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1500 questions
70
votes
3 answers
Why is the ampere a base unit and not the coulomb?
I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the ampere that is the base unit and not the coulomb. Why is this? It seems to me that charge can exist without current, but…
Greg
- 1,790
70
votes
9 answers
What are the reasons to expect that gravity should be quantized?
What I am interested to see are specific examples/reasons why gravity should be quantized. Something more than "well, everything else is, so why not gravity too". For example, isn't it possible that a quantum field theory on curved space-time would…
MBN
- 3,875
70
votes
9 answers
Can the photoelectric effect be explained without photons?
Lamb 1969 states,
A misconception which most physicists acquire in their formative years is that the photoelectric effect requires the quantization of the electromagnetic field for its explanation. [...] In fact we shall see that the photoelectric…
user4552
70
votes
6 answers
Why isn't my calculation that we should be able to see the sun well beyond the observable universe valid?
I recently read an interesting article that states that a human being can perceive a flash of as few as 5 or so photons, and the human eye itself can perceive even a single photon. The brain will filter this out, however.
I wanted to calculate how…
Reggie Simmons
- 691
70
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
- 30,008
70
votes
5 answers
How do we know the LHC results are robust?
Nature article on reproducibility in science.
According to that article, a (surprisingly) large number of experiments aren't reproducible, or at least there have been failed attempted reproductions. In one of the figures, it's said that 70% of…
Allure
- 23,373
70
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
- 705
70
votes
6 answers
How does light combine to make new colours?
In computer science, we reference colours using the RGB system and TVs have pixels which consist of groups of red, green and blue lines which turn on and off to create colours.
But how does this work? Why would certain amounts of red, blue and green…
Isky Mathews
- 1,965
70
votes
3 answers
What makes cheese so effective at absorbing microwaves?
Whenever I put a meal in the microwave which contains cheese, why does the cheese get hot before the rest of the meal is heated through?
Robert Frost
- 851
70
votes
9 answers
What is the difference between the magnetic $H$-field and $B$-field?
From Wikipedia:
"The term (Magnetic Field) is used for two distinct but closely related fields denoted by the symbols $B$ and $H$, where $H$ is measured in units of amperes per meter in the SI. $B$ is measured in teslas in the SI."
So, the two are…
Sergei Gorbikov
- 1,125
70
votes
5 answers
Why do atomic clocks only use caesium?
Modern atomic clocks only use caesium atoms as oscillators. Why don't we use other atoms for this role?
Pinki
- 701
70
votes
2 answers
What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?
The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the fundamental constants of nature than were possible when…
Emilio Pisanty
- 137,480
69
votes
7 answers
How is the Schroedinger equation a wave equation?
Wave equations take the form:
$$\frac{ \partial^2 f} {\partial t^2} = c^2 \nabla ^2f$$
But the Schroedinger equation takes the form:
$$i \hbar \frac{ \partial f} {\partial t} = - \frac{\hbar ^2}{2m}\nabla ^2f + U(x) f$$
The partials with respect…
user28823
- 1,110
- 3
- 12
- 16
69
votes
1 answer
Status of experimental searches for tachyons?
Now that the dust has settled on the 2011 superluminal neutrino debacle at OPERA, I'm interested in understanding the current status of experimental searches for tachyons. Although the OPERA claim was a fun puzzle for theorists trying to find…
user4552
69
votes
2 answers
Why was water freezing almost instantaneously when shaking a bottle that spent the night outside during a frosty night?
Due to the forecasted frost last night, I placed yesterday evening, some 1.5l standard PET bottles filled up to 90% with warm tap water(+60°C) close to some vegetables that I wanted to protect in my garden. The temperature dropped to roughly -3 ~…
s.k
- 799