Ask a physicist

[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?

 

[cookyy2k]

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

I assume you mean centrifugal force at the equaters reducing your weight by producing a force upwards? The yes, you do. You also weigh more if you live near a fault line because faul lines ae very rich in iron and other heavy metals so have a higher gravitaional field in that area.

This is a map of the gravity of the Earth.

 

[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.

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.

Hmm, I see what you mean. Does this also go for imposing a particle's (un)known spin state on another (destroying the original state). Not doubting or anything, but "powerful magnet" sounds rather crude.

I used to have a fairly crude understanding of NMRi, but it's been slipping. I do remember something about a strong magnetic field to allign the hydrogen nuclei, and a smaller, fluctuating field superimposed on that one to cause vibration/precession or something like that.

 

[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.

Hmm, I see what you mean. Does this also go for imposing a particle's (un)known spin state on another (destroying the original state). Not doubting or anything, but "powerful magnet" sounds rather crude.

I used to have a fairly crude understanding of NMRi, but it's been slipping. I do remember something about a strong magnetic field to allign the hydrogen nuclei, and a smaller, fluctuating field superimposed on that one to cause vibration/precession or something like that.

Well for the actual teleportation of the spin state its not really known the causation of this, for imposing the spin on the "transmitting" electron it would be with a powerful magnet mainly. And the MRI description is more or less how one works, the nuclii aligned against the powerful magnet are in a higher enegy state, causing them to precess to align with the magnetic field means they have to lose energy, this is in the form of a radio frequency wave tht can be detected.

 

[RA92]

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...

Aaah... so, I'm guessing, the hypothetical tidal forces generating heat within the interior of Europa might make the satellite crash into Jupiter one day?

Thanks for taking the time to answer, much appreciated.

Edit: Oh wait.

how often do people make Gordon Freeman/Half-Life jokes?

Not heard one yet.

http://rickatnight11.com/wp-content/uploads/2008/09/gman.jpg


http://www.blogcdn.com/www.joystiq.com/media/2008/09/largehadron.jpg



CERN has a <url=http://www.1up.com/news/real-gordon-freeman-saves-cern>sense of humor.

 

[Woodsey]

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...

Aaah... so, I'm guessing, the hypothetical tidal forces generating heat within the interior of Europa might make the satellite crash into Jupiter one day?

Thanks for taking the time to answer, much appreciated.

Edit: Oh wait.

how often do people make Gordon Freeman/Half-Life jokes?

Not heard one yet.

http://rickatnight11.com/wp-content/uploads/2008/09/gman.jpg


http://www.blogcdn.com/www.joystiq.com/media/2008/09/largehadron.jpg



CERN has a <url=http://www.1up.com/news/real-gordon-freeman-saves-cern>sense of humor.

The G-Man one must be a fucking plant xD The guy doesn't even look human!

 

[cookyy2k]

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...

Aaah... so, I'm guessing, the hypothetical tidal forces generating heat within the interior of Europa might make the satellite crash into Jupiter one day?

Thanks for taking the time to answer, much appreciated.

Yeah should do, though I doubt we'll be observing it in our lives.

Edit: Oh wait.

how often do people make Gordon Freeman/Half-Life jokes?

Not heard one yet.

snip

CERN has a <url=http://www.1up.com/news/real-gordon-freeman-saves-cern>sense of humor.

You sir, are the first to show me that

 

[Shakomaru]

Since the Hadron collider CAN make black holes that will die out very quickly, Would it be possible to see it, and what would it look like? Also, how much do you know about antimatter? besides the obvious of course.

 

[cookyy2k]

Since the Hadron collider CAN make black holes that will die out very quickly, Would it be possible to see it, and what would it look like?

I imagine it would only be vissible as a short burst of Hawking radiation. I don't think it would be visible in anyother way as we observe blackholes in space from their acretion disks, the ring of matter being accelerated towards thew singularity producing radiateion.

Also, how much do you know about antimatter? besides the obvious of course.

Only the basics really. The anti-particle had the same mass as, but opporsite charge to the particle. For example the positron is the electron's anti-particles.

When an anti-particle and it's particle partner come into contact they are both converted into 2 gamma photons at the rate E=mc[sup]2[/sup]. Also if you bring 2 gamma photons at the correct energy we can pair produce a particle/anti-particle pair.

When you create matter, you always must produce anti-matter. Even in radioactive decay, for example beta- decay, a neutron decays into a proton ejecting an electron and a anti-neutrino. and in beta+ decay a proton decays into a neutron ejecting a positron and a neutrino.