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1500 questions
202
votes
16 answers
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 been writing a c++ dimensional analysis library…
Nicolas Holthaus
- 1,931
198
votes
15 answers
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 expanding when we talk about the speed of galaxies…
SoulmanZ
- 2,095
193
votes
2 answers
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 seen (see Bailin and Love, Supersymmetry) that we…
James
- 2,941
190
votes
5 answers
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 of a pseudo-random quantum circuit. Random…
Bridgeburners
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- 2
- 13
- 17
189
votes
8 answers
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 near the pages of the book, joined up with the…
turnip
- 3,768
188
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,…
SMUsamaShah
- 5,507
184
votes
5 answers
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 calculations.
Regarding this I have a few questions:
Why…
Revo
- 17,504
183
votes
3 answers
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?
wtoh
- 2,087
- 2
- 14
- 9
181
votes
7 answers
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 polarizations) meet at a point such that they're 180…
user22862
180
votes
2 answers
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 cookie sides.
Jiminion
- 2,673
179
votes
15 answers
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 stuff until I understand the fundamentals (which I,…
Dan
- 1,949
178
votes
1 answer
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 explain the basic concept of why gold foil is so…
Smith
- 1,603
178
votes
21 answers
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 and atomic clocks. These instruments are many…
Vilx-
- 3,501
173
votes
2 answers
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 the result that the position of the light source…
user56903
170
votes
9 answers
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 horizon the result is infinite. This implies that the…
John Rennie
- 367,598