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1500 questions
110
votes
5 answers
How do we know that radioactive decay rates are constant over billions of years?
A friend and I recently discussed the idea that radioactive decay rates are constant over geological times, something upon which dating methods are based.
A large number of experiments seem to have shown that decay rate is largely uninfluenced by…
Pertinax
- 1,007
110
votes
3 answers
Why are some people are claiming that the Big Bang never happened?
A news story is going viral on social media networks claiming that two physicists have found a way to eliminate the Big Bang singularity, or in layman's terms (as claimed by many sensationalist news articles): The Big Bang never happened at all.
The…
Janus Boffin
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109
votes
4 answers
Why does nature favour the Laplacian?
The three-dimensional Laplacian can be defined as $$\nabla^2=\frac{\partial^2}{\partial x^2}+\frac{\partial^2}{\partial y^2}+\frac{\partial^2}{\partial z^2}.$$ Expressed in spherical coordinates, it does not have such a nice form. But I could define…
Sam Jaques
- 1,397
109
votes
6 answers
Why is oil a better lubricant than water?
How come mineral oil is a better lubricant than water, even though water has a lower viscosity?
When two surfaces slide over each other with a gap filled with a fluid, the different layers of the fluid are dragged at different speeds. The very top…
DK2AX
- 4,860
109
votes
10 answers
What is time, does it flow, and if so what defines its direction?
This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address issues such as:
What do physicists mean by…
John Rennie
- 367,598
109
votes
12 answers
Why are differential equations for fields in physics of order two?
What is the reason for the observation that across the board fields in physics are generally governed by second order (partial) differential equations?
If someone on the street would flat out ask me that question, then I'd probably mumble something…
Nikolaj-K
- 8,873
108
votes
11 answers
If gravity isn't a force, then why do we learn in school that it is?
I have studied some of Einstein's Theory of General Relativity, and I understand that it states that gravity isn't a force but rather the effects of objects curving space-time. If this is true, then why are we instructed in middle school that it is…
Peter Hall
- 1,225
107
votes
8 answers
Why does the Sun's (or other stars') nuclear reaction not use up all its "fuel" immediately?
The temperature and pressure everywhere inside the Sun reach the critical point to start nuclear reactions - there is no reason for it to take such a long time to complete the reaction process.
Just like a nuclear bomb will complete all the reaction…
user.3898215
- 1,196
106
votes
11 answers
What justifies dimensional analysis?
Dimensional analysis, and the notion that quantities with different units cannot be equal, is often used to justify very specific arguments, for example, you might use it to argue that a particular formula cannot possibly be the correct expression…
Jack M
- 2,079
106
votes
2 answers
Is there such thing as imaginary time dilation?
When I was doing research on General Relativity, I found Einstein's equation for Gravitational Time Dilation. I discovered that when you plugged in a large enough value for $M$ (around $10^{19}$ kilograms), and plugged in $1$ for $r$, then the…
John Dumancic
- 4,077
105
votes
5 answers
Why do same/opposite electric charges repel/attract each other, respectively?
I know plus pushes another plus away, but why, really, do they do that? On the other hand, molecules of the same type are attracted to each other. I find that weird.
I do know some stuff about four universal forces. But why in general the general…
Muhammad Umer
- 2,733
105
votes
6 answers
Explain it to me like I'm a physics grad: Greenhouse Effect
What is the mechanism by which increasing $\rm CO_2$ (or other greenhouse gases) ends up increasing the temperature at (near) the surface of the Earth?
Mostly what I'm looking for is a big-picture explanation of how increasing $\rm CO_2$ affects the…
Dave
- 4,203
105
votes
18 answers
Why do most formulas in physics have integer and rational exponents?
I mean, why is $F=ma$? Why not $m^{0.123}$, $a^{1.43}$ or some random non-integers or irrational?
I hope you understand that my question isn't limited just to force, energy, velocity, etc.; it also extends to the area of a square, circle, etc. and…
dushyanth
- 1,304
105
votes
7 answers
Why is there a scarcity of lithium?
One of the major impediments to the widespread adoption of electric cars is a shortage of lithium for the batteries. I read an article a while back that says that there is simply not enough lithium available on the entire planet to make enough…
Mason Wheeler
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104
votes
7 answers
How did Einstein know the speed of light was constant?
I often hear the story of how Einstein came up to the conclusion that time would slow down the faster you move, because the speed of light has to remain the same.
My question is, how did Einstein know that measuring the speed of light wouldn't be…
Pablo Fernandez
- 1,065