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39
votes
6 answers
What constitutes an observation/measurement in QM?
Fundamental notions of QM have to do with observation, a major example being The Uncertainty Principle.
What is the technical definition of an observation/measurement?
If I look at a QM system, it will collapse. But how is that any different from…
ThisIsNotAnId
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39
votes
2 answers
Are gravitational waves longitudinal or transverse?
Waves are generally classified as either transverse or longitudinal depending on the they way the propagated quantity is oriented with respect to the direction of propagation. Then what is a gravitational wave? It doesn't make sense to me that a…
Ignacio
- 645
39
votes
6 answers
Formalizing Quantum Field Theory
I'm wondering about current efforts to provide mathematical foundations and more solid definition for quantum field theories. I am aware of such efforts in the context of the simpler topological or conformal field theories, and of older approaches…
user566
39
votes
4 answers
Why exactly do atomic bombs explode?
In atomic bombs, nuclear reactions provide the energy of the explosion. In every reaction, a thermal neutron reaches a plutonium or a uranium nucleus, a fission reaction takes place, and two or three neutrons and $\gamma$ radiation are produced. I…
L.Gyula
- 810
39
votes
4 answers
Can units be plural?
I was in a conversation with my senior engineer where he kept on insisting that we can use plural when we write down any unit. I argued that it is not the 'common' practice or even throughout my whole academic career (unfortunately) I haven't found…
Ahmed
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39
votes
10 answers
Does any particle ever reach any singularity inside the black hole?
I am not a professional physicist, so I may say something rubbish in here, but this question has always popped in my mind every time I read or hear anyone speak of particles hitting singularities and "weird things happen".
Now to the question at…
user1549
39
votes
5 answers
Does old light contain clues to its age?
Light from celestial objects is old. In the case of galaxies, it's millions of years old. It seems plausible to me that light might show signs of its age.
I was surprised that a Google search only turned up one study in this area: Measurement of the…
Lambda
- 4,721
39
votes
3 answers
Why do the neutrinos (with mass) from a supernova arrive before the light (no mass)?
I've already read the below questions (and their answers) regarding neutrinos vs. electromagnetic waves propagating through space, but I'm still not clear on something.
Neutrinos arrived before the photons (supernova)
The delay between neutrinos…
pr1268
- 1,837
39
votes
1 answer
Why does wind direction significantly affect sound propagation?
I live about 2 miles from a race track.
Some days the noise from testing is extremely noticeable, Other days despite the exact same testing activity, there is little-to-no noise at all.
This is mostly effected by the direction of the wind.
How does…
39
votes
2 answers
What are the units or dimensions of the Dirac delta function?
In three dimensions, the Dirac delta function $\delta^3 (\textbf{r}) = \delta(x) \delta(y) \delta(z)$ is defined by the volume integral:
$$\int_{\text{all space}} \delta^3 (\textbf{r}) \, dV = \int_{-\infty}^{\infty} \int_{-\infty}^{\infty}…
Andrew
- 1,143
39
votes
11 answers
Discreteness and Determinism in Superstrings?
So Gerard 't Hooft has a brand new paper (thanks to Mitchell Porter for making me aware of it) so this is somewhat of a expansion to the question I posed on this site a month or so ago regarding 't Hoofts work.
Now he has taken it quite a big step…
QuestionAnswers
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39
votes
3 answers
Phase space volume and relativity
Much of statistical mechanics is derived from Liouville's theorem, which can be stated as "the phase space volume occupied by an ensemble of isolated systems is conserved over time." (I'm mostly interested in classical systems for the moment.)
It's…
N. Virgo
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39
votes
4 answers
Why is this vector field curl-free?
The curl in cylindrical coordinates is defined:
$$\nabla \times \vec{A}=\left({\frac {1}{\rho }}{\frac {\partial A_{z}}{\partial \varphi }}-{\frac {\partial A_{\varphi }}{\partial z}}\right){\hat {\boldsymbol {\rho }}}+\left({\frac {\partial A_{\rho…
forky40
- 761
39
votes
5 answers
Why do we assume, in dimensional analysis, that the remaining constant is dimensionless?
Walter Lewin's first lecture (at 22:16) analyzes the time $t$ for an apple to fall to the ground, using dimensional analysis. His reasoning goes like this:
It's natural to suppose that height of the apple to the ground ($h$), mass of the apple…
Fine Man
- 1,493
39
votes
3 answers
Why does water pouring from a glass sometimes travel down the side of the glass?
If you have a glass of water, say, three quarters full and you pour it at an angle of say, $45^{\circ}$ with respect to the the table, the water comes out of the glass and goes directly down towards the floor.
However, when the glass is more full,…
ODP
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