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1500 questions
38
votes
7 answers
What exactly is a bound state and why does it have negative energy?
Could you give me an idea of what bound states mean and what is their importance in quantum-mechanics problems with a potential (e.g. a potential described by a delta function)?
Why, when a stable bound state exists, the energies of the related…
Charlie
- 1,084
38
votes
5 answers
Why is current a scalar quantity?
Current has both magnitude and direction. As per the definition of vector defined in encyclopedia, current should be a vector quantity. But, we know that current is a scalar quantity. What is the reason behind it?
user36159
- 381
38
votes
6 answers
Why does the density of electric field lines make sense, if there is a field line through every point?
When we're dealing with problems in electrostatics (especially when we use Gauss' law) we often refer to the density of electric field lines, which is inversely proportional to the radius in the case of a single point charge (all field lines are…
odg
- 471
38
votes
4 answers
Are neutrino stars theoretically possible?
Since neutrinos
have a small mass and
are affected by gravity,
wouldn't it be theoretically possible to have such a large quantity of them so close to each other, that they would form a kind of a stellar object, i.e. one that would keep itself…
miikkas
- 765
38
votes
3 answers
Is it possible to speak about changes in a physical constant which is not dimensionless?
Every so often, one sees on this site* or in the news† or in journal articles‡ a statement of the form "we have measured a change in such-and-such fundamental constant" (or, perhaps more commonly, "we have constrained the rate of change of...").…
Emilio Pisanty
- 132,859
- 33
- 351
- 666
38
votes
13 answers
If water is nearly as incompressible as ground, why don't divers get injured when they plunge into it?
I have read that water (or any other liquid) cannot be compressed like gases and it is nearly as elastic as solid. So why isn’t the impact of diving into water equivalent to that of diving on hard concrete?
Hitarth Vyas
- 515
38
votes
3 answers
Is it really proper to say Ward identity is a consequence of gauge invariance?
Many (if not all) of the materials I've read claim Ward identity is a consequence of gauge invariance of the theory, while actually their derivations only make use of current conservation $\partial_\mu J^\mu=0$ (which is only equivalent to a global…
Jia Yiyang
- 3,985
38
votes
5 answers
Can the Hubble constant be measured locally?
The Hubble constant, which roughly gauges the extent to which space is being stretched, can be determined from astronomical measurements of galactic velocities (via redshifts) and positions (via standard candles) relative to us. Recently a value of…
burgerking
- 425
38
votes
6 answers
How can a Kestrel hover in the wind?
Kestrels are birds of prey commonly found in Europe, Asia, Africa, and the North America. They belong to the falcon family but have a unique ability to hover in the air. You can find a whole bunch of videos (See 1,2,3,4, for example) about these…
AlphaLife
- 11,871
38
votes
5 answers
Why does canola oil heat up in the microwave?
Introduction: I read on Wikipedia's list of common misconceptions that microwaves work not by emitting the resonant frequency of water, but as a result of dielectric heating. As I understand it, this process heats a substance by emitting a…
i like math
- 589
38
votes
4 answers
Did LIGO measurements prove that the speed of gravity equals the speed of light?
This question about the speed of light prompted my own question. In the linked question it is asked if there is experimental proof that the speed of gravity equals the speed of light. I was surprised not to see the LIGO measurements mentioned.
The…
38
votes
8 answers
When snow falls, temperature rises. Is this due to entropy?
A friend of mine told me that temperature rises when snow falls. And this is because condensation of water in snowflakes reduces entropy and the temperature of the air rises to compensate for this.
Is this explanation correct?
38
votes
4 answers
When can a global symmetry be gauged?
Take a classical field theory described by a local Lagrangian depending on a set of fields and their derivatives. Suppose that the action possesses some global symmetry. What conditions have to be satisfied so that this symmetry can be gauged? To…
Tomáš Brauner
- 1,992
- 13
- 15
38
votes
8 answers
Is there a symbol for "unitless"?
I'm making a table where columns are labelled with the property and the units it's measured in:
Length (m) |||| Force (N) |||| Safety Factor (unitless) ||| etc...
I'd like not to write "unitless" on several columns...and I'm quite surprised I…
Ben
- 981
38
votes
9 answers
Why don't we prove that functions used in physics are continuous and differentiable?
I have studied physics up to 12th grade and I noticed that whenever new equations are introduced for certain entities, such as a simple harmonic wave, we never prove that it's continuous everywhere or differentiable everywhere before using these…
FoundABetterName
- 797