All Questions
219,889
questions
790
votes
24
answers
257k
views
Cooling a cup of coffee with help of a spoon
During breakfast with my colleagues, a question popped into my head:
What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon?
A qualitative answer would be ...
- 7,617
520
votes
7
answers
61k
views
Can I compute the mass of a coin based on the sound of its fall?
The other day, I bumped my bookshelf and a coin fell down. This gave me an idea. Is it possible to compute the mass of a coin, based on the sound emitted when it falls?
I think that there should be a ...
- 5,087
479
votes
6
answers
64k
views
How does light bend around my finger tip?
When I close one eye and put the tip of my finger near my open eye, it seems as if the light from the background image bends around my finger slightly, warping the image near the edges of my blurry ...
- 4,549
471
votes
20
answers
50k
views
How does gravity escape a black hole?
My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to ...
- 4,829
379
votes
7
answers
59k
views
Did the Big Bang happen at a point?
TV documentaries invariably show the Big Bang as an exploding ball of fire expanding outwards. Did the Big Bang really explode outwards from a point like this? If not, what did happen?
365
votes
29
answers
141k
views
Why are mirror images flipped horizontally but not vertically?
Why is it that when you look in the mirror left and right directions appear flipped, but not the up and down?
- 3,837
331
votes
34
answers
52k
views
Do we know why there is a speed limit in our universe?
This question is about why we have a universal speed limit (the speed of light in vacuum). Is there a more fundamental law that tells us why this is?
I'm not asking why the speed limit is equal to $c$ ...
- 7,261
314
votes
11
answers
47k
views
What experiment would disprove string theory?
I know that there's big controversy between two groups of physicists:
those who support string theory (most of them, I think)
and those who oppose it.
One of the arguments of the second group is ...
- 3,719
306
votes
1
answer
125k
views
Resource recommendations [closed]
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. ...
300
votes
2
answers
28k
views
What is Chirped Pulse Amplification, and why is it important enough to warrant a Nobel Prize?
The 2018 Nobel Prize in Physics was awarded recently, with half going to Arthur Ashkin for his work on optical tweezers and half going to Gérard Mourou and Donna Strickland for developing a technique ...
- 127k
297
votes
10
answers
57k
views
Why don't metals bond when touched together?
It is my understanding that metals are a crystal lattice of ions, held together by delocalized electrons, which move freely through the lattice (and conduct electricity, heat, etc.).
If two pieces ...
- 2,621
296
votes
18
answers
77k
views
Why does kinetic energy increase quadratically, not linearly, with speed?
As Wikipedia says:
[...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $\frac{1}{2}mv^2$.
Why does this not increase linearly with speed? Why does it take so ...
- 3,079
295
votes
2
answers
27k
views
Why do ballpoint pens write better on pages that have pages below them?
If I write on the starting page of a notebook, it will write well. But when there are few or no pages below the page where I am writing, the pen will not write well. Why does this happen?
- 2,059
276
votes
17
answers
78k
views
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 ...
- 74.9k
265
votes
12
answers
54k
views
Why are four-legged chairs so common?
Four-legged chairs are by far the most common form of chair. However, only three legs are necessary to maintain stability whilst sitting on the chair. If the chair were to tilt, then with both a four-...
- 2,833
265
votes
5
answers
32k
views
If I sliced the universe in half, would the slice go through a star?
This question is based on a discussion with a 10-year old. So if it is not clear how to interpret certain details, imagine how a 10-year old would interpret them.
This 10-year old does not know about ...
user78574
262
votes
11
answers
63k
views
Don't heavier objects actually fall faster because they exert their own gravity?
The common understanding is that, setting air resistance aside, all objects dropped to Earth fall at the same rate. This is often demonstrated through the thought experiment of cutting a large object ...
- 2,858
247
votes
8
answers
39k
views
How do towels stay on hooks?
Towels (and coats) are often stored on hooks, like this:
To the untrained eye, it looks like the towel will slide off from its own weight. The hook usually angles upwards slightly, but a towel does ...
- 1,949
236
votes
22
answers
39k
views
What exactly is a photon?
Consider the question, "What is a photon?". The answers say, "an elementary particle" and not much else. They don't actually answer the question. Moreover, the question is flagged as a duplicate of, "...
- 10.7k
234
votes
8
answers
39k
views
Does Earth really have two high-tide bulges on opposite sides?
The bit that makes sense – tidal forces
My physics teacher explained that most tidal effect is caused by the Moon rotating around the Earth, and some also by the Sun.
They said that in the Earth - ...
- 2,950
212
votes
1
answer
18k
views
Strange ice found in my garden
This morning I found a really strange ice formation in my garden. I can't figure out how it appeared, because there was nothing above. The night was particularly cold (Belgium).
To give an idea, it ...
- 1,647
205
votes
10
answers
20k
views
Why is the detection of gravitational waves so significant?
LIGO has announced the detection of gravitational waves on 11 Feb, 2016. I was wondering why the detection of gravitational waves was so significant?
I know it is another confirmation of general ...
- 5,201
204
votes
14
answers
92k
views
Why don't electrons crash into the nuclei they "orbit"?
I'm having trouble understanding the simple "planetary" model of the atom that I'm being taught in my basic chemistry course.
In particular,
I can't see how a negatively charged electron can stay ...
- 4,969
203
votes
10
answers
260k
views
If photons have no mass, how can they have momentum?
As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects ...
- 2,706
200
votes
3
answers
57k
views
Surviving under water in air bubble
An incredible news story today is about a man who survived for two days at the bottom of the sea (~30 m deep) in a capsized boat, in an air bubble that formed in a corner of the boat. He was ...
- 4,297
199
votes
8
answers
16k
views
Why do we bend a book to keep it straight?
I noticed that I have been bending my book all along, when I was reading it with one hand.
This also works for plane flexible sheets of any material.
Illustration using an A4 sheet
Without bending ...
- 8,455
198
votes
4
answers
80k
views
Why doesn't matter pass through other matter if atoms are 99.999% empty space?
The ghostly passage of one body through another is obviously out of the question if the continuum assumption were valid, but we know that at the micro, nano, pico levels (and beyond) this is not even ...
- 3,745
192
votes
7
answers
232k
views
How do moving charges produce magnetic fields?
I'm tutoring high school students. I've always taught them that:
A charged particle moving without acceleration produces an electric as well as a magnetic field.
It produces an electric field ...
- 6,975
190
votes
5
answers
24k
views
Why is Google's quantum supremacy experiment impressive?
In the Nature paper published by Google, they say,
To demonstrate quantum supremacy, we compare our quantum processor against state-of-the-art classical computers in the task of sampling the output ...
- 2,036
189
votes
16
answers
19k
views
Are units of angle really dimensionless?
I know mathematically the answer to this question is yes, and it's very obvious to see that the dimensions of a ratio cancel out, leaving behind a mathematically dimensionless quantity.
However, I've ...
- 1,801
189
votes
13
answers
29k
views
Why does space expansion not expand matter?
I have looked at other questions on this site (e.g. "why does space expansion affect matter") but can't find the answer I am looking for.
So here is my question: One often hears talk of space ...
- 2,005
183
votes
15
answers
51k
views
What's the point of Hamiltonian mechanics?
I've just finished a Classical Mechanics course, and looking back on it some things are not quite clear. In the first half we covered the Lagrangian formalism, which I thought was pretty cool. I ...
- 27.1k
183
votes
2
answers
28k
views
Why do we not have spin greater than 2?
It is commonly asserted that no consistent, interacting quantum field theory can be constructed with fields that have spin greater than 2 (possibly with some allusion to renormalization). I've also ...
- 2,793
180
votes
21
answers
157k
views
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 ...
- 1,843
179
votes
2
answers
19k
views
When separating an Oreo cookie, why does the cream stick to just one side only?
There is probably some reason for this, but I can't figure out what it is. I agree that it probably doesn't happen 100% of the time, but most all of the time, the cream is clinging to just one of the ...
- 2,609
178
votes
5
answers
23k
views
Gauge symmetry is not a symmetry?
I have read before in one of Seiberg's articles something like, that gauge symmetry is not a symmetry but a redundancy in our description, by introducing fake degrees of freedom to facilitate ...
- 16.4k
176
votes
1
answer
91k
views
Why does NASA use gold foil on equipment and gold-coated visors?
I've read several websites about equipment covered with gold foil and astronaut helmet visors are coated with gold. However, their explanations are devoid of almost all physics content. Can someone ...
- 1,583
176
votes
3
answers
46k
views
How does a knife cut things at the atomic level?
As the title says. It is common sense that sharp things cut, but how do they work at the atomical level?
- 1,997
175
votes
7
answers
14k
views
Why do we actually see the sun?
I haven't yet gotten a good answer to this: If you have two rays of light of the same wavelength and polarization (just to make it simple for now, but it easily generalizes to any range and all ...
user22862
174
votes
21
answers
12k
views
How do you make more precise instruments while only using less precise instruments?
I'm not sure where this question should go, but I think this site is as good as any.
When humankind started out, all we had was sticks and stones. Today we have electron microscopes, gigapixel cameras ...
- 2,733
174
votes
6
answers
28k
views
Why do shadows from the sun join each other when near enough?
I was laying on my bed, reading a book when the sun shone through the windows on my left. I happened to look at the wall on my right and noticed this very strange effect. The shadow of my elbow, when ...
- 3,558
173
votes
14
answers
74k
views
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 ...
- 5,177
167
votes
2
answers
37k
views
Why do sunbeams diverge even though the sun is much more than a few kilometers away?
Consider this picture of sun beams streaming onto the valley through the clouds.
Given that the valley is only (at a guess) 3km wide, with simple trigonometry and the angles of the beams, this gives ...
user56903
167
votes
9
answers
24k
views
Could Legolas actually see that far?
The video “How Far Can Legolas See?” by MinutePhysics recently went viral. The video states that although Legolas would in principle be able to count $105$ horsemen $24\text{ km}$ away, he shouldn't ...
- 5,865
165
votes
11
answers
16k
views
What makes a theory "Quantum"?
Say you cook up a model about a physical system. Such a model consists of, say, a system of differential equations. What criterion decides whether the model is classical or quantum-mechanical?
None ...
165
votes
15
answers
41k
views
What is a field, really?
There was a reason why I constantly failed physics at school and university, and that reason was, apart from the fact I was immensely lazy, that I mentally refused to "believe" more advanced ...
- 1,809
160
votes
9
answers
37k
views
Does someone falling into a black hole see the end of the universe?
This question was prompted by Can matter really fall through an event horizon?. Notoriously, if you calculate the Schwarzschild coordinate time for anything, matter or light, to reach the event ...
- 343k
159
votes
3
answers
35k
views
Why does Stephen Hawking say black holes don't exist?
Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he ...
- 1,489
157
votes
27
answers
23k
views
Simple check for the global shape of the Earth
I have been on a date recently, and everything went fine until the moment the girl has told me that the Earth is flat. After realizing she was not trolling me, and trying to provide her with a couple ...
- 1,421
157
votes
9
answers
25k
views
Why are the wet patches on these floor tiles circular?
My friend's 3-year-old daughter asked "Why are there circles there?"
It had either rained the night before or frost had thawed. What explains the circles?
Follow-up question: Ideally, are these ...
- 1,515