# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: benm on 05/04/2019 15:57:49

Title: Why do neutrons weigh more than protons?
Post by: benm on 05/04/2019 15:57:49
Jeff has three partic-ularly puzzling problems:

1) I would expect protons to have greater mass than neutrons, because protons have a positive charge, which should involve mass to confer the charge, and neutrons have none.  Why do neutrons then have greater mass?

2) The speed of light is always given "in a vacuum".  Why that qualification?  What causes light to move at a slower speed when passing through matter?  Don't the photons mostly pass through empty space between nuclei and electron rings?  Is light actually slower, or just bouncing around until it's absorbed?

3) What happens to the electrons that are stripped off atoms in order to make a plasma?  Where do they go?

Title: Re: Why do neutrons weigh more than protons?
Post by: chiralSPO on 05/04/2019 17:32:11
1) A neutron can decompose into a proton and an electron (also releasing a neutrino). It's actual mass is determined by the masses of the quarks and binding energy (a proton is made of two up quarks and a down quark, while a neutron is made of two down quarks and an up quark).

2) This is a very tricky question. We can use different models to account for the different speeds of light in different materials--one model essentially views the photon as being absorbed and re-emitted repeatedly, and one model just looks at the effective permittivities (both electric and magnetic), and uses that to calculate the speed of light. Both models predict the same speeds (as far as I know), but sometimes it is better to use one model than the other.

3) The free electrons stay in the plasma in a gaseous state. (overall the plasma is neutral, with an equal number of protons and electrons in it)
Title: Re: Why do neutrons weigh more than protons?
Post by: Janus on 05/04/2019 17:49:34
Jeff has three partic-ularly puzzling problems:

1) I would expect protons to have greater mass than neutrons, because protons have a positive charge, which should involve mass to confer the charge, and neutrons have none.  Why do neutrons then have greater mass?
Charge, per say, doesn't add mass.  A proton is made of two Up quarks, each with a charge of +2/3 and one down quark with a charge of -1/3 , which leads to a net charge of 2/3+2/3 -1/3 = 1
Neutrons are made of of 1up and 2 down quarks, which gives 2/3 -1/3-13 =0 for the net charge
It is the energy configuration of the quarks that make the mass difference.  Free neutrons will decay in a Proton and electron. Since it natural for things to go from a higher to lower energy state, it then makes sense that the Neutron is at the higher state and it is this extra energy that is the source of the mass difference.
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2) The speed of light is always given "in a vacuum".  Why that qualification?  What causes light to move at a slower speed when passing through matter?  Don't the photons mostly pass through empty space between nuclei and electron rings?  Is light actually slower, or just bouncing around until it's absorbed?
The qualification is important as the constant c, of which Relativity makes heavy use, doesn't change, while the measured speed of light can through different material.  c is the speed at which light would travel through a vacuum, And this is the value which is important. So when you say that the speed of light is a constant for all inertial reference frames, you need the "in vacuum" qualifier, because the speed of light through other mediums is not constant between reference frame.
The important thing is the value c and not whether or not the light you are dealing with is actually traveling at that speed. It's not the light that is responsible Relativity's effects, its that value of c, which is built into the universe.
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3) What happens to the electrons that are stripped off atoms in order to make a plasma?  Where do they go?[/b][/i]
They generally don't go anywhere. A plasma is a mixture of those stripped atoms and the stripped away electrons. They just aren't bound to each other.   You can separate them, but you would still have the electrons somewhere.  You can pass a current through it, but then you would have electrons coming in to replace those going out.

Title: Re: Why do neutrons weigh more than protons?
Post by: syhprum on 05/04/2019 18:13:15
At school we were taught that the nucleus of an atom contained both protons and electrons, neutrons had been discovered but our text books were rather old and nuclear physics like the British invention radar were military secrets.
Title: Re: Why do neutrons weigh more than protons?
Post by: Janus on 05/04/2019 19:08:41
At school we were taught that the nucleus of an atom contained both protons and electrons, neutrons had been discovered but our text books were rather old and nuclear physics like the British invention radar were military secrets.
The funny thing about "military secrets", sometimes, they aren't all that secret.  I for instance, have a Physics text, copyright 1939, which talks of neutrons and the possibility of releasing atomic energy.   Many science fiction writers of the early 40's were writing stories about atomic bombs and getting the basics pretty much correct during the same time that the US was trying to develop the bomb.   Some of them even got knocks of the door from government agents, asking them how they knew so much and would they please stop giving the info away.
The writers were able to convince them that the information was readily available to anyone with a library card and the willingness to learn it.  They also pointed out that if US science fiction writers suddenly stopped writing stories about atomic energy/bombs, that it would be a dead give away that the US was working on the bomb and had put the lid on.
Title: Re: Why do neutrons weigh more than protons?
Post by: flummoxed on 06/04/2019 15:21:03
2) This is a very tricky question. We can use different models to account for the different speeds of light in different materials--one model essentially views the photon as being absorbed and re-emitted repeatedly, and one model just looks at the effective permittivities (both electric and magnetic), and uses that to calculate the speed of light. Both models predict the same speeds (as far as I know), but sometimes it is better to use one model than the other.

This is a little like something I was mulling over a little while ago, and never found any conclusive answers on. Can a photon passing through free space, be viewed as being absorbed and re-emitted by virtual photons? Thus maybe re-producing a wave particle effect via a jittery movement through space?  :-\
Title: Re: Why do neutrons weigh more than protons?
Post by: syhprum on 06/04/2019 19:27:03
All the basic "secrets" of how to make nuclear bombs were well known to the axis powers what they lacked was the vast amount of Industrial capacity that could be spared for such a project
We are told that the Germans miscalculated the amount of U235 required for a warhead, I find that hard to believe and I am sure as soon as they had samples to test they would have arrived at the correct figure.
One of the so called secrets was the need of a Neutron source to speed things up after the critical mass has been assembled but pictures published in the newspapers just after the first bombs were used showed lust such a source
Title: Re: Why do neutrons weigh more than protons?
Post by: evan_au on 06/04/2019 22:59:34
Quote from: chiralSPO
a proton is made of two up quarks and a down quark, while a neutron is made of two down quarks and an up quark
The down quarks (at 4.8MeV/c2) have more mass than an up-quark (at 2.3 MeV/c2), so you might expect that the neutron weighs more than a proton.

However, the mass of the quarks only adds up to 9.4 MeV/c2 for the proton and 11.9 MeV/c2 for the neutron.

Their true masses are about 100 times greater, at 938.2 MeV/c2 for the proton and 939.6 MeV/c2 for the neutron.

Around 99% of the mass of the proton and neutron is made up of the gluon field that "glues" the quarks together.

See: https://en.wikipedia.org/wiki/Quark#Table_of_properties