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1500 questions
41
votes
4 answers
If both radio waves and gamma rays can travel through walls
and they are on opposite ends of the electromagnetic spectrum, then why can't light travel through walls which is right in the middle of the spectrum?
This question has already been asked here. However, I am not entirely satisfied by the answer…
morpheus
- 548
41
votes
7 answers
Does a photon interfere only with itself?
I sometimes hear statements like:
Quantum-mechanically, an interference pattern occurs due to quantum interference of the wavefunction of a photon. The wavefunction of a single photon only interferes with itself. Different photons (for example from…
Kostya
- 19,992
41
votes
7 answers
Three polarizers, 45° apart
If light is passed through two polarizing filters before arriving at a target, and both of the filters are oriented at 90° to each other, then no light will be received at the target. If a third filter is added between the first two, oriented at a…
vaindil
- 567
41
votes
8 answers
Why do small patches of snow remain on the ground many days or weeks after all the other snow has melted?
I often notice small patches of snow that remain on the ground in seemingly random locations, many days or even weeks after all other snow in an area has melted, and even when temperatures have been well above freezing for some time. What makes…
user3091
- 519
41
votes
6 answers
What would happen if a 10-kg cube of iron, at a temperature close to 0 kelvin, suddenly appeared in your living room?
What would be the effect of placing an object that cold in an environment that warm? Would the room just get a little colder? Would it kill everyone in the room like some kind of cold bomb? What would happen?
Don't think about how the cube got…
Mark Morales II
- 944
41
votes
6 answers
Why does ponytail-style hair oscillate horizontally, but not vertically when jogging?
Many people with long hair tie their hair to ponytail-style:
Closely observing the movement of their hair when they are running, I have noticed that the ponytail oscillates only horizontally, that is, in "left-right direction". Never I have seen…
Zeick
- 1,223
41
votes
12 answers
What does it mean when people say "Physics break down"?
So I keep hearing people talking about how physics break down at for example the center of a black hole. And maybe I am just to stupid but, why? How can we say that? For all we know a black hole could just be a very dense sphere. Kind of like a…
Erik Hall
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41
votes
1 answer
Why is the period of a geostationary satellite not exactly 1440 minutes?
When reading about Astra satellites on Wikipedia (https://en.wikipedia.org/wiki/Astra_1KR), I saw that the period of the Astra 1KR satellite, positioned at 19.2° E, is 1,436.1 minutes (source: NORAD data).
That is 3.9 minutes short of a day (1440…
fbitterlich
- 471
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41
votes
4 answers
Validity of naively computing the de Broglie wavelength of a macroscopic object
Many introductory quantum mechanics textbooks include simple exercises on computing the de Broglie wavelength of macroscopic objects, often contrasting the results with that of a proton, etc.
For instance, this example, taken from a…
Mark Allen
- 840
41
votes
3 answers
Is 13 really the answer for the "Devil's problem" in physics (a rolling tube with a rod)?
Recently I chewed the fat with a physics student and got intrigued by him mentioning "the Devil's problem," which he described as a simply worded mechanics problem that is extremely difficult to solve and has an answer of exactly 13 despite the…
Mitsuko
- 1,559
41
votes
6 answers
Can we do better than "a spinor is something that transforms like a spinor"?
It's common for students to be introduced to tensors as "things that transform like tensors" - that is, their components must transform in a certain way when we change coordinates. However, we can do better by defining a tensor as a multilinear map…
Jacob Drori
- 596
41
votes
2 answers
Was X17 predicted before it was observed?
Articles, with very little detail, have made their rounds about an X17 boson (16.7 MeV) being observed in tests of decaying beryllium-8 and perhaps once in a test with helium.
Most of the undiscovered particles that are searched for in CERN or other…
userLTK
- 5,658
41
votes
5 answers
Why does wrapping aluminium foil around my food help it keep warm, even though aluminium is a good conductor?
Aluminium being such a good conductor, how is it possible that it is helping me keep my food warm ?? Because ultimately it should conduct the heat that is inside to the outside for exchange and should have no effect (maybe even cool it faster by…
user235329
41
votes
3 answers
Will the volt, ampere, ohm or other electrical units change on May 20th, 2019?
When watching a video by Veritasium about the SI units redefinition (5:29), a claim that the volt and unit of resistance (presumably the ohm) will change by about 1 part in 10 million caught my attention:
[...] I should point out that a volt will…
user231851
41
votes
5 answers
What is the usefulness of the Wigner-Eckart theorem?
I am doing some self-study in between undergrad and grad school and I came across the beastly Wigner-Eckart theorem in Sakurai's Modern Quantum Mechanics. I was wondering if someone could tell me why it is useful and perhaps just help me understand…
Cogitator
- 1,075