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How close does a particle-antiparticle pair need to be for annihilation to happen?
I've most often seen the statement that the annihilation of a particle and its antiparticle occurs when they 'collide' with one another. So in other words when they get very close to one another right?
How close do they need to be (for annihilation…
docscience
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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 any time at all, since the law itself is…
ayuanx
- 587
40
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2 answers
In atomic bomb tests under ground, where does the displaced volume of the rocks go?
Underground atomic bomb tests are done in a deep, sealed hole. Not all underground tests eject material on the surface. In this case, they are only noticeable as earthquakes, according to German Wikipedia on nuclear tests. There seems to be no…
Volker Siegel
- 3,883
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2 answers
How to take partial trace?
$L$ is a linear operator acting on hilbert space $V$ of dimension $n$, $L: V \to V$. The trace of a linear operator is defined as sum of diagonal entries of any matrix representation in same input and output basis of $V$. But if $L$ is a linear…
advocateofnone
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40
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6 answers
Is the spring constant $k$ changed when you divide a spring into parts?
I've always been taught that the spring constant $k$ is a constant — that is, for a given spring, $k$ will always be the same, regardless of what you do to the spring.
My friend's physics professor gave a practice problem in which a spring of…
nhinkle
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4 answers
Why are stars white?
That is may be an easy question, but I am not a professional. The Sun is a star, and when I look at the Sun it is usually yellow. Why are stars in the sky at night white?
I suppose it could be due to their distance. What is the explanation?
Alejandro
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40
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4 answers
Is there an infinite amount of wavelengths of light? Is the EM spectrum continuous?
The electromagnetic spectrum is a continuum of wavelengths of light, and we have labels for some ranges of these and numerical measurements for many.
Question: Is the EM spectrum continuous such that between two given wavelengths (e.g. 200nm and…
toothandsticks
- 597
40
votes
3 answers
What are galactic speeds measured against?
The Earth moves through space at 67,000 MPH. The Milky Way travels through a local group at 2,237,000 MPH.
Wouldn't you need a fixed point to be able to measure velocity against? After all, compared to the total speed of our Milky Way, the Earth…
Sean Long
- 503
40
votes
3 answers
Why did "tickling the dragons tail" by Louis Slotin not cause an explosion?
I have been reading the excellent Command and Control by Eric Schlosser and discovered more about Louis Slotin's experiment with "tickling the dragons tail" and the infamous Demon Core.
What I don't understand; and please excuse my naivety, is when…
dooburt
- 503
40
votes
8 answers
"Falling upward" - how far you have to be from Earth to start falling to the Moon?
Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus makes you accelerate towards it, and how to get…
rafb3
- 525
40
votes
3 answers
How can interstellar space have a temperature of 2-3K?
Several different sources online state that the average temperature of interstellar space (or the universe in general) is around 2-3K.
I learned that temperature is basically the wiggling of matter, and I find it somewhat counterintuitive that the…
Phaptitude
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votes
6 answers
Which ball touches the ground first?
This is a very well known problem, but I can't find an answer in the specific case I'm looking for.
Let's consider two balls :
Ball 1 weighs 10 kg
Ball 2 weighs 1 kg
Balls have identical volumes (so Ball 1 is much more dense)
Balls have identical…
FlipFlapFlop
- 503
40
votes
6 answers
If there were fundamental forces weaker than gravity, would we know about it?
We know that gravity is a very weak force compared to electromagnetic forces and the nuclear forces. We know about the other forces because they're necessary to explain atoms, and we can detect gravity easily, because unlike the other forces it is…
N. Virgo
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8 answers
Is (rest) mass quantized?
I learned today in class that photons and light are quantized. I also remember that electric charge is quantized as well. I was thinking about these implications, and I was wondering if (rest) mass was similarly quantized.
That is, if we describe…
Justin L.
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6 answers
How can the Earth keep spinning with a liquid core?
In regards to the 'conservation of angular momentum' being the explanation of why celestial objects spin...
If you fill a ball or any other container with a liquid and try to spin it, you will not see any more than 5 or 6 revolutions because of the…
user50507
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