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1500 questions
67
votes
10 answers
Is the uncertainty principle a property of elementary particles or a result of our measurement tools?
In many physics divulgation books I've read, this seems to be a commonly accepted point of view (I'm making this quote up, as I don't remember the exact words, but this should give you an idea):
Heisenberg's uncertainty principle is not a result of…
Saturnix
- 805
66
votes
6 answers
What keeps mass from turning into energy?
I understand the energy and mass can change back and forth according to Einstein. It is fluid; it can go from one to the other. So, what keeps mass from just turning into energy? Is there some force holding a subatomic particle together? What keeps…
Moo
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66
votes
2 answers
Why do we expect our theories to be independent of cutoffs?
Final edit: I think I pretty much understand now (touch wood)! But there's one thing I don't get. What's the physical reason for expecting the correlation functions to be independent of the cutoff? I.e. why couldn't we just plump for one "master…
Edward Hughes
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66
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
- 6,111
66
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
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66
votes
5 answers
Would touching a black hole of a small mass (the mass of an apple) cause you to spiral in and get dead?
I know that a typical stellar black hole would spaghettify someone who crosses its event horizon.
Is this also true for a hypothetical tiny black hole with a small mass (the mass of an apple)? Would someone touching such a black hole spiral into it…
user9343456
- 1,278
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
- 1,321
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
- 1,729
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
Why don't you get burned by the wood benches in a sauna?
When you go to the sauna you may sit in a room with 90°C+. If it is a "commercial" sauna it will be on for the whole day. How does it come that when you sit on the wood you don't get burned?
I believe this question is different than the "classical"…
famfop
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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
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66
votes
9 answers
Why is the observable universe so big?
The observable universe is approximately 13.7 billion years old. But yet it is 80 billion light years across. Isn't this a contradiction?
Thomas O
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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
- 5,111
66
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
- 938
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
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