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1500 questions
97
votes
9 answers
What is a manifold?
For complete dummies when it comes to space-time, what is a manifold and how can space-time be modelled using these concepts?
Richard971
- 1,085
96
votes
4 answers
Why don't miners get boiled to death at $4$ km deep?
The Mponeng Gold Mine is nearly $4$ km deep. It has the largest elevators in the world and is considered one of the most dangerous mines in the world.
The geothermal gradient is $25$ degrees Celsius per kilometer, which would be $100$ degrees.…
Ambrose Swasey
- 4,474
95
votes
12 answers
Why are radiators always placed under windows?
I don't know if anyone else has noticed this but in most buildings and most rooms, radiators are predominantly placed under a window.
Now, in my eyes, that is the worst place to put them; hot air rises, reaches the window (which no matter how well…
turnip
- 3,648
95
votes
10 answers
Why bother buying efficient lights if you are already heating your house?
Assume I live in a location where at any time of day and any time of year, I need to heat my house. Assume further that I have a room with no windows. In this case, does it make sense for me to buy efficient light bulbs, considering that any…
Mathew
- 1,123
95
votes
10 answers
Quantum Entanglement - What's the big deal?
Bearing in mind I am a layman - with no background in physics - please could someone explain what the "big deal" is with quantum entanglement?
I used to think I understood it - that 2 particles, say a light-year apart spatially, could affect each…
Pete Oakey
- 1,544
95
votes
4 answers
Physical meaning of Legendre transformation
I would like to know the physical meaning of the Legendre transformation, if there is any? I've used it in thermodynamics and classical mechanics and it seemed only a change of coordinates?
gsAllan
- 1,217
95
votes
3 answers
Why was carbon-12 chosen for the atomic mass unit?
The atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Was there any physical reason for such a definition? Were they trying to include electrons in the atomic mass unit?
Why not define the amu as the mass of one proton or neutron…
Dieblitzen
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95
votes
4 answers
If Earth had rings?
If Earth had rings, would they center on the equator like Saturn's rings do on its equator?
Muze
- 1
95
votes
1 answer
If we had a "perfectly efficient" computer and all the energy in the Milky-way available, what number could it count to?
The idea for this question comes from an example in cryptography, where supposedly 256-bit symmetric keys will be enough for all time to come (brute-forcing a 256-bit key is sort-of equivalent to counting to $2^{255}$, with some constant in front of…
cooky451
- 1,069
94
votes
8 answers
Does juggling balls reduce the total weight of the juggler and balls?
A friend offered me a brain teaser to which the solution involves a $195$ pound man juggling two $3$-pound balls to traverse a bridge having a maximum capacity of only $200$ pounds. He explained that since the man only ever holds one $3$-pound…
adamdport
- 1,147
94
votes
6 answers
What is more fundamental, fields or particles?
My confusion about quantum theory is twofold:
I lack an adequate understanding of how the mathematics of quantum theory is supposed to correspond to phenomena in the physical world
I still have an incomplete picture in my mind of how cause and…
jpbrooks-user153707
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- 1
- 8
- 6
93
votes
4 answers
Why does a yellow object turn white under a yellow light? Shouldn't it turn yellow instead?
Recently I was eating a yellow rice for lunch in a restaurant with only yellow lights. But the rice looked white! I was intrigued by this because I always thought it should look yellow since the yellow pigment reflects only yellow light, but the…
David A.
- 1,029
93
votes
5 answers
Does a gun exert enough gravity on the bullet it fired to stop it?
My question is set in the following situation:
You have a completely empty universe without boundaries.
In this universe is a single gun which holds one bullet.
The gun fires the bullet and the recoil sends both flying in opposite directions.…
JadaLovelace
- 1,419
92
votes
3 answers
First and second order phase transitions
Recently I've been puzzling over the definitions of first and second order phase transitions. The Wikipedia article starts by explaining that Ehrenfest's original definition was that a first-order transition exhibits a discontinuity in the first…
N. Virgo
- 33,913
92
votes
3 answers
Why does the LIGO observation disprove higher dimensions?
I recently read this article which claims that last year’s LIGO observation of gravitational waves is proof that, at least on massive scales, there cannot be more than three spatial dimensions.
I don’t understand the physics fully, so could someone…
DonielF
- 952