Ask a Theoretical Physicist

[CVT Temp]

Okay, so the title is pretentious and I'm not quite there yet (only in graduate school), but this could be a good opportunity to test my knowledge and my capacity to explain concepts.

Ask away, and I'll try to answer what I'm able to.

[Ethel mermania]

can god make a rock so big even he cant move it, or is Archimedes right?

[Alyekra]

Start basic.

What is theoretical physics and why does it matter?

[CVT Temp]

can god make a rock so big even he cant move it, or is Archimedes right?

It depends on which god you're talking about.

[Ostroeuropa]

You should probably clear up what we should ask you or you are going to get some very strange que-

WHAT IS THE COLOUR OF MY DOG?!

[Franklin Delano Bluth]

Shouldn't this be an AMA on Reddit?

[CVT Temp]

Start basic.

What is theoretical physics and why does it matter?

Physics as a discipline has become arcane and esoteric enough that division of labor is now necessary. Some physicists work on understanding and developing theories, whereas others work on testing those theories. Theoretical physicists understand theories very well but don't necessarily have the greatest understanding of the inner mechanisms of various experimental procedures and devices. Experimentalists know very well how to test things and how to do all sorts of data analysis, but they often lack a real understanding of the theory proper.

As for why it matters, it matters because the development of physics matters, and the development of physics requires this division of labor.

[Olivaero]

http://en.wikipedia.org/wiki/Alcubierre_drive
http://www.wired.com/wiredscience/2012/09/warp-drive-plausible/

How plausible is going FTL with this type of technology? or is it still impossible?

[CVT Temp]

You should probably clear up what we should ask you or you are going to get some very strange que-

WHAT IS THE COLOUR OF MY DOG?!

Green (plus or minus 300nm)

[Ostroeuropa]

You should probably clear up what we should ask you or you are going to get some very strange que-

WHAT IS THE COLOUR OF MY DOG?!

Green (plus or minus 300nm)

Very good. Very good.
...
Suspiciously good.

[Divair]

Green (plus or minus 300nm)

Very good. Very good.
...
Suspiciously good.

He's a robot from the future.

[Tubbsalot]

I'm going to basically ignore the strictures of the thread and ask what the deal is with light, because I don't understand it.

I at least know it has oscillating magnetic and electric fields which are described by vectors perpendicular to each other, but that's pretty much all. Do the fields exhibit a sinusoidal oscillation in space perpendicular to the direction of movement for the light, or do the changing vectors represent varying magnitude of the fields and the only physical movement is as part of the light?

And if the latter, how is the size of the field defined? Proportionally to the energy of the photon?

[CVT Temp]

http://en.wikipedia.org/wiki/Alcubierre_drive
http://www.wired.com/wiredscience/2012/09/warp-drive-plausible/

How plausible is going FTL with this type of technology? or is it still impossible?

These drives never allow you to actually violate the speed of light. You just cheat by stretching space, the same way that the expanding universe allows there to be places expanding away from us faster than light.

They seem possible in principle without necessarily being a problem, but they also seem technically almost impossible due to the need to precisely manipulate enormous amounts of energy.

Stable wormholes, on the other hand, seem to be completely impossible since they require negative mass, which certain positive energy theorems in quantum gravity take issue with.

[New Sapienta]

Are you Four-Sided Triangles?

[Alyekra]

Who pays you for theoretical Physicisting?

[The Land Fomerly Known as Ligerplace]

Is there any possible end to entropy?

Are there any clues to why matter seemingly spontaneously came from absolutely nothing?

[Ostroeuropa]

Is there any possible end to entropy?

Are there any clues to why matter seemingly spontaneously came from absolutely nothing?

If the universe is closed and begins to collapse in on itself to the singularity again, then yes, there is a possible end to entropy.

[CVT Temp]

I'm going to basically ignore the strictures of the thread and ask what the deal is with light, because I don't understand it.

I at least know it has oscillating magnetic and electric fields which are described by vectors perpendicular to each other, but that's pretty much all. Do the fields exhibit a sinusoidal oscillation in space perpendicular to the direction of movement for the light, or do the changing vectors represent varying magnitude of the fields and the only physical movement is as part of the light?

And if the latter, how is the size of the field defined? Proportionally to the energy of the photon?

Classically, the wave represents increases and decreases in the strength of electric and magnetic fields. A fundamental EM wave is a plane wave:

E = cos(kz - wt)i

B = cos(kz - wt)j/c

where E is the electric field vector, B is the magnetic field vector, k is the wave number (2*pi over the wavelength), w is the angular frequency (2*pi times frequency), and i and j are the unit vectors in the x and y directions, respectively.

The electric and magnetic fields only change with time and with z. They does not depend on x or y, so they are constant everywhere along a cross-section of the wave. The notion of the wave as being like a physical displacement of water or a string is just an analogy to help people think about it.

Quantum mechanically, it's a bit harder to describe, since transverse polarization is not the natural way to describe things. Instead, circular polarization is, and then, the two different circular polarizations coefficients represent the probability amplitudes for finding photons with spin up or down along the direction of motion, where as the functional part of the polarizations represents the probability of finding photons in given points in space.

Basically, quantum mechanically, fields can only have quantized levels of excitation. These quantized excitations are particles. Thus, in quantum field theory, it is fields which are the real fundamental object, with particles just being excited states of the fields.

[CVT Temp]

Are you Four-Sided Triangles?

No. From what I've been told, he was insane and anti-sexual.

[Greed and Death]

Can you get my brother an internship ?

[New Sapienta]

Are you Four-Sided Triangles?

No. From what I've been told, he was insane and anti-sexual.

I remain skeptical.

[CVT Temp]

Is there any possible end to entropy?

You can reach a state of maximum entropy such that you can't go any higher.

Are there any clues to why matter seemingly spontaneously came from absolutely nothing?

The origins of matter aren't really a problem. Explaining why inflation happened is one of the real problems remaining in cosmology.

[CVT Temp]

Who pays you for theoretical Physicisting?

Universities, national labs, and on occasion, industries that may find your skills useful.

[Greed and Death]

Who pays you for theoretical Physicisting?

Universities, national labs, and on occasion, industries that may find your skills useful.

I actually met a theoretical Physicist back in 2007, who was employed by AIG to produce models that were more accurate models to predict the value of sub-prime mortgages.

[Olivaero]

http://en.wikipedia.org/wiki/Alcubierre_drive
http://www.wired.com/wiredscience/2012/09/warp-drive-plausible/

How plausible is going FTL with this type of technology? or is it still impossible?

These drives never allow you to actually violate the speed of light. You just cheat by stretching space, the same way that the expanding universe allows there to be places expanding away from us faster than light.

They seem possible in principle without necessarily being a problem, but they also seem technically almost impossible due to the need to precisely manipulate enormous amounts of energy.

Stable wormholes, on the other hand, seem to be completely impossible since they require negative mass, which certain positive energy theorems in quantum gravity take issue with.

Ah okay, so regarding wormholes is exotic matter theoretically impossible then? I have great respect for your field btw Without physicists there would be no computers for me to program!