, there is almost no friction effect.

So for example, if you deep space ship got itself out of our solar
system and headed for Alpha Centauri, (or maybe you're really
ambitious and went for something like the Andromeda Galaxy), then once
it was on the right route, it would just sail along, with only the
occasional tiny boost to maintain it's speed.

The fine print: I've never tried this. Please don't launch into space
with only my theory that you don't need a lot of fuel.

-sparky4ca-ga

Yes, you're right about that in general, Sparky. And in fact there are
four spacecraft on interstellar trajectories at this time. But they
are, of course, excrutiatingly slow! ;) The question of increasing an
interstellar craft's momentum is the question of traveling fast enough
to get to a destination in a timely manner. At present we have no
established way of carrying along the energy & reaction mass to
accelerate a crew carrying spaceship to extremely high speeds,
allowing them to reach, shall we say Proxima Centauri, before calling
the undertaker. :)

And so schemes are proposed to either propell the ship remotely, such
as by ground based lasers, which would presumably suck off the Sun's
wattage, or propulsion by living off the land, so to speak. This would
include the noteworthy Bussard Ramjet, which assumes a reliable fusion
reactor. The spacecraft ionises the thin hydrogen gas out front, then
funnels it with a powerful magnetic field into the fusion engine,
which uses the hydrogen to both generate useful energy and also spits
the fusion by-products out the rear for reaction mass.

Of course, the Bussard will ultimately experience significant
resistance between the interstellar gas & the magnetic field. It'll
reach a terminal velocity, just like a skydiver falling through the
air. The only way for the ship to go faster is to throttle up the
motor, but that increases the acceleration, and the crew has to live
with a higher g. That's not unthinkable, but natuarally it still has
some upper limit.

And as I mentioned, there are those who think that there may be
sources of ambient energy in deep space sufficient to supliment a
ship's initial launch conditions. The only such reliable energy
sources I can think of are the radiations present from distant
origins: microwave background, starlight, cosmic rays. As anyone
should be able to easily assess without any hard numbers, the power
level of this ambient radiation is miniscule. As it turns out, since
my original post I have gotten hold of a rough, order of magnitude
figure for the wattage of the sky. It's approximately 1 microwatt
incident per square meter of collecting surface, divided just about
evenly between the three sources.

But even without this specific knowledge, it's possible to accurately
figure out that the wattage of the sky is quite low. For instance,
existing space probes traveling beyond the orbit of Mars necessarily
use carry-on nuclear energy, rather than solar panels, since the
sunlight at that distance is so weak. In order to generate enough
power from sunlight, the probes' solar panels would need much greater
surface areas, and thus much greater masses. If the power available
from the Sun is that weak even within the near planetary neighborhood,
how much weaker must the all-sky radiation be that it can't even make
a difference in the operation of a small instrument package. It
certainly isn't enough to meaningfully help accelerate a mass as large
as a starship.

What prompted me on this originally was stumbling upon the website of
Dr. Stanton Freidman, the Roswell incident researher. He posted a
letter on the New York Times chastising an article by Lawrence Krauss
in which Krauss states that it's possible to know absolutely how much
energy a starship will require to accelerate it to a meaningful
fraction of the speed of light. Friedman accused Krauss of being
shortsighted, and that resourcefulness should allow energy for a
voyage to be harvested enroute. I honestly haven't the slighest idea
where Friedman thinks this much energy is going to come from in deep
space. At one microwatt/square meter(actually less than that, since
there will be heat loss.), it's possible to surround a ship with a
large enough collecting surface to provide the caloric needs of the
crew, but it still doesn't allow for a wattage high enough to
accelerate the payload mass to a high fraction of lightspeed; if the
collector grows in area, it also grows in mass. The wattage needed to
accelerate the ship grows exponentially.

Since you've blown way past my knowledge and pretty much approached
even my level of comprehension, I'm going to stick with science
fiction (which can often also be known as near science future.)

To eliminate the super long travel time, travel near the speed of
light will be necessary. Once that is achieved, or even light speed or
warp (greater then light speed) travel is attained, localized time
dilation effects will need to be compensated for.

ie. As the vessel approaches light speed, time onboard travels slower
and slower, until light speed is reached. beyond light speed, time
would, in theory, move backwards. So travelling a few light years, at
close to light speed, might take a decade, but have only a few months
pass on the ship. ( a side effect would be the usable lifespan of the
ships themselves would be ghreatly increased since during that 10 year
period, the ship only saw a few months useage.)

I've read "proof" before that approaching the speed of light requires
an exponentially increasing amount of energy until light speed itself,
which would require an infinite amount of energy. I don't buy that. I
think it's just an easy excuse because we cannot currently produce or
comprehend the amount of energy needed.

On that matter, (pun not intended, but in hindsight not mad) I think
that matter/anti-matter reaction could prove the key. Again from
sci-fi theory, the annihilation of matter and antimatter when they
come into contact with each other, could produce the great energy
needed. Except that currently the reaction releases so little energy
that it takes more to create the antimatter then it generates. Either
we aren't very efficient at making anti-matter, or energy is being
released in a form that we just don't understand yet.
Maybe if we could channel the energy through a dilithium crystal. (OK,
so I don't have a clue what purpose these serve either. They just
sound really cool. I don't think the writers ever really figured it
out either, nor is it explained well in technical manuals. It's a
handy placeholder for some sort of technology that we haven't even
consodered yet.)

Getting on to the g- effect of the increased acceleration. Again
referring to SF. "inertial dampeners" some sort of technology that
generates a reverse g-effect equal to that being felt onboard the
ship. SO the occupants aren't turned into fresh salsa.

I believe we will get there someday ( to the stars). It just makes
sense.

basically, if there is a God, why would he bother making all the stars
and stuff if there was never a way for us to get to them. Why wouldn't
he jsut decorate the sky with pinpoints of light that aren't really
anything?

And if there is no God, then there is a rest of the universe that is
out there, and someday we'll find a way to visit it. Simply
inevitable.

Hello Sparky,

   You said before:
"I've read "proof" before that approaching the speed of light requires
an exponentially increasing amount of energy until light speed itself,
which would require an infinite amount of energy. I don't buy that. I
think it's just an easy excuse because we cannot currently produce or
comprehend the amount of energy needed."

   The reason that this is asserted is because that's how it comes out
in the math related to the theory of relativity. Now, for a long time
it was generally accepted that the relationship E = mc^2 implied that
the intrinsic mass of a moving body increases with velocity. That is
to say, as the speed grows, so does the kinetic energy. But if
energy(E) is equal to mass(m) multiplied by the square of the speed of
light(c^2), then it was reasoned that, since c is a constant, the only
way for E to grow is for m to grow proportionately. It was then
figured that the growing mass of the moving object means growing
inertia, and thus the energy mandated to further increase velocity by
some increment is increased. By this reasoning, as velocity approaches
the speed of light, the inertia approaches infinity, and energy
required for that last infinitesimal increment in velocity is
infinite.

   In fact, this is a misconception due to the fact that most people
aren't aware of Einstein's whole equation for energy. In its entirety
it goes:

E^2 = p^2c^2 + m^2c^4

   The 2nd term on the right (m^2c^4) gives the energy equivalent to
an object's internal mass, while the 1st term (p^2c^2) gives the
object's energy due to its momentum(p), which itself depends on both
mass and velocity. Thus, it's this term which represents all energy
due to motion. The mass energy is independent on velocity, and the
mass of an object stays the same no matter how fast it's going. The
real reason that it gets harder to increase one's speed as lightspeed
is approached is the well known time dilation effect that you mention.
As I sit here on Earth watching an interstellar spaceship accelerate,
I observe that all processes associated with the ship's frame of
reference get slower compared with otherwise identical prcesses right
here on Earth, at rest with respect to me. The spacecraft increases
its velocity by some mechanical process, and whatever that process is,
from here I observe that mechanism slowing down, and as the ship's
speed approaches lightspeed, its rate of increasing speed approaches
zero. Time dilation is what keeps things below lightspeed in special
relativity. But interestingly enough, it's still equivalent to needing
infinite energy to get up to the speed of light, since the ship now
needs to chug away at it infinitely long into the future! :)

   Of course, this is all in relation to the mutual spacetime axes. As
I move through space at some speed, my coordinate system gets tilted
to that of some other frame of reference. How far it's tilted depends
on how fast the two reference frames are moving toward/away from each
other. As the spacetime axes tilt, their space & time components get
mixed, so that space gets turned into time, time gets turned into
space. In this way nature conspires to keep the speed of light always
the same within any reference frame, and time dilation rules.

   Now, this is all strictly true within "flat" spacetime, meaning
that there's no curvature to the spacetime continuum within one's
frame of reference. As a matter of realistic circumstances, spacetime
in our very dynamic universe gets truly flat only as the dimensions of
your frame of reference shrink and approach zero size! That's why it's
called "special" relativity. It's only absolutely, literally true
within a certain special, restricted  domain. For more "general"
circumstances, you need (yep, you guessed it) general relativity,
which deals with frames of reference in curved spacetime. As it turns
out, it's entirey allowed by general relativity for two regions of
space to fly apart at greater than lightspeed. I may be under the
thumb of special relativity within my small, local frame of reference,
but if that box I'm stuck in is attached to expanding space, then I
can travel along with that region of space as fast as it's expanding.
Two regions of space can be receeding from each other at, shall we
say, ten times lightspeed, and I'm being carried along for the ride at
10c!

   So the trick seems to be to selectively manipulate the differential
geometry of space so that it expands & contracts the way that we want
it to, so that it carries us at great velocities overall. This of
course is how the starship Enterprise propells itself, but as it turns
out some actual theoretical work has been done on this. A few years
ago the relativist Migel Alcubierre studied the formal requirements
for a Star Trek warp drive within the constraints of general
relativity. What he came up with amounts to selectively "gripping" the
space out front, flattening it, while also expanding the region at the
rear. Thus, your spaceship makes its way at some arbitrarily high
speed between two pieces of the Universe, while locally it's
travelling at zero speed, in total agreement with special relativity.
Cool, so far.

   As always, there are problems known with this scheme. As a purely
technical problem, the theory as it now stands predicts that the warp
field surrounding the spaceship will become a stable envelope, and you
won't be able to shut it off!! I've no idea if it's possible to
overcome this or not, but it is a roadblock at least now. On the other
hand, the most pressing obstacle is that the theory postulates a
source of negative mass/energy. As things stand, there's nowhere in
existing physical theory or observation that features genuinely,
intrinsically negative mass/energy. It just doesn't fit in with
anything understood. So what it amounts to right now is what's called
a "non-physical solution". It's consistent with the mathematics of
relativity, but nature doesn't seem to use it. However, you never
know...

   But one way or another, there's nothing absolutely preventing
people who are dedicated from interstellar travel. As a guy I know on
the 'net always puts it, it's not physics that keeps us here, but
economics. Make it affordable, and there'll be ships to Proxima
Centauri sailing with the tide. From an engineering standpoint, it's
possible to sail to another star system even today. It's just too
darned expensive for anyone to even consider yet. But if we can get
the cost of a kilowatt-hour of energy to some very cheap level, then
it's no big deal, as long as we're patient enough to make the voyage
to nearby stars.

   Getting back to more conventional schemes of high velocity space
travel, you might be interested in a great sci-fi novel called Tau
Zero, by Poul Anderson. It was wriiten over thirty years ago, but the
story is terrific, and has to do with the crew of a Bussard ramjet. It
gets damaged enroute to some nearby solar system and they can't shut
off the motor to repair it. They're moving so very fast that if they
shut down the motor, the magnetic field will disappear and they'll be
fried by the thin interstellar gas. So they become doomed to fly
through the Universe, always accelerating. By the end of the book
they've been traveling so long, at such high velocity, that back on
Earth billions of years have passed. It's a good read.

There was a theory that if you pause the movements of high speed
sub-atomic energy particles, like photons, for fractions of a second,
it could "charge" and allow more energy dispursement. But storing raw
solar energy is hard enough by todays standards, let alone being able
to automatically break it down to use it as a method of propulsion
with a deep-space machine. And I doubt many would risk billions of
dollars and many years of time just to have it turn out to be the
first real life photon torpedo test run.