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1500 questions
93
votes
9 answers
What really causes light/photons to appear slower in media?
I know that if we solve the Maxwell equation, we will end up with the phase velocity of light being related to the permeability and the permittivity of the material. But this is not what I'm interested in - I want to go deeper than that. We know…
Emitabsorb
- 2,502
93
votes
8 answers
Why is the application of probability in Quantum Mechanics fundamentally different from application of probability in other areas?
Why is the application of probability in Quantum Mechanics (QM) fundamentally different from its application in other areas? QM applies probability according to the same probability axioms as in other areas of physics, engineering, etc.
Why is there…
Nikos M.
- 5,302
93
votes
6 answers
What is spin as it relates to subatomic particles?
I often hear about subatomic particles having a property called "spin" but also that it doesn't actually relate to spinning about an axis like you would think. Which particles have spin? What does spin mean if not an actual spinning motion?
Nick
- 1,457
92
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
- 1,007
92
votes
4 answers
What is the mechanism behind the slowdown of light/photons in a transparent medium?
So light travels slower in glass (for example) than in a vacuum. What causes light to slow down? Or: How does it slow down? If light passes through the medium, is it not essentially traveling in the "vacuum" between the atoms?
Henry
- 1,133
92
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
- 1,431
92
votes
5 answers
How do laser "tape measures" work?
There is a device available for about $\$40$, which fits in the palm of the hand, runs on two AA batteries, and can measure distances up to $50\,{\rm ft}$ to an accuracy of $\sim \frac{1}{8}''$ ($\sim 3\,{\rm mm}$).
Light travels $300,000\,{\rm…
Anthony X
- 3,666
91
votes
9 answers
Why doesn't the Moon fall onto the Earth?
Why doesn't the Moon fall onto the Earth? For that matter, why doesn't anything rotating a larger body ever fall onto the larger body?
Adir Peretz
- 1,061
91
votes
2 answers
Why is a laserpointer able to "erase" a glow-in-the-dark sticker?
a while ago I tried to charge a glow-in-the-dark sticker using a simple red laser pointer. It was a large sticker, of the type used to mark emergency exits and fire extinguishers here in Germany. I thought I could draw a picture this way, with…
Toastor
- 791
91
votes
16 answers
What are examples of Lagrangians that not of the form $T-U$?
My Physics teacher was reluctant to define Lagrangian as Kinetic Energy minus Potential Energy because he said that there were cases where a system's Lagrangian did not take this form. Are you are aware of any such examples?
Update: Here I'm of…
ZAC
- 1,367
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
- 18,043
91
votes
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
- 53,961
91
votes
3 answers
How does this baby rattle work?
Here's a gif showing how the balls move when I move the rattle. The circular tube hangs vertically, with the balls on the bottom. There are more images in the bottom.
The balls roll freely inside the tube
The inner diameter of the tube is larger…
Stewie Griffin
- 823
91
votes
11 answers
Why must a physical theory be mathematically self-consistent?
I always read in modern physics textbooks and articles about the need for physical theories to be mathematically self-consistent, which implies that the theories must not produce contradictions or anomalies. For example, string theorists are proud…
Maxis Jaisi
- 1,311
91
votes
11 answers
What is the physical significance of dot & cross product of vectors? Why is division not defined for vectors?
I get the physical significance of vector addition & subtraction. But I don't understand what do dot & cross products mean?
More specifically,
Why is it that dot product of vectors $\vec{A}$ and $\vec{B}$ is defined as $AB\cos\theta$?
Why is it…
claws
- 7,493