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1500 questions
106
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
- 1,186
105
votes
5 answers
How and why do accelerating charges radiate electromagnetic radiation?
Let's consider it case by case:
Case 1: Charged particle is at rest. It has an electric field around it. No problem. That is its property.
Case 2: Charged particle started moving (it's accelerating). We were told that it starts radiating EM…
claws
- 7,255
105
votes
12 answers
How can time dilation be symmetric?
Suppose we have two twins travelling away from each other, each twin moving at some speed $v$:
Twin $A$ observes twin $B$’s time to be dilated so his clock runs faster than twin $B$’s clock. But twin $B$ observes twin $A$’s time to be dilated so…
John Rennie
- 355,118
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
- 4,153
105
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
- 355,118
105
votes
12 answers
Why is filling a balloon from your mouth much harder initially?
Why is it that when you first fill up a balloon, it's hard to get air through, but after inflating it a bit, it becomes much easier to further inflate the balloon?
yuritsuki
- 1,638
104
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
- 1,979
104
votes
4 answers
Would a pin head heated to 15 million degrees Celsius kill everyone in a 1000 mile radius?
The YouTube video How Hot Can it Get? contains, at the 2:33 mark, the following claim:
A pin head heated to 15 million degrees will kill everyone in a 1000 miles radius.
On what basis can this claim be true?
Some of the things I can think…
Jus12
- 3,423
104
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
- 3,937
104
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
- 8,443
104
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
- 1,293
104
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
- 2,162
103
votes
9 answers
Which is stronger, a rope without knots or a rope with knots?
Suppose I have two identical ropes, one of which I manually tie some knots in. If I use them to hang clothes, which one is more durable?
Personally I think that rope with knots will be more durable, but I can't come up with a satisfactory reason.
an offer can't refuse
- 6,035
103
votes
7 answers
What do spacelike, timelike and lightlike spacetime interval really mean?
Suppose we have two events $(x_1,y_1,z_1,t_1)$ and $(x_2,y_2,z_2,t_2)$. Then we can define
$$\Delta s^2 = -(c\Delta t)^2 + \Delta x^2 + \Delta y^2 + \Delta z^2,$$
which is called the spacetime interval. The first event occurs at the point with…
Gold
- 35,872
103
votes
7 answers
What is the speed of sound in space?
Given that space is not a perfect vacuum, what is the speed of sound therein? Google was not very helpful in this regard, as the only answer I found was $300\,{\rm km}\,{\rm s}^{-1}$, from Astronomy Cafe, which is not a source I'd be willing to…
Josh Glover
- 1,103