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38
votes
1 answer
How can dark matter collapse without collisions or radiation?
I understand that dark matter does not collapse into dense objects like stars apparently because it is non-interacting or radiating and thus cannot lose energy as it collapses. However why then does it form galactic halos? Isn't that also an example…
Astrobuf
- 381
38
votes
5 answers
Shine a beam of light horizontally, drop a stone from same height - would both hit the ground at the same time?
If a beam of light was shone horizontally, and simultaneously a stone was dropped from the same height, would they both hit the ground a the same time?
Of course on Earth they would not, but let's imagine a land mass large enough for the light not…
user59315
38
votes
8 answers
Will we ever be able to view the past?
By analogy with the sun, whose light is apparently reaching us in 8 minutes, it means that we are only viewing the sun 8 minutes ago when we look up at it. However, what if we were eventually able to build powerful machines that could view light…
user45220
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38
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1 answer
What is the theoretical lower mass limit for a gravitationally stable neutron star?
I ask here intentionally not for the size of the smallest possible observed size of neutron stars, which corresponds approximately to the well-known Chandrasekhar-limit for the upper limit of the white dwarfs. This is defined by the minimal size of…
peterh
- 8,208
38
votes
4 answers
Why is Earth's climate so stable?
Earth wasn't always the only water-world in the solar system. Mars also appear to have started out wet
but, as conditions changed, Mars lost its oceans.
So, how has Earth managed to avoid a similar fate?
Doesn't the Giant impact hypothesis…
Achmed
- 1,139
38
votes
5 answers
What are washers for?
When you attach a bolt to something using a nut, it is clear what the roles of the nut and bold are.
The more you tighten the bolt the more secure your fastening. However, you are often also told to use a washer as well.
I know this somehow…
Simd
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38
votes
1 answer
What are the alternative theories of dark energy? ($w \neq -1$)
There has been a lot of related questions about dark energy around here but these are usually 2-4 years old and the closest question to mine hasn't really been answered, so I am going to proceed. Experts may skip to the last paragraph, but I am…
Void
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38
votes
5 answers
Why isn't it $E \approx 27.642 \times mc^2$?
Why is it that many of the most important physical equations don't have ugly numbers (i.e., "arbitrary" irrational factors) to line up both sides?
Why can so many equations be expressed so neatly with small natural numbers while recycling a…
badroit
- 499
38
votes
3 answers
Why is spacetime curved by mass but not charge?
It is written everywhere that gravity is curvature of spacetime caused by the mass of the objects or something to the same effect. This raises a question with me: why isn't spacetime curved due to other forces or aspects of bodies?
Why isn't it that…
Rijul Gupta
- 5,463
38
votes
4 answers
Birds sitting on electric wires: potential difference between the legs
We have seen birds sitting on uninsulated electric wires of high voltage transmission lines overhead without getting harmed, because sitting on only one wire doesn't complete any circuit.
But what about the potential difference between their legs?…
Tea is life
- 2,726
38
votes
3 answers
The formal solution of the time-dependent Schrödinger equation
Consider the time-dependent Schrödinger equation (or some equation in Schrödinger form) written down as
$$
\tag 1 i\hbar \partial_{t} \Psi ~=~ \hat{H} \Psi .
$$
Usually, one likes to write that it has a formal solution of the form
$$
\tag 2 \Psi (t)…
Andrew McAddams
- 6,494
37
votes
4 answers
Is it pions or gluons that mediate the strong force between nucleons?
From my recent experience teaching high school students I've found that they are taught that the strong force between nucleons is mediated by virtual-pion exchange, whereas between quarks it's gluons. They are not, however, taught anything about…
qftme
- 1,810
37
votes
7 answers
Why is the contribution of a path in Feynmans path integral formalism $\sim e^{(i/\hbar)S[x(t)]}$?
In the book "Quantum Mechanics and Path Integrals" Feynman & Hibbs state that
The probability $P(b,a)$ to go from point $x_a$ at the time $t_a$ to the point $x_b$ at the time $t_b$ is the absolute square $P(b,a) = \|K(b,a)\|^2$ of an amplitude…
asmaier
- 9,794
37
votes
5 answers
What is so special about speed of light in vacuum?
I will try to be as explanatory as possible with my question. Please also note that I have done my share of googling and I am looking for simple language preferable with some example so that I can get some insight in this subject.
My question is…
java_doctor_101
- 653
37
votes
9 answers
Why do disk brakes generate "more stopping power" than rim brakes?
Bicycle's brakes generally fall into two broad categories: rim brakes where the braking pads pinch against the wheel's rim, and disk brakes where the pads press against a disk (rotor) that is rigidly attached to the wheel's hub.
General wisdom is…
Kevin Kostlan
- 6,463