1. SRH
Established Member
Join Date
Jul 2012
Posts
216

## Question about angular momentum in a vacuum

If i am floating in space in a vacuum and i am holding a baseball,
And i throw the baseball as a curveball,

Would the baseball spin at the same rotation speed for infinite time,
Or would the rotation speed slow down over time?

Is there inertia for rotation speed?

Second question, if i throw the curveball through a hydrogen gas medium, would friction cause the rotation speed to slow over time?

thanks.

2. Yes, the ball will continue to spin. And it won't curve or lose linear velocity, either. (Ignoring the fact that there's no PERFECT vacuum.)

And yes, there's inertial for rotation.

In an atmosphere of any kind, it'll slow down, both in spin and linear velocity, and tend to curve. Hydrogen probably less so than air, at a given pressure.

3. Order of Kilopi
Join Date
Sep 2005
Location
Metrowest, Boston
Posts
4,055
Originally Posted by Trebuchet
Yes, the ball will continue to spin. And it won't curve or lose linear velocity, either. (Ignoring the fact that there's no PERFECT vacuum.)

And yes, there's inertial for rotation.

In an atmosphere of any kind, it'll slow down, both in spin and linear velocity, and tend to curve. Hydrogen probably less so than air, at a given pressure.

Trebuchet. Not quite. True, the MLB pitcher will not be able to throw his standard curve which relies upon the aerodynamics of the atmosphere. But, removing the atmosphere is necessary but not sufficient. You must also assume there is no neutrino sea, which "empty"space possess...no solids, no liquids, no gases, no plasma. If a ball is thrown knuckle ball towards me in isotropic space at a low velocity....and I nick it with my fingernail as it passes by on a seam, it will begin to rotate, and during it's transitory contact will slow slightly and change it's vector velocity slightly towards my finger...kind of like grabbing a bicycle by the handlebar on one side as it goes by.
With an isotropic neutrino sea, a similar effect occurs. The cross-sections (sigma....) for nucleon-neutrino interactions vary as the square of the average energy. From the ball's point of view, the rotation causes the incoming neutrinos on the forward rotating side to have a Doppler- blue shifted energy spectrum, whereas on the receding side, they are Doppler redshifted. Neutral currents, which are believed to occur at all energy levels, thus having a greater probability of occurring on the blue side vs. the red....like so many little fingernail nickings of the ball's passage. So, it veers towards the blue-shift side ever so slowly but inexorably.
A similar argument applies to a satellite in space orbiting in an elliptical orbit around a central body. Let's say Mercury around the Sun. The rotation of the central body changes the isotropy of the neutrino sea passing through it, such that if you were on Mercury, the sea from the receding edge of the sun would be slightly redshifted due to a higher neutral current interaction rate vs the sea from the other approaching limb of the sun. The vector sum of these two effects is such that the motion of the satellite as it travels from aphelion to perihelion has a tiny shift towards the receding edge......contributing to the precession of the perihelion of Mercury.
SEE:http://cupp.oulu.fi/neutrino/nd-cross.html

Note that by this argument, the precession of the perihelion must rotate in the same direction as the central body involved...which means the Venus,Earth,Mars...etc also rotate their perihelia.

Last edited by trinitree88; 2012-Jul-30 at 07:08 PM. Reason: link

4. Yes, and in principle the ball would interact with the cosmic microwave background as well. However, the neutrino interaction cross section is fantastically small even for highly energetic neutrinos. For the cosmic neutrino background, which consists of very low energy neutrinos, these effects would be sufficiently small that I doubt we could even come close to measuring them. I'm not sure why you feel compelled to bring it up; it seems like it's most likely to confuse what is a relatively straightforward question.

5. It may be worth noting that it isn't just "inertia" of angular momentum. There is a conservation law so, when the ball slows down, the angular momentum hasn't just "disappeared", it has been transferred.

6. And the conservation laws means that you, the thrower, will not only start drifting backwards because you threw the ball, you'll also be rotating very slowly in the opposite direction, because angular momentum is one of the conserved properties and the sum of the balls and your momentums is the same before and after the throw.
Last edited by HenrikOlsen; 2012-Jul-30 at 08:08 PM.

7. Order of Kilopi
Join Date
Sep 2005
Location
Metrowest, Boston
Posts
4,055
Yep and yep. pete

8. SRH
Established Member
Join Date
Jul 2012
Posts
216
Originally Posted by Grey
the neutrino interaction cross section is fantastically small even for highly energetic neutrinos.
What is a neutrino cross section?

9. Originally Posted by TOEfetish
What is a neutrino cross section?
It's a measure of how likely two particles are to interact. Think instead about macroscopic objects. If you throw a baseball at a target, the likelihood of the baseball hitting a target is directly related to how big the target is. Subatomic particles don't really behave like little balls aimed at targets (i.e., the chance of two particles interacting does not really depend just on them being aimed precisely so that they hit head on; there are many other factors involved), but you can still create a number that has the dimensions of area that lets you compare how likely a particle is to interact with another particle. Amusingly, the typical unit used to measure cross sections is the barn (as in "hitting the broad side of a"), which is 100 fm2.

So neutrinos having a very small interaction cross section is just another way to say that they are very unlikely to interact with other particles as they go by, as though they had to hit a very small target or they would just pass right by without doing anything.

10. Order of Kilopi
Join Date
Sep 2005
Location
Metrowest, Boston
Posts
4,055
Originally Posted by TOEfetish
What is a neutrino cross section?
TOE. Grey is succinct and correct as usual. The relevance of cross-section is that the probability of an event taking place (P) is the product of the number of particles flowing per unit time ,flux,(phi) times the cross-sectional area (sigma) in barns, (or sometimes nanobarns). The tricky business with low energy neutrinos/antineutrinos is that we know neither very well...so P is still a very challenging thing to determine. Typically in all neutrino experiments, they strive for very large fluxes, enormous detector volumes, and long periods of time such that the probability of seeing a statistically meaningful event is enhanced. This also often involves removing background sources such as entrained radioactive nuclides, cosmic rays...etc. or developing computer algorithms and shielding materials that screen out unwanted data (coincidence/anti-coincidence stuff). pete

When you go to the Ultimate Neutrino Page...they give the calculations for known cross-sections in the higher energy regime.
SEE:http://cupp.oulu.fi/neutrino/nd-cross.html
Last edited by trinitree88; 2012-Jul-31 at 02:14 PM. Reason: link

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•