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1500 questions
92
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20 answers
What software programs are used to draw physics diagrams, and what are their relative merits?
Undoubtedly, people use a variety of programs to draw diagrams for physics, but I am not familiar with many of them. I usually hand-draw things in GIMP which is powerful in some regards, but it is time consuming to do things like draw circles or…
Mark Eichenlaub
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92
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Orbits of maximally entangled mixed states
It is well known (Geometry of quantum states by Bengtsson and Życzkowski) that the set of $N$-dimensional density matrices is stratified by the adjoint action of $U(N)$, where each stratum corresponds to orbits with a fixed type of degeneracy…
David Bar Moshe
- 30,503
92
votes
7 answers
If dark matter only interacts with gravity, why doesn't it all clump together in a single point?
I'm a complete layperson. As I understand, dark matter theoretically only interacts with the gravitational force, and doesn't interact with the other three fundamental forces: weak nuclear force, strong nuclear force, and electromagnetism.
Those are…
user151841
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92
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9 answers
Are matrices and second rank tensors the same thing?
Tensors are mathematical objects that are needed in physics to define certain quantities. I have a couple of questions regarding them that need to be clarified:
Are matrices and second rank tensors the same thing?
If the answer to 1 is yes, then…
Revo
- 16,956
92
votes
9 answers
Why is there a controversy on whether mass increases with speed?
Some people say that mass increases with speed while others say that the mass of an object is independent of its speed.
I understand how some (though not many) things in physics are a matter of interpretation based on one's definitions. But I can't…
User 17670
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91
votes
12 answers
What makes running so much less energy-efficient than bicycling?
Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why?
According to the law of conservation of energy, bicycling should be more intensive because you have to move a higher mass, requiring more kinetic energy to…
Steven Roose
- 997
91
votes
10 answers
Why can't the Schrödinger equation be derived?
Honestly, I don't get it. People say it's because it's a postulate. But, I mean, I see people deriving the Schrödinger equation with the help of the wave function, $T+U$ and partial differentials in three space coordinates and one time coordinate.…
anon
91
votes
7 answers
Why does a billiard ball stop when it hits another billiard ball head on?
(I'm repeating myself a lot here, but it's because I want to make my confusion clear.)
If 2 billiard balls are the same exact mass, and one hits another stationary one head on, I have heard that the hitting ball will often stop entirely while the…
joshuaronis
- 3,055
91
votes
10 answers
Is there a symmetry associated to the conservation of information?
Conservation of information seems to be a deep physical principle.
For instance, Unitarity is a key concept in Quantum Mechanics and Quantum Field Theory.
We may wonder if there is an underlying symmetry, in some space, which may explain this…
Trimok
- 17,567
91
votes
10 answers
Why do scientists think that all the laws of physics that apply in our galaxy apply in other galaxies?
I like watching different videos about space. I keep seeing all these videos saying scientists found so and so at 200 billion light years away or this happened 13 billion years ago.
My question is why do scientists think that all the physics that…
andre chancellor
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91
votes
11 answers
Quantum Field Theory from a mathematical point of view
I'm a student of mathematics with not much background in physics. I'm interested in learning Quantum field theory from a mathematical point of view.
Are there any good books or other reference material which can help in learning about quantum field…
user774025
- 1,403
91
votes
9 answers
Why does matter exist in 3 states (liquids, solid, gas)?
Why does matter on the earth exist in three states? Why cannot all matter exist in only one state (i.e. solid/liquid/gas)?
Kiran Kumar
- 987
91
votes
6 answers
Am I attracting Pluto?
My question is simple: as the title says, do I exert a gravitational force on distant objects, for example, Pluto? Although it is a very small force, it is there, right?
This leads me to the question, am I exerting a gravitational force on…
Antonio Aguilar
- 1,309
91
votes
1 answer
Why does my wooden door disperse light into a rainbow color spectrum?
On a clear morning, light comes through this window (viewed from the inside)
It then hits the door on the opposite side (so viewing from the outside the door straight on the other side)
There is nothing particularly special going on.
However, if…
Zach Saucier
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90
votes
10 answers
What is a complete book for introductory quantum field theory?
There's a fairly standard two or three-semester curriculum for introductory quantum field theory, which covers topics such as:
classical field theory background
canonical quantization, path integrals
the Dirac field
quantum…
Andy Bale
- 1,967