105,677 reputation
4156285
bio website ratsauce.co.uk
location Chester, United Kingdom
age 53
visits member for 3 years, 8 months
seen 4 hours ago

Semi retired old time computer nerd who started programming on a Commodore Pet.

Since I'm also active in the Physics forum I should add that I started as a theoretical chemist, moved into solid state photochemistry and finally worked in industry as a colloid scientist. I only became a full time computer nerd in 1997.


2h
awarded  Explainer
4h
comment When does a singularity start to exist during a black hole formation?
@pabouk: we are using a simplified model that assumes the collapsing object is homogeneous and made of non-interacting matter i.e. dust. In this case the object remains homogeneous at all stages of the collapse, so the singularity forms at a single point in (proper) time everywhere inside the collapsing body. A real star would have some density profile i.e. it would b denser at the centre than at the surface. In this case it's likely the singularity would form while the outer layers were still at non-zero $r$.
6h
reviewed No Action Needed Is the Moon freely falling towards the Earth?
6h
reviewed Close Question on pressure required on a fire hose pump
6h
reviewed Close Motivating the importance of energy
6h
revised How can we minimize the coefficient of static/kinetic friction?
Add more links
6h
answered How can we minimize the coefficient of static/kinetic friction?
7h
comment Lorentz factor, derivation from 4-velocity vector
The four velocity is only $(c, \vec{v})$ in one frame. More generally it's $(c\tfrac{dt}{d\tau}, \tfrac{d\vec{x}}{d\tau})$. It's the fact the norm is always unity in all coordinates that you could use as a way to calculate $\gamma$.
7h
comment What happens when a photon reaches the “edge” of the universe?
@CoilKid: I think (I haven't actually sat down and worked it out) that in a closed universe if you shine a light out from your current position the photons will slow down, come to a stop then start moving back towards you as the recollapse starts. So from your perspective the light does indeed move in a curved path. But this is routine in general relativity, and indeed it's the cause of gravitational lensing. In this case it's the whole universe acting as the lens!
7h
answered FRW metric and its validity througout the age of the universe
11h
comment What happens when a photon reaches the “edge” of the universe?
@CBauer: the views that Neil attributes to Hawking are not mainstream. If there is a Big Crunch (and current evidence suggests the universe will not recollapse) then most of us expect entropy to increase smoothly during the recollapse phase. Time won't run backwards or indeed do anything screwy. See the Wikipedia article on the Big Crunch for more.
12h
comment How to calculate heat generated from cosmic dust in space?
possible duplicate of Would a fast inter-stellar spaceship benefit from an aerodynamic shape?
14h
reviewed Leave Closed What will be the time taken by a stone to reach the ground if it is thrown with velocity $v$?
14h
reviewed Leave Closed Can beamer presentation can be used at APS meetings?
14h
comment Intensity of Brightest Fringe of a multi-slit Diffraction
Possibly, but it can't contain more energy. The integral of the intensity of the central fringe must be less that the integral of the intensity over all the slits that form the diffraction pattern. If it was that would mean there was more energy per second in the central fringe than was passing through the diffraction grating. However it's certainly true that if there are many slits, so a high overall area, the intensity at the central fringe can be higher than the intensity of the light hitting the diffraction grating.
14h
comment Intensity of Brightest Fringe of a multi-slit Diffraction
The integral of the intensity over all the fringes will be equal to the total intensity of the light coming through the slits.
14h
answered On solving a two lens system
16h
comment Can time be interacted with?
Zack, it's a good question, but it can't be answered in any useful way without getting into the maths. I dont know of any non-mathematical explanation that isn't ultimately misleading.
16h
revised What happens when a photon reaches the “edge” of the universe?
Tweak
17h
comment Can time be interacted with?
The downvote seems harsh. Zack is basically asking what the mechanism of time dilation is, and I suspect that's a question that has puzzled most of us at some point.