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Re: 1489: "Fundamental Forces"

Posted: Sat Feb 21, 2015 1:46 pm UTC
by Plutarch
Aiwendil wrote: Edit: A more pedagogical link for Feynman diagrams (haven't really read through it yet, but it looks rather friendly): Let's draw Feynman diagrams!

Thanks for that link, I found it really interesting. Now at least I have some idea what these Feynman diagrams mean.

Re: 1489: "Fundamental Forces"

Posted: Sat Feb 21, 2015 2:11 pm UTC
by Copper Bezel
Drooling Iguana wrote:
mathmannix wrote:Let's see... the four forces are Thrust, Summer, Famine, and Phlegm.

And Venus de Milo in that one TV show that we don't talk about.

Where is that damned Like button?

Re: 1489: "Fundamental Forces"

Posted: Sat Feb 21, 2015 8:16 pm UTC
by Pfhorrest
Khaz wrote:
A possible analogy for electro-weak unification is the relationship between diamond and graphite - two substances with very different physical properties, but if you get them both hot enough, they become indistinguishable blobs of atomic carbon - which, when it cools again, could become diamond, graphite, soot, or any of the exotic allotropes.

I like this. Following that line of thought, does it make sense to suppose that all these subatomic particles are really just different states of the same fundamental thing? Perhaps this thing is simply the physical manifestation of energy itself. I dunno. I would love to know what matter actually is someday.

Yeah, that's pretty much it. There are various quantities (energy, charge, spin, etc) that must be conserved, but so long as they are conserved basically every kind of thing that we know exists is interconvertible with every other kind of thing, so they're really all just different forms of the same substance. And all particles, be they 'matter' particles (fermions) or 'energy' particles (bosons) are all just exitations of underlying energetic fields.

My question for the physicists here is whether the unification of the electromagnetic and nuclear forces means that W and Z bosons and gluons are all just different kinds of excitations of the same field as photons, i.e. The electromagnetic field. Or are the fields themselves somehow split at lower energies? How does that work?

Re: 1489: "Fundamental Forces"

Posted: Sat Feb 21, 2015 8:29 pm UTC
by rmsgrey
Pfhorrest wrote:
Khaz wrote:
A possible analogy for electro-weak unification is the relationship between diamond and graphite - two substances with very different physical properties, but if you get them both hot enough, they become indistinguishable blobs of atomic carbon - which, when it cools again, could become diamond, graphite, soot, or any of the exotic allotropes.

I like this. Following that line of thought, does it make sense to suppose that all these subatomic particles are really just different states of the same fundamental thing? Perhaps this thing is simply the physical manifestation of energy itself. I dunno. I would love to know what matter actually is someday.

Yeah, that's pretty much it. There are various quantities (energy, charge, spin, etc) that must be conserved, but so long as they are conserved basically every kind of thing that we know exists is interconvertible with every other kind of thing, so they're really all just different forms of the same substance. And all particles, be they 'matter' particles (fermions) or 'energy' particles (bosons) are all just exitations of underlying energetic fields.

My question for the physicists here is whether the unification of the electromagnetic and nuclear forces means that W and Z bosons and gluons are all just different kinds of excitations of the same field as photons, i.e. The electromagnetic field. Or are the fields themselves somehow split at lower energies? How does that work?


I'm not a physicist, but my understanding is that there isn't an electromagnetic field, but a fog of virtual photons that interact with charged particles and each other, producing much the same effects as the classical EM field - at short ranges, you encounter more of the virtual photons, so the effects are stronger, producing the inverse-square effect. So when photons split off from other exchange particles, the "fields" also become distinct.

Re: 1489: "Fundamental Forces"

Posted: Sat Feb 21, 2015 10:24 pm UTC
by OP Tipping
Between pairs of objects in which the products of the mass:charge ratio is greater than the ratio of Coulomb's constant to the Gravitational constant, ie

|M1 * M2 / Q1 / Q2 |> k/G

... gravitation is a stronger force than electromagnetism.

For those pairs in which |M1 * M2 / Q1 / Q2 |< k/G, electromagnetism is a stronger force than gravitation.

It just straight up doesn't mean anything to say that one of them is stronger than the other without further qualification.

(I've used || in case one of you smarties points out that Q can be negative.)

k/G is about 1.34E20 kg^2/C^2, so a pair of objects each with a mass to charge ratio of positive 11.6 million tonnes per coulomb would have gravitational attraction and electromagnetic repulsion just about balanced.

EDIT: tonnes, not tons.

Re: 1489: "Fundamental Forces"

Posted: Sat Feb 21, 2015 11:49 pm UTC
by orthogon
... whereas a proton has 10-8kg/C. I guess that's what people mean when they say gravity is much weaker than electromagnetism. I always wondered whether it was really a meaningful statement, comparing two things with different dimensions like that. But it makes some sense to say that that the particles we see, when they have any charge, have a shitload of it compared to the puny amount of mass they manage.

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 1:32 am UTC
by schapel
You can easily demonstrate that electromagnetism is far stronger than gravity. Just get some magnets (or a magnet and a ferromagnetic object) and bring them close together. You can see and feel the force they generate. You can't see or feel the gravitational force between objects that you can manipulate, unless you have sensitive equipment. The statement that electromagnetism is stronger than gravity isn't a deep statement -- it's something that virtually everyone commonly experiences.

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 2:10 am UTC
by OP Tipping
".. whereas a proton has 10-8kg/C. I guess that's what people mean when they say gravity is much weaker than electromagnetism. I always wondered whether it was really a meaningful statement, comparing two things with different dimensions like that. But it makes some sense to say that that the particles we see, when they have any charge, have a shitload of it compared to the puny amount of mass they manage"


Great, except subatomic particles are not the only objects in the universe worth looking at. The behaviour of macroscopic, even gigantic, objects are also important in our understanding of everything, and they have enormous mass to charge ratios.


"You can't see or feel the gravitational force between objects that you can manipulate, unless you have sensitive equipment. The statement that electromagnetism is stronger than gravity isn't a deep statement -- it's something that virtually everyone commonly experiences."

Great, that means in that particular case, at that particular scale, electromagnetism is stronger. In the analysis of the behaviour of galaxies, solar systems or planetary systems, it turns out gravitation is stronger. The earth is a charged object, not chargeless, and so is the sun, but their mass/charge ratios are large, so gravitation is stronger between those particular objects.

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 2:18 am UTC
by rmsgrey
schapel wrote:You can easily demonstrate that electromagnetism is far stronger than gravity. Just get some magnets (or a magnet and a ferromagnetic object) and bring them close together. You can see and feel the force they generate. You can't see or feel the gravitational force between objects that you can manipulate, unless you have sensitive equipment. The statement that electromagnetism is stronger than gravity isn't a deep statement -- it's something that virtually everyone commonly experiences.


Or you can generate enough charge through surface friction - just rubbing the loose electrons off something - to pick up small objects just by inducing a dipole. To pick things up by gravity using a sphere with 1m diameter (the size of a large beach ball) would require that sphere to have a mass over 10^10 kg

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 3:26 am UTC
by NaturalSpin
This comic would have been accurate before 2012. Today there are five known fundamental forces: gravity, electromagnetism, the strong force, the weak force, and the Higgs force.

The Higgs particle doesn't just give mass to some fundamental particles; it also mediates a short-range fundamental attractive force. See, for example, "The Strengths of the Known Forces" by particle physicist Matt Strassler. Now, the Higgs force is very tricky to measure directly, so we know about it indirectly from its interaction effects, but the same is true of the weak force.

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 6:04 am UTC
by Copper Bezel
OP Tipping wrote:".. whereas a proton has 10-8kg/C. I guess that's what people mean when they say gravity is much weaker than electromagnetism. I always wondered whether it was really a meaningful statement, comparing two things with different dimensions like that. But it makes some sense to say that that the particles we see, when they have any charge, have a shitload of it compared to the puny amount of mass they manage"


Great, except subatomic particles are not the only objects in the universe worth looking at. The behaviour of macroscopic, even gigantic, objects are also important in our understanding of everything, and they have enormous mass to charge ratios.


"You can't see or feel the gravitational force between objects that you can manipulate, unless you have sensitive equipment. The statement that electromagnetism is stronger than gravity isn't a deep statement -- it's something that virtually everyone commonly experiences."

Great, that means in that particular case, at that particular scale, electromagnetism is stronger. In the analysis of the behaviour of galaxies, solar systems or planetary systems, it turns out gravitation is stronger. The earth is a charged object, not chargeless, and so is the sun, but their mass/charge ratios are large, so gravitation is stronger between those particular objects.

A particular particle can have a much stronger charge than it can have gravitational force of its own. That's a meaningful statement.

I don't think gravity is offended when we say that it's "weaker," so I'm really not sure who you're sticking up for, here. = /

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 9:10 am UTC
by x7eggert
Copper Bezel wrote:
Mokurai wrote:The constancy of the speed of electromagnetic waves gives us Special Relativity.

Either you meant this sentence in a very different sense than the others around it, or it's really not at all right. All of those other statements were physical consequences of the thing you were describing. This one was a historical consequence that led to a discovery, but EM definitely doesn't make c be what it is in the actual physics of the universe. EM waves move at c, but everything that isn't a massy particle does, so there's nothing special about that. EM is a thing which obeys c.

(...)


I think SRT is a consequence from EM (Maxwell's equations) like EM is from SRT. You can't have one without the other.

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 9:47 am UTC
by Copper Bezel
But EM is definitely not the only thing that follows SRT. Even if you somehow didn't have EM, massless particles other than photons would still move at c. SRT was historically discovered as a consequence of Maxwell's equations, but the former is more "fundamental" than the latter.

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 3:35 pm UTC
by SuicideJunkie
Consider the amount of energy you have to spend to get it.

If you want 1N of force at 1mm from electromagnetic effects, you will end see a respectable bill from the electric company to run your collider and generate some fresh electrons and positrons.
If you want 1N of force at 1mm from gravitational effects, your bill to generate the required particles will be much higher.

When you start considering large objects, then you run into engineering problems. A planetary scale test mass works OK, but the equivalent test charge has a "tendency to explode".

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 4:13 pm UTC
by schapel
OP Tipping wrote:Great, that means in that particular case, at that particular scale, electromagnetism is stronger. In the analysis of the behaviour of galaxies, solar systems or planetary systems, it turns out gravitation is stronger. The earth is a charged object, not chargeless, and so is the sun, but their mass/charge ratios are large, so gravitation is stronger between those particular objects.

All sorts of things in science depend at one particular case at one particular scale. Ever heard of "standard temperature and pressure"? That's atmospheric pressure at the surface of the Earth and a temperature humans regularly experience. Even the atomic weights on a periodic chart give the average weights according to the relative isotopes found on Earth... one particular case. Why do we have units like the second, the meter, and the gram, which are at just the right scale for humans to relate to? Cause we're humans on Earth, dammit!

The decay of the neutron also reminds me of what it would be like to actually have a teaspoon of a neutron star. It wouldn't be pleasant.

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 6:48 pm UTC
by rmsgrey
Electromagnetism does appear to have a fixed strength - a fundamental constant that determines it - the fine structure constant. Sadly, none of the other four appear to have a natural scale - the best you can do is compare them in specific instances. When it comes to gravity and EM, depending on whether you look at the mass/charge ratio of the electron or the proton, your measured ratio of the strengths of the two forces varies by a couple of orders of magnitude. Since it's between 16 orders of magnitude and 18 orders of magnitude, it's not terribly significant to the conclusion that one is much stronger than the other...

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 8:08 pm UTC
by sevenperforce
Copper Bezel wrote:A particular particle can have a much stronger charge than it can have gravitational force of its own. That's a meaningful statement.

That depends. Can you consider a black hole to be a "particle"? If so, there is a limit on the charge/mass ratio for that particle which ensures it will always have a much greater gravitational force than coulomb force.

Saying that gravity is weaker than EM is like saying "paperclips are less than drywall". Just because that sort of makes sense (the average paperclip IS less heavy than an average sheet of drywall) doesn't mean it's an entirely meaningful statement, because it's not really specified. Most paperclips are longer than an average sheet of drywall is thick, for example. A steel paperclip has a greater density than certain types of drywall. And so forth.

Re: 1489: "Fundamental Forces"

Posted: Sun Feb 22, 2015 11:49 pm UTC
by Aiwendil
Pfhorrest wrote:My question for the physicists here is whether the unification of the electromagnetic and nuclear forces means that W and Z bosons and gluons are all just different kinds of excitations of the same field as photons, i.e. The electromagnetic field. Or are the fields themselves somehow split at lower energies? How does that work?


There is a single electroweak field, and photons, W bosons, and Z bosons are all excitations of that field. However, at low energies, we can treat it as being two different fields - the electromagnetic field and the weak field, with photons being excitations of the EM part and the W and Z of the weak part. It's kind of like how in classical E&M, there is a single electromagnetic field, but you can also talk sensibly about just the 'electric field' or just the 'magnetic field'.

But remember, the strong force is not unified with the electroweak in the Standard Model, so gluons are excitations of a different field. There are theories in which the strong and electroweak forces are unified, called Grand Unified Theories or GUTs, but so far we haven't seen direct evidence (like proton decay or supersymmetric particles) for any of them.

The next level beyond that is what is called a Theory of Everything, which would be a theory in which those three forces and gravity are all unified into a single field. Note, though, a difference between gravity and the strong force: in the case of the strong force, we do have a quantum theory of the force (QCD), it just isn't unified with the electroweak as a single field; whereas we don't have a separate quantum theory of gravity at all.

rmsgrey wrote:I'm not a physicist, but my understanding is that there isn't an electromagnetic field, but a fog of virtual photons that interact with charged particles and each other, producing much the same effects as the classical EM field - at short ranges, you encounter more of the virtual photons, so the effects are stronger, producing the inverse-square effect. So when photons split off from other exchange particles, the "fields" also become distinct.


Actually, there are fundamental fields. In a quantum field theory, each particle is an excitation of a quantized field. One of the slightly confusing things about modern physics is that you can switch back and forth between thinking of things as particles and as fields.

Re: 1489: "Fundamental Forces"

Posted: Mon Feb 23, 2015 12:48 am UTC
by Aiwendil
rmsgrey wrote:Electromagnetism does appear to have a fixed strength - a fundamental constant that determines it - the fine structure constant. Sadly, none of the other four appear to have a natural scale - the best you can do is compare them in specific instances. When it comes to gravity and EM, depending on whether you look at the mass/charge ratio of the electron or the proton, your measured ratio of the strengths of the two forces varies by a couple of orders of magnitude. Since it's between 16 orders of magnitude and 18 orders of magnitude, it's not terribly significant to the conclusion that one is much stronger than the other...


All of the forces have fundamental 'strength constants' associated with them. Interestingly, going just by these constants, the weak force is stronger than electromagnetism. It ends up being effectively weaker because the bosons that mediate it are massive, whereas the photon is massless.

Re: 1489: "Fundamental Forces"

Posted: Mon Feb 23, 2015 1:24 am UTC
by Copper Bezel
But the thing is, I actually don't think it would be a surprising statement to say, based on that fact, that the weak force is stronger than EM. It hasn't really come up, but I think I would say that. I mean, it has a built-in range, which is different from the less-built-in, more structural limitations on EM's influence on the universe at large, but I'm not sure that's important. That it's called the "weak force" is relative in the first place and historical besides.

Re: 1489: "Fundamental Forces"

Posted: Mon Feb 23, 2015 2:10 am UTC
by Qaanol
Copper Bezel wrote:A particular particle can have a much stronger charge than it can have gravitational force of its own. That's a meaningful statement.

Even aside from trying to compare “charge” with “force”, I’m not convinced it’s a particularly useful statement. For example, another specific particle—the neutron—has a mass-to-charge ratio of ∞. Let me know when you find a massless charged particle.

Re: 1489: "Fundamental Forces"

Posted: Mon Feb 23, 2015 3:50 am UTC
by Copper Bezel
Well, that's certainly true.

Re: 1489: "Fundamental Forces"

Posted: Mon Feb 23, 2015 3:44 pm UTC
by sevenperforce
Qaanol wrote:
Copper Bezel wrote:A particular particle can have a much stronger charge than it can have gravitational force of its own. That's a meaningful statement.

Even aside from trying to compare “charge” with “force”, I’m not convinced it’s a particularly useful statement. For example, another specific particle—the neutron—has a mass-to-charge ratio of ∞. Let me know when you find a massless charged particle.

Gluons carry color charge but no mass (at least we don't think they have mass). But of course that's not EM at all. Just another example of how it's not really meaningful to compare the fundamental forces in this way.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 4:28 am UTC
by addams
xokocodo wrote:While I'd like to think I know a good amount of physics, the last two panels pretty much summarize my understanding of the Strong and Weak forces.

That's so funny.
I have spoken to a Physicist or three.

That's the way they explained it, too.
Right before they ducked under a Dog Pile of Math.

It Has to be Real!
It works on paper.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 5:55 am UTC
by billyswong
I start to think that "weak force" and "electromagnetic force" mingle together in "high energy" is an side-effect of "buffer overflow"/"integer overflow", assuming our world is like the one in http://xkcd.com/505/

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 11:02 am UTC
by Kit.
mathmannix wrote:Let's see... the four forces are Thrust, Summer, Famine, and Phlegm.

Spoiler:
Or how about Drag, Fall, Death, and Blood?

Almost.

They are Conquest, War, Famine and Death.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 11:50 am UTC
by drachefly
schapel wrote:I looked up just exactly how the weak force causes beta decay. Really, a down quark in a neutron decays into an up quark and a W boson. Then the W boson decays into an electron and an electron antineutrino. Because the W boson is the mediator of the weak force, you can kind of think of the weak force as being responsible for beta decay. I think really the appropriate terminology is that this decay is a weak interaction. There isn't really any force involved.


Beta decay produces kickback, so yeah there's a force. The proton that comes out has momentum in the rest frame of the original neutron.

Qaanol wrote:
Copper Bezel wrote:A particular particle can have a much stronger charge than it can have gravitational force of its own. That's a meaningful statement.

Even aside from trying to compare “charge” with “force”, I’m not convinced it’s a particularly useful statement. For example, another specific particle—the neutron—has a mass-to-charge ratio of ∞. Let me know when you find a massless charged particle.


The neutron has a magnetic dipole. It's coupled to the EM field at least a little. I suspect that if you put two neutrons next to each other, either their dipoles or their weak force (decay) will dominate their interaction compared to gravity (i.e. the weak force takes over except at very short ranges, so gravity having a merely-inverse-square force law doesn't let it win at long ranges). I might be wrong. I think that's an interesting question to check. Later.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 1:11 pm UTC
by schapel
drachefly wrote:Beta decay produces kickback, so yeah there's a force. The proton that comes out has momentum in the rest frame of the original neutron.

Well, sure, any time new particles are emitted they have energy and momentum and can provide a force. When an electron goes into a lower level orbit and emits a photon, that photon carries away energy. But it's not really an electromagnetic force between the electron and photon... the photon is not repelled because it's negatively charged!

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 2:55 pm UTC
by mathmannix
Drooling Iguana wrote:
mathmannix wrote:Let's see... the four forces are Thrust, Summer, Famine, and Phlegm.

And Venus de Milo in that one TV show that we don't talk about.

OK, I don't get this comment, please explain. My first thought was Salute Your Shorts, followed by The Simpsons...?

Kit. wrote:
mathmannix wrote:Let's see... the four forces are Thrust, Summer, Famine, and Phlegm.

Spoiler:
Or how about Drag, Fall, Death, and Blood?

Almost.

They are Conquest, War, Famine and Death.

Actually, those are their riders. The only one whose name I know is Binky.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 3:36 pm UTC
by Neil_Boekend
Nah, War drives a cherry red 1965 Mustang Fastback, Famine drives a Cadillac Escalade, Pestilence drives a green '72 AMC Hornet and Death drives a pale 1959 Cadillac Eldorado.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 4:14 pm UTC
by da Doctah
Kit. wrote:
mathmannix wrote:Let's see... the four forces are Thrust, Summer, Famine, and Phlegm.

Spoiler:
Or how about Drag, Fall, Death, and Blood?

Almost.

They are Conquest, War, Famine and Death.


Not to be confused with Bread, Cream, Raspberries and Vanilla Fudge. That's the Four Food Groups.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 5:06 pm UTC
by Klear
Neil_Boekend wrote:Nah, War drives a cherry red 1965 Mustang Fastback, Famine drives a Cadillac Escalade, Pestilence drives a green '72 AMC Hornet and Death drives a pale 1959 Cadillac Eldorado.


Are you sure Famine doesn't drive a Trabant?

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 5:20 pm UTC
by Neil_Boekend
Not according to the Supernatural wiki.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 6:54 pm UTC
by Pfhorrest
schapel wrote:
drachefly wrote:Beta decay produces kickback, so yeah there's a force. The proton that comes out has momentum in the rest frame of the original neutron.

Well, sure, any time new particles are emitted they have energy and momentum and can provide a force. When an electron goes into a lower level orbit and emits a photon, that photon carries away energy. But it's not really an electromagnetic force between the electron and photon... the photon is not repelled because it's negatively charged!

I'm a little fuzzy on this myself (someone else please clarify or correct!), but it's my understanding that the interaction between electrically charged particles attracting or repelling each other can actually be modeled as an exchange of (virtual?) photons transferring momentum between them. So the photon that that electron emits really isn't so different from the attraction or repulsion of charged particles at all.

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 7:58 pm UTC
by rmsgrey
Pfhorrest wrote:
schapel wrote:
drachefly wrote:Beta decay produces kickback, so yeah there's a force. The proton that comes out has momentum in the rest frame of the original neutron.

Well, sure, any time new particles are emitted they have energy and momentum and can provide a force. When an electron goes into a lower level orbit and emits a photon, that photon carries away energy. But it's not really an electromagnetic force between the electron and photon... the photon is not repelled because it's negatively charged!

I'm a little fuzzy on this myself (someone else please clarify or correct!), but it's my understanding that the interaction between electrically charged particles attracting or repelling each other can actually be modeled as an exchange of (virtual?) photons transferring momentum between them. So the photon that that electron emits really isn't so different from the attraction or repulsion of charged particles at all.


When it comes to quantum mechanics, we have wonderful theoretical models that predict what happens when we do experiments to fantastic degrees of accuracy (just getting measurements to that degree of accuracy is an achievement in itself) but which don't actually explain what's happening - and any model that obeys the mathematics is a possible explanation. So there are several different models, each of which relates a different familiar thing to the quantum realm in ways our primate brains can grasp, but probably none of them is exactly what's happening - they're just stories we make up to make the maths easier to remember...

Re: 1489: "Fundamental Forces"

Posted: Tue Feb 24, 2015 10:29 pm UTC
by sevenperforce
drachefly wrote:
Qaanol wrote:
Copper Bezel wrote:A particular particle can have a much stronger charge than it can have gravitational force of its own. That's a meaningful statement.

Even aside from trying to compare “charge” with “force”, I’m not convinced it’s a particularly useful statement. For example, another specific particle—the neutron—has a mass-to-charge ratio of ∞. Let me know when you find a massless charged particle.

The neutron has a magnetic dipole. It's coupled to the EM field at least a little. I suspect that if you put two neutrons next to each other, either their dipoles or their weak force (decay) will dominate their interaction compared to gravity (i.e. the weak force takes over except at very short ranges, so gravity having a merely-inverse-square force law doesn't let it win at long ranges). I might be wrong. I think that's an interesting question to check. Later.

Let's see. The absolute value of the maximum potential energy of an interaction between two magnetic dipole moments is given by PEm = μ0m1m2/2πr3. The absolute value of the maximum gravitational potential energy between two massive objects is given by PEG = GM1M2/r.

The gravitational potential energy is proportional to r-1 but the magnetic potential energy is proportional to r-3, so it's going to come down completely to the range of distances you're dealing with. Setting these potential energies equal to each other actually allows us to find this critical distance numerically:

μ0m1m2/2πr3 = GM1M2/r

Remembering that m1 is the same as m2 and the same for M, and doing a little algebra...

r2 = μ0m2/2πGM2

And solving for r with a bit more algebra...

r = mM(μ0/2πG)1/2

Now, μ0 is 1.3e-6 N/A2, and of course G is 6.7e-11 J/kg2. In the case of a neutron, the magnetic moment is 9.7e-27 Am2 and the mass is 1.7e-27 kg. So it's just simple math...

r = 9.7e-27 Am2 * 1.7e-27 kg * (1.3e-6 N/A2 / 2 * π * 6.7e-11 J/kg2)1/2

r = 1.65e-53 A*kg*m2 (3090 kg2N/A2J)1/2

r = 9.17e-52 m

Assuming that I haven't made any math mistakes...that's a really small number. Given that this number is 18 million billion times smaller than the Planck length, I think we can say without reservation that the gravitational potential energy between two neutrons is always going to be greater than the potential energy of their magnetic dipole interactions.

Re: 1489: "Fundamental Forces"

Posted: Wed Feb 25, 2015 1:20 am UTC
by addams
rmsgrey wrote:
Pfhorrest wrote:
schapel wrote:
drachefly wrote:Beta decay produces kickback, so yeah there's a force. The proton that comes out has momentum in the rest frame of the original neutron.

Well, sure, any time new particles are emitted they have energy and momentum and can provide a force. When an electron goes into a lower level orbit and emits a photon, that photon carries away energy. But it's not really an electromagnetic force between the electron and photon... the photon is not repelled because it's negatively charged!

I'm a little fuzzy on this myself (someone else please clarify or correct!), but it's my understanding that the interaction between electrically charged particles attracting or repelling each other can actually be modeled as an exchange of (virtual?) photons transferring momentum between them. So the photon that that electron emits really isn't so different from the attraction or repulsion of charged particles at all.


When it comes to quantum mechanics, we have wonderful theoretical models that predict what happens when we do experiments to fantastic degrees of accuracy (just getting measurements to that degree of accuracy is an achievement in itself) but which don't actually explain what's happening - and any model that obeys the mathematics is a possible explanation. So there are several different models, each of which relates a different familiar thing to the quantum realm in ways our primate brains can grasp, but probably none of them is exactly what's happening - they're just stories we make up to make the maths easier to remember...

Yes.
I believe you.

They are stores we make up that fit The Math, are inside our understanding, and they entertain our Primate Minds.
"We all look like a bunch of Big Blond Monkeys to you. Don't we?" paraphrased from Penny on The Big Bang Theory.

Re: 1489: "Fundamental Forces"

Posted: Wed Feb 25, 2015 5:48 am UTC
by schapel
Pfhorrest wrote:I'm a little fuzzy on this myself (someone else please clarify or correct!), but it's my understanding that the interaction between electrically charged particles attracting or repelling each other can actually be modeled as an exchange of (virtual?) photons transferring momentum between them. So the photon that that electron emits really isn't so different from the attraction or repulsion of charged particles at all.

I think you're right, but I think it's more correct to say that an electron emitting a photon is an "electromagnetic interaction" rather than an example of the electromagnetic force. The latter term seems to imply there's a force acting between the electron and the photon. Well, there is actually a force between them, because the photon shoots one way, so the electron must experience an equal force in the opposite direction. But the electron and photon are not actually repelling each other electrically or magnetically. The force between them is more like the radiation pressure light exerts when shined on an object. We can even use this kind of force to shine a laser on a gas to cool it down.

But I have only the equivalent of an A.S. degree in physics. Any physics majors are welcome to correct anything I got wrong.

Re: 1489: "Fundamental Forces"

Posted: Wed Feb 25, 2015 5:50 am UTC
by Eternal Density
mathmannix wrote:Let's see... the four forces are Thrust, Summer, Famine, and Phlegm.

Spoiler:
Or how about Drag, Fall, Death, and Blood?
The four Summers are Time, Court, Holiday, and Glau.
mathmannix wrote:Actually, those are their riders. The only one whose name I know is Binky.
Inky, Pinky, and Clyde.

Re: 1489: "Fundamental Forces"

Posted: Wed Feb 25, 2015 7:18 am UTC
by PM 2Ring
drachefly wrote:The neutron has a magnetic dipole. It's coupled to the EM field at least a little. I suspect that if you put two neutrons next to each other, either their dipoles or their weak force (decay) will dominate their interaction compared to gravity (i.e. the weak force takes over except at very short ranges, so gravity having a merely-inverse-square force law doesn't let it win at long ranges). I might be wrong. I think that's an interesting question to check.


There's some residual strong interaction between a pair of neutrons, but it's not enough to create a bound state. From Neutronium
Wikipedia wrote:Dineutron: The dineutron, containing two neutrons was unambiguously observed in the decay of beryllium-16, in 2012 by researchers at Michigan State University.[6][7] It is not a bound particle, but had been proposed as an extremely short-lived state produced by nuclear reactions involving tritium. It has been suggested to have a transitory existence in nuclear reactions produced by helions that result in the formation of a proton and a nucleus having the same atomic number as the target nucleus but a mass number two units greater. There had been evidence of dineutron emission from neutron-rich isotopes such as beryllium-16 where mononeutron decay would result in a less stable isotope.

The dineutron hypothesis had been used in nuclear reactions with exotic nuclei for a long time.[8] Several applications of the dineutron in nuclear reactions can be found in review papers.[9] Its existence has been proven to be relevant for nuclear structure of exotic nuclei.[10] A system made up of only two neutrons is not bound, though the attraction between them is very nearly enough to make them so.[11] This has some consequences on nucleosynthesis and the abundance of the chemical elements.[9][12]


Also see Isospin singlet state of the deuteron.

It's sometimes said that neutronium is held together by gravity, but that's not quite right. The formation of neutron-degenerate matter requires very high pressure, gravity is merely a way of achieving that pressure.