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1500 questions
88
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
- 1,001
- 1
- 9
- 5
88
votes
6 answers
How exactly do you avoid fooling yourself?
In cargo cult science Feynman writes:
"Millikan measured the
charge on an electron by an experiment with falling oil drops, and
got an answer which we now know not to be quite right. It's a
little bit off, because he had the incorrect value…
Trevor Andrade
- 841
88
votes
10 answers
Can black holes form in a finite amount of time?
One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an infinite amount of time for the object to reach…
Itai Bar-Natan
- 1,453
88
votes
4 answers
Why does rainwater form moving waves on the ground? Is there a name for this effect?
A while ago it was raining and I noticed that, on sloped pavement, water was flowing in very regular consistent periodic waves, as you see below.
However, I realized I had no idea why this should be happening. There was nothing uphill actually…
user541686
- 4,311
88
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
- 3,119
88
votes
8 answers
Will a hole cut into a metal disk expand or shrink when the disc is heated?
Take a metal disc and cut a small, circular hole in the center.
When you heat the whole thing, will the hole's diameter increase or decrease? and why?
What will happen to the diameter of disc?
jojo
- 897
87
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
- 2,253
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
11 answers
What determines color -- wavelength or frequency?
What determines the color of light -- is it the wavelength of the light or the frequency?
(i.e. If you put light through a medium other than air, in order to keep its color the same, which one would you need to keep constant: the wavelength or the…
user541686
- 4,311
87
votes
6 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
- 3,011
86
votes
7 answers
Why is the Earth so fat?
I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy?
The calculation I did was to imagine placing a ball down on…
Mark Eichenlaub
- 53,961
86
votes
5 answers
Why doesn't water actually perfectly wet glass?
According to many high school textbook sources, water perfectly wets glass. That is, the adhesion between water and glass is so strong that it is energetically favorable for a drop of water on glass to spread out and coat the entire surface.…
knzhou
- 107,105
86
votes
2 answers
Can photons be detected without being absorbed?
I am thinking about a detector that would beep if light passes through it. Is it possible?
Arik
- 871
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
- 7,257
86
votes
7 answers
Which way does the scale tip?
I found the problem described in the attached picture on the internet. In the comment sections there were two opposing solutions. So it made me wonder which of those would be the actual solution.
So basically the question would be the following.…
Peter Raeves
- 973