I have just finished reading "Dragon's Egg" by Robert Forward and I find myself confused by some of the physics involved.

The molecular creatures (the Cheela) living on a neutron star find it much easier to travel along the intense magnetic field lines rather than having to travel across them. But it would seem that the qv X B force on the protons in the nuclear material of their bodies would be radial not rearward? Plus, it seems there would be as many electrons as protons and therefore the net force would be zero. Anybody know what's going on?

They also find it a lot easier to see in the direction of field lines. Why would that be so?

Cool book; I need to pick up the sequel someday. I don't know much about the physics of a neutron star, so I can't make any statements about the magnetic field lines making movement more difficult. I thought charged particles moved along magnetic field lines because any velocity tangental to them would produce a force parallel to the lines. But I could be terribly wrong; it's been a while since I studied magnetism in any serious fashion.

However, the reason that the bodies of the Cheela were not neutral is because the engeries required to sustain the nuclear reactions within their bodies are much higher than are allowed for the electrons to bind with the nuclei and form atoms. That's why they were alive on the surface of the neutron star: they used nuclear rather than chemical reaction for all of their life processes.

All in all, a very cool premise for a story.

__________________
If E = MC², why do I have less energy the more mass my body acquires?
That is all.

--Azpod... Formerly known as James Justin

I think you're right. The force on an electric charge should be perpendicular to the magnetic field. This seems to imply they would be going in circles alot.

Of course if they had magnetic charge, they would find moving in the direction of the field easier (or harder). Just because we have never detected a magnetic monopole, does not mean it does not exist.

It's been a while since I read it, but I seem to remember something about the effect of the magnetic field on the surface itself. I don't think the Cheela fought the field itself so much as the ground they travelled on. The crystalline structure of the surface tended to align with the east-west field, making it easier to travel in that direction than across it. Also I remember when the expedition to the east(?) pole started getting near it, they had a hard time moving in any direction because the crystals were aligning in a vertical direction.

That's my thinking, I'm no expert. Actually, I don't even fully understand the point of the OP.

Apzod, I have a paperback rinting that combines both Dragon's Egg and Starquake in one volume. Starquake is just as interesting in it's own way, but I was a little dissatisfied by the ending. I don't want to give it away for people who haven't read the book, but I think even Forward kind of forgot one or two of the basic premises of his creation.

__________________
...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

[quote]
On 2001-12-01 14:16, Azpod wrote:
Yes, but the electrons would still be there. They'd just be degenerate, and therefore the qv X b force would act both away from the surface and towards the surface, in other words it would be compressive in nature.

At another forum someone suggested that eddy currents might be the answer, however I don't see where the change in magnetic flux would come from. It would seem that the magnetic field would be uniform at the surface of the star.

Good book, I thought; long time since I hace read it.

In the Lorentz force formula F = q(E + VxB) if the angle between the V and B vectors is zero, then so is the value of VxB zero. Assuming there is no E either, then moving along the field line produces no resulting force. Moving in any other direction will produce a force, and it really does not matter in principle what the direction of the force is; it's fair to say (I think) that "no force" is always the preferable situation. However, if the V vector is parallel to the surface of the star, then it will be coplanar to the magnetic field lines, which are dipolar and more or less parallel to the surface as well (or so I recall the story model). In that case, using the right hand rule for the direction of VxB (curl the fingers of your right from V to B), then the force will be either up or down, depending on the charge q. However, since q for electrons and protons is of opposite signs, so will the force VxB be in opposite directions. Hence, trying to move in the wrong direction, will subject the creature not to a net zero force, but to internal stress that tends to tear it apart.

<font size=-1>[ This Message was edited by: Tim Thompson on 2001-12-02 17:08 ]</font>

I agree that there would be two forces that would tend to pull the Cheela apart rather than compress them as I erroneously stated earlier. But it would seem that the net force would still be zero. I.e. the up force equals the down force, and adding the two together equals zero.

However, this is still not to the point. The book clearly says that there exists a force that impedes forward progress when moving across the field lines. The only thing I can figure is that somehow there is an induced eddy current with its concomitant drag. I just can't figure out what causes the changing magnetic flux.

I also don't see why seeing in the field direction is easier than seeing in the direction across the field lines.

The change in the magnetic flux is caused by the movement across the magnetic field lines. Just like a current can be induced in a wire by moving the field OR by moving the wire.The Cheela feel the force only when they try to move across the the field.

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

I think that the characteristics of a neutron star would preclude anything from living on one. The gravity would crush anything and everything. Note that most of its material has already been compressed to form neutrons (whereas with white dwarf stars, the electron degeneracy prevents this). Of course, if much more mass is added (as with an accretion disk pulling sufficient matter from the outer envelope of a nearby red giant star), it might end up as a black hole where nothing (including light) escapes the singularity.

Science fiction is not science and the plot line could not get anywhere if it were true to science. I would not look to science for answers for science fiction where none can possibly exist.

ljbrs [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]

<font size=-1>[ This Message was edited by: ljbrs on 2001-12-02 20:55 ]</font>

I don't think that's the way it works. If you move a coil of wire through a *uniform* magnetic field there's no d(phi)/dt and therefore no induced EMF. And as far as I can see the magnetic field would be constant at surface of the star.

<font size=-1>[ This Message was edited by: Ring on 2001-12-02 21:47 ]</font>

<font size=-1>[ This Message was edited by: Ring on 2001-12-02 22:04 ]</font>

Rober L. Forward is a Phd physicist and he is known for his accurate physical descriptions.

As I understand it, Forward's story concept was taken from an actual paper on the possibility of life on a Neutron star.

The creatures can exist in that environment because they themselves are as dense as their world, and their life systems are based on nuclear, not chemical, reactions. Imagine a human being crushed into the size of a sesame seed (and still living and thinking) and you have an idea of the size and density of the Cheela.

__________________
...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

I think he was writing science fiction there.

I think that it is highly improbable that anything like this has actually been studied under scientific conditions. Where have such creatures been studied?

When someone is writing science fiction, he (or she) is hardly doing science. He (or she) is having fun (and making a lot of money if it is good). Fun is fine, but it sometimes makes for a good old "roll on the floor, laughing".

Then again, the science fiction discussed here must have been very, very entertaining.

ljbrs [img]/phpBB/images/smiles/icon_eek.gif[/img]

ljbrs,
I very seriously suggest that you read the book (Dragon's Egg by Robert L. Forward Ph.D) before you criticize the physics of Dr. Forward. He is a full time physicist and part time science fiction writer. The hallmark of all of his science fiction is his adherence to currently known principals of physics. In other words, it may be "science fiction", but it violates none of the laws of physics as they are currently known.

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Yes. The force would depend on latitude. It would mostly be horizontal, either up or down. However, they may have to fight forces that are similiar to our frictional forces: kinetic friction and static friction. These forces increase with normal force. If these creatures were pushed down, it would increase the normal force. They would have difficulty moving across the lines. If up, they could skid.
There is also the point that the magnetic field won't be EXACTLY horizontal. The small component of the field parallel to the surface could also cause difficulties in most latitudes. The books never described an expedition to the pole. There, the effect would be MUCH different than Forward describes.

I think that they would have to be charged just because of the heat emanating from the neutron star through them. Friction from air currents create a separation in charge. Our own atmosphere has an electric field caused by air currents that whose friction creates charge separation. We have a slight electric charge too. However, our magnetic field is not nearly strong enough to interfere with us.
The thermocouple effect could also contribute to charge separation. The surface of Dragonsegg is different from the atmosphere, almost like two dissimiliar metals. The surface is heated, so it would be at a higher temperature than the atmosphere. There would be a strong thermocouple effect. That could also create a charge on the Cheela.
The Cheela live in a stronger magnetic field and they move much faster than we do. This would increase the effect of a magnetic field. The Cheela wouldn't need a strong electric charge to be affected.
This "latitude" effect would be the only situation on the Cheela world where electromagnetic forces are strong enough to actually feel. However, given the fact it would be strong, they may develop electromagnetic theory sooner than Forward says they would.


The atmosphere of Dragonsegg is probably birefringent due to the Faraday effect. A hot plasma is oriented by a strong magnetic field. When they look across the field lines, they are looking partly at extraordinary rays which move in curved lines. When they look with the magnetic field, they see ordinary rays which move in a straight line.


Some explanation of the premise should be given. The baryons in nucleii (i.e., neutrons and protons) interact by strong and weak nuclear forces.
The Cheela have a biology based mostly on the strong nuclear force. Lets consider what goes on in a very heavy nucleus. For the outermost baryons, there are quantum shells very similiar to the electronic shells in atoms. The innermost baryons of the nucleous form a compact mass, similiar to a droplet of liquid. The outermost baryons orbit the innermost baryons in quantized orbits. This system resembles an atom, at least superficially. It could even form "bonds" with other systems like it. There could be complex "molecules" made of nucleii with no electrons bound to them.
Experiments with colliding nucleii show that there are nuclear reactions analogous to chemical reactions. So the premise is plausible in a wild sort of way.
On a neutron star, the conditions could be just right for "biochemistry" to occur with baryons rather than with electrons, but the scale of these reactions would be much smaller and much faster. What we call chemical reactions could not occur at the temperature and pressure of a neutron star, but nuclear reactions at this temperature and pressure are possible.
Forward used all this in the Dragonegg series.

Note that these creatures disintegrate "slowly" in a vacuum. Their skin peels off. This would be because of the "weak" nuclear force. Since they move by the strong force, the disintegration looks slow to them. However, it would be a rapid explosion to us creatures with a chemistry based on electromagnetic forces.


<font size=-1>[ This Message was edited by: Rosen1 on 2001-12-04 12:31 ]</font>

Where does Dr. Forward do his physics? Where has he published? It would be interesting to read what he has written in science before I attempt his science fiction.

ljbrs [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_smile.gif[/img]

ljbrs wrote:At Forward Unlimited, a company he founded in 1962.

His home page,
http://www.whidbey.com/forward/index.html has a link titled "technical papers" that lists 168 publications.
That should be sufficient to sate your curiosity.


_________________
TANSTAAFL!

<font size=-1>[ This Message was edited by: Kaptain K on 2001-12-06 05:32 ]</font>

That is a .com site. For my understanding of science, I only visit .edu and .gov (the sites for governmental science laboraties). He seems to be a great science fiction writer and a great businessman (more lucrative). This does not mean that what he necessarily writes agrees completely with science. If he did, he would never had made any money or had any followers.

I am certain that his science education can only help him avoid blatent scientific errors in his science fiction. I do not get my science from science fiction. If you take your science from science fiction, that is all right with me.

ljbrs [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_biggrin.gif[/img]

ljbrs said:

<blockquote>"That is a .com site. For my understanding of science, I only visit .edu and .gov (the sites for governmental science laboraties)."</blockquote>

If I understand you correctly, I think you are using ".edu" and ".gov" as a criterion or filter for considering the validity of science information. I believe you are arbitrarily and unnecessarily limiting your sources of information.

Consider, for example, that Nature, which is undoubtedly a top class primary source of scientific information, is a .com site. (Even our very own GrapesofWrath has had a letter published there.)

Then there is Science, or The Proceedings of the National Academy of Sciences, both of which are ".org" sites. Science has published wonderful papers and review articles on astronomy topics. (PS, ljbrs, I know you're a subscriber [img]/phpBB/images/smiles/icon_wink.gif[/img] ). PNAS has free access to its articles on-line after 6 months, BTW. Actually, you can still get this year's articles after 4 weeks -- they're changing policy. What a fantastic resource!

Through my Canadian ISP I have free access to all the journals published by Canada's National Research Council Press. This is a ".ca" site.

And don't forget our host. Phil's Bad Astronomy is a site devoted to explaining things in very understandable fashion about our branch of science.

And though the following is drifting from astronomy, I want to underscore the necessity of using our baloney detection tools rather than an arbitrary ".edu" domain as a criterion for evaluating sites. Consider this "technical paper" from this ".edu" site. Read the first paragraph of the summary. There's science for ya! [/sarcasm]

I hope I have misunderstood your reasoning. If so, please forgive my rant.