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1500 questions
167
votes
9 answers
Could Legolas actually see that far?
The video “How Far Can Legolas See?” by MinutePhysics recently went viral. The video states that although Legolas would in principle be able to count $105$ horsemen $24\text{ km}$ away, he shouldn't have been able to tell that their leader was very…
Ali
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166
votes
11 answers
What makes a theory "Quantum"?
Say you cook up a model about a physical system. Such a model consists of, say, a system of differential equations. What criterion decides whether the model is classical or quantum-mechanical?
None of the following criteria are valid:
Partial…
AccidentalFourierTransform
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163
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9 answers
Does someone falling into a black hole see the end of the universe?
This question was prompted by Can matter really fall through an event horizon?. Notoriously, if you calculate the Schwarzschild coordinate time for anything, matter or light, to reach the event horizon the result is infinite. This implies that the…
John Rennie
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161
votes
6 answers
Why would spacetime curvature cause gravity?
It is fine to say that for an object flying past a massive object, the spacetime is curved by the massive object, and so the object flying past follows the curved path of the geodesic, so it "appears" to be experiencing gravitational acceleration.…
user1648764
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160
votes
4 answers
Why are the harmonics of a piano tone not multiples of the base frequency?
I was trying to figure out which piano keys were being played in an audio recording using spectral analysis, and I noticed that the harmonics are not integer multiple of the base note. What is the reason for this?
Take a look at the spectrogram of…
Szabolcs
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158
votes
3 answers
Why does Stephen Hawking say black holes don't exist?
Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he said.
References:
Article in Nature News: Stephen…
Devesh Saini
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157
votes
27 answers
Simple check for the global shape of the Earth
I have been on a date recently, and everything went fine until the moment the girl has told me that the Earth is flat. After realizing she was not trolling me, and trying to provide her with a couple of suggestions why that may not be the case, I've…
SBF
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157
votes
9 answers
Why are the wet patches on these floor tiles circular?
My friend's 3-year-old daughter asked "Why are there circles there?"
It had either rained the night before or frost had thawed. What explains the circles?
Follow-up question: Ideally, are these really circles or some kind of superellipse?
user138719
- 1,515
156
votes
1 answer
What is an "attosecond pulse", and what can you use it for?
The 2023 Nobel Prize in Physics was announced today, and it was awarded to Pierre Agostini, Ferenc Krausz and Anne L’Huillier, for
“experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”.
The…
Emilio Pisanty
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156
votes
7 answers
Why are there only derivatives to the first order in the Lagrangian?
Why is the Lagrangian a function of the position and velocity (possibly also of time) and why are dependences on higher order derivatives (acceleration, jerk,...) excluded?
Is there a good reason for this or is it simply "because it works".
Sam
- 2,426
156
votes
8 answers
Is $\pi^2 \approx g$ a coincidence?
In spite of their different dimensions, the numerical values of $\pi^2$ and $g$ in SI units are surprisingly similar,
$$\frac{\pi^2}{g}\approx 1.00642$$
After some searching, I thought that this fact isn't a coincidence, but an inevitable result of…
nalzok
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154
votes
14 answers
How do I explain to a six year old why people on the other side of the Earth don't fall off?
Today a friend's six year old sister asked me the question "why don't people on the other side of the earth fall off?". I tried to explain that the Earth is a huge sphere and there's a special force called "gravity" that tries to attract everything…
Amal Murali
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154
votes
7 answers
Is temperature a Lorentz invariant in relativity?
If an observer starts moving at relativistic speeds will he observe the temperature of objects to change as compared to their rest temperatures?
Suppose the rest temperature measured is $T$ and the observer starts moving with speed $v$. What will be…
Sahil Chadha
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153
votes
7 answers
A list of inconveniences between quantum mechanics and (general) relativity?
It is well known that quantum mechanics and (general) relativity do not fit well. I am wondering whether it is possible to make a list of contradictions or problems between them?
E.g. relativity theory uses a space-time continuum, while quantum…
Gerard
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152
votes
8 answers
Why is nuclear waste more dangerous than the original nuclear fuel?
I know the spent fuel is still radioactive. But it has to be more stable than what was put in and thus safer than the uranium that we started with. That is to say, is storage of the waste such a big deal? If I mine the uranium, use it, and then bury…
Anthony B
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