Gravity or Electromagnetism? Which will win?

[robinson]

Gravity or Electromagnetism(EM)? Which is stronger? Which matters most?

In another topic, which is about Plasma Cosmology, whatever that may be, the statement came up again, about Gravity being "stronger" than Electromagnetism.

You failed to understand the point. Both gravity and plasma can in principle be "scaled". That is, we know what the laws of physics are and so we can deduce what the situation will be when we scale everything up in size. One of the immediate and obvious consequences is that gravity is much stronger than electromagnetism (for large objects which are uncharged on average).

The first time I saw that I simply checked, and EM is 10³⁹ times stronger than gravity. Which is, in case you can't see it, is a whole lot stronger. It is so much stronger, it is hard to fathom. For small objects, gravity hardly matters at all. It is all about the EM.

Then we get to large objects, which are usually electrically neutral, because electric charges tend to balance out, rather than stay charged. This will no doubt be explained in great detail later in the thread, by somebody who is way smarter than me, with maths and everything.

So we had this conversation (at the link above) which led to this topic, because while contemplating an example, I came up with a good one. OK, I think it is a good one. A practical example of EM VS gravity, and it is one that actually happens, not just theory.

And, it involves both a large body, and a small one. Both of which are supposed to be neutral, in regards to EM.

The sun every now and then ejects a whole bunch of plasma. A CME. And it accelerates towards us. Really really fast.

But let us set the stage for our collision, which is where the rubber meets the road, so to speak.

A few billion tons of plasma has just been ejected from the sun. It is heading towards us at 2000 kilometers per second. It isn't photons, it is plasma. The fourth state of matter. And not only is it fast, and heavy, due to the velocity of the mass, it has a huge mass.

Because it is heading towards us, our gravity is increasing the speed, because gravity does that to mass. It is going to hit us, and really hard.

What is more important? Our planet is a neutral body, according to Sol, and our 3,000,000,000 Tons of matter heading towards us is neutral.

That is 6,000,000,000,000 pounds of stuff heading our way. Six trillion pounds of matter, moving at about a third of the speed of light, and it is going to hit us.

For you metric dudes, lets say it is 1.6 x 10¹⁵ g, an average CME. To understand how much matter we are looking it, that is about the mass of 107 Camilla, aan asteroid about 11 km in diameter.

If it was heading for us at a third the speed of light, nobody would doubt gravity is the most important.

But we have this CME, and it is going to hit us.

Which matters most in that case? Gravity? Or EM?

If you said EM, because you are smart and stuff, how can that be? Two neutral masses, getting ready to collide. Why wouldn't gravity be more important?

Of course after this affair is hashed out, we are going to scale this up to large objects and structures. But our small practical example makes it clear that gravity, while obviously a big player, isn't the most important thing in the Universe.

 

[Olowkow]

I'm pretty sure Sol know the relative strengths of gravity and EM. He meant the "total impact" of gravity vs. Em.
By the way, 2000 km/sec is not 1/3 the speed of light. ETA. Sorry, perhaps you didn't mean that.

 

[lenny]

One of the immediate and obvious consequences is that gravity is much stronger than electromagnetism (for large objects which are uncharged on average).

pretty powerful parenthetical there. and one easily turned against you, no:

equally obviously EM is much stronger (for light objects that are highly charged!)

isn't the issue shielding, not "scaling": one can shield charge, one cannot shield mass.

 

[robinson]

I'm pretty sure Sol know the relative strengths of gravity and EM. He meant the "total impact" of gravity vs. Em.
By the way, 2000 km/sec is not 1/3 the speed of light. ETA. Sorry, perhaps you didn't mean that.

I was using multiple sources, which conflict. A CME can vary greatly in speed. Either way, it is one big hunk of matter coming fast. You know why gravity doesn't matter? In this case?

equally obviously EM is much stronger (for light objects that are highly charged!)

isn't the issue shielding, not "scaling"ne can shield charge, one cannot shield mass.

Not really. The discussion turns on the belief that at great distances, gravity is all that matters, because EM is neutral in large bodies. I think it started with Galactic rotations or something.

A related point, is how absolutely rare it is for two regular stars to collide with each other. Considering the number of stars, and how they all are attracted to each other, because of gravity, you would think two of them would hit each other now and then. Or even a whole bunch would collide.

Not a single case of two non-neutron stars colliding, in the entire Universe. Much less multiple collisions.

I told you it would get more complicated.

 

[robinson]

As to the OP scenario, the Electromagnetism of our planet will shield us from disaster. Canada might lose it's power grid, but it is Canada, eh?

What? How can two neutral masses be influenced by EM instead of gravity? Wouldn't gravity draw all that mass right down onto the planet?? HELP!!!

This is to illustrate the point (and really piss off a couple of know-it-alls in that other topic). These are small neutral bodies of matter we are talking about here. According to Sol, both plasma and planets are neutral. No charge.

If that was so, how come a CME doesn't slam into the planet, ending all life as we know it?

 

[Terry]

Not a single case of two non-neutron stars colliding, in the entire Universe. Much less multiple collisions.

What about the Blue Stragglers in globular clusters? The current best model for those is that they are collision products, as I understand it.

 

[robinson]

Nobody has observed two normal stars colliding. And we can see a lot of stars.

We don't even have much theory on the matter. Because it has not been observed, we don't know what would happen.

What is even stranger, is that most stars are binary stars.

If they are the result of binary collisions, it would be cool. The lack of collisions between stars is a very great mystery, if you don't consider EM in stellar evolution.

The cause of this is not yet clearly known, but the leading hypothesis is that they are current or former binary stars that are in the process of merging or have already done so. The merger of two stars would create a single star with larger mass, making it hotter and more luminous than stars of a similar age. If this theory is correct, then blue stragglers would no longer cause a problem for stellar evolution theory; the resulting star would have more hydrogen in its core making it behave like a much younger star. There is evidence in favor of this view, notably that blue straggler stars appear to be much more common in dense regions of clusters, especially in the cores of globular clusters. Since there are more stars per unit volume, collisions and close-encounters are far more likely in clusters than among field stars.

http://en.wikipedia.org/wiki/Blue_stragglers

 

[lenny]

Considering the number of stars, and how they all are attracted to each other, because of gravity, you would think two of them would hit each other now and then. Or even a whole bunch would collide.

why would i think that? unless i mistakenly thought that they all started out at rest or some such?

Newton worried about this, in letters to Bentley (?i think?); making sure the initial conditions included non-zero angular momentum was one plank in the argument enabling him to infer the existance of God...

 

[lenny]

What is even stranger, is that most stars are binary stars.

again, why is this unexpected or even related to collisions?

(without assuming near zero net angular momentum in the region of star formation?)

 

[NoisyAstronomer]

If that was so, how come a CME doesn't slam into the planet, ending all life as we know it?

This is so NOT my field, but I'm going to take a stab. A CME is considered electrically neutral because it has equal numbers of positively charges particles and negatively charged particles. A CME is mostly made of electrons and protons that are not bound to each other in a neutral atom, they stay separate. Charged particles interact with the magnetosphere and must follow field lines, which deflect them around the Earth or towards the magnetic poles. Therefore, we see beautiful auroras as these particles interact with the atoms in our atmosphere. This can mess with the Earth's ionosphere, the layer of the atmosphere that contains charged particles, which then disrupts radio transmissions, which are at low enough frequencies to be disturbed by the plasma that is the ionosphere. I don't know how that relates to power outages, though.

I think the lesson from this is that even though a large object, be it a CME or the Earth, is considered electrically neutral as a whole, the charged particles within that body may still interact in ways for the EM force to be important.

Also, you brought up how very few stars collide. That is because of the immense distances between stars, even within a seemingly crowded galaxy. Gosh, I don't remember the number, but when you take into account the sizes of stars and the distances between them, the chances of two colliding is some ridiculously tiny number. That's why it is said that when galaxies collide, the stars do not. This is not a statement about gravity but about the stellar density of galaxies, which are supported by rotation and random motions inherited from the initial clouds of gas. Gravity does have a HUGE influence, however, since it determines where the stars move, even if they do not collide. When two galaxies collide, the changing gravitational well deflects the stars' paths until they form a new shape... a new galaxy. Yes, I know that the gravitational well is shaped by the stars which are moved by gravity... but that is how incremental changes in galactic structure are made.

and sorry for the rambling...

 

[NoisyAstronomer]

Oo found the number... the volume filling factor of a galaxy, on average, is 10^-21. The path length for a collision in this case is on the order of 10⁹ orbits around our galaxy, which would take 10¹⁷ years... and that's much longer than the age of the universe.

(I love hand-wavy math. An error on the order of a factor of 10 is not uncommon for astronomical explanations. Don't try and publish it though.)

 

[Ziggurat]

isn't the issue shielding, not "scaling": one can shield charge, one cannot shield mass.

The shielding issue is indeed quite important, but "scaling" actually matters too. And it matters because gravity is nonlinear, but electromagnetism is linear. When mass/energy densities become large enough, the charge becomes irrelevant: collapse to a black hole is inevitable, and no amount of Coulomb repulsion can prevent it. I'm not sure that's the sense in which he meant it, though, because scaling is also relevant to shielding, since shielding tends to happen within certain scales, and if you get large enough, you're pretty much guaranteed to be close to charge neutral.

 

[lenny]

The shielding issue is indeed quite important, but "scaling" actually matters too.

agreed.

When mass/energy densities become large enough, the charge becomes irrelevant: collapse to a black hole is inevitable, and no amount of Coulomb repulsion can prevent it.

If the mass density is high enough then collapse is inevitable, but are you sure you can get it high enough?

do you think you could make a black hole out of just electrons? or would the energy required to get the density high enough result in...

out of protons?

alpha particles?

(i honestly do not know, but expect someone has worked it out)

one could always try to charge up an isolated black hole after you had one, but making one out of charged particles might present some nontrivial challenges.

 

[geni]

Long term dark energy wins.

 

[shadron]

Oo found the number... the volume filling factor of a galaxy, on average, is 10^-21. The path length for a collision in this case is on the order of 10⁹ orbits around our galaxy, which would take 10¹⁷ years... and that's much longer than the age of the universe.

(I love hand-wavy math. An error on the order of a factor of 10 is not uncommon for astronomical explanations. Don't try and publish it though.)

Bring this a little more down to Earth (so to speak): If the distance between the Sun and Alpha Centaurii were scaled down to the distance from New York to LA (about 3000 miles), then the sun would be about 6.5 inches in diameter. The average orbit of Pluto would be about 2000 feet from the scaled sun.

Now, an awful lot of other soccer balls can pass between those two stars without affecting either the stars or their planetary systems.

At the center of a dense star cluster (say M80, the densest in our galaxy) the mean distance between stars is about .1 lightyear. Scaled, that is about 70 miles, still a lot of room for soccer balls. Antares, he largest nearby red giant, has a radius of 300e6 km, (beyond Mars but not Jupiter orbit size), scales to 117 feet or so.

The odds of two galaxies with a billion stars each passing through each other having a single pair of colliding stars is very, very small. Earth scales to a 20th of an inch (1.3mm); a neutron star would be invisibly small at 2 micrometers.

 

[NoisyAstronomer]

Bring this a little more down to Earth (so to speak)

Heehee, thanks. I get all excitable and forget to come out of numbers mode.

 

[sol invictus]

The sun every now and then ejects a whole bunch of plasma. A CME. And it accelerates towards us. Really really fast.

Your example is idiotic, and has nothing to do with the topic of that thread. Let's see - suppose some alien in a distant galaxy is playing with her toy catapult, and launches a bit of alien goo straight up off her planet. It just so happens that the bit of goo is on precisely the right trajectory that it hits robinson between the eyes. There - we've just proven that alien goo is much more important than gravity at long distances.

We were talking about "plasma cosmology". Among the ridiculous things PC posits is that galactic rotation curves, and the large-scale motions of stars and galaxies in general, are strongly influenced by EM forces. This claim is absurd and easily disposed of.

As an example, let's compare the electric force between the sun and the earth to the gravitational force. Since both forces fall off as one over distance squared, we just need to compare the coefficients. Those are the charge of the sun times the charge of the earth in the case of electric force, and the mass of the sun times the mass of the earth for gravity. Multiplying by the appropriate constants in my head (so check the numbers if you don't believe them), I get 10^43 for gravity, and 10^22 for the electric force (that's using an extremely generous estimate for the charges, namely that they are the square root of the number of protons times the charge of the proton, so about 10^4 C for the earth and 10^7 for the sun). So gravity is a billion trillion times stronger in this case.

Try the same thing for a larger object, and gravity's advantage will grow like the square root of the product of the masses. So for two galaxies, gravity will be a billion trillion trillion times stronger.

 

[sol invictus]

A related point, is how absolutely rare it is for two regular stars to collide with each other. Considering the number of stars, and how they all are attracted to each other, because of gravity, you would think two of them would hit each other now and then. Or even a whole bunch would collide.

No, not at all. Such collisions are extremely rare for an obvious reason - space is big. Moreover, there is very little "friction" - so if two stars are not already on a collision course, conservation of angular momentum prevents them from colliding (they just orbit or scatter). That's why in 5 billion years the earth hasn't fallen into the sun, by the way.

As for binaries, they are almost certainly the result of several stars forming nearby in a gravitationally bound state. All but two get ejected, leaving the two in a binary.

 

[sol invictus]

pretty powerful parenthetical there. and one easily turned against you, no:

No, not in the context the quote was taken out of.

equally obviously EM is much stronger (for light objects that are highly charged!)

Sure. But we were discussing cosmology.

isn't the issue shielding, not "scaling": one can shield charge, one cannot shield mass.

Yes, more or less, but the shielding leads to a scaling law. Ignore EM forces for a moment, and imagine a universe in which half the particles are positive and half negative. When a star forms in such a universe, its typical charge will be the square root of the number of particles making it up (that's one standard deviation from the mean). A star with 10 times that charge will be incredibly rare. So the charge scales as the square root of the mass.

Now add back EM forces. If a star ended up with any significant charge, it would eject like-charged ions from its surface and attract oppositely charged ones. Hence the estimate above is a large overestimate of the net charge. But even if we use it, gravity is still vastly stronger for these objects.

 

[The Man]

As to the OP scenario, the Electromagnetism of our planet will shield us from disaster. Canada might lose it's power grid, but it is Canada, eh?

What? How can two neutral masses be influenced by EM instead of gravity? Wouldn't gravity draw all that mass right down onto the planet?? HELP!!!

This is to illustrate the point (and really piss off a couple of know-it-alls in that other topic). These are small neutral bodies of matter we are talking about here. According to Sol, both plasma and planets are neutral. No charge.

If that was so, how come a CME doesn't slam into the planet, ending all life as we know it?

Technically it is Earth’s magnetosphere (or the “M” of Earth’s “EM”) that protects us form charged particles emitted by the Sun (solar wind and CME). Although equal numbers of those charged particles are emitted by the sun (making the emission over all charge neutral) the particles are still charged and relatively unbound, so their motions are separately influenced by magnetic fields. So indeed for each highly charged small, unbound and light (low mass) particle the influence of Earth’s magnetic field dominates the influence of Earth’s gravitational field. But that issue has already been agreed upon, that when considering small and highly charged bodies, electromagnetism tends to dominate gravity. However we are suppose to be discussing the interaction of large bodies. Suppose we consider some relatively not so large bodies such as asteroids and comets. Again essentially charge neutral, but in this case Earth’s magnetosphere does not protect us from them. That is because in this case the charges are in a bound state and the objects charged particles can not be separately influenced by magnetic fields (as in the case of the unbound charged particles). So the effects on say the positive charges by the magnetic field are canceled by the opposite effects on the negative charges by that same field. The result being that although Earth’s magnetosphere can protect us from an equal number of unbound opposing charged particles (charge neutral solar wind and CME) it can not protect us from an equal number of bound opposing charged particles (charge neutral asteroids and meteors). In the latter case gravity would dominate as opposing EM effects would cancel, thus such a neutral bound collection of opposing charged particle could impact the Earth once within its gravitational field. Although I do not recall any specific incidence of a mass extinction thought to be due to a CME, mass extinction from impacts are quite possible and considered evident (Cretaceous-Tertiary boundary).

Oh, and just in case someone is proposing to suggest that asteroids or comets are significantly charged (such that the charge has a greater influence then gravitational attraction), well then in that case the Earths magnetosphere would protect us from them (which it does not).

ETA: