# All Questions

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### Ideal four-level Gain Medium (or just any old gain medium)

What exactly is happening in a laser's gain cell when the irradiance is greater than the saturation irradiance? Also can someone offer a clear conceptual treatment of the gain-coefficient and its ...
672 views

### Missing transverse energy, exact definition

This might seem basic, but it is a bit confusing. You hear about missing transverse energy a lot in SUSY searches due to the LSP which cannot be detected. Let's say I have the 4-vector for the LSP. ...
146 views

### Black out the sun?

How many observers would it take to cancel out the number of photons produced in a medium sized star like our Sun, thus making it dark? (I want to consume more photons than the star is generating)
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### Minkowski diagram and length contraction

The length contraction means that an object is the longest in the frame in which it is at rest. Lets assume i have a meter stick with length $\Delta x$ in my rest frame which is $x,ct$ and i want to ...
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### Is there perfect vacuum 10000 billion lightyears away?

Is there perfect vacuum about some 10,000 billion light years away? With perfect vacuum, I mean that there are no particles, not even virtual photons! I had this idea by assuming that all particle ...
1k views

### Find total energy and momentum of an moving electron in a rest frame

I have an electron moving with speed $u'$ in a frame $S'$ moving with speed $v'$ relative to a rest frame $S$. How do I find the total energy and momentum of the electron in the rest frame $S$? I ...
12k views

### Why does the balloon pop?

When we pierce a balloon with a sharp needle, it pops and produce a great sound. But, It doesn't pop when we open the mouth of the balloon (through which we have blown air)... So, Why doesn't the gas ...
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1k views

### Free falling objects

My teacher and I are in the middle of an argument because she says that if you were to drop two objects at the same time and the same height, but with different initial velocities, both of them would ...
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### What is the pressure on an empty pneumatic tube?

At the residence I'm constructing a kitchen digester. The gas is to be collected into an old bicycle/scooter tube - initially as empty as may be after wringing it out.. Will the digester (total ...
232 views

### How many pieces of toast would you need to make a black hole?

I am trying to find the following: How many pieces of toast would you need to make a black hole? From what I've learnt so far I need to find an equation for the compression force the massive amount ...
2k views

### Why does an infrared thermometer display very low temperature when being directed to the outer air?

I'm toying with an infrared thermometer - one which you point onto an object, press the button and it instantly measures the temperature by estimating the infrared radiation from the object. It shows ...