Ask a physicist

[Chromatic Aberration]

Hiho - A fellow physicist here.

I'am currently about to start working for my Master Thesis and plan to be done with that in about a year from now on and then go on and do my phD somewhere. I was wondering if you could perhaps share your experiences in finding/getting your current phD student position. Anything in that regard would be quite helpful

 

[cookyy2k]

There was one bit I didn't understand. Can you explain again what the second problem with CMD is?

Ah right, for the main galactic DM halo and the sub halo DM fragments their is a theoretical density function that fits current observations, however at the centre of each of these the density function become infinate and so the gravitational potential should be infinate, obviously this is unphysical but then other proposed density functions that do not have this problem do not recreate what we observe well enough to be considered correct.

Hiho - A fellow physicist here.

I'am currently about to start working for my Master Thesis and plan to be done with that in about a year from now on and then go on and do my phD somewhere. I was wondering if you could perhaps share your experiences in finding/getting your current phD student position. Anything in that regard would be quite helpful

The most important things are do your research on the exact research fields of each department you wish to apply to to make sure what you want to do fits in, next find a PhD supervisor at that uni that researches into the area you want to go into and send an email explaining what you want to do and asking if they think it would be a good project. They will probably contact you back and adjust it sightly and say to apply. Then when you apply you are able to put a name in the "proposed supervisor" box then the application will go right to that supervisor to decide instead of just a departmental admisions tutor.

If they give you an interview, some do some just go off applications, then make sure you know your project inside and out. My interview was them asking questions about my project until I couldn't answer anylonger to see just how deep your understanding goes. They will ask job interview style questiong but they concentrate mostly on your understanding of your project because that tells them how suited to research you are.

 

[infohippie]

Special relativity assumes there is no privileged reference frame and therefore no universal time or absolute motion. How do we know there is no privileged reference frame? Have there been experiments done to demonstrate this?

 

[cookyy2k]

Special relativity assumes there is no privileged reference frame and therefore no universal time or absolute motion. How do we know there is no privileged reference frame? Have there been experiments done to demonstrate this?

I assume by this you're asking if their is an "ether" an absolute rest frame in which everything else moves? This was the thinking before relativity and was actually what relitivity set out to disprove, the speed of light stuff came later. Hence the name relitivity, everything is relitive to everything else, their is no absolute standard of rest.

An experiment was done on board a train travelling at constant velocity, the nice thing about this was it was a light experiment called the michelson interferometer, this is a fairl basic experiment but it showed the results are the same nomatter which frame you're in and this experiment plus maxwell's equations for light are what lead to the constant speed of light idea.

Things that show this are thought experiments like the twin paradox which I've described in this thread earlier. But these have problems in special relativity that are corrected by general relivity.

If their was an absolute standard of rest light would always have a constant velocity relative to this as opposed to all rest frames, so you'd be going back to Galilean relativity not the Eintein model.

 

[Valkyrie101]

What happens if a laser beam collides with a mirror? Would it burn through (I'm talking about proper, exciting lasers here) or be totally reflected?

 

[Salad Is Murder]

Can god make yo mama so fat that even he can't lift her?

 

[LordOrin]

There was one bit I didn't understand. Can you explain again what the second problem with CMD is?

Ah right, for the main galactic DM halo and the sub halo DM fragments their is a theoretical density function that fits current observations, however at the centre of each of these the density function become infinate and so the gravitational potential should be infinate, obviously this is unphysical but then other proposed density functions that do not have this problem do not recreate what we observe well enough to be considered correct.

Interesting stuff. Thanks!

 

[Dr_Matt]

Now in the case of superconductivity the electrons pair up into cooper's pairs which have spin 0 and so pauly exclusion no longer applies. Due to quantum mechanical effects the cooper pairs require a minimum amount of energy to excite them, if this energy is greater than the thermal energy (kT where k is boltzman's constant and T is temperature) the electron pairs will not be scattered by the lattice, thus having 0 resistance.

Superconductors display zero DC resistance, but they still have resistance for alternating currents due to the inertia of the Cooper pairs. Another bizarre feature is the heat transport properties of superconductors - although heat transfer in metals is by electrons, metals in the superconducting state actually display very low thermal conductivity, which is useful if rather counterintuitive.

Why is Pluto no longer a planet?

At the risk of igniting that argument again, Pluto technically never was a planet. Until the IAU meeting a couple of years ago where all this was discussed and settled, there wasn't a proper definition of what a planet actually is.

Oh, and I'm also a physicist. Good luck with the studies, guys.

 

[cookyy2k]

What happens if a laser beam collides with a mirror? Would it burn through (I'm talking about proper, exciting lasers here) or be totally reflected?

If the mirror is not 100% reflective at the laser's wavelength, a small amount of the light will be absorbed causing heating in the surface of the mirror, this will begin to tarnish the mirror, as it tarnishes it absrobs more energy which tarnises it futher and so on, until the laser makes a hole through the mirror.

This is similar to using a YAG laser to cut alluminium, to start with the metal is shiny so reflects a lot of the beam but it begins to tarnish and then is cut by the laser.

 

[The Heik]

Feel free to ask any physics related questions you want, if I answer too simplistically or too advanced call me out on it and I'll try again. If you want you can message the question instead or ask follow ups by message. However you ask I will do my best to get you an accesible answer as fast as possible.

Can you give me the basics behind string theory as it applies to the universe? Every time I've read articles about it, it swiftly becomes difficult to read, and on two occasions has given me a migraine.

 

[cookyy2k]

Feel free to ask any physics related questions you want, if I answer too simplistically or too advanced call me out on it and I'll try again. If you want you can message the question instead or ask follow ups by message. However you ask I will do my best to get you an accesible answer as fast as possible.

Can you give me the basics behind string theory as it applies to the universe? Every time I've read articles about it, it swiftly becomes difficult to read, and on two occasions has given me a migraine.

Not really unfortunately. I'm fairly well in a similar possition. I just don't know it well enough to summarise it accurately and I just don't have the sort of time required to learn the field enought.

 

[Wyes]

A Classical Mechanics question: How on Earth can you tell what quantities will be conserved from a Lagrangian (i.e. L = T - V, where T = Kinetic Energy, V = Potential Energy)?

Doing my physics undergrad at the moment, doing an advanced unit, and our lecturer didn't explain this as well as I'd like. All I know is that it has to do with dependency on the variables found in the Lagrangian.

EDIT: Also, how do Hamiltonians differ from Lagrangians? I know they have the form T + V, but where does this come from?

 

[RA92]

Feel free to ask any physics related questions you want, if I answer too simplistically or too advanced call me out on it and I'll try again. If you want you can message the question instead or ask follow ups by message. However you ask I will do my best to get you an accesible answer as fast as possible.

So far, awesome thread.

To business: I've been obsessed with making a perpetual motion machine since I was a kid, and the Laws of Thermodynamics telling me I can't haven't deterred me. Here's a theory: let's say that I've created a giant deformable spherical satellite and put it in an elliptical orbit around the Earth. Because of the elliptical shape of the orbit, the gravitational force exerted by the Earth will not be constant throughout, and will, I'm assuming, deform the shape of the satellite depending on its distance from the Earth. So, let's say with various contraptions (pistons and dynamos) I utilize these deformations to create electricity.

Does this even qualify as a perpetual motion machine? Where's the hole in the plan? Will the resistance to the deformations provided by the dynamos somehow effect its orbit and make it crash into the planet? If I succeed, will I speed up the heat death of the universe? 'Been slacking at my Physics class, so help would be appreciated. Thanks!

 

[RA92]

Double post, disregard.

 

[Chromatic Aberration]

A Classical Mechanics question: How on Earth can you tell what quantities will be conserved from a Lagrangian (i.e. L = T - V, where T = Kinetic Energy, V = Potential Energy)?

Doing my physics undergrad at the moment, doing an advanced unit, and our lecturer didn't explain this as well as I'd like. All I know is that it has to do with dependency on the variables found in the Lagrangian.

EDIT: Also, how do Hamiltonians differ from Lagrangians? I know they have the form T + V, but where does this come from?

In short: Noether's Theorem [http://en.wikipedia.org/wiki/Noether's_theorem]. It states that if your action is invariant under continuous symmetry transformations you will have certain conserved quantities depending on what symmetry transformations you apply. For instance, Energy conservation follows from invariance under time-translations and momentum-conservation from spatial translational invariance. In other words: conserved quantities follow from symmetries in nature. You are now free to contemplate the awesomeness of this statement

A conserved quantity can be easily spotted by using the Euler-Lagrange equations: if your Lagrangian does not depend on your generalized coordinate (the coordinate is then called a cyclic coordinate [http://en.wikipedia.org/wiki/Cyclic_coordinate#.22Cyclic_coordinates.22_and_conservation_laws]) your Euler Lagrange equations reduce to: (d/dt)dL/dk=0 where k is the time derivative of your generalized coordinate. This tells you that the quantity dL/dk (the generalized momentum) will be conserved. Thus, if you have a cyclic coordinate dL/dk is a conserved quantity.

Hamiltonian and Lagrangian are connected by means of a Legendre Transformation [http://en.wikipedia.org/wiki/Legendre_transformation]. The Lagrangian depends on your generalized coordinate and its time derivative while the Hamiltonian depends on the generalized coordinate and the generalized momentum. A Legendre transform lets you substitute those variables. As for why one uses the Hamiltonian instead of the Lagrangian is its property of representing the total energy of your system (E = T + V = H) or just easier calculations by virtue of the different variables. The energy aspect will come in handy in Quantum Mechanics where you will want to calculate the Energy spectrum of your model under study - there, you will use quantized versions of your classical Hamiltonians to obtain it.

Hiho - A fellow physicist here.

I'am currently about to start working for my Master Thesis and plan to be done with that in about a year from now on and then go on and do my phD somewhere. I was wondering if you could perhaps share your experiences in finding/getting your current phD student position. Anything in that regard would be quite helpful

The most important things are do your research on the exact research fields of each department you wish to apply to to make sure what you want to do fits in, next find a PhD supervisor at that uni that researches into the area you want to go into and send an email explaining what you want to do and asking if they think it would be a good project. They will probably contact you back and adjust it sightly and say to apply. Then when you apply you are able to put a name in the "proposed supervisor" box then the application will go right to that supervisor to decide instead of just a departmental admisions tutor.

If they give you an interview, some do some just go off applications, then make sure you know your project inside and out. My interview was them asking questions about my project until I couldn't answer anylonger to see just how deep your understanding goes. They will ask job interview style questiong but they concentrate mostly on your understanding of your project because that tells them how suited to research you are.

Thank you. I really appreciate it.

 

[Nocta-Aeterna]

If an infinite force collides with an inmovable, undeformable, unbreakable object...
Nah j/k.

If it's already possible: how does one force a specific spin state to a 1/2 spin particle? For example, when using two entangled fermions to quantum teleport the spin state of a specific particle.

 

[Wyes]

A Classical Mechanics question: How on Earth can you tell what quantities will be conserved from a Lagrangian (i.e. L = T - V, where T = Kinetic Energy, V = Potential Energy)?

Doing my physics undergrad at the moment, doing an advanced unit, and our lecturer didn't explain this as well as I'd like. All I know is that it has to do with dependency on the variables found in the Lagrangian.

EDIT: Also, how do Hamiltonians differ from Lagrangians? I know they have the form T + V, but where does this come from?

In short: Noether's Theorem [http://en.wikipedia.org/wiki/Noether's_theorem]. It states that if your action is invariant under continuous symmetry transformations you will have certain conserved quantities depending on what symmetry transformations you apply. For instance, Energy conservation follows from invariance under time-translations and momentum-conservation from spatial translational invariance. In other words: conserved quantities follow from symmetries in nature. You are now free to contemplate the awesomeness of this statement

A conserved quantity can be easily spotted by using the Euler-Lagrange equations: if your Lagrangian does not depend on your generalized coordinate (the coordinate is then called a cyclic coordinate [http://en.wikipedia.org/wiki/Cyclic_coordinate#.22Cyclic_coordinates.22_and_conservation_laws]) your Euler Lagrange equations reduce to: (d/dt)dL/dk=0 where k is the time derivative of your generalized coordinate. This tells you that the quantity dL/dk (the generalized momentum) will be conserved. Thus, if you have a cyclic coordinate dL/dk is a conserved quantity.

Hamiltonian and Lagrangian are connected by means of a Legendre Transformation [http://en.wikipedia.org/wiki/Legendre_transformation]. The Lagrangian depends on your generalized coordinate and its time derivative while the Hamiltonian depends on the generalized coordinate and the generalized momentum. A Legendre transform lets you substitute those variables. As for why one uses the Hamiltonian instead of the Lagrangian is its property of representing the total energy of your system (E = T + V = H) or just easier calculations by virtue of the different variables. The energy aspect will come in handy in Quantum Mechanics where you will want to calculate the Energy spectrum of your model under study - there, you will use quantized versions of your classical Hamiltonians to obtain it.

Thanks, that was incredibly useful! Our lecturer was trying to say something to this effect, but he couldn't quite get the idea across. Would help if we knew anything about transformations...

 

[cookyy2k]

Feel free to ask any physics related questions you want, if I answer too simplistically or too advanced call me out on it and I'll try again. If you want you can message the question instead or ask follow ups by message. However you ask I will do my best to get you an accesible answer as fast as possible.

So far, awesome thread.

To business: I've been obsessed with making a perpetual motion machine since I was a kid, and the Laws of Thermodynamics telling me I can't haven't deterred me. Here's a theory: let's say that I've created a giant deformable spherical satellite and put it in an elliptical orbit around the Earth. Because of the elliptical shape of the orbit, the gravitational force exerted by the Earth will not be constant throughout, and will, I'm assuming, deform the shape of the satellite depending on its distance from the Earth. So, let's say with various contraptions (pistons and dynamos) I utilize these deformations to create electricity.

Does this even qualify as a perpetual motion machine? Where's the hole in the plan? Will the resistance to the deformations provided by the dynamos somehow effect its orbit and make it crash into the planet? If I succeed, will I speed up the heat death of the universe? 'Been slacking at my Physics class, so help would be appreciated. Thanks!

Here's your problem; all the deformations and pistons etc would take some of the energy out of the orbit causing the sattelite to spiral inwards unless you use boosters to keep putting the satellite in higher orbits but this will take more energy than you're getting back since boosters ain't 100% efficient and you know, the 2nd law of thermodynamics.

If you got it to work the speed of the death of the universe wouldn't be effected since a perptual motion machine requires the same amount of entropy in the system before and after the process, assuming no creation of energy, this is how the 2nd law puts pay to this idea since it states entropy must always increase.

Even gravitational sling shots are not free energy or acceleration since you don't get something for nothing. Using a planet as a sling shot actually takes energy out of it's orbit and moves the planet closer to the sun, it's just minute because the relative mass of the space craft is tiny compared to the planet.

I don't really see a way to making a perpetual motion machine since the 1st and 2nd laws team up on you, you always violate atleast one. Though you can violate the first with quantum mechanics so maybe its a matter of time til someone thinks something up.

 

[cookyy2k]

If an infinite force collides with an inmovable, undeformable, unbreakable object...
Nah j/k.

If it's already possible: how does one force a specific spin state to a 1/2 spin particle? For example, when using two entangled fermions to quantum teleport the spin state of a specific particle.

The main way in which this is done is with large magnets, the fact an electron has a non-zero spin means it has a magnetic moment so will be repelled/attracted by magnetic forces. An MRI scanner works on this principal to detect the proton in the nucleus of hydrogen atoms around your body.

 

[Baneat]

Do you weigh differently near the poles of the earth due to Central Force?