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1500 questions
67
votes
6 answers
Why does a ping pong ball bounce higher when it is dropped together with a cup of water?
Setup: an official ping pong ball is floating inside a party plastic cup filled with clean water, which is then dropped from a certain height onto a soft mat.
Observation: the ping pong ball shoots up to a height which is much higher than its…
user6760
- 12,980
67
votes
5 answers
Is there a Lagrangian formulation of statistical mechanics?
In statistical mechanics, we usually think in terms of the Hamiltonian formalism. At a particular time $t$, the system is in a particular state, where "state" means the generalised coordinates and momenta for a potentially very large number of…
N. Virgo
- 33,913
67
votes
7 answers
Do photons bend spacetime or not?
I have read this question:
Electromagnetic gravity
where Safesphere says in a comment:
Actually, photons themselves don't bend spacetime. Intuitively, this is because photons can't emit gravitons, because, as any massless particles not…
Árpád Szendrei
- 28,452
67
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
- 817
67
votes
6 answers
Why aren't particles constantly "measured" by the whole universe?
Let's say we are doing the double slit experiment with electrons. We get an interference pattern, and if we put detectors at slits, then we get two piles pattern because we measure electrons' positions when going through slits. But an electron…
FunkyLoiso
- 675
67
votes
5 answers
Why do we need large particle accelerators?
The LHC is much larger than its predecessors, and proposed successors much larger still. Today, particle accelerators seem to be the main source of new discoveries about the fundamental nature of the world.
My lay interpretation is that particle…
Lemma
- 683
67
votes
2 answers
What explanations are there for a rug steadily creeping at about a millimeter/day rate across a room?
I have bought a handmade rug of size 1.5 $\times$ 2m. About 1-2 weeks ago I noticed the rug was not in the center of my room and it had moved a bit. I thought maybe because I walked on it, it has moved. I put it in its place and it happened again…
Joe
- 793
67
votes
15 answers
Why is ascending some stairs more exhausting than descending?
I have been asked this question by school kids, colleagues and family (usually less formally):
When ascending a flight of stairs, you exchange mechanical work to attain potential Energy ($W_\text{ascend} = E_\text{pot} = m gh$).
However, when…
Daniel
- 831
67
votes
5 answers
What does Enthalpy mean?
What is meant by enthalpy? My professor tells me "heat content". That literally makes no sense. Heat content, to me, means internal energy. But clearly, that is not what enthalpy is, considering: $H=U+PV$ (and either way, they would not have had two…
PhyEnthusiast
- 2,908
67
votes
10 answers
Would time freeze if you could travel at the speed of light?
I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit.
So, if I were to travel in a spacecraft at (practically) the speed of light, would I freeze and stop moving?…
67
votes
8 answers
Why not use the Lagrangian, instead of the Hamiltonian, in nonrelativistic QM?
Undergraduate classical mechanics introduces both Lagrangians and Hamiltonians, while undergrad quantum mechanics seems to only use the Hamiltonian. But particle physics, and more generally quantum field theory seem to only use the Lagrangian, e.g.…
Revo
- 16,956
67
votes
6 answers
Path integral vs. measure on infinite dimensional space
Coming from a mathematical background, I'm trying to get a handle on the path integral formulation of quantum mechanics.
According to Feynman, if you want to figure out the probability amplitude for a particle moving from one point to another, you…
Nick Alger
- 2,730
67
votes
7 answers
Does gravity exist in a vacuum?
My understanding has always been that it does from conventional science courses, but really thinking about it, I was wondering if this is really the case.
To my limited understanding there is a theory that there are gravitons that act as particles…
user4779
- 781
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
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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
- 935