Time : Expansion of The Universe

slade_x

maybe the following may be of use to read. in the terminology spin does actually mean spin and describes the angular momentum of a particle

http://en.wikipedia.org/wiki/Spin_quantum_number paying attention to #Electron_spin, #detection_of_spin and #dirac_equation_solves_spin

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

NB. the # denotes a heading under the article eg.
http://en.wikipedia.org/wiki/Spin_quantum_number#Electron_spin

Wh1stler

Okay, let me rephrase the question: Does a photon experience a change in spin when its path becomes curved due to a strong gravitational field?

A photon travelling along a curved path will see circumpolar motion in the stars; does this not mean that it is rotating about its own axis in the same way that a tidally locked object is deemed to do?

Anonymo

Wh1stler wrote: »

Okay, let me rephrase the question: Does a photon experience a change in spin when its path becomes curved due to a strong gravitational field?

A photon travelling along a curved path will see circumpolar motion in the stars; does this not mean that it is rotating about its own axis in the same way that a tidally locked object is deemed to do?

Ok I'll give a shot at this. Spin is comprised of two things: orbital angular momentum and intrinsic spin. The latter is a quantum thing and won't change due to the strong gravitational field (*unless you're talking about the quantum gravity domain that we don't understand anyway). The angular momentum can change due to the 'geodetic effect' which is because the direction of the angular momentum is changing due to the increased curvature of spacetime. There also is a frame-dragging effect but this is very small so you can ignore that. There's a description here http://einstein.stanford.edu/SPACETIME/spacetime4.html.

Anyway a photon has zero orbital angular momentum (unless you start talking about twisted photons - i.e. a corkscrew type configuration of photons). So for the standard case there will not be an effect on the photons. You will however see the effect on light (but non-zero mass) particles. This is one of the ways that they test general relativity.

Justin1982

shizz wrote: »

Yeah I was wondering about this problem myself, but any time I seen a description of a particle spinning they always showed it actually spinning and said that for example one might spin clockwise and the other anti-clockwise (with regards to quantum entanglement). So unless they are dumbing down the term I don't see what they could mean.

As you might know, electrons orbit about the atomic nucleus so that means they have orbital angular momentum due to that motion. Early experiments measured this orbital angular momentum and then they noticed the values were slightly shifted. To explain these slight shifts they imagined the electrons spinning about their centre of mass axis while also orbiting about the nucleus. Bit like the earth orbits the sun but also spins on its axis. Thing was that the orbital angular momentum of an electron and its spin is quantized compared to the earths which seems continuous.

Above analogy is how its taught and how most early physicists thought it worked. Spin is really just a quantum number. Its actually more accurate to think of spin as just a mathematical number assigned to the electron than to think that the electron actually spins on its axis. Similarly for the orbital angular momentum (although most working physicists still have it in their heads to a degree that the electron is actually orbiting the electron)

To think of it conceptually though it helps physicists to draw diagrams of an electron spinning about an axis. It can be easier to conceptualize and work with than thinking of just numbers is all.

Wh1stler

Anonymo wrote: »

Ok I'll give a shot at this. Spin is comprised of two things: orbital angular momentum and intrinsic spin. The latter is a quantum thing and won't change due to the strong gravitational field (*unless you're talking about the quantum gravity domain that we don't understand anyway). The angular momentum can change due to the 'geodetic effect' which is because the direction of the angular momentum is changing due to the increased curvature of spacetime. There also is a frame-dragging effect but this is very small so you can ignore that. There's a description here http://einstein.stanford.edu/SPACETIME/spacetime4.html.

Anyway a photon has zero orbital angular momentum (unless you start talking about twisted photons - i.e. a corkscrew type configuration of photons). So for the standard case there will not be an effect on the photons. You will however see the effect on light (but non-zero mass) particles. This is one of the ways that they test general relativity.

I'm sorry but you seem to be saying that photons have and don't have angular momentum.

Is it theoretically possible for a photon to orbit a black hole, say?

If it is then a photon that starts with zero angular momentum before it enters such an orbit will acquire one apparent spin on its own axis per orbit; it would be effectivelly 'tidally locked', wouldn't it? And its angular momentum would be entirely due to the space-time curvature.

Or would it be the case that the photon would appear to have retrograde motion from the perspective of the black hole? (This would cause a serious problem in physics I think.)

And consider the photon whose path is curved temporarily due to its passing close to a star; while its path is curved, it would have some angular momentum. As it re-enters 'flat space', would it retain axial rotation acquired during the 'curved' part of its journey?

Well, the orbit of the moon is caused by space-time curvature so if the moon entered into orbit around the earth possessing zero angular momentum to begin with then why should it be considered to be spinning any more than the photon does in orbit around a black hole?

I mean, the moon does not spin relative to the line along which it travels whereas the earth does; the left-hand side of the moon is always on the left-hand side of its path and the right-hand side is always on the right-hand side of the path.

The far side of the moon always travels a little faster, and a little further, than the near side.

If the moon considers itself travelling in a straight line as a photon does then it is not actually spinning is it?

Cú Giobach

Wh1stler wrote: »

Well, the orbit of the moon is caused by space-time curvature so if the moon entered into orbit around the earth possessing zero angular momentum to begin with then why should it be considered to be spinning any more than the photon does in orbit around a black hole?

The question above makes no sense.
If an object enters orbit with zero angular momentum of course it isn't spinning and without tidal effects it will not start spinning (like Uranus isn't spinning perpendicular to the plane of the solar system). I keep saying this to you over and over again so I'll put it in bold this time, there is no friction in space.

You keep ignoring these so please address them this time.
If a body is spinning at a set rate and is then put into orbit do you think it suddenly loose one rotation without any change in rotational speed?
Or is a non rotating body actually somehow making exactly one retrograde rotation per orbit?

Anonymo

Wh1stler wrote: »

I'm sorry but you seem to be saying that photons have and don't have angular momentum.

Nope I'm not. My answer gave you the general case. Any particle has both orbital and spin angular momentum. For a photon the orbital angular momentum is zero (because it's massless). Spin is quantum so it won't change. Cu Giobach has described to you why something with zero orbital angular momentum will not acquire it.

Electrons have non-zero mass. This is why they are used to test for general relativistic effects on orbital angular momentum.