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scourge
2005-Feb-26, 12:32 PM
I have some burning questions about the interstellar medium, and I'd very much appreciate some discussion about this stuff. I have an endless supply of questions, take your pick:

Do we know or have a good idea what the energy density of interstellar space is?

Can we extrapolate how much energy is coming at us in the form of Cosmic rays based on our current knowledge…can we even detect the highest energy cosmic rays passing through us?

If our eyes could see the full electromagnetic spectrum, would space be dark or bright?

If our Sun is emitting a million tons of energy per second, then how much energy exists in any given volume of space from all the stars in the universe emitting this enormous quantity of energy?

If we could make a perfect one-way glass for all electromagnetic wavelengths in interstellar space, how much force would it experience?



(And I'm having trouble searching the topics to see if this thread already exists, but is there a thread about the Sun? I have some questions about that too...thank you!)

crosscountry
2005-Feb-26, 01:58 PM
one at a time.




Do we know or have a good idea what the energy density of interstellar space is?
[quote]

first as you point out, the energy density of space is in large part to Cosmic background radiation and current emmission by living stars. but that really means ones that were living to emit those rays.




Can we extrapolate how much energy is coming at us in the form of Cosmic rays based on our current knowledge…can we even detect the highest energy cosmic rays passing through us?

yes. I'm not sure of the data, but we have detectors from long radio up to gamma. that question is answerable, at least locally.



If our eyes could see the full electromagnetic spectrum, would space be dark or bright?


I'd suspect dim would be the word to describe, expecially in the higher and lower frequencies



If our Sun is emitting a million tons of energy per second, then how much energy exists in any given volume of space from all the stars in the universe emitting this enormous quantity of energy?


light propogates with a 1/r^2 factor. so the further away from a light source the less energy/intensity you get. So of course the density goes down too, also as a factor of 1/r^2

answer: depends on where you are!



If we could make a perfect one-way glass for all electromagnetic wavelengths in interstellar space, how much force would it experience?


not much I suspect. remember Pressure = Force/Area. it would take a large area to feel any real force from a given source. from the sun the earth gets several pounds of sunlight in 24 hours, but all other sources combined don't give us that much.

to answer your question more, it(the total EM of space) would probably average to 0 if we were a significant distance from our sun.

jnik
2005-Feb-27, 02:11 AM
If our Sun is emitting a million tons of energy per second, then how much energy exists in any given volume of space from all the stars in the universe emitting this enormous quantity of energy?
Do some reading on Obler's Paradox (assuming you haven't already); it would likely inform the sorts of thinking you're doing.

scourge
2005-Feb-27, 05:52 AM
Thanks for your thoughts gentlemen. I read somewhere that some cosmic rays have so much energy, that a single photon could knock a cup of coffee off the table. But since they can pass through an enormous amount of matter without interacting with it, this pretty much never happens.

I’m just trying to understand what we would find in a snapshot of an average cubic centimeter of space, preferably with real numbers. Olber’s Paradox is interesting, but given the limited size of the apparent universe, and intervening nonluminous matter, and the cosmological redshift and all, the optically-dark perception of the sky doesn’t really surprise me (though maybe it should…). I’m more interested in the actuality of space than the theory, for the purpose of these questions.

How about matter in interstellar space? I’ve heard that there are at least 75 charged particles in every cubic inch of space, even in the most remote regions of deep space…I presume that this indicates that the mass in free space is a form of ‘cold’ plasma…that most of the mass is not atomic or diatomic hydrogen.

Since vacuum fluctuations can’t be used to induce momentum, I’m looking for other aspects of free space that we could interact with to impart momentum to a spacecraft.

For example, thermal propulsion. Perhaps if we maintained a temperature of near-absolute zero on one side of a craft, and an extremely high temperature on the other side, we could absorb the momentum of the interstellar free particles on one side, and impart it on the other side to produce thrust. Or what if we surrounded a craft with an intense, ionizing electrical field, and used a magnetic field to accelerate those ionized particles to relativistic velocities to the rear of the craft?

And I suppose that if dark matter turns out to be something we can interact with, we might be able to impart momentum to that.

Because no matter how you cut it, trying to use an on-board source of fuel for interstellar probe propulsion is not going to do the trick (aside from ideas like Project Orion, which have significant impracticalities). But if we can interact with the interstellar medium in some way, maybe we can get somewhere…

Jpax2003
2005-Feb-27, 07:36 AM
Scourge, are you thinking of the Oh My God (http://www.fourmilab.com/documents/ohmygodpart.html) Particle detected hitting earth's atmosphere a decade ago? It could knock a cat off a table.

At the same site is a suggestion for a space screw AKA Vacuum propeller (http://www.fourmilab.com/documents/vprop/). It's not a design or concept just a dream. I'm trying to put together a hypothesis for this to use in my fiction stories.

scourge
2005-Feb-27, 09:41 AM
Scourge, are you thinking of the Oh My God (http://www.fourmilab.com/documents/ohmygodpart.html) Particle detected hitting earth's atmosphere a decade ago? It could knock a cat off a table.

At the same site is a suggestion for a space screw AKA Vacuum propeller (http://www.fourmilab.com/documents/vprop/). It's not a design or concept just a dream. I'm trying to put together a hypothesis for this to use in my fiction stories.

Excellent links Jpax, thanks! I didn't have a source on the nature of this kind of high-energy 'cosmic ray,' that must be it. For some reason I thought it was a photon, not a proton. I've been curious about the energy passing through a given volume of space for some time since.

The 'vacuum propeller' is perfectly illustrative of the line of reasoning I'm interested in. At first, I looked into the vacuum energy as a reaction medium, but after reading a good paper about it, I think by Puthoff, I became convinced that vacuum fluctuations won't serve the propulsion purpose, because, apparently, it can't transfer momentum to a physical object. Although, according to Michael LaPointe at the MSFC, the vacuum may be a source of antimatter by utilizing the Casimir effect: link (http://gltrs.grc.nasa.gov/reports/2001/CR-2001-211116.pdf).

The key to instellar flight is in the interstellar medium...whether that's photons, the tenuous matter of space, or in the properties of spacetime itself, I don't know. If there's a Higgs field, maybe we could interact asymmetrically with that.

I tend to think that the missing piece of the puzzle we need, is in the nature of inertia itself. One intriguing idea that has been running through my mind, is some way to use the Wheeler-Feynman model...generate a cancellation wave for the inertia-imparting waves returning from distant masses that create the inertia effect. But a juxtaposition of Olbers' paradox with the Wheeler-Feynman model suggests, in my mind anyway, a challenge to that model of inertia. And the idea of photons travelling backward through time just seems fishy.

eburacum45
2005-Feb-27, 11:54 AM
If you haven't read the NASA study on weird and wondeful propulsion concepts, try this;
http://www.grc.nasa.gov/WWW/bpp/TM-107289.htm

The idea of using the energy in empty space for propulsion is explored in the various Collision Drive Sail concepts;
more detail is given here
http://www.zamandayolculuk.com/cetinbal/NASAwarpdrive.htm

Hypothetical Differential Sail: Analogous to the principles of an ideal radiometer vane, a net difference in radiation pressure exists across the reflecting and absorbing sides. It is assumed that space contains a background of some form of isotropic medium (like the vacuum fluctuations or Cosmic Background Radiation) that is constantly impinging on all sides of the sail.

Hypothetical Diode Sail: Analogous to a diode or one-way mirror, space radiation passes through one direction and reflects from the other creating a net difference in radiation pressure.

Hypothetical Induction Sail: Analogous to creating a pressure gradient in a fluid, the energy density of the impinging space radiation is raised behind the sail and lowered in front to create a net difference in radiation pressure across the sail.

I also like the Diametric drive, which could produce thrust if only negative inertia were available (which in theory it might be, if negative energy is available in useful amounts).
The other reactionless drives (Pitch, Disjunction and Bias drive) are too baffling for my limited understanding.

scourge
2005-Feb-27, 02:25 PM
Negative mass, I love this one. At first I assumed that it was just a kind of mathematical/theoretical thought exercise, but the term ‘negative energy’ seems to equate to ‘negative mass.’

The region between Casimir plates is often referred to as a region of ‘negative energy’ with respect to the mean energy value of the quantum vacuum’s virtual particle sea. This may be a layman’s error, but if this form of negative energy is just that, then it should be equivalent to some vanishing small quantity of negative mass, right?

Which makes me question whether a pair of Casimir plates has less net mass when the plates are proximal than when they’re apart. After all, when uncharged particles experience the nuclear strong force, their net mass drops (i.e., binding energy/mass defect). And the same happens at a chemical level when molecules form, and the energy differential is radiated as light, and the new mass is lower than the sum of the parts (though this mass change is probably way too small to directly measure).

So here’s a crazy idea—maybe if we could surround a device with a series of microscopically-separated concentric shells of ultrathin metal, we could create a region of negative energy intense enough to nullify the positive mass of the entire device. I have no idea what I’m talking about here, btw. I’m assuming that this ‘negative energy’ also has negative inertial mass, but, that does seem to be supported by nuclear and chemical physics. And I’d expect electromagnetism to be no exception…in any case, it would take a whole lotta of negative energy to have a noticeable effect.

At first I thought this contradicted a passage in Heisenberg’s book ‘Nuclear Physics,’ wherein he states that potential energy has positive mass (the example was that a wound clock weighs some superminiscule amount more than an unwound clock). However, it looks like there’s a difference between positive potential energy and negative potential energy, with respect to the nature of the appertaining mass differential. When we wind a clock, we’re introducing energy into the system, and the restoring force is repulsive, viz, positive potential energy…and positive mass. Whereas attractive force interactions impart negative potential energy. This seems to be the case presented regarding the Equations Of State described in this Berkeley book: http://www.nuc.berkeley.edu/courses/classes/E-115/Reader/chap2.pdf

Now, the terrible conclusion I come to at this point, which undoubtedly reveals the unbridled expanse my profound failure to comprehend basic physics, is this—doesn’t that mean that if you had a device with strongly attracting magnetic fields on one side, and strongly repelling magnetic fields on the other, that the center of gravity of the device would shift slightly toward the repulsive field interaction, where the potential energy is positive? I mean, with more positive potential energy on one side representing positive mass, and negative potential energy (mass) on the other, then spacetime would slope down where the thing is heavier and up where it’s lighter, right? Isn’t that situation a fair representation of the Diametric Drive idea—spacetime warped positively on one side and negatively on the other? It can’t be that simple…and it would violate conservation of momentum. So where’d I go wrong? Is negative potential energy different than negative energy or negative mass?

TravisM
2005-Feb-27, 06:42 PM
Well, the average density of the universe appears to be far less than the required billionth of a billionth of a billionth of a gram per cubic centemeter. (Heavy on the word average :D )

scourge
2005-Feb-28, 01:30 AM
Well, I've been coming across the idea that the universe is flat, in the context that all of the positive mass of matter and kinetic energy is equivalent to an equal magnitude of gravititional energy, which is measured in negative terms, for a net mass of zero. Some argue that this is why the conservation of energy wasn't violated by the BB, which seems like a good argument.

Jpax2003
2005-Feb-28, 04:02 AM
I don't know nearly enough about physics to speak intelligently on some of these ideas. But, I wonder if a hypothetical construct could make use of neutrinos streaming out of stars. It might be a super dense material or some special force or field effect. Instead of a wide and shallow sail used for photons and solar wind, this might be a long thin cone allowing for diretionality of capture of neutrinos. A neutrino passing through the side would not encounter the field, but one coming into the cone might become trapped. Or perhaps we could determine sources of "Oh my God" particles and sail on that.

Another concept that is under development is the Mini-Magnetospheric Plasma Propulsion (http://en.wikipedia.org/wiki/Mini-magnetospheric_plasma_propulsion) which is a variation on the Magnetic Sail (http://en.wikipedia.org/wiki/Magnetic_sail). What I am wondering is if this concept could be tweaked to make a propeller using a rotating ring superstructure that creates elongated elliptical blades via centripetal force which might create a more dynamic system. Perhaps the variable pitch of the rotating blades might enable the craft to actually fly into the solar wind. Furthermore, a more powerful reactor might enable it to work like a turbine, actually accelerating the solar wind behind it, in order to move toward the star. However, I think the particles are already at high relativistic speeds and may not be accelerable by a realistic spacecraft.

scourge
2005-Feb-28, 06:16 PM
The Mini-Magnetospheric Plasma Propulsion idea sounds exciting...though it sounds like it would encounter serious limitations beyond the heliopause. And if we tried to use this tangential force to gain velocity, centripetal acceleration would force the ship out into lower thrust regions before much velocity could be acquired, leaving the problem of unreasonable flight times. It sounds terrific for interplanetary flight though.

Neutrinos pose a host of practicality problems for propulsion...their chargeless aspect and their ability to pass through vast amounts of matter make coupling a big problem. And I'm not sure how much force they could generate...they -seem- to be massless, like photons.

I've been unable to find anything online to directly address the question of negative potential energy and negative mass. It does seem that two magnets will weigh less when they're allowed to join attractively, and more if they're bound together against their repulsive forces. But I can't tell if this would apply to electromagnets yet, since an input of energy is required to generate the fields. And the field densities would have to be enormous...however...even the case of permanent magnetic fields illustrates that it's possible to generate a spacetime distortion, vis-a-vis- the Diametric Drive...albeit a vanishingly small one. It seems so anyway.

So the big question is--can an input of energy generate a net negative potential energy, and thereby a decrease of inertial mass, or would the additional kinetic (electrical) energy result in zero change of mass? Hmm...

Does anyone know the answer to this question?

publiusr
2005-Mar-02, 06:55 PM
Forward spoke a lot about Negative matter (not Dark Matter or anti-matter).

I would suggest these websites for more realistic drives:

http://www.starshipmodeler.net/cgi-bin/phpBB2/viewtopic.php?p=319112&
http://www.npl.washington.edu/AV/altvw56.html