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1500 questions
37
votes
3 answers
Can quantum entanglement travel faster than the speed of light?
Recently I was watching a video on quantum computing where the narrators describe that quantum entanglement information travels faster than light!
Is it really possible for anything to move faster than light? Or are the narrators just wrong?
Vineet Menon
- 2,863
37
votes
5 answers
What do we see while watching light? Waves or particles?
I'm trying to understand quantum physics. I'm pretty familiar with it but I can't decide what counts as observing to cause particle behave (at least when it's about lights). So the question is what do we see with our eye-balls?
We point a laser (or…
martintrapp
- 505
37
votes
5 answers
Is it possible to focus the radiation from a black body to make something hotter than that black body?
My previous question wasn't specific enough. I'll try to be more specific.
Let's imagine we have a hot body let's say 6000K hot that emits lots of thermal radiation. Let's assume 1kW of radiative power falls on on a m². Now if we use a lens with 1m²…
Calmarius
- 8,428
37
votes
6 answers
Motion described by $m \frac{\mathrm{d}^2 x}{\mathrm{d}t^2}=-k\frac{\mathrm{d}^{\frac12 }x}{\mathrm{d}t^{\frac12}}$
What kind of motion would a (preferably dimensionless for simplicity) body do if the force acted on it was proportional to the semi-derivative of displacement, i.e.
$$m \frac{\mathrm{d}^2…
user148432
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37
votes
4 answers
Any good resources for Lagrangian and Hamiltonian Dynamics?
I'm taking a course on Lagrangian and Hamiltonian Dynamics, and I would like to find a good book/resource with lots of practice questions and answers on either or both topics.
So far at my university library, I have found many books on both…
JackReacher
- 201
37
votes
6 answers
Blowing your own sail?
How it this possible? Even if the gif is fake, the Mythbusters did it and with a large sail it really moves forward. What is the explanation?
mathematiccian
- 479
37
votes
8 answers
Is the Big Bang defined as before or after Inflation?
Is the Big Bang defined as before or after Inflation? Seems like a simple enough question to answer right? And if just yesterday I were to encounter this, I'd have given a definite answer. But I've been doing some reading while writing up my thesis…
Jim
- 24,676
37
votes
5 answers
Why can't we feel the Earth turning?
The Earth turns with a very high velocity, around its own axis and around the Sun.
So why can't we feel that it's turning, but we can still feel earthquake.
lamwaiman1988
- 519
37
votes
5 answers
Is the graviton hypothetical?
Wikipedia lists the graviton as a hypothetical particle. I wonder whether graviton is indeed hypothetical or does its existence directly follow from modern physics? Does observation of gravitational waves amount to the discovery of graviton or could…
Anixx
- 11,524
37
votes
7 answers
Why does a system try to minimize potential energy?
In mechanics problems, especially one-dimensional ones, we talk about how a particle goes in a direction to minimize potential energy. This is easy to see when we use cartesian coordinates: For example, $-\frac{dU}{dx}=F$ (or in the multidimensional…
user35734
- 795
37
votes
7 answers
If we suddenly lost track of time, how would we know what time is now?
If we hypothetically lost all watches and all devices that keep track of time, how would we say what is the current time? Or we actually don't and time is just a convention?
matt_zarro
- 489
37
votes
3 answers
Do photons occupy space?
Total noob here.
I realize that photons do not have a mass. However, they must somehow occupy space, as I've read that light waves can collide with one another.
Do photons occupy space? and if so, does that mean there is a theoretically maximum…
LanceLafontaine
- 799
37
votes
3 answers
Best Sets of Physics Lecture Notes and Articles
This post is inspired by this math.se post.
Let me start by apologizing if there is another thread on phys.se that subsumes this.
I often find that I learn best from sets of lecture notes and short articles. There are three particular reasons that…
Hakim
- 1,031
36
votes
1 answer
How to define orbital angular momentum in other than three dimensions?
In classical mechanics with 3 space dimensions the orbital angular momentum is defined as
$$\mathbf{L} = \mathbf{r} \times \mathbf{p}.$$
In relativistic mechanics we have the 4-vectors $x^{\mu}$ and $p^{\mu}$, but the cross product in only defined…
asmaier
- 10,250
36
votes
7 answers
Is it possible to "cook" pasta at room temperature with low enough pressure?
It is known fact, that boiling point of water decreases by decreasing of pressure. So there is a pressure at which water boils at room temperature.
Would it be possible to cook e.g. pasta at room temperature in vacuum chamber with low enough…
Viliam
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