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1500 questions
37
votes
1 answer
Nuclear bomb mushroom cloud with trumpet formation
I have found this specific image here (Loong found out that it is the Soviet Joe 4 test of the 400 kiloton RDS-6 warhead at the Semipalatinsk test site on August 12, 1953):
Also an impressive Youtube Video of the same explosion
As you see, an…
Thorsten S.
- 2,020
37
votes
5 answers
Why do people rule out local hidden variables?
I bet the automatic response to my question would be "Bell's theorem" and of course I am not disputing Bell's proof. I am however uncertain of one of his assumptions.
The so called "no conspiracy" assumption states that we somehow posses this…
QuestionAnswers
- 913
37
votes
2 answers
Before a once-warm lake starts to freeze, must its temperature be 4°C throughout at some point?
This is a problem I just started puzzling over, and I felt this would be a good forum to check my reasoning. So here are the relevant observations followed by my question:
Water achieves its maximum density at roughly 4°C. That is, water (including…
Excellll
- 477
37
votes
6 answers
Can a balloon be inflated with electrons?
If I use a Van De Graff generator to pump electrons into a deflated balloon, eventually negative charge will start to build up inside the balloon. Assume the mouth of the balloon is sealed so air cannot enter or exit the balloon. The electrons will…
Andrew
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37
votes
1 answer
What does it mean to say that "the fundamental forces of nature were unified"?
It is said that immediately after the Big Bang, the fundamental forces of nature were unified. It is also said that later they decoupled, becoming separate forces.
Indeed, if we look at the list of states of matter on Wikipedia we see:
Weakly…
Gold
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37
votes
5 answers
Why is the potential energy equal to the negative integral of a force?
Why is the potential energy equals to the negative integral of a force? I am really confused with this negative sign. For example, why there is a negative sign in the gravitational potential energy and what does it mean?
I read that the negative…
Omar Ali
- 766
37
votes
1 answer
State of Matrix Product States
What is a good summary of the results about the correspondence between matrix product states (MPS) or projected entangled pair states (PEPS) and the ground states of local Hamiltonians? Specifically, what "if and only if" type of…
Kaveh_kh
- 543
37
votes
3 answers
Can two heavy objects circling around their C.M. be separated because of the speed of gravity?
Imagine two massive objects, with the same mass (M) circling around their center of mass (C.M.). Let's assume that the distance between them is 1 light hour. Don´t the two bodies get accelerated and move away from each other because they feel the…
Deschele Schilder
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37
votes
3 answers
What is the Kosterlitz -Thouless transition?
I couldn't find any simple texts explaining the Kosterlitz-Thouless transition. More specifically can someone explain the role of vortices in the transition.
edit: links explaining the transition in a simple manner are also appreciated. Also the…
MomoTheSir
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37
votes
4 answers
Why have our eyes not evolved to see "gluons"?
The photons are the propagators for QED, and we rely on photons to see the world around us.
The gluon is the propagator in QCD. Why have our eyes not evolved to see gluons (either on top of being able to "see" photons, or instead of)?
DarthPlagueis
- 1,654
37
votes
2 answers
Were the Michelson-Morley results a surprise?
How unexpected were the Michelson-Morley experiment results?
Did physicists have theoretical reasons to predict that the speed of light would result to be invariant?
Marco Disce
- 673
37
votes
4 answers
Give a description of Loop Quantum Gravity your grandmother could understand
Of course, assuming your grandmother is not a theoretical physicist.
I'd like to hear the basics concepts that make LQG tick and the way it relates to the GR. I heard about spin-networks where one assigns Lie groups representations to the edges and…
Marek
- 23,981
37
votes
3 answers
Free electron can't absorb a photon
Why can't a free electron absorb a photon? But a one attached to an atom can.. Can you explain to me logically and by easy equations? Thank you..
user65035
37
votes
8 answers
Why can't we see light travelling from point A to B?
Let's say we have a cloud of dust which is a lightyear across and someone shoots a beam of light from point A to B , why it is not possible for an observer far far away to see the light while it travels through the cloud at the speed of light?
Zer01
- 405
37
votes
3 answers
Intuitive understanding of the elements in the stress-energy tensor
There is an image in the Wikipedia about the stress-energy tensor:
I have a rough understanding of the stress tensor: you imagine cutting out a tiny cube from the fluid and form a matrix out of the forces on each side of it: it's not hard to see…
Calmarius
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