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2080173
bio website ellipsix.net
location Wuhan, China
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visits member for 4 years, 5 months
seen 3 hours ago

I'm a postdoc doing research in high-energy particle physics. I also have a hobby interest in computer programming.

You can find me on Twitter, or check out my blog and personal website! Or you can email me at stack@ellipsix.net.

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3h
comment What is a body's momentum really equal to?
I'm intentionally calling it outdated even though I know some people still use it - the implication is partly that it should be outdated, not that nobody uses it.
20h
comment How to determine which object in a special relativity physics problem has proper time?
@Brionius the modern terminology, as I know it, is that what you learned to call "rest mass", formerly denoted $m_0$, is simply called "mass" and denoted $m$, and "relativistic mass", formerly denoted $m$, is now just "energy" or "total energy" $E$. Or more precisely, instead of talking about relativistic mass, we talk about energy, which differs by a factor of $c^2$. For example, the (total) energy of an object is its mass energy plus its kinetic energy, and the total energy increases with velocity, but the mass never changes. (My main complaint is with the use of $m$ for relativistic mass.)
1d
comment How to determine which object in a special relativity physics problem has proper time?
My one complaint about this answer is that it's not clear about the difference between relativistic mass (an outdated concept) and rest mass. Otherwise, very nicely explained.
Apr
23
comment How fast would you have to go to claim you saw the red light as green due to the Doppler Effect?
What have you done to try to figure this out yourself?
Apr
23
comment Why don't things get destroyed by gas molecules flying around?
Yes, but even intermolecular forces are strong enough to resist most collisions with air molecules. I mean, I don't know the numbers offhand, but it must work out that way, otherwise objects as we know them wouldn't exist ;-) So perhaps you only need a sphere of air the size of, I don't know, the moon to get one air molecule moving fast enough to break an intermolecular bond, instead of 16 Earth-orbit-sized spheres. And besides, breaking one molecule off something hardly counts as destroying it.
Apr
23
comment How do I choose the right value of $r$ to find where the electric field is zero?
Ah, well I left out the absolute value signs in my previous comment because we were only talking about a point in the region where $x>0$. The more general expression is that the distances are $|x|$ and $|x+2|$, in your convention, or $|x|$ and $|x-2|$ in the other convention. So if you use the other example's definition of $x$, which has $x>0$ to the right of the origin, then the distance from $x=-3$ to $Q_2$ is $|-(-3)+2|=5$. Note that $|x-2|=|-x+2|=|2-x|$, so you can use any of these interchangeably in this situation. (PS the fact that $Q_2$ is negative is completely irrelevant.)
Apr
23
comment How do I choose the right value of $r$ to find where the electric field is zero?
Your diagram shows that $x$ is defined as distance to the left of the positive charge, as I mentioned in the answer. So, using your definition, points to the left of the positive charge correspond to positive values of $x$. If you think about it a bit, you should be able to understand that this corresponds to your equation as well: take a point 3 units to the left of the positive charge, for example. Its distance from the positive charge ($x$) is 3 and from the negative charge ($x+2$, in your convention) is 5. If $x$ were $-3$, that wouldn't work out.
Apr
23
comment How do I choose the right value of $r$ to find where the electric field is zero?
@DevinCrossman That's how they defined $x$, not how you defined $x$ - at least, not according to your diagram or your equation.
Apr
23
comment special theory of relativity-time dilation
A web search for "time dilation" or "special relativity" or any of various similar terms would lead you to several online resources that explain this; therefore I don't think it's a good question.
Apr
22
comment How do I maniptulate the Fourier transform of a field using the delta function?
I'm voting to close this question because it's been cross-posted to Mathematics.
Apr
22
comment Units for displacement current density
You can't delete it because it has an upvoted answer. But in this case, it's your answer, so you can delete the answer and then you should be able to delete the question. That being said, I think this is a good question and answer - even though it was caused by a minor mistake, someone else could easily make the same mistake, and leaving this post here would help them. (Plus, free rep :-P)
Apr
21
comment How do you become a theoretical physicist?
I'm voting to close this question as off-topic because it is about career advice.
Apr
20
comment Circular Motion
I've deleted some comments which were against our homework policy. (Don't give complete or near-complete answers to homework-like questions. Also don't answer questions in the comments.)
Apr
20
comment Where does an LED use energy other than emitting light?
@Floris if this were a solid state physics question, I'd expect to see some mention of the materials involved, as solid state physics is all about the properties of materials, isn't it? My view is that this falls under the category of how some device works, and those kinds of questions I consider off topic in general (except for advanced physics experiments). Also, I was (perhaps mis)remembering this meta post.
Apr
20
comment Where does an LED use energy other than emitting light?
Let me record my thoughts from chat here: I don't think this is on topic because it's a question about the internal workings of an electronic device (an LED). We handle some questions about simple circuits - resistors, capacitors, and inductors - to the extent that the questions are about the physics of electric current flow, but not diodes. (Though considering we have a tag for them, maybe it's not so clear....) I think the first part of this, about the equation, might be on topic if it can be divorced from LEDs.
Apr
20
comment Strong and Weak limits in Quantum Mechanics
@PrakrutChaubal that's the reference you add, then. In other words, it would help if you edit your question to say that you encountered this in your professor's notes but found nothing about it in Sakurai or Gottfried and Yan.
Apr
18
comment Event Horizon of Black Hole
@ACuriousJim that should be an answer
Apr
18
comment Is it possible to travel ONLY through Time and not Space?
@DreSam911 if you want to complain about downvotes, do it in a comment ;-) It's possible the reason you're getting downvoted is that the question shows that you don't understand the basic idea of special relativity, namely the idea that all motion is relative. This is an expert-level site, which means we expect people to have some basic competence in the area they're asking about. You might get a better response to this question on another physics Q&A site.
Apr
18
comment In Gauss's law, why do we use an infinte long wire or long plane to calculate electric field intensity?
@santiago emphasize much? ;-) In any case, I don't think this is really a homework question; Mohit isn't asking us to solve the problems mentioned, or to check solutions. It appears to be a conceptual question that is just organized in a confusing manner. I'll see if I can clarify it. Mohit, could you please check whether the edit is accurate?
Apr
17
comment Why does moving through time not require energy?
@OrangeDog that was my first thought as well, but I suppose the question is unclear about whether it's saying a moving body has energy (which is true) or expends energy (which is false).