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1500 questions
55
votes
10 answers
Is Schrodinger's Cat itself an observer?
In Schrodinger's Cat thought experiment,
why doesn't the cat itself qualify as an observer?
Reading through the replies there seem to be two suggestions for what can take the role of observer:
any "large" body
any "living" thing (or should that…
Malcolm Storey
- 721
55
votes
5 answers
Why is matter-antimatter asymmetry surprising, if asymmetry can be generated by a random walk in which particles go into black holes?
My understanding is the early universe was a very "hot" (ie energy dense) environment. It was even hot enough for black holes to form from photons.
My second point of understanding is that black holes can lose mass due to hawking radiation, which…
Livid
- 870
55
votes
7 answers
Why does Taylor’s series “work”?
I am an undergraduate Physics student completing my first year shortly. The following question is based on the physical systems I’ve encountered so far. (We mostly did Newtonian mechanics.)
In all of our analyses of the physical systems (up till…
Atom
- 1,931
55
votes
3 answers
Why are relativistic quantum field theories so much more restrictive than non-relativistic ones?
Part of the reason that relativistic QFT is so hard to learn is that there are piles of 'no-go theorems' that rule out simple physical examples and physical intuition. A very common answer to the question "why can't we do X simpler, or think about…
knzhou
- 101,976
55
votes
3 answers
Why don't electric fish shock themselves?
Fish like electric eels and torpedoes have specially designed nerve cells that allow them to discharge hundreds of volts of electricity.
Now, while pure water is usually nonconductive, the dissolved salts and other stuff in both sea and fresh water…
user172
55
votes
7 answers
Is a hard drive heavier when it is full?
Browsing Quora, I saw the following question with contradicting answers.
For the highest voted answer:
The bits are represented by certain orientations of magnetic fields
which shouldn't have any effect on gravitational mass.
But, another answer…
Casebash
- 2,784
55
votes
7 answers
How many atoms does it take for us to perceive colour?
Atoms individually have no colors, but when there is a large collection of atoms we see objects colorful, which leads to a question: at least how many atoms are required for us to see the color?
Syed Ilyas
- 917
55
votes
1 answer
What causes the Sun to appear to be in front of a building in this picture?
I took this photograph a few days ago, during sunrise, using my smartphone camera and digital zoom:
It seems that the sun is in front of the building. How is this explained?
Lior
- 3,309
55
votes
4 answers
Why doesn't hydrogen gas exist in Earth's atmosphere?
The root mean square velocity of hydrogen gas at room temperature is:
Gas constant: $R=8.31\ \mathrm{J\ K^{-1}\ mol^{-1}}$
Molar mass of hydrogen gas: $M=2.02\times10^{-3}\ \mathrm{kg/mol}$
$$\begin{align}
v &= \left(\frac{3\times8.31\ \mathrm{J\…
Bhushan Misal
- 579
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- 4
55
votes
2 answers
2002 research: speed of light slowing down?
Back in 2002 there was some research published hinting that $c$ may have been faster at some distant point. It was based on measurements of the fine-structure constant,
$$
\alpha = \frac1{4\pi\epsilon_0} \frac{e^2}{\hbar c} \approx \frac 1{137},
$$
…
Errol Hunt
- 1,106
55
votes
8 answers
What is behind the definitions of work and energy?
My knowledge of physics comes down to kinematics and Newton's laws. I would like to get answers which do not deviate from this basic knowledge.
I am currently questing for exact definitions of work and energy, and what is behind these definitions.…
Michael Haddad
- 825
55
votes
4 answers
GR and my journey to the centre of the Earth
[General Relativity] basically says that the reason you are sticking to the floor right now is that the shortest distance between today and tomorrow is through the center of the Earth.
I love this, not the least because it sounds…
Lloeki
- 643
55
votes
4 answers
Maximum theoretical data density
Our ability to store data on or in physical media continues to grow, with the maximum amount a data you can store in a given volume increasing exponentially from year to year. Storage devices continue to get smaller and their capacity gets bigger.…
tylerl
- 684
55
votes
8 answers
Why CAN we see the new moon at night?
I understand that the Moon's phases are determined by its position in orbit relative to the Sun. (See: Full Story on the Moon). The "shadow" is not cast by the Earth (a common misconception - this is actually a lunar eclipse), but by the moon's body…
ArchitectofAges
- 703
55
votes
6 answers
What symmetry causes the Runge-Lenz vector to be conserved?
Noether's theorem relates symmetries to conserved quantities. For a central potential $V \propto \frac{1}{r}$, the Laplace-Runge-Lenz vector is conserved. What is the symmetry associated with the conservation of this vector?
Dan
- 5,685