Ask a Theoretical Physicist

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Re: Ask a Theoretical Physicist

Postby Readersprite » Sat Sep 20, 2014 5:58 pm

Sorry for asking so much. I'm very ignorant but interested in your field.

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Joseph Stalin wrote:
I followed up on those wishes by going to a math specialized high school and later going to university and majoring in physics.
When I finished my undergraduate studies I got a job as a research assistant and did my PhD. I didn't choose my particular field rather I choose which group I would work with (and which wanted me of course). My requirement was that it was a theoretical group and did high-quality research.


If you don't mind, could you elaborate on what it means to "choose which group I would work with?" I assume there is some sort of application process; if there is, would you mind detailing that? How long does employment in a group last? Does it require constant readmission?

Joseph Stalin wrote:Today theoretical physics is highly specialized - you have people doing high energy (i.e., string theory), gravity, cosmology, condensed matter, biophysics/soft matter, quantum information theory, mathematical physics even econophysics, etc. etc.
I'm quite happy that I found my niche and also got to dabble in several of these interesting fields because I want to have a broad overview of physics as a whole.


When you say "dabble in several of these interesting fields", do you mean working in several of these (a la various groups as mentioned in the prior section)?

Joseph Stalin wrote: No matter how smart you are there will always be some problem that is too difficult for you to solve (obviously or we would know everything) so the trick is to find the hardest possible (and most interesting) problem that you can solve and then solve it. This means that the greatest geniuses are doing stellar research but they still have to work as hard as the average physicist doing mediocre stuff.


Joseph Stalin in an earlier post wrote:Or, I can invent some new problem I can work on, which I think would be interesting. In physics, you can always come up with interesting questions, and you can always come up with questions you can solve. The real trick is to come up with questions which are interesting AND you can also solve!


When you invent a problem, is this in the same way a textbook publisher creates practice questions (though, of course, far more advanced) or is there some other meaning I am missing? Is there a way you can give an example simple enough for those who haven't been trained in theoretical physics?

Joseph Stalin wrote:What ones does afterwards is not so easy either. You are almost always expected to find a low paying temporary 2-4 year position as a "postdoctoral researcher" (postdoc). Permanent positions are almost impossible to find these days and if you get one it will likely be somewhere you don't want to be.


How do you mean, "afterwards"? After you finish working in a group? Is there a point past which you can no longer be allowed to work in one, or disallowed from applying to others? Is postdoctoral research not a pleasant form of work? How does it differ from the group research you mentioned before?
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Re: Ask a Theoretical Physicist

Postby Joseph Stalin » Sun Sep 21, 2014 2:00 pm

Readersprite wrote:Sorry for asking so much. I'm very ignorant but interested in your field.

Not at all. Your questions are good.

Readersprite wrote:
Joseph Stalin wrote:
I followed up on those wishes by going to a math specialized high school and later going to university and majoring in physics.
When I finished my undergraduate studies I got a job as a research assistant and did my PhD. I didn't choose my particular field rather I choose which group I would work with (and which wanted me of course). My requirement was that it was a theoretical group and did high-quality research.


If you don't mind, could you elaborate on what it means to "choose which group I would work with?" I assume there is some sort of application process; if there is, would you mind detailing that? How long does employment in a group last? Does it require constant readmission?

Yes, there is an application process. It depends on where you want to enroll. For instance in the US you have to take the GRE, submit your grades, some CV, list your accomplishments, etc. In the EU the application process is more or less a formality. There are a lot of PhD positions these days. When you're a undergrad you learn what groups are doing interesting things related to what you're professors are doing or something. That's the best way to apply because then you will get a recommendation from someone who knows the group you're applying to. Of course, you're technically applying to the University, but the decision is in the end in the hands of the head of the group, who will also most likely be your thesis adviser. Or if you really want you can apply blind to some place else and then figure out what you want to do later on. This will work if you're really really good and have no problem being accepted anywhere you want.
Readersprite wrote:
Joseph Stalin wrote:Today theoretical physics is highly specialized - you have people doing high energy (i.e., string theory), gravity, cosmology, condensed matter, biophysics/soft matter, quantum information theory, mathematical physics even econophysics, etc. etc.
I'm quite happy that I found my niche and also got to dabble in several of these interesting fields because I want to have a broad overview of physics as a whole.


When you say "dabble in several of these interesting fields", do you mean working in several of these (a la various groups as mentioned in the prior section)?

I meant working in things which are of interest or somewhat related to each of these fields. For instance, there is a lot of mathematical dualities in physics. The AdS/CFT duality first discovered in the context of high-energy physics is now applied to the study of condensed matter systems.

Readersprite wrote:
Joseph Stalin wrote: No matter how smart you are there will always be some problem that is too difficult for you to solve (obviously or we would know everything) so the trick is to find the hardest possible (and most interesting) problem that you can solve and then solve it. This means that the greatest geniuses are doing stellar research but they still have to work as hard as the average physicist doing mediocre stuff.


Joseph Stalin in an earlier post wrote:Or, I can invent some new problem I can work on, which I think would be interesting. In physics, you can always come up with interesting questions, and you can always come up with questions you can solve. The real trick is to come up with questions which are interesting AND you can also solve!


When you invent a problem, is this in the same way a textbook publisher creates practice questions (though, of course, far more advanced) or is there some other meaning I am missing? Is there a way you can give an example simple enough for those who haven't been trained in theoretical physics?

I remember wondering the same thing when I was an undergrad. No, it's not the same thing. When you think of a practice question for students you usually already know what the answer is or how it can be found. When you're doing research you usually don't know that, unless there is some really strong indication what the answer should be then you can try to prove that that's the answer.
Usually, when you start your PhD, your thesis adviser will give you a problem to work on. Later, you know the field so well that you can come up with new research directions on your own. For instance, a few days ago I read a paper which did something similar to what I did in a previous paper (but didn't cite me :x ) and now I immediately see that I can probably combine both ideas to do something even more interesting than either.

Readersprite wrote:
Joseph Stalin wrote:What ones does afterwards is not so easy either. You are almost always expected to find a low paying temporary 2-4 year position as a "postdoctoral researcher" (postdoc). Permanent positions are almost impossible to find these days and if you get one it will likely be somewhere you don't want to be.


How do you mean, "afterwards"? After you finish working in a group? Is there a point past which you can no longer be allowed to work in one, or disallowed from applying to others? Is postdoctoral research not a pleasant form of work? How does it differ from the group research you mentioned before?

After you finish your PhD you typically are no longer funded. Postdocs last for 2 years on average and it takes 1 year at least to find a new job. So you will waste half of your time just looking for another job. If you get a 1 year postdoc then you will start looking for a new position immediately. It is low paid and you have to move all over the world. It is quite stressful to do work knowing if you don't publish something interesting you will be out of a job very soon, especially if you have a family. The low pay makes it almost impossible to support your husband/wife and it is almost impossible for him/her to find a new job in a new part of the world every couple of years - even if they're in the postdoc cycle they won't get a position in the same place you get one.
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Re: Ask a Theoretical Physicist

Postby Readersprite » Sun Sep 21, 2014 10:52 pm

Joseph Stalin wrote:
Readersprite wrote:
Joseph Stalin wrote:What ones does afterwards is not so easy either. You are almost always expected to find a low paying temporary 2-4 year position as a "postdoctoral researcher" (postdoc). Permanent positions are almost impossible to find these days and if you get one it will likely be somewhere you don't want to be.


How do you mean, "afterwards"? After you finish working in a group? Is there a point past which you can no longer be allowed to work in one, or disallowed from applying to others? Is postdoctoral research not a pleasant form of work? How does it differ from the group research you mentioned before?

After you finish your PhD you typically are no longer funded. Postdocs last for 2 years on average and it takes 1 year at least to find a new job. So you will waste half of your time just looking for another job. If you get a 1 year postdoc then you will start looking for a new position immediately. It is low paid and you have to move all over the world. It is quite stressful to do work knowing if you don't publish something interesting you will be out of a job very soon, especially if you have a family. The low pay makes it almost impossible to support your husband/wife and it is almost impossible for him/her to find a new job in a new part of the world every couple of years - even if they're in the postdoc cycle they won't get a position in the same place you get one.


However, once heavily saturated with the optimism of the ignorant, that career does sound excellent for a single person who likes to travel. Is that grossly inaccurate summation of the above fair?
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Re: Ask a Theoretical Physicist

Postby Joseph Stalin » Sun Sep 21, 2014 11:02 pm

Readersprite wrote:
Joseph Stalin wrote:
Readersprite wrote:
Joseph Stalin wrote:What ones does afterwards is not so easy either. You are almost always expected to find a low paying temporary 2-4 year position as a "postdoctoral researcher" (postdoc). Permanent positions are almost impossible to find these days and if you get one it will likely be somewhere you don't want to be.


How do you mean, "afterwards"? After you finish working in a group? Is there a point past which you can no longer be allowed to work in one, or disallowed from applying to others? Is postdoctoral research not a pleasant form of work? How does it differ from the group research you mentioned before?

After you finish your PhD you typically are no longer funded. Postdocs last for 2 years on average and it takes 1 year at least to find a new job. So you will waste half of your time just looking for another job. If you get a 1 year postdoc then you will start looking for a new position immediately. It is low paid and you have to move all over the world. It is quite stressful to do work knowing if you don't publish something interesting you will be out of a job very soon, especially if you have a family. The low pay makes it almost impossible to support your husband/wife and it is almost impossible for him/her to find a new job in a new part of the world every couple of years - even if they're in the postdoc cycle they won't get a position in the same place you get one.


However, once heavily saturated with the optimism of the ignorant, that career does sound excellent for a single person who likes to travel. Is that grossly inaccurate summation of the above fair?


Traveling is different than living somewhere, mind you. But if you enjoy not being to afford as much as most people much younger than you can and living in constant financial insecurity and spending most of your time looking for another job while doing mind bendingly hard stuff under stress then yes it is excellent.
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Re: Ask a Theoretical Physicist

Postby Gen.Knowledge » Fri Oct 03, 2014 6:13 am

Just ignore this, wrong thread, srry.
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Re: Ask a Theoretical Physicist

Postby Learned Nand » Sat Nov 08, 2014 10:11 am

I had these questions after doing some research watching Interstellar. None of this questions really contain spoilers, but I'll put them in a spoiler tag just in case somebody is afraid of accidentally inferring plot information:

Spoiler: show
If it takes infinite time (from our perspective) to enter a black hole due to gravitational time dilation, shouldn't the inside of a black hole just be nothing, and the mass of a black hole contained entirely in the accretion disk approaching the schwarzchild radius? And if you were to try to enter a black hole, and were somehow able to resist the tidal forces and not be spaghettified, wouldn't you just experience stronger and stronger tidal forces, and then weaker ones as time accelerated so much that it accelerated towards the heat death of the universe, when black holes were no more, at which point you'd be freed? Basically, how is the mass of a black hole concentrated in a singularity at the center and not in the accretion disk?

How can black holes have a charge? Isn't the photon the carrier of the electromagnetic force? And photons can't escape a black hole, hence the blackness.
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Re: Ask a Theoretical Physicist

Postby NathanLoiselle » Sat Nov 08, 2014 9:37 pm

Photons don't carry a charge.
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Re: Ask a Theoretical Physicist

Postby Learned Nand » Sat Nov 08, 2014 10:01 pm

I know photons don't have a charge, but they are the carrier bosons of the electromagnetic force.
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Terry Pratchett wrote:The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it.

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OrangeEyebrows wrote:There once was a guy, Aviel,
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He tested with Turing,
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Re: Ask a Theoretical Physicist

Postby CarrieVS » Sat Nov 08, 2014 10:04 pm

They don't carry a charge but they are the particles that carry the force that a charged particle exerts on other charged particles. If I remember my A-level physics aright. When a charged particle attracts or repels stuff it doesn't expend charge to do so, so the carrier particles of the force don't have a charge.
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Re: Ask a Theoretical Physicist

Postby Joseph Stalin » Mon Nov 10, 2014 4:16 am

aviel wrote:I had these questions after doing some research watching Interstellar. None of this questions really contain spoilers, but I'll put them in a spoiler tag just in case somebody is afraid of accidentally inferring plot information:

Spoiler: show
If it takes infinite time (from our perspective) to enter a black hole due to gravitational time dilation, shouldn't the inside of a black hole just be nothing, and the mass of a black hole contained entirely in the accretion disk approaching the schwarzchild radius? And if you were to try to enter a black hole, and were somehow able to resist the tidal forces and not be spaghettified, wouldn't you just experience stronger and stronger tidal forces, and then weaker ones as time accelerated so much that it accelerated towards the heat death of the universe, when black holes were no more, at which point you'd be freed? Basically, how is the mass of a black hole concentrated in a singularity at the center and not in the accretion disk?

How can black holes have a charge? Isn't the photon the carrier of the electromagnetic force? And photons can't escape a black hole, hence the blackness.


The coordinate time taken to fall into the black hole is indeed infinite, however the proper time (the one which is physically relevant) of the in-falling observer is finite. Time is tricky in relativity, especially in GR. There is no absolute time. If you are far, far away from the black hole then your proper time coincides with the coordinate time, but then it will take a long time for information to reach you (via photons or something). And if you are near the black hole you are falling in unless you are accelerating away from it. But then you are in very special coordinate system which is not inertial and your perspective will be altered compared to an observer who is falling in.

I don't understand the disk question. The black hole outside the event horizon is just an object with the same mass. There is nothing special about it outside the event horizon.

The photons mediating EM forces are not real, they're virtual aka off shell. They can't be measured and they are not physical in the same sense ordinary photons are. They are also not bound by having to propagate along light cones so they can leave the black hole.
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Re: Ask a Theoretical Physicist

Postby Learned Nand » Mon Nov 10, 2014 6:42 am

Wait, virtual particles can leave black holes? Then why is there hawking radiation? Isn't that the result of some virtual particles falling into black holes while their anti-particles don't?

Also, the reason I was asking about the concentration of the black hole's mass in the accretion disk is because I don't see how a black hole every absorbs anything in coordinate time. Given that we are more or less in coordinate time relative to many black holes we've looked at, then how can those black holes have mass if the things that would get inside them to give them mass are never able to get inside them in our time?
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Terry Pratchett wrote:The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it.

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Re: Ask a Theoretical Physicist

Postby Joseph Stalin » Mon Nov 10, 2014 7:15 am

aviel wrote:Wait, virtual particles can leave black holes? Then why is there hawking radiation? Isn't that the result of some virtual particles falling into black holes while their anti-particles don't?

Also, the reason I was asking about the concentration of the black hole's mass in the accretion disk is because I don't see how a black hole every absorbs anything in coordinate time. Given that we are more or less in coordinate time relative to many black holes we've looked at, then how can those black holes have mass if the things that would get inside them to give them mass are never able to get inside them in our time?


Yes, Hawking radiation is virtual pair production followed by one (anti)-particle in the pair falling in the other escapes. If neither escaped there would be no radiation. A charged black hole is basically a static charge and virtual photons mediating static charge fields mediating it have zero frequency so the following discussion about things falling into black holes will not impact their existence.

There is no contradiction here. The time taken for something to cross the event horizon in coordinate time is infinite but from the point of view of an observer whose proper time is approximately coordinate time an object falling into a black hole will seem to have its clock tick slower and slower as it approaches event horizon. That also means that the time between photons emitted by the object will go to infinity and the photons will also be infinitely red-shifted, that is for all intents and purposes the black hole will be completely black and there will be no way to distinguish from an object actually crossing an event horizon and the rest of the black hole.
Now you're probably thinking how can a black hole form at all then from our point of view. Well, this is more complicated. In GR coordinates are something which are arbitrary - you can choose any set you like. Schwarzschild coordinates are not the best choice to understand what is going on. Now, instead of writing the explanation here I will refer you to this hour and a half lecture by Leonard Susskind instead:
http://theoreticalminimum.com/courses/g ... /lecture-8

:)
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Re: Ask a Theoretical Physicist

Postby Learned Nand » Mon Nov 10, 2014 8:07 am

I'll have to take a look at the lecture, but I think that answers my question. Thank you.
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Terry Pratchett wrote:The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it.

Click for a Limerick
OrangeEyebrows wrote:There once was a guy, Aviel,
whose arguments no one could quell.
He tested with Turing,
his circuits fried during,
and now we'll have peace for a spell.
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Re: Ask a Theoretical Physicist

Postby Joseph Stalin » Mon Nov 10, 2014 8:26 am

aviel wrote:I'll have to take a look at the lecture, but I think that answers my question. Thank you.

No problem. :D
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Re: Ask a Theoretical Physicist

Postby Queen_Elizabeth » Mon Feb 09, 2015 6:34 pm

Would you describe the Half-Life series as an accurate representation of your daily routine?
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