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Physics, Astronomy & Cosmology / Re: Do antiparticles have opposite charge and spin?

« on: 08/12/2016 17:35:37 »

particle and antiparticle must have:
- the same mass m
- the same spin state J
- opposite electric charges q and -q.

See: https://en.wikipedia.org/wiki/Antiparticle#Properties

Protons and antiprotons are both spin=1/2 particles (fermions).
- Ignoring antiprotons (which are quite rare in our world), your body has lots of protons in Hydrogen atoms.
- Normally, these proton spins are randomly aligned, but it is possible to align them in an external magnetic field (this is what an MRI scanner does).
- It is possible to measure the state of protons as being "spin up" or "spin down", when measured relative to an external magnetic field.
- So "opposite" spins can occur in normal matter, without needing to invoke antimatter

PS: While checking my answer, I discovered that physicists can't fully account for where the spin of a proton comes from. Surprisingly little (maybe none) comes from the spin of the component quarks. This is called the proton spin crisis, and is one of the major unsolved puzzles in physics.

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Physics, Astronomy & Cosmology / Re: Is there a dark matter halo around EVERY galaxy?

« on: 28/05/2016 13:07:43 »

There are claims from NASA that in the Bullet Cluster, a high-velocity impact of Matter (which interacts strongly) has separated from the expected accompanying Dark Matter (which interacts weakly, if at all, with Matter or Dark Matter).

They used the technique of gravitational lensing to detect the invisible Dark Matter by its gravitational impact on light from more distant galaxies.
See: http://chandra.harvard.edu/press/06_releases/press_082106.html

There have been some contentious claims that certain galaxies show no signs of Dark Matter, for example:
https://www.newscientist.com/article/dn13280-galaxy-without-dark-matter-puzzles-astronomers/

But for now, most cosmologists think that the universe could only get the way we see it today if there is far more Dark Matter in the universe than the Matter we can see.

Most astronomers think that most galaxies show signs of a Dark Matter halo through internal evidence (rotation curves) or external evidence (gravitational lensing), among others.

It's just that we don't know what Dark Matter is, as yet - but that won't stop scientists dreaming up theories (that might win them a Nobel Prize, someday).

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Physics, Astronomy & Cosmology / Re: Is there a dark matter halo around EVERY galaxy?

« on: 28/05/2016 09:09:00 »

This is a very good question, and a hard one to answer. In fact, I have asked more broadly for some input here...

Who can help?

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Physics, Astronomy & Cosmology / Re: What is the difference between Electron, Muon and Tau Neutrinos?

« on: 18/02/2016 22:58:23 »

Neutrinos are some of the least-well understood elementary particles. Although we are constantly bathed in a stream of neutrinos, they interact so weakly with matter that they are very hard to detect, let alone measure.

They are predicted to have different masses, but so far I am unaware of any direct measurement of mass of a neutrino. There are some experimental data that place upper bounds on the mass, but even so, they are significantly less massive than electrons.

I don't know how much is known about the neutrino oscillation, but it was only fairly recently predicted, and even more recently shown to occur.

There are other members here with more knowledge than I have on the subject. Hopefully they will add to this discussion soon.

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Physics, Astronomy & Cosmology / Re: Neutrino mass could have been discovered without Neutrino Oscillations?

« on: 15/02/2016 21:02:20 »

So, if positrons, anti-muons and anti-taus have different masses, the three neutrinos need to have different masses as well for the law of conservation of energy to work.

The energy carried away from a nuclear decay is in the form of:
- the particles' "rest-mass", for which you have to bring the particle to rest. You can consider this mass to have an equivalent energy, according to Einstein's iconic E=mc²
- the particles' "kinetic energy". You can consider this energy to have an equivalent mass, according to Einstein's iconic E=mc². (...although physicists prefer to talk about momentum rather than kinetic energy)

The energy of the decay is divided among the debris in a somewhat random way, with the kinetic energy of the neutrino mostly far exceeding it's rest mass.

Take the example of neutron decay, which is better-understood than Higgs decay, with much lower energy: n⁰ → p⁺ + e⁻ + ν_e + 0.78MeV
In some cases, the electron carries off almost all the energy; in other cases, the neutrino carries off almost all the energy.

However, the mass of the neutrino is thought to be somewhere around 0.1-0.25eV (it's really hard to measure).

So a neutron decay where the neutrino rest mass is 0.1eV, and the kinetic energy is anywhere up to 782,343eV±13,000eV makes it really hard to pin down the rest mass of the neutrino. The error bars are far larger than the thing you are trying to measure!

Trying to pin down the rest mass of 3 neutrinos from the very infrequent (and very high energy: 126GeV) decay of the Higgs is even more problematic.

Other experiments have constrained the difference in mass of the different neutrino types, and it appears that they are fairly similar to each other. So distinguishing the masses via different paths of Higgs decay is especially tricky!

For current guesstimates of neutrino mass, see: http://en.wikipedia.org/wiki/Neutrino#Mass
It is hoped that the KATRIN & MARE experiments should pin down the mass of the electron neutrino more accurately, over the next couple of years.

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Physics, Astronomy & Cosmology / Re: What does it mean to say an Electron Neutrino is associated with an electron?

« on: 30/01/2016 09:24:42 »

Why do you say a neutrino travels, other than deafly repetere everyone, instead of goes?

The other generations of neutrinos are isomers of the same neutrino, like the isomers of positronium or dihydrogen.  Note there are three generations like there are three bodies in the π-e bond.

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Physics, Astronomy & Cosmology / Re: What does it mean to say an Electron Neutrino is associated with an electron?

« on: 29/01/2016 21:52:42 »

Neutrinos were first detected in radioactive Beta capture (which absorbs an electron) and Beta decay (which produces a positron = anti-electron) - but there was some missing energy and momentum in the reaction. Fermi suggested that the missing energy was carried away by an unseen particle.

There were similar anomalies in later reactions involving Muons, and similarly when the Tau particle was discovered. This gave the three known neutrino types their names.

We now know that these associations are not "fixed", and that a given neutrino will oscillate between all three types as it travels through space at slightly less than the speed of light in a vacuum.

See: http://en.wikipedia.org/wiki/Neutrino#Neutrino_flavor

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Physics, Astronomy & Cosmology / Re: Can nuclear fission be a result of the colision of two particles?

« on: 17/12/2015 12:13:52 »

The collision of two nuclei can certainly result in fission of one or both of the nuclei. However, the nuclei must be large enough and unstable enough to undergo fission.

For instance, it is impossible for a hydrogen nucleus to undergo nuclear fission (there is only one proton). And while nuclei like that of ¹²C could hypothetically break into smaller pieces, this is highly unlikely because ¹²C is so stable.

Realistically only nuclei heavier than iron are likely to undergo any type of fission event, and you still need a lot of energy. If your theory is about the early universe, you might want to rethink the importance of fission, because in the beginning there was really only hydrogen and helium. It is only through the action of stars and supernovae (over billions of years) that we have any heavier elements at all.

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Physics, Astronomy & Cosmology / Does sound have mass?

« on: 17/09/2012 19:30:01 »

Candice asked the Naked Scientists:
   
Hi, my name is Candice and I have a quick question.

Sound travels slower than light, and light is the speed limit of the universe because it doesn't have mass. Does this mean that sound has mass?

Thank you

What do you think?