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1500 questions
115
votes
12 answers
How can time dilation be symmetric?
Suppose we have two twins travelling away from each other, each twin moving at some speed $v$:
Twin $A$ observes twin $B$’s time to be dilated so his clock runs faster than twin $B$’s clock. But twin $B$ observes twin $A$’s time to be dilated so…
John Rennie
- 367,598
115
votes
6 answers
Could we send a man safely to the Moon in a rocket without knowledge of general relativity?
I'm wondering if it's possible to send a man to the Moon using equations consistent with Newtonian gravity and without the elaborate tools of Einstein gravity. Are the predictions made by Newtonian gravity sufficiently precise to plan a successful…
Dargscisyhp
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114
votes
5 answers
What is the actual significance of the amplituhedron?
The news that physicists have discovered a geometrical object that simplifies a lot our models of quantum physics has recently became viral. For an outsider like me, it is difficult to actually understand the significance of this finding.
Is it…
MaiaVictor
- 1,992
114
votes
7 answers
What do spacelike, timelike and lightlike spacetime interval really mean?
Suppose we have two events $(x_1,y_1,z_1,t_1)$ and $(x_2,y_2,z_2,t_2)$. Then we can define
$$\Delta s^2 = -(c\Delta t)^2 + \Delta x^2 + \Delta y^2 + \Delta z^2,$$
which is called the spacetime interval. The first event occurs at the point with…
Gold
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113
votes
1 answer
Why exactly do sometimes universal covers, and sometimes central extensions feature in the application of a symmetry group to quantum physics?
There seem to be two different things one must consider when representing a symmetry group in quantum mechanics:
The universal cover: For instance, when representing the rotation group $\mathrm{SO}(3)$, it turns out that one must allow also…
ACuriousMind
- 132,081
113
votes
8 answers
Thought experiment - would you notice if you fell into a black hole?
I've heard many scientists, when giving interviews and the like, state that if one were falling into a black hole massive enough that the tidal forces at the event horizon weren't too extreme, that you wouldn't "notice" or "feel" anything, and so…
alzee
- 1,274
112
votes
15 answers
About the complex nature of the wave function?
1.
Why is the wave function complex? I've collected some layman explanations but they are incomplete and unsatisfactory. However in the book by Merzbacher in the initial few pages he provides an explanation that I need some help with: that the de…
yayu
- 4,982
112
votes
5 answers
How and why do accelerating charges radiate electromagnetic radiation?
Let's consider it case by case:
Case 1: Charged particle is at rest. It has an electric field around it. No problem. That is its property.
Case 2: Charged particle started moving (it's accelerating). We were told that it starts radiating EM…
claws
- 7,493
112
votes
19 answers
What is a good introductory book on quantum mechanics?
I'm really interested in quantum theory and would like to learn all that I can about it. I've followed a few tutorials and read a few books but none satisfied me completely. I'm looking for introductions for beginners which do not depend heavily on…
PhaDaPhunk
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112
votes
17 answers
What is the proper way to explain the twin paradox?
The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible.
There are a thousand explanations out there for why this doesn't happen, but they all end up…
John Rennie
- 367,598
112
votes
8 answers
Is it possible to start fire using moonlight?
You can start fire by focusing the sunlight using the magnifying glass.
I searched the web whether you can do the same using moonlight. And found this and this - the first two in Google search results.
What I found is the thermodynamics argument:…
Calmarius
- 8,428
111
votes
4 answers
Why does a sticker slowly peel off, but if it is pulled quickly it tears?
Anyone who has removed a sticker, knows that often they must be pulled off slowly, otherwise they tear. Why is this?
Ambrose Swasey
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111
votes
15 answers
Why quantum mechanics?
Imagine you're teaching a first course on quantum mechanics in which your students are well-versed in classical mechanics, but have never seen any quantum before. How would you motivate the subject and convince your students that in fact classical…
Jonathan Gleason
- 8,834
111
votes
7 answers
How can we see an atom now? What was the scale of this equipment?
I've just seen this on the news - Single Trapped Atom Captures Science Photography Competition's top prize.
Credit: David Nadlinger via EPSRC
I am not a Physics major but I believe I do know the basics. I have always believed that we can't really…
Hanky Panky
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110
votes
7 answers
Why do tuning forks have two prongs?
I believe the purpose of a tuning fork is to produce a single pure frequency of vibration. How do two coupled vibrating prongs isolate a single frequency? Is it possible to produce the same effect using only 1 prong? Can a single prong not…
acadien
- 1,060