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162135
bio website lightandmatter.com
location Fullerton, California
age
visits member for 3 years, 5 months
seen Dec 7 at 20:09

I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.


Nov
21
comment Why is the energy of particles in accelerators much higher than the energy of the particles they are trying to find?
The multiplicity of charged hadrons produced in pp collisions at the LHC seems to be $\sim100$: dspace.mit.edu/handle/1721.1/68876 . If you're producing on the order of a hundred particles, maybe it makes sense that you have to put in on the order of a hundred times the energy of the particle you're hoping to create.
Nov
20
comment Center of mass of two $\gamma$ rays moving in opposite directions
Nice. Another way of looking at this is that the energy-momentum vector of the system as a whole is $(E,p)=(\omega_1+\omega_2,\omega_1-\omega_2)$, in units where $\hbar=1$. The boost needed in order to make this vector purely timelike is $v=p/E$.
Nov
20
comment Light emitted from galaxies receding faster than $c$
Thus, there truly is a limit to the volume of the universe that we will ever observe. The observable volume will keep growing. However, objects will keep passing out of the observable volume.
Nov
20
answered What does the exponential decay constant depend on?
Nov
20
comment Expansion rate in matter dominated era
When you ask for the expansion rate, are you asking for the Hubble constant?
Nov
20
comment Was the Big Bang actually cold?
With traditional Big Bang model (which doesn't contain Inflation), the universe started out with zero energy No, see physics.stackexchange.com/questions/2838/…
Nov
20
comment Was the Big Bang actually cold?
In the standard homogeneous cosmological models the total energy in an expanding volume is zero. No, there is nothing "standard" about this claim. The standard interpretation of GR is that we can't measure the total energy-momentum in a region of space in a cosmological spacetime, if the region is cosmological in size. See physics.stackexchange.com/questions/2838/…
Nov
20
answered Does light slow down (for an external observer) as it approaches the event horizon?
Nov
20
comment What is the identity of Dark Matter?
Isn't supersymmetry looking dead at this point?
Nov
20
comment Did spacetime start with the Big bang?
This answer is nonsense, starting with the second sentence.
Nov
19
answered How is strong time dilation consistent with weak tidal forces?
Nov
19
answered What does it mean to say that “6 tons of dark energy would be found within the radius of Pluto's orbit”?
Nov
19
comment What does it mean to say that “6 tons of dark energy would be found within the radius of Pluto's orbit”?
What do you mean by "system of inertia?"
Nov
19
comment Is an atom charged after undergoing beta emission?
related: physics.stackexchange.com/questions/146549/… . Note that there are two types of beta decay. You seem to be referring to $\beta^-$ decay. the electron number is unchanged This is probably not true. Beta decay is a violent process, so other electrons are likely to be knocked out.
Nov
19
comment No-hair theorems for naked singularities?
@MBN: I could be wrong, but I think the assumptions of the black-hole no-hair theorems are stationarity, electrovac, and a horizon (which implies asymptotic flatness). I don't think any of these indirectly implies cosmic censorship. I'm basing this on relativity.livingreviews.org/Articles/lrr-1998-6/fulltext.html
Nov
19
comment How to theoretically define a concrete operation to perform in order to measure the length of an object?
It is not clear (to me) why using a stick you have a problem of simultaneity, if you juxtapose a metre CD to your object AB why should be there a problem of simultaneity? end A will touch C at the very time when end B is touching end D. The coincidence of A with C is only simultaneous with the coincidence of B with D in one particular frame. Simultaneity depends on the frame of reference.
Nov
19
comment Why are black hole singularities stable?
The Friedmann equations says that huge matter densities lead to huge expansion rates. Not true. And irrelevant in any case because the Friedmann equations don't apply to a black hole. In Newtonian gravity, two massive point particles separated by an infinitesimal distance will experience an enormous force. The force is attractive, so what's the problem?
Nov
19
comment Gravitational Redshift and Length Contraction
Length contraction doesn't have anything to do with the metric.
Nov
19
comment Is it possible to prove that units can be manipulated algebraically?
Re the final paragraph, Leibniz notation is designed to make it manifest that when you differentiate or integrate, the dimensional validity of the equation remains valid at every step. For example, in $\int v dt$, the units work out to be distance if you take $dt$ to have units of time.
Nov
19
comment No-hair theorems for naked singularities?
@zibadawatimmy: He's saying that if you take the Kerr–Newman metric and put in the mass and charge of an electron, you get a naked singularity. That doesn't mean that an electron is a naked singularity.