99,116 reputation
4145255
bio website ratsauce.co.uk
location Chester, United Kingdom
age 53
visits member for 3 years, 7 months
seen 16 mins 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.


Aug
16
comment Why wire filament shakes inside a light bulb?
Why the close votes? I've seen this myself and always assumed it was related to the alternating magnetic field created as current flows through the filament.
Aug
14
comment Why does the Sun's (or other stars') nuclear reaction not use up all its “fuel” immediately?
@Wildling: Yes, although I wouldn't lose any sleep over the prospect.
Aug
13
comment Space within galaxies. Is it stuck by the gravity of the galaxy or expanding and “slipping past”?
Also see What is the difference between matter & spacetime?. Although this has been closed as a duplicate of the question ACuriousMind linked, it is relevant to your question about space sliding past the coins.
Aug
13
comment Can a wave propagate in any substance? Aren't there any prerequisites?
@tinchito: the EM field is elastic in the sense that it costs energy to change its value (i.e. deform it) and it will relax back to its equilibrium value afterwards. This is basically no different to a block of rubber.
Aug
13
comment If we could reach very high speeds, could we measure the velocity of the Earth this way?
I would guess you're thinking the speed of light represents an absolute maximum velocity for the whole universe, so if you can measure the Earth's velocity relative to the speed of light you'd get a figure for the Earth's velocity. If so, I'm afraid your initial assumption is wrong. The difference between any observer and the speed light travels is always $c$, regardless of that observer's motion. So the speed of light relative to the Earth is just $c$.
Aug
13
comment Computing average force exerted by ground when jumping
You're almost there. Your mistake is that the force only acts while the mans feet are in contact with the ground, so the work done by the force isn't $F(h_3 - h_1)$.
Aug
13
comment Robertson-Walker metric and stable orbits
Yes. The manifold used for the embedding is a mathematical device and is not intended to represent anything real.
Aug
13
comment What is the status of massless photons traveling through a medium?
See this answer. In a medium photons are no longer just photons. The interaction with the medium means they need to be described by a wavefunction covering both the photon and the medium. This combined state has a mass and a non-zero proper time.
Aug
13
comment What is the status of massless photons traveling through a medium?
Sorry yes, careless commenting!
Aug
13
comment Robertson-Walker metric and stable orbits
That shows the embedding of the spatial part of the metric within a 4D spatial manifold. Embedding is quite a common way to illustrate hypersurfaces, e.g. embeddings are frequently used to illustrate the de Sitter and AdS metrics.
Aug
13
comment Beginner question: timelessness of massless particles
Or How can photons interact with anything?, or searching for photon time returns 2000 hits.
Aug
13
comment Beginner question: timelessness of massless particles
possible duplicate of How does a photon experience space and time?
Aug
13
comment Robertson-Walker metric and stable orbits
Is the Robertson-Walker metric different to the FLRW metric or FRW metric or Friedmann equations or however you want to attribute it? If not, it has 3 spacelike and 1 timelike dimension.
Aug
12
comment The universe appears to have a lower bound in the time dimension, why not an upper bound?
@Jim: the point is that there was (probably) a past singularity, and there probably isn't a future singularity. This seems to me the answer to the question the OP meant to ask.
Aug
12
comment Pressure at a bend in a pipe
Actually I take back my earlier comment. After some Googling it appears the pressure is dominated by inertial terms. It's higher on the outside of the bend because that's where the liquid is being forced inwards. See this article and this paper. Note that in figure 2 the inside of the pipe is to the right.
Aug
12
comment How can gravity affect light?
A downvote? Why was that? I didn't think I'd said anything controversial. If you'd like to say why you downvoted I'll have a look at editing my answer accordingly.
Aug
12
comment How far can something travel in a straight line?
MHOOS: I've attempted a rewording that retains the spirit of your question, but it's quite a radical change. If you don't like my edit please roll it back.
Aug
12
comment How far can something travel in a straight line?
I don't agree with David's objection. Your question is asking what the geodesic looks like for a body moving at some velocity relative to the comoving frame, and this seems to me a perfectly reasonable question and quite an interesting one.
Aug
12
comment How to test that a flat metric represents a global three-torus geometry
Related, and possibly a duplicate: Is topology of universe observable?
Aug
12
comment A clock in freefall
@aepryus: post a question if you need a clue. If you do ask emphasise you're trying to understand how time dilation derives from a metric so it doesn't get flagged as homwork!