# All Questions

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### Variable wavelength LED

Does there exist a type of LED (or other type of light source) that can change it's wavelength depending on the current/voltage it receives on a second wire? If not, would something like this even be ...
11 views

### Numerical of capacitance [on hold]

what is the equivalent capacitance between A and B ?
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### The difference between $R(r)$ vs $r$ graph for $4s$ and $5p$ orbital

Both orbitals have equal number of radial nodes (number of radial nodes = $n-l-1=3$). So the curve cuts the $x$-axis in 3 points. If it was $\psi^2$ vs $r$, then the curve would touch the $x$-axis at ...
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### For infinitesimal Canonical Transformations, what functions are allowed for this to be a canonical transformation?

Consider two infinitesimal transformation: $$q_{i} \rightarrow Q_{i} =q_{i} + \alpha F_{i}(q,p)$$ $$p_{i} \rightarrow P_{i} = p_{i} + \alpha E_{i}(q,p)$$ where $α$ is considered to be ...
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### Do we regularly measure and update our physical constants (By using the LHC)?

This question is motivated by sheer curiosity. I certainly do not expect that the free parameters we use in the standard model have changed in value since we started measuring them with a "modern" ...
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### Can you tell your absolute speed in space?

Normally in relativity your speed can only be known relative to another object, given that as one approaches light speed more energy is required to accelerate faster, based on the energy consumption ...
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### Can matter be formed from nothing?

Well, I wanted to know whether matter can be created from nothing? Could matter be created in pure vacuum or does it require some energy? If energy is required, How is energy converted to matter? If ...
3k views

### Are Hubble Telescope Images in true color?

Like many others, I have marveled at the images made available from the Hubble Space Telescope over the years. But, I have always had a curiosity about the color shown in these images. An example is ...
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### If I touch an object, am I touching the atoms on its surface? [duplicate]

If I hit an object with a pen for example, does the pen touch the atoms on the surface of the object? Won't it damage the atoms? If I can't touch it, then where does the sound come from?
22 views

### Planetary alignment of the gas giants?

There is this nice and new applet of the orbits of the planets around the sun: Solar System Orrery You can click and drag any planet and watch their relative orbits evolve in time. I was curious ...
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### Mechanism for visible light frequency mixing in storm clouds

So I know that when red and blue light (or the frequencies/wavelengths we percieve as such) hit our eyes with the correct proportions, our eyes and brains interpret that as the color purple. In ...
59 views

### With respect to what does time itself change? [duplicate]

We all have an intuitive notion that time itself "changes" - before it was the past, now it is the present, later it will be the future, and as time "changes" or moves forward what was the present ...
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### Can a nozzle increase the thrust of a water rocket?

First of all, what is the correct equation to determine the thrust produced by a water rocket? $F_T = ṁ * V_e$ (1) $F_T = ṁ * V_e + (p_e - p_0)A_e$ (2) Using the first equation it should be ...
4k views

### Do all the conservation laws of Physics take no time to propagate?

For instance, conservation of momentum, does it take time to propagate between two or more objects? If it does, then there would be some moment that the momentum is not conserved. If it doesn't take ...
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### Planetary Gravity and its effects [duplicate]

I live in Tunisia, exactly in Gafsa .. and there's a weird effect is happening, i really tried to ask people but no one had a clue, also i have seen that in other places on the internet. If you put a ...
23 views

### What's the best way to avoid slipping on ice? [migrated]

Where I live, winter is still a good bit away, but last year I slipped on ice and ended up with a black eye and a broken wrist. This year I sincerely want to reduce the chances of that recurring. I ...
52 views

### Quantum Gravity [on hold]

By the Pauli exclusion principle fermions cannot occupy the same space, cannot have identical quantum numbers. The bosons however can. We also know that the universe, and in particular the various ...
25 views

### How much do we know about Quantum Gravity at energies larger than Planck energy?

What kind of predictions do current approaches to quantum gravity (superstrings/M-theory, loop quantum gravity,...) make at energies LARGER than Planck scale? Or is that a meaningless question? Does ...
31 views

### Why topological strings have to be closed or infinite?

Let's assume spontaneously broken global $U(1)$ group. During phase transition global topological strings are formed. Why they have to be infinite or closed (there doesn't exist finite strings)?
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### Thomas precession, Lie algebra of the Lorentz group and the conservation of energy

If you read this post Thomas Precession, you will see a very good answer by WetSavannaAnimal, on the subject of Thomas Precession, which I am currently working my through, in conjunction with some ...
53 views

### Why is there no change in speed of a charged particle in a uniform perpendicular magnetic field?

When the initial velocity is perpendicular to the magnetic field, the force acting on the charged particle F = qvBsin(theta) ...
29 views

### Is circular masking a laser beam an acceptable way to get the beam diameter you require?

I am wondering what are the down sides to just masking a laser to get the beam diameter you require? For example if you have a collimated eliptical beam of say 2mm width and 4mm height, but you ...
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### Blazed phase profile to generate laguerre gaussian beam

How to add blazed grating to a fork grating to achieve maximum diffraction efficiency in the first order while generating laguerre gaussian beam?
63 views

### In QFT do we always use normal-ordered Hamiltonian? [duplicate]

In quantization of the Dirac field I learned that we use normal ordering to get rid of negative energy vacuum state. From this point is there any reason we would use original Hamiltonian?
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### Should i buy National Geographic Telescope NG 114 / 500mm DOBSONIAN [on hold]

I want to buy my first telescope. And want to go forNational Geographic Telescope NG 114 / 500mm DOBSONIAN.Is it a good telescope? What can i see with it? It has 114 mm. Aperture. It is coming for ...
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### Modelling discrete spacetime

Supposed space and time were to be discrete, then how would i go about modelling this inside a computer simulation? In a simple 2D world, taking a square for example with side length $A$, then if ...
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### How is classical information put into and retrieved from a quantum computer?

How, if you had to find the factors of say a 256 digit number, would you go about inputting the data and how do the proper answers "drop out" of a quantum computer. Other than terms such as qubits, ...
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### Why we can't define energy in accelerating frame?

In my old unanswered question about accelerating frame, I have found (later after editing that question), that one of the keys is that energy is not defined in accelerating frame. What I do not ...
81 views

### Why can't people just pump water from ground and flood deserts to make them habitable? [on hold]

I know this may be the wrong site, but I'm a curious man and I want to solve world's problems. One of big problems is deserts. They're so dry that nothing grows there. I was thinking about this and ...
29 views

### Finding the V(speed) of a 12KG bucket in pulley with the law of conversation of energy [on hold]

A system of two paint buckets connected by a lightweight rope is released from rest with 12kg bucket 2m above the ground floor. Use the principle of conservation of energy to find the speed with which ...