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1500 questions
66
votes
2 answers
Why is the ampere a base unit and not the coulomb?
I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the ampere that is the base unit and not the coulomb. Why is this? It seems to me that charge can exist without current, but…
Greg
- 1,740
66
votes
9 answers
What are the reasons to expect that gravity should be quantized?
What I am interested to see are specific examples/reasons why gravity should be quantized. Something more than "well, everything else is, so why not gravity too". For example, isn't it possible that a quantum field theory on curved space-time would…
MBN
- 3,785
66
votes
3 answers
What causes insects to cast large shadows from where their feet are?
I recently stumbled upon this interesting image of a wasp, floating on water:
Assuming this isn't photoshopped, I have a couple of questions:
Why do you see its image like that (what's the physical explanation; I'm sure there is an interesting…
OmnipresentAbsence
- 1,311
66
votes
5 answers
Why isn’t CERN afraid of a fusion reaction in the LHC?
Given that they can reach terrifying energies and temperatures, why isn’t fusion of protons a concern? After all, they start with a plasma and ram protons into each other.
At some point the strong force will overcome the proton-proton electric…
aquagremlin
- 1,721
66
votes
10 answers
Why doesn't a bike/bicycle fall if going with a high speed?
Why does a bike/bicycle fall when its speed is very low or close to zero and is balanced when going with a high speed?
Ahmad Farid
- 777
66
votes
4 answers
What's the physics behind XKCD #2027 (time between lightning flash and radio wave burst)?
XKCD usually has solid (and often contemporary) science behind it. Lightning Difference, #2027 one says:
Q: What’s that trick for telling how many miles away lightning is?
A: Just count the seconds between the visible flash and the radio wave…
uhoh
- 6,308
66
votes
10 answers
Do all black holes have a singularity?
If a large star goes supernova, but not enough mass collapses to form a black hole, it often forms a neutron star. My understanding is that this is the densest object that can exist because of the Pauli exclusion principle: It's made entirely of…
Carson Myers
- 5,041
66
votes
4 answers
From where (in space-time) does Hawking radiation originate?
According to my understanding of black hole thermodynamics, if I observe a black hole from a safe distance I should observe black body radiation emanating from it, with a temperature determined by its mass. The energy from this radiation comes from…
N. Virgo
- 33,913
66
votes
12 answers
How long a straw could Superman use?
To suck water through a straw, you create a partial vacuum in your lungs. Water rises through the straw until the pressure in the straw at the water level equals atmospheric pressure. This corresponds to drinking water through a straw about ten…
Mark Eichenlaub
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66
votes
6 answers
Why is jumping into water from high altitude fatal?
If I jump from an airplane straight positioned upright into the ocean, why is it the same as jumping straight on the ground?
Water is a liquid as opposed to the ground, so I would expect that by plunging straight in the water, I would enter it…
Conrad C
- 911
65
votes
5 answers
What exactly is regularization in QFT?
The question.
Does there exist a mathematicaly precise, commonly accepted definition of the term "regularization procedure" in perturbative quantum field theory? If so, what is it?
Motivation and background.
As pointed out by user drake in his…
joshphysics
- 57,120
65
votes
3 answers
Do sunrises and sunsets look the same in a still image?
A question that popped into my head: if I see a picture of the sun close to the horizon, in an unknown place, can I know if it was taken at sunset or sunrise?
Do sunrises and sunsets look the same in a still image? Can one tell them apart?
AlonMln
- 633
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65
votes
6 answers
How long can you survive 1 million degrees?
I asked my Dad this once when I was about 14, and he said that no matter how short the amount of time you were exposed to such a great temperature, you would surely die. The conversation went something like:
Me: What about a millisecond?
Dad: Of…
Briguy37
- 991
65
votes
6 answers
Why does the same proportion of a radioactive substance decay per time period? (half life)
Just wondering, if decay is random, why does the activity half every half life, as in, why does it have to reduce by the same proportion in the same time period?
Saharsh Aanand
- 979
65
votes
5 answers
What does it mean for a Hamiltonian or system to be gapped or gapless?
I've read some papers recently that talk about gapped Hamiltonians or gapless systems, but what does it mean?
Edit: Is an XX spin chain in a magnetic field gapped? Why or why not?
Jordan
- 1,583