Most Popular
1500 questions
77
votes
1 answer
Why does a window become a mirror at night?
In day, when you look in the room through the window out, you can clearly see what happens outside. At night when it's dark outside but there's light inside you can look in the window but it becomes a mirror.
Why?
Alon Gubkin
- 873
- 1
- 6
- 6
77
votes
10 answers
Is 3+1 spacetime as privileged as is claimed?
I've often heard the argument that having 3 spatial dimensions is very special. Such arguments are invariably based on certain assumptions that do not appear to be justifiable at all, at least to me. There is a summary of arguments on Wikipedia.
For…
Roman Starkov
- 2,265
76
votes
3 answers
Why don't the Earth's oceans generate a magnetic field?
Many questions have been asked here about why the Earth has a magnetic field, e.g.,
What is the source of Earth's magnetic field?
How does Earth's interior dynamo work?
How can an electrically neutral planetary core be geodynamo?
Why does the…
Thorondor
- 4,110
- 2
- 23
- 46
76
votes
2 answers
Do "almost black holes" exist?
The only things I read about so far in astrophysics are either black holes, developing black holes or not black holes at all.
So I am wondering, is it physically possible to have an object that is almost a black hole, but not a black hole. What I…
Winston
- 3,276
76
votes
8 answers
If I stood next to a piece of metal heated to a million degrees, but in a perfect vacuum, would I feel hot?
A friend of mine told me that if you were to stand beside plate of metal that is millions of degrees hot, inside a 100% vacuum, you would not feel its heat. Is this true? I understand the reasoning that there is no air, thus no convection, and…
Peter_Browning
- 919
76
votes
12 answers
Is the wave-particle duality a real duality?
I often hear about the wave-particle duality, and how particles exhibit properties of both particles and waves. However, I wonder, is this actually a duality? At the most fundamental level, we 'know' that everything is made up out of particles,…
user14445
- 1,511
- 5
- 17
- 25
76
votes
13 answers
If visible light has more energy than microwaves, why isn't visible light dangerous?
Light waves are a type of electromagnetic wave and they fall between 400-700 nm long. Microwaves are less energetic but seem to be more dangerous than visible light. Is visible light dangerous at all and why not?
suse
- 1,298
76
votes
3 answers
Is there 'friction' in spacetime?
So, if all the bodies are embedded in space-time and moves through it, is there some kind of 'friction' with space time of the planets? For example, the Earth suffers friction when moving near the sun due the curvature and General Relativity and…
Jose Javier Garcia
- 4,847
76
votes
4 answers
How does water evaporate if it doesn't boil?
When the sun is out after a rain, I can see what appears to be steam rising off a wooden bridge nearby. I'm pretty sure this is water turning into a gas.
However, I thought water had to reach 100 degrees C to be able to turn into a gas.
Is there an…
Malcolm Crum
- 1,053
- 1
- 7
- 11
76
votes
5 answers
Why does a candle blow out when we blow on it? Our breath is 16% oxygen and only 4% CO2
Don't say that a layer of carbon dioxide covers the flame, because our breath has more oxygen than carbon dioxide.
Also, our breath does not extinguish the flame by cooling it as it is itself warmer than the coolness required to extinguish it.
So…
Prem
- 2,356
75
votes
12 answers
What is a tensor?
I have a pretty good knowledge of physics, but couldn't deeply understand what a tensor is and why it is so fundamental.
0x90
- 3,456
75
votes
4 answers
Why correlation functions?
While this concept is widely used in physics, it is really puzzling (at least for beginners) that you just have to multiply two functions (or the function by itself) at different values of the parameter and then average over the domain of the…
Kostya
- 20,288
75
votes
6 answers
If like charges repel, why doesn't a charge break itself apart?
How can it be that, if like charges repel, they don't repel themselves? In other words, why don't charges break apart?
About the possible duplicate: I want to know about charges in general, not just that of an electron.
My response to Lawrence B.…
Kevin
- 857
75
votes
5 answers
Why is it cold on the sea floor if pressure heats things?
I was reading this and it says that Microsoft put a server farm at the bottom of the ocean because it's cooler there. Particularly it seems to imply that it get's colder as you go deeper, "Since ocean water gets pretty cold toward the sea floor..."…
user273872
- 2,623
- 2
- 18
- 30
75
votes
7 answers
Why isn't Higgs coupling considered a fifth fundamental force?
When I first learned about the four fundamental forces of nature, I assumed that they were just the only four kind of interactions there were. But after learning a little field theory, there are many other kinds of couplings, even in the standard…
user542
- 1,100