Frequently asked questions. Whenever the same question winds up getting asked over and over again, this tag should designate the original one, which we close all the others as duplicates of.
Questions tagged [faq]
29 questions
304
votes
1 answer
Resource recommendations
Every once in a while, we get a question asking for a book or other educational reference on a particular topic at a particular level. This is a meta-question that collects all those links together. If you're looking for book recommendations, this…
David Z
- 78,470
301
votes
17 answers
What really allows airplanes to fly?
What aerodynamic effects actually contribute to producing the lift on an airplane? I know there's a common belief that lift comes from the Bernoulli effect, where air moving over the wings is at reduced pressure because it's forced to travel further…
David Z
- 78,470
204
votes
21 answers
Given Newton's third law, why are things capable of moving?
Given Newton's third law, why is there motion at all? Should not all forces even themselves out, so nothing moves at all? When I push a table using my finger, the table applies the same force onto my finger like my finger does on the table just with…
user16458
- 2,083
190
votes
14 answers
Why does holding something up cost energy while no work is being done?
I read the definition of work as $$W ~=~ \vec{F} \cdot \vec{d}$$ $$\text{ Work = (Force) $\cdot$ (Distance)}.$$ If a book is there on the table, no work is done as no distance is covered. If I hold up a book in my hand and my arm is stretched, if no…
SMUsamaShah
- 5,547
170
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
- 7,471
143
votes
11 answers
How fast does gravity propagate?
A thought experiment: Imagine the Sun is suddenly removed. We wouldn't notice a difference for 8 minutes, because that's how long light takes to get from the Sun's surface to Earth. However, what about the Sun's gravitational effect ? If gravity…
Stefano Borini
- 3,988
128
votes
16 answers
Is it possible for information to be transmitted faster than light by using a rigid pole?
Is it possible for information (like 1 and 0s) to be transmitted faster than light? For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them starts pulling and pushing on his/her end. The person…
Jonathan.
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72
votes
17 answers
What is the simplest way to prove the Earth is round?
Assume you've come in contact with a tribe of people cut off from the rest of the world, or you've gone back in time several thousand years, or (more likely) you've got a numbskull cousin. How would you prove that the Earth is, in fact, round?
tQuarella
67
votes
10 answers
Why doesn't a bike/bicycle fall if going with a high speed?
Why does a bike/bicycle fall when its speed is very low or close to zero and is balanced when going with a high speed?
Ahmad Farid
- 787
62
votes
4 answers
How does gravity work underground?
Would the effect of gravity on me change if I were to dig a very deep hole and stand in it? If so, how would it change? Am I more likely to be pulled downwards, or pulled towards the edges of the hole? If there would be no change, why not?
Mia Clarke
- 723
52
votes
5 answers
Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?
Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?
user68
51
votes
3 answers
How are classical optics phenomena explained in QED (Snell's law)?
How is the following classical optics phenomenon explained in quantum electrodynamics? Reflection and Refraction Are they simply due to photons being absorbed and re-emitted? How do we get to Snell's law, for example, in that case? Split by request:…
Sklivvz
- 13,925
50
votes
4 answers
What if the LHC doesn't see SUSY?
A question in four parts. What are the main problems which supersymmetry purports to solve? What would constitute lack of evidence for SUSY at the proposed LHC energy scales (e.g. certain predicted superpartners are not in fact observed)? Are there…
Nigel Seel
- 3,424
38
votes
3 answers
Is it possible to speak about changes in a physical constant which is not dimensionless?
Every so often, one sees on this site* or in the news † or in journal articles ‡ a statement of the form "we have measured a change in such-and-such fundamental constant" (or, perhaps more commonly, "we have constrained the rate of change of...").…
Emilio Pisanty
- 139,063
36
votes
6 answers
Is time travel possible? Is it possible to go back in time?
I read somewhere that according to relativity, it is possible - involving black holes and other stuff - to jump into the past. Is it possible for anything to go back in time either continuously or by jumping?
SMUsamaShah
- 5,547