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1500 questions
111
votes
6 answers
What is the physical meaning of commutators in quantum mechanics?
This is a question I've been asked several times by students and I tend to have a hard time phrasing it in terms they can understand. This is a natural question to ask and it is not usually well covered in textbooks, so I would like to know of…
Emilio Pisanty
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111
votes
3 answers
What conservation law corresponds to Lorentz boosts?
Noether's Theorem is used to relate the invariance of the action under certain continuous transformations to conserved currents. A common example is that translations in spacetime correspond to the conservation of four-momentum.
In the case of…
Warrick
- 9,625
110
votes
11 answers
Is there something similar to Noether's theorem for discrete symmetries?
Noether's theorem states that, for every continuous symmetry of an action, there exists a conserved quantity, e.g. energy conservation for time invariance, charge conservation for $U(1)$. Is there any similar statement for discrete symmetries?
Tobias Kienzler
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110
votes
7 answers
Why do tuning forks have two prongs?
I believe the purpose of a tuning fork is to produce a single pure frequency of vibration. How do two coupled vibrating prongs isolate a single frequency? Is it possible to produce the same effect using only 1 prong? Can a single prong not…
acadien
- 1,060
110
votes
7 answers
How can we see an atom now? What was the scale of this equipment?
I've just seen this on the news - Single Trapped Atom Captures Science Photography Competition's top prize.
Credit: David Nadlinger via EPSRC
I am not a Physics major but I believe I do know the basics. I have always believed that we can't really…
Hanky Panky
- 1,109
110
votes
8 answers
Is it possible to start fire using moonlight?
You can start fire by focusing the sunlight using the magnifying glass.
I searched the web whether you can do the same using moonlight. And found this and this - the first two in Google search results.
What I found is the thermodynamics argument:…
Calmarius
- 8,100
109
votes
6 answers
Are Newton's "laws" of motion laws or definitions of force and mass?
If you consider them as laws, then there must be independent definitions of force and mass but I don't think there's such definitions.
If you consider them as definitions, then why are they still called laws?
user5402
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109
votes
15 answers
Why quantum mechanics?
Imagine you're teaching a first course on quantum mechanics in which your students are well-versed in classical mechanics, but have never seen any quantum before. How would you motivate the subject and convince your students that in fact classical…
Jonathan Gleason
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109
votes
5 answers
How do we know that radioactive decay rates are constant over billions of years?
A friend and I recently discussed the idea that radioactive decay rates are constant over geological times, something upon which dating methods are based.
A large number of experiments seem to have shown that decay rate is largely uninfluenced by…
Pertinax
- 997
109
votes
19 answers
What is a good introductory book on quantum mechanics?
I'm really interested in quantum theory and would like to learn all that I can about it. I've followed a few tutorials and read a few books but none satisfied me completely. I'm looking for introductions for beginners which do not depend heavily on…
PhaDaPhunk
- 387
109
votes
3 answers
Why are some people are claiming that the Big Bang never happened?
A news story is going viral on social media networks claiming that two physicists have found a way to eliminate the Big Bang singularity, or in layman's terms (as claimed by many sensationalist news articles): The Big Bang never happened at all.
The…
Janus Boffin
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108
votes
6 answers
Why is oil a better lubricant than water?
How come mineral oil is a better lubricant than water, even though water has a lower viscosity?
When two surfaces slide over each other with a gap filled with a fluid, the different layers of the fluid are dragged at different speeds. The very top…
DK2AX
- 4,779
108
votes
15 answers
What is the proper way to explain the twin paradox?
The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible.
There are a thousand explanations out there for why this doesn't happen, but they all end up…
John Rennie
- 355,118
107
votes
11 answers
If gravity isn't a force, then why do we learn in school that it is?
I have studied some of Einstein's Theory of General Relativity, and I understand that it states that gravity isn't a force but rather the effects of objects curving space-time. If this is true, then why are we instructed in middle school that it is…
Peter Hall
- 1,215
106
votes
4 answers
Why does nature favour the Laplacian?
The three-dimensional Laplacian can be defined as $$\nabla^2=\frac{\partial^2}{\partial x^2}+\frac{\partial^2}{\partial y^2}+\frac{\partial^2}{\partial z^2}.$$ Expressed in spherical coordinates, it does not have such a nice form. But I could define…
Sam Jaques
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