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1500 questions
78
votes
4 answers

What reference clock is an atomic clock measured against?

I looked at a few of the other posts regarding the accuracy of atomic clocks, but I was not able to derive the answer to my question myself. I've seen it stated that atomic clocks are accurate on the order of $10^{-16}$ seconds per second. However,…
zrbecker
  • 937
78
votes
7 answers

How does faster than light travel violate causality?

Let's say I have two planets that are one hundred thousand lightyears away from each other. I and my immortal friend on the other planet want to communicate, with a strong laser and a tachyon communication device. I record a message on the tachyon…
markovchain
  • 1,501
78
votes
14 answers

Does throwing a penny at a train stop the train?

If I stand in front of a train and throw a penny at it, the penny will bounce back at me. For the penny to reverse its direction, at some point its velocity must go to zero. This is the point it hits the train. Two objects in contact have the same…
Whose
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78
votes
4 answers

How exactly is a normal force exerted, at the molecular level?

I've been surfing the web for quite a while, finding the answers I would need, but couldn't find a convincing one. First of all I need to remind you that this a very long/continuous question, so please kindly take your time. I'll provide some…
user226894
78
votes
2 answers

Can an atom orbit the Sun?

Is it possible for a molecule or atom to orbit a star (e.g. the Sun)? Or is there always too much outward force imparted by solar radiation compared to the inward force of gravitational attraction?
Nicole Sharp
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78
votes
6 answers

Is the total energy of the universe zero?

In popular science books and articles, I keep running into the claim that the total energy of the Universe is zero, "because the positive energy of matter is cancelled out by the negative energy of the gravitational field". But I can't find anything…
user1265
78
votes
5 answers

What is information?

We're all familiar with basic tenets such as "information cannot be transmitted faster than light" and ideas such as information conservation in scenarios like Hawking radiation (and in general, obviously). The Holographic Principle says, loosely,…
Mitchell
  • 995
78
votes
6 answers

How can we see planets thousands of light years away but don't know if there are more planets in the solar system?

That is basically my question, it arose when I saw an article (here is the scientific paper, which should be free to read) saying two Caltech scientists might have found the 9th planet of the solar system.
Suriya
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78
votes
8 answers

Does quantum mechanics play a role in the brain?

I'm interested in whether the scale of processes that occur in the brain is small enough to be affected by quantum mechanics. For instance, we ignore quantum mechanics when we analyze a game of tennis because a tennis ball is much too large to be…
QCD_IS_GOOD
  • 6,710
78
votes
7 answers

Does a charged particle accelerating in a gravitational field radiate?

A charged particle undergoing an acceleration radiates photons. Let's consider a charge in a freely falling frame of reference. In such a frame, the local gravitational field is necessarily zero, and the particle does not accelerate or experience…
Sergio
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78
votes
7 answers

Why do we think there are only three generations of fundamental particles?

In the standard model of particle physics, there are three generations of quarks (up/down, strange/charm, and top/bottom), along with three generations of leptons (electron, muon, and tau). All of these particles have been observed experimentally,…
Scott Carnahan
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78
votes
11 answers

Why is the charge naming convention wrong?

I recently came to know about the Conventional Current vs. Electron Flow issue. Doing some search I found that the reason for this is that Benjamin Franklin made a mistake when naming positive and negative charges. There is even this little comic…
GetFree
  • 1,271
77
votes
3 answers

Why can we see the dust particles in a narrow beam of light (and not in an all lighted area)?

Let us say that I am sitting in a room with all the drapes open. Bright sunlight is coming through the window. The whole room is brilliantly lighted. I will not be able to see the dust particles suspended in air. Now, if I draw the drapes close,…
Masroor
  • 759
77
votes
21 answers

Comprehensive book on group theory for physicists?

I am looking for a good source on group theory aimed at physicists. I'd prefer one with a good general introduction to group theory, not just focusing on Lie groups or crystal groups but one that covers "all" the basics, and then, in addition, talks…
Lagerbaer
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77
votes
5 answers

Do the weights of two liquids not add when mixed?

I was given an interesting dilemma today. A co-worker saw me adding a liquid (Diisopropyl ethylamine AKA DIPEA) to a flask filled with another liquid (Tetrahydrofuran AKA THF). I needed to weigh out exactly 5 grams of DIPEA into the THF and so I…
Brian
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