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4 fundamental forces 
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#1
May703, 10:14 PM

P: 321

In my textbk, it says that
The four fundamental forces are : a) Gravitational force b) Electric and magnetic forces c) Weak force responsible for beta decay d) Strong force which holds the nucleus together What does (c) mean? I've heard that scientists think the 4 forces were seperated from 1 force shortly after the bb, that's why they are eager to find the GUT. Why they think the 4 forces were seperated at the very beginning? (simple explanation please ) 


#2
May703, 10:40 PM

P: n/a

c) is just what it says. There is a force that is really weak but causes certain types of particles to interact (electrons and neutrinos for instance.) It happens to be responsible for a certain type of radioactive decay, beta decay.
Reasons for unifaction: 1) The history of physics has been one of unifying various things: for example the electric and magnetic forces; more recently the weak and electromagnetic forces (b&c above.) 2) The coupling constants  a measure of the strengths of the various forces  are all equal at small distances. This would be a really strange coincidence, unless the four (three) forces were just different manifestations of the same thing, in which case it would be expected. This is the most persuasive argument. 3) One force is much simpler than three/four. 


#3
May703, 11:47 PM

P: n/a

"c" is an acronym for "celeritas" which is Latin for "swift". The speed of light, ~186,000mi/sec is symbolized with c. 'c' is a constant, as light is a natural constant according to special relativity.



#4
May703, 11:57 PM

Emeritus
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PF Gold
P: 1,817

4 fundamental forces
The strong force is what holds the protons together when they are reeeaally close together.
The weak force is what pushes them apart once they get past a certain distance. It is what is responsible for nuclear explosions (among other things) 


#5
May803, 12:22 AM

P: n/a

What is the force called, enigma?



#7
May803, 02:32 PM

P: n/a

Fusion bomb uses rearrangement of nucleons thus difference in nuclear potential as a source of energy  again not the weak force. 


#8
May803, 04:53 PM

Emeritus
PF Gold
P: 8,147

The weak force is now thought to be unified with elecromagnetism in the electroweak theory, part of the Standard Model. The theory has four force carrying particle, one massless, the photon of EM, and the other three massive, a positive and a negatively charged W and a neutral Z. The weak force is the only one that can "break flavor" that is, change one kind of quark into another.



#9
May803, 05:30 PM

Emeritus
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PF Gold
P: 1,817

That's what I get for trying to swim outside my realm of expertise...



#10
May803, 05:35 PM

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PF Gold
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 Warren 


#11
May803, 05:52 PM

Mentor
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#12
May903, 03:41 AM

P: 161




#13
May903, 11:35 AM

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PF Gold
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Beta decay produces, in fact, an electron antineutrino.  Warren 


#14
May903, 01:12 PM

PF Gold
P: 621

Hm,..It was mentioned that at small distances the 4 "coupling constants" are all the same...What does that mean? Are those constants like "G" u mean? 


#15
May903, 03:48 PM

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#16
May903, 04:00 PM

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PF Gold
P: 23,271

http://math.ucr.edu/home/baez/constants.html G is not a coupling constant in this picture so much as something set equal to one to determine the underlying scales. And gravity is not part of the Standard Model but rather its geometrical backdrop. I dont question Baez he is expert and a good explainer. I just dont understand. The four numbers he seems to be talking about are the electromagnetic coupling (1/137...) and the strong force coupling (....) plus two higgs things: higgs mass and higgs field expectation value. And maybe he does not mean 1/137...when he says "fine structure constant"maybe there is a bare or small distance version of it and he's talking about that! Many possibilities for confusion. He says weak force coupling is not fundamental because it can be calculated from the others. He says that in place of THAT choice of four you can make a logically or algebraically equivalent choice of another four, namely U(1) coupling const, and SU(2) coupling const, and again higgs mass and higgs field expectation value. Any way if there really are some coupling constants in the prevailing model of things, what are the numbers? I know: 42, but what's the question? "What does (c) mean?" But KLKam was not asking about the speed of light so, in fact, Majin's comment was disconnected, as you suggest. 


#17
May903, 10:37 PM

P: n/a

Naa, the EM coupling constant alpha_e is the usual ~1/137. Technically the coupling constants are, well, constants that appear in the vertex term when you do Feynman diagram calculations. Remember Feynman diagrams are just a type of perturbation / power series expansions.... the number of vertices in a diagram is what you are expanding on.
So the amplitude for a simple QED process has two vertices, hence a factor of (1/137)^2; the amplitude for a fourvertex process has a factor of (1/137)^4 in it. So we can usually get really good values just by doing the simple diagrams and ignoring the more complicated ones. The weak coupling constant is what you use for weak interactions, eg leptonneutrinoW vertices (plus some others.) The constants basically represent the strength of the force  the likelihood of an interaction sorta. It turns out they all 'run' with energy, eg for higherenergy particles you need to use a different 'constant.' The intuitive explanation for this is that the vacuum particleantiparticle pairs do a sort of polarization shielding  exactly like the presence of matter alters the permeability/permittivity, and hence the fine structure constant. Higherenergy particles come closer together, hence see less of the shielding and interact differently. For the weak and EM (I think) interactions, this means the coupling gets stronger at close distances; for the strong force it turns out the coupling gets *weaker.* When you extrapolate the the measured coupling constants vs. energy graphs, it turns out they almost meet at ~10^14 GeV. When you add in SUSY, they meet exactly (within the error margins.) Incidentally, because the strong force is so strong, at low energies its coupling constant becomes greater than 1. At this point you can't use traditional Feynman diagrams because the series is obviously divergent. This is a pain in the ***, and there are crazy 'nonperturbative QCD' techniques used to model these interactions. At higher interaction energies  highQ deep inelastic scattering for example  the coupling constant is low enough that we can do the usual Feynman diagram calculations. That help? 


#18
May1003, 10:45 AM

Astronomy
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PF Gold
P: 23,271

particularly useful way to think of it besides the point where these couplings happen to jibe? One landmark, namely Planck energy, is 2 billion joulesso more like 10^19 GeV, way out of the 10^14 GeV ballpark. There must be some different perspective on 10^14 GeV. You say the EM coupling (1/137) gets stronger (you think) and I seem to remember some number like 1/128not that precisely but vaguely like thatfor a bare unshielded real close high energy fine structure constant. I'm pretty sure you are right it does get stronger. The woods are full of things people call "constants" which change and run around and aren't constant. I wish I could mentally bridge between the practical experimentally useful constants that are used in perturbative (power series/successive approximation) calculation with Feynmann diagrams and the 26 REAL constants of the Standard Model summarized here: http://math.ucr.edu/home/baez/constants.html In this brief exposition Baez seems not to be talking about constants that "run" but about 26 definite dimensionless numbers. His list does not include 1/137, for instance, because it is calculable from four numbers he calls theoretically more basic. I expect or hope that the "running" of 1/137 is also calculable from these four theoretically basic numbers. Here's a quote: [[Instead of the electromagnetic coupling constant together with the masses of the W, Z, and Higgs, we could have used 4 other constants: the U(1) coupling constant, the SU(2) coupling constant, the mass of the Higgs, and the expectation value of the Higgs field. These are the numbers that actually show up in the fundamental equations of the Standard Model. The idea is that the photon, the W and the Z are described by an U(1) x SU(2) gauge theory, which involves two coupling constants. The beautiful symmetry of this theory is hidden by the way it interacts with the Higgs particle. The details of this involve two further constants  the Higgs mass and the expectation value of the Higgs field  for a total of 4. If we know these 4 numbers we can calculate the numbers that are easier to measure in experiments: the masses of the W and Z, the electromagnetic coupling constant, and the mass of the Higgs. In practice, we go back backwards and use the constants that are easy to measure to determine the theoretically more basic ones.]] It may not be fair to ask for help out of this particular patch of quicksand so I will just say thankyou for your earlier reply (but any further insights would be welcome!) 


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