100,606 reputation
4146258
bio website ratsauce.co.uk
location Chester, United Kingdom
age 53
visits member for 3 years, 7 months
seen 7 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.


1h
comment If a photon has no mass, how can it be attracted by the Sun?
@Py-ser: Newton didn't derive the equation, but in Optiks he did say Do not Bodies act upon Light at a distance, and by their action bend its Rays, and is not this action strongest at the least distance?. However your point is taken and I've edited my answer accordingly.
2h
comment Is there a way to calculate the photoelectric effect in QED via a Feynman diagram?
For photoemission to occur the primary electron has to transfer energy to the lattice, and the lattice then transfer enough energy back to a different electron near the surface to eject it. It's this seconday process that has the extremely low quantum efficiency, which is hardly surprising given its haphazard nature. It's also the complexity of this secondary process that would make it hard to handle using QED. The overall quantum efficiency is in the range I cited.
2h
comment Is there a way to calculate the photoelectric effect in QED via a Feynman diagram?
@CuriousOne: A photomultiplier tube uses a thin metal film and the electron is ejected in the same direction as the incident photon - the quantum efficiency for this process is indeed very high. In photoelectron emission from a metal surface the photon also generates a fast moving electron with a high quantum efficiency, but that electron is moving in the same direction as the photon, just as in a PMT, so it's moving down into the bulk of the metal.
12h
comment Is there a way to calculate the photoelectric effect in QED via a Feynman diagram?
@CuriousOne: the question asks light is shone onto a metal and the quantum efficiency for photoelectron ejection from a metal is in the range I gave. Neither CCDs nor solar panels eject electrons, they work by exciting electrons between bands. This happens in a metal too, but that's just the first step in a process leading to ejection of an electron from the metal surface.
15h
comment How far has a black hole to be in order for its tidal forces to disintegrate earth?
@AbanobEbrahim: the Earth will break up far, far outside the event horizon. At about $10^5$ times the event horizon distance in fact. That means the pieces will occupy stable orbits because they'll never get close enough to the event horizon to encounter the unstable region. It's really hard to get that close to a black hole because you have to shed an enormous amount of angular momentum. For all everyday purposes orbits around a black hole are perfectly stable.
15h
comment Is the Big Bang defined as before or after Inflation?
@jim: if you want to use my post as a basis for an extended answer let me know and I'll turn it into a community wiki. However my personal belief remains that Big Bang strictly means the FLRW Big Bang, and using the term in an inflationary universe is a potential source of confusion. I wouldn't use the term in any context where its precise meaning matters.
15h
comment Does car tire pressure change with weight of car load?
@RobJeffries: well my answer does say: However any deformation of the tyre will reduce its volume and therefore tend to increase the pressure inside. My point is simply that for constant $M$ we have $P \propto A^{-1}$. I'll look at the wording and see if I can make it clearer.
16h
comment How far has a black hole to be in order for its tidal forces to disintegrate earth?
@AbanobEbrahim: That's a general rule for all black holes, though of course the event horizon depends on the mass of the black hole. Specifically $r_s = 2GM/c^2$.
16h
comment Can I throw a stone through my window without breaking the glass?
For this to be possible wouldn't the wavefunction of the stone need to remain coherent for a time at least as long as it takes to traverse the glass? For a macroscopic object like the stone the coherence time would be too small for it to travel far enough.
16h
comment Infinite Atwood (pulley) system - what's wrong with this?
See the question Infinite Atwood's machine
16h
comment How far has a black hole to be in order for its tidal forces to disintegrate earth?
@AbanobEbrahim: yes, the pieces of the Earth would form a ring much like Saturn's rings. As long as you stay more that 3 times the event horizon radius away you will stay in a stable orbit just like orbiting a star. Once you're closer than $3 r_s$ there are no stable orbits, but that's a question for another day!
16h
comment Which of these two textbook equations of geodesic deviation is correct?
Peter has faithfully reproduced the equation from Lambourne. I note gj255 now suggests this is a mistake in Lambourne's book.
16h
comment Which of these two textbook equations of geodesic deviation is correct?
@JerrySchirmer: oops, yes, sorry I didn't read your comment carefully enough. I have both books but don't know them well enough to find the relevant sections without a prolonged search. I guess I'll admit defeat :-)
16h
comment Which of these two textbook equations of geodesic deviation is correct?
@JerrySchirmer: Lambourne uses +--- and MTW use -+++
17h
comment Is the Big Bang defined as before or after Inflation?
@Jim: thanks :-) Sadly there is no extra privilege on reaching 100K, though you do get a freebie of your choice from the Stack Exchange shop.
20h
comment What happens to light at sharp points?
@Nick: searchng for duplicates is very hit and miss. I only found the duplicate because I remembered it as being an interesting question and knew what to search for.
20h
comment What happens to light at sharp points?
possible duplicate of Fate of light ray
21h
comment Angular magnification of a two-lens system
Presumably related to Magnification of an astronomical telescope not in normal adjustment?
22h
comment Why is acceleration constant in this example?
A downvote? Not that I'm fussed, but you need to tell me why you downvoted or I won't know how I can improve the answer.
1d
comment Unsatisfactory explanation for the EMF measurement of a battery
@Just_a_fool: you'll have to draw a diagram of the three resistor circuit as I'm not sure what you're measuring in it. Incidentally CuriousOne is correct that there exist voltmeters that have effectively infinite resistance.