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1500 questions
35
votes
7 answers
Why are leap seconds needed so often?
In Coordinated Universal Time (UTC), leap seconds are added to account for the slowing down of Earth's rotation. But the slowing down is said to be of the order of milliseconds in a century. Then why there were more than 25 leap seconds added to UTC…
dsvthampi
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35
votes
3 answers
Clarifying the actual definition of elasticity. Is steel really more elastic than rubber?
Yes, I know it's steel. It's everywhere on the web and I did google. But I seek enlightenment.
My physics textbook defines elasticity as:
Property by virtue of which a material regains its shape.
Or, the ability of material to resist change in…
Sarthak123
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35
votes
4 answers
What really is a fluid?
My question may be pretty basic, but I feel it is important to ask this as I've gone through several texts and none offer me the clarity I seek.
The question is: What is a fluid? What is flow? If we say that a fluid is something that flows, the next…
stoic-santiago
- 1,881
35
votes
1 answer
Intuition for when the replica trick should work and why it works
I am a graduate student in mathematics working in probability (without a very good background in physics honestly) and I've started to see arguments based on computations derived from the replica trick. I understand that it is non-rigorous but it…
Chris Janjigian
- 593
35
votes
2 answers
Why does the Earth even have a magnetic field?
From my knowledge of magnetism, if a magnet is heated to a certain temperature, it loses its ability to generate a magnetic field. If this is indeed the case, then why does the Earth's core, which is at a whopping 6000 °C — as hot as the sun's…
Rumplespaceking
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35
votes
5 answers
How were the Navier-Stokes equations found in the first place if we can't solve them?
I was reading up on the Clay Institute's Millenium prizes in mathematics.
And I noticed the Navier-Stokes equations were described as minimally understood.
As far as I was taught in physics a few weeks ago(SCQF Level 6), they are used but solutions…
Disgusting
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35
votes
3 answers
Does QED really break down at the Landau pole?
In QED, the fine structure constant $\alpha$ runs upwards in the UV, with a loop calculation (involving a geometric series of the vacuum polarisation diagram) indicating a divergence in $\alpha$ at $\sim 10^{286}\,\text{eV}$. It is often claimed…
gj255
- 6,375
35
votes
3 answers
Why does snow stay after a snowfall?
I have just experienced a snowfall and I am not so clear on how it works.
Three days after a short day of snowfall, and having 2 min | 17 max degrees Celsius, full sunny scarcely cloudy each day, there is still some snow persisting in shadow and…
Brethlosze
- 715
35
votes
6 answers
Why can't we produce gravitational waves in the lab?
Fairly straightforward. I just thought it wouldn't be too hard to produce a ripple in spacetime several times smaller than a proton radius in a particle accelerator or something. It seems like it should be going on all the time.
Sam Cottle
- 1,544
35
votes
3 answers
Why does a blackboard dry very quickly?
When you have made some stupid mistakes on a blackboard, you quickly want to erase it with a wet sponge before anyone sees them. So you clean the blackboard and within a minute the blackboard is clean and dry again!
I was wondering why the board is…
SjonTeflon
- 2,229
35
votes
7 answers
Is the light from the Sun the same as the light from a bulb?
I'm arguing with a friend of mine on whether the light emitted from the sun is of the same type of that emitted by a bulb. Her insistent ignorance is laughable, unless I'm wrong...
She's talking about how light from the bulb is "artificial" ...
I've…
Tobi
- 1,819
35
votes
11 answers
Why can't the outcome of a QM measurement be calculated a-priori?
Quantum Mechanics is very successful in determining the overall statistical distribution of many measurements of the same process.
On the other hand, it is completely clueless in determining the outcome of a single measurement. It can only describe…
Sklivvz
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35
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2 answers
Is there any hard scientific evidence that the alpha particle is tetrahedral?
I'm writing a piece on the nuclear force, and I'm struggling with something. I always thought of the alpha particle as something with a tetrahedral disposition. If you search the internet on this there's plenty of hits. Ditto if you search for…
John Duffield
- 11,097
35
votes
2 answers
Basic buoyancy question: Man in a boat with a stone
This comes from a brain teaser but I'm not sure I can solve it:
You are in a rowing boat on a lake. A large heavy rock is also in the boat. You heave the rock overboard. It sinks to the bottom of the lake. What happens to the water level in the…
lezebulon
- 453
35
votes
5 answers
Do massless particles really exist?
I was in doubt, so I went to wikipedia. There it says "the photon has zero rest mass", but on the side description it says the mass is $<1.10^{-18} \:\mathrm{eV}/c^2$. So is the mass of the photon really zero or do we just consider it to be zero…
Patrick
- 1,835