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1500 questions
64
votes
12 answers
Are we inside a black hole?
I was surprised to only recently notice that
An object of any density can be large enough to fall within its own
Schwarzschild radius.
Of course! It turns out that supermassive black holes at galactic centers can have an average density of less…
Marcos
- 908
64
votes
7 answers
Why does everything spin?
The origin of spin is some what a puzzle to me, everything spin from galaxies to planets to weather to electrons.
Where has all the angular momentum come from? Why is it so natural?
I was also thinking do photons spin? we always think of the wave as…
Ben
- 649
64
votes
15 answers
Why do we say that the Earth moves around the Sun?
In history we are taught that the Catholic Church was wrong, because the Sun does not move around the Earth, instead the Earth moves around the Sun.
But then in physics we learn that movement is relative, and it depends on the reference point that…
HappyDeveloper
- 1,387
63
votes
17 answers
How can momentum but not energy be conserved in an inelastic collision?
In inelastic collisions, kinetic energy changes, so the velocities of the objects also change.
So how is momentum conserved in inelastic collisions?
user36604
- 633
63
votes
4 answers
How exact is the analogy between statistical mechanics and quantum field theory?
Famously, the path integral of quantum field theory is related to the partition function of statistical mechanics via a Wick rotation and there is therefore a formal analogy between the two. I have a few questions about the relation between the two…
user26866
- 3,442
63
votes
5 answers
Dimensionless Constants in Physics
Forgive me if this topic is too much in the realm of philosophy. John Baez has an interesting perspective on the relative importance of dimensionless constants, which he calls fundamental like alpha, versus dimensioned constants like $G$ or $c$ […
Michael Luciuk
- 5,754
63
votes
7 answers
If I run along the aisle of a bus traveling at (almost) the speed of light, can I travel faster than the speed of light?
Let's say I fire a bus through space at (almost) the speed of light in vacuum. If I'm inside the bus (sitting on the back seat) and I run up the aisle of the bus toward the front, does that mean I'm traveling faster than the speed of light?…
ed209
- 765
63
votes
7 answers
Does a constantly accelerating charged particle emit EM radiation or not?
The Abraham-Lorentz force gives the recoil force, $\mathbf{F_{rad}}$, back on a charged particle $q$ when it emits electromagnetic radiation. It is given by:
$$\mathbf{F_{rad}} = \frac{q^2}{6\pi \epsilon_0 c^3}\mathbf{\dot{a}},$$
where…
John Eastmond
- 5,917
63
votes
7 answers
Rigor in quantum field theory
Quantum field theory is a broad subject and has the reputation of using methods which are mathematically desiring. For example working with and subtracting infinities or the use of path integrals, which in general have no mathematical meaning (at…
MBN
- 3,785
63
votes
3 answers
How can a butterfly dodge the windshield of a fast moving car?
While driving, an unlucky butterfly was about to hit my windshield. But instead of splattering, it sort of glided smoothly upwards across the surface of my windshield. The butterfly was clearly not skillfully dodging the car because the required…
AlphaLife
- 11,871
63
votes
2 answers
Why is gravity such a unique force?
My knowledge on this particular field of physics is very sketchy, but I frequently hear of a theoretical "graviton", the quantum of the gravitational field. So I guess most physicists' assumption is that gravity can be described by a QFT?
But I…
Nick
- 2,949
63
votes
8 answers
Ball hits curve of same curvature
I was doing some physics problems for homework and, while procrastinating, I came up with a theoretical scenario that I couldn't figure out the result of.
The following is from a side view and in a frictionless, ideal environment:
A ball moves…
pzach
- 628
63
votes
11 answers
What equation describes the wavefunction of a single photon?
The Schrödinger equation describes the quantum mechanics of a single massive non-relativistic particle. The Dirac equation governs a single massive relativistic spin-½ particle. The photon is a massless, relativistic spin-1 particle.
What is the…
nibot
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63
votes
11 answers
Why don't solar panels contribute to global warming?
I've been wondering this for a while but I have not yet encountered an explanation.
This is from my understanding of physics, which is by no means expert, so sorry for my crude explanation:
Energy within earth can be considered a closed system; it…
Joeri Hendrickx
- 741
63
votes
5 answers
The Sun is giving us a low entropy, not energy
While I was watching a popular science lecture on YouTube, I came across this sentence
"Sun is giving us a low entropy, not energy"
which was said by Prof. Krzysztof Meissner.
I am not a physicist, but this sounds to me like a huge leap.
I would…
janusz
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