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1500 questions
88
votes
9 answers
What really causes light/photons to appear slower in media?
I know that if we solve the Maxwell equation, we will end up with the phase velocity of light being related to the permeability and the permittivity of the material. But this is not what I'm interested in - I want to go deeper than that. We know…
Emitabsorb
- 2,422
88
votes
8 answers
Why is the application of probability in Quantum Mechanics fundamentally different from application of probability in other areas?
Why is the application of probability in Quantum Mechanics (QM) fundamentally different from its application in other areas? QM applies probability according to the same probability axioms as in other areas of physics, engineering, etc.
Why is there…
Nikos M.
- 5,142
87
votes
3 answers
Why can Hiroshima be inhabited when Chernobyl cannot?
There was an atomic bomb dropped in Hiroshima, but today there are residents in Hiroshima. However, in Chernobyl, where there was a nuclear reactor meltdown, there are no residents living today (or very few). What made the difference?
user14154
- 991
87
votes
6 answers
If gravity is a pseudoforce in general relativity, then why is a graviton necessary?
As far as I’m aware, gravity in general relativity arises from the curvature of spacetime and is equivalent to an accelerated reference frame. Objects accelerating in a gravitational field are in fact inertial and are moving through geodesics in…
Thatpotatoisaspy
- 2,133
87
votes
7 answers
Why is it "bad taste" to have a dimensional quantity in the argument of a logarithm or exponential function?
I've been told it is never seen in physics, and "bad taste" to have it in cases of being the argument of a logarithmic function or the function raised to $e$. I can't seem to understand why, although I suppose it would be weird to raise a…
sangstar
- 3,192
87
votes
1 answer
What happened to David John Candlin?
This is an ultra-soft question about relatively recent history. While reading some of Mandelstam's papers, I noticed that he cites David John Candlin consistenly whenever he does anything with Grassman path-integral. Everyone else cites Berezin.
So…
87
votes
4 answers
Why do travelling waves continue after amplitude sum = 0?
My professor asked an interesting question at the end of the last class, but I can't figure out the answer. The question is this (recalled from memory):
There are two travelling wave pulses moving in opposite directions along a rope with equal and…
Dylan
- 1,051
87
votes
2 answers
In the earth's crust, why is there far more uranium than gold?
In parts per million in the Earth's crust Uranium is around 1.8ppm and Gold 0.003ppm. Given that it takes far more energy to create Uranium than Gold, why is this?
user56903
87
votes
6 answers
What is spin as it relates to subatomic particles?
I often hear about subatomic particles having a property called "spin" but also that it doesn't actually relate to spinning about an axis like you would think. Which particles have spin? What does spin mean if not an actual spinning motion?
Nick
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86
votes
7 answers
Why isn't an infinite, flat, nonexpanding universe filled with a uniform matter distribution a solution to Einstein's equation?
In Newtonian gravity, an infinite volume filled with a uniform distribution of mass would be in perfect equilibrium. At every point, the gravitational forces contributed by masses in one direction would be exactly counterbalanced by those in the…
D. Halsey
- 2,143
86
votes
4 answers
What does one second after big bang mean?
Consider the following statement:
Hadron Epoch, from $10^{-6}$ seconds to $1$ second: The temperature of the
universe cools to about a trillion degrees, cool enough to allow
quarks to combine to form hadrons (like protons and neutrons).
What…
Yashas
- 7,163
86
votes
8 answers
Could a "living planet" alter its own trajectory only by changing its shape?
In Stanislaw Lem's novel Solaris the planet is able to correct its own trajectory by some unspecified means. Assuming its momentum and angular momentum is conserved (it doesn't eject or absorb any mass), would this be possible (in Newtonian…
Petr
- 3,069
86
votes
7 answers
Do Maxwell's Equations overdetermine the electric and magnetic fields?
Maxwell's equations specify two vector and two scalar (differential) equations. That implies 8 components in the equations. But between vector fields $\vec{E}=(E_x,E_y,E_z)$ and $\vec{B}=(B_x,B_y,B_z)$, there are only 6 unknowns. So we have 8…
Warrick
- 9,625
86
votes
3 answers
Why didn't the Big Bang create heavy elements?
In the case of a supernova explosion it is possible to create heavy elements through fusion. Supernovae have a tremendous amount of energy in a very small volume but not as much energy per volume as there was in our early universe. So, what is the…
Alex
- 5,973
86
votes
4 answers
Is the Planck length the smallest length that exists in the universe or is it the smallest length that can be observed?
I have heard both that Planck length is the smallest length that there is in the universe (whatever this means) and that it is the smallest thing that can be observed because if we wanted to observe something smaller, it would require so much…
George Smyridis
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