Ask a physicist

[IvoryTowerGamer]

This thread is my dream come true! I've been wiki surfing about astrophysics and quantum theory for a while, but some things still aren't clear to me:

What exactly is Quantum Field Theory?

Sometimes time is referred to as "the fourth dimension", but then there are theories that refer to additional spacial dimensions (past the first 3 I mean). What's the deal with this? Is time actually a dimension, or is it separate?

How exactly do particles "mediate" a force? What does that mean?

What's the deal with virtual particles. More specifically, how do they make Hawking Radiation and black hole evaporation possible?

If the universe is currently expanding, does that mean that more empty space is being created everywhere, or are things like galaxies etc just flying farther apart from each other? In other words, will the acceleration of the expansion of the universe mean that one day the universe will be expanding so quickly that it'll overcome things like the strong nuclear force?

Sorry if that's too many questions. If you only want to answer one or two the QFT and the Hawking Radiation questions are the ones that have been bothering me the most.

 

[cookyy2k]

I'm confused as to how Hawking Radiation works.

I will admit this is laziness on my part as I have not researched into it. But I do not understand how a black hole can emit energy and eventually evaporate. Once matter crosses the event horizon of a black hole it can not escape. Since matter is a form of energy, shouldn't it be impossible for black holes to emit energy?

This can be through a process called quantum tunneling, this is where a particle can travel through an unpassable potential aslong as that potential is thin enough.

A matter/antimatter pair can be produced just inside thet even horizon of the blackhole from some of the vaccuum energy within. One of these particles will fall into the blackhole but the other will tunnel outside the event horizon and escape. Since both masses were produced from the blackhole's energy this loss of a particle translates to a loss of energy and therefor mass for the blackhole. The particle that is emitted can then be detected as a wave (due to wave particle duality) of radiation which can be detected.

 

[Soluncreed]

I have no idea what anyone's talking about. I should really start learning more.

 

[thethingthatlurks]

What's the title of your dissertation going to be? And what made you want to research galactical dynamics?
On a completely different note, h or h-bar?

 

[TheIronRuler]

I have a question, not sure if you deal with this sort of thing.
Is it possible for matter to exist in 0 K?
Since its the 'ultimate zero', the electrons couldn't have any motion, but that's not possible. If you could try and explain this to me I'll be very thankful.

 

[cookyy2k]

Which idea of universe 'creation' i.e how it started, do you favor?
And isn't the big bang theory just on how it expanded 'n stuff after creation, not creation itself?

...

I haven't done advanced physics.

Well their isn't really many theories of creation if we discount the big bang theory as one and say it's just expansion, getting into the relms of where theists bring out the "G word".

I do favour the big bang theory though there are a few holes here and there. The universe would have had to undergo very fast expansion indeed to look the way it does today however mass fluctuations were needed to produce anythinjg we see since a universe of constant density everywhere would never end up with structure since everything would have experienced equal force in every direction and inflation periods would be problamatic to this. Plus since Steven Hawking himself now disagrees with the big bang theory (even though it's his theory) so who am I to argue?

 

[SckizoBoy]

Centrifugal and centripetal forces are not the same thing. Centrifugal is the force outwards when an object travels a curved trajectory, centripetal is the force inwards to make the object follow a curved trajectory.

I stand corrected, and thanks for the clarification. I just had a memory of my A-Level physics teacher recalling how riled up he gets when he hears the word 'centrifugal' being misused.

*shrug* (anyway, hence the disclaimer)

 

[Three Eyed Cyclops]

Can you not form a 5/6-membered ring? If you could do that then the substitution would be fairly simple. Otherwise it's probably just some sort of Zn/Pt/Pd-catalysed substitution (that's soft enough that it doesn't interact with the ester--or alternatively picked cleverly enough that interaction with the ester leads to regiospecifity).

Around an alkyne core?! Talk about ring-strain (even if 6-membered). Currently, the only thing I have in mind is using TBDS to protect the carbonyl, then assaulting it with a base to take off the terminal proton. My supervisor will probably come up with something a little more elegant than that.

One option might be to deprotonate with t-butyl lithium, then methylate with methyl iodide.

 

[cookyy2k]

This thread is my dream come true! I've been wiki surfing about astrophysics and quantum theory for a while, but some things still aren't clear to me:

What exactly is Quantum Field Theory?

Sometimes time is referred to as "the fourth dimension", but then there are theories that refer to additional spacial dimensions (past the first 3 I mean). What's the deal with this? Is time actually a dimension, or is it separate?

How exactly do particles "mediate" a force? What does that mean?

What's the deal with virtual particles. More specifically, how do they make Hawking Radiation and black hole evaporation possible?

If the universe is currently expanding, does that mean that more empty space is being created everywhere, or are things like galaxies etc just flying farther apart from each other? In other words, will the acceleration of the expansion of the universe mean that one day the universe will be expanding so quickly that it'll overcome things like the strong nuclear force?

Sorry if that's too many questions. If you only want to answer one or two the QFT and the Hawking Radiation questions are the ones that have been bothering me the most.

Quantum field theory is the name given to a way of constructing quantum mechanical theoretical models, it is called quantum field theory simply because it was invented to be the quantum mechanics model of electromagnetic fields.

The dimentions thing is all dependant what field you're in, quantum mechanics is upto 11 (I believe) so far. This is really not something I'm well up on or that's that easy to find good level information on. All I know is I hated 11D matrix algebra!

In the standard model particles are said to be responsible for forces, for example the photon is responcible for the electromagnetic force. It can be though of as two people stood on skate boards, one throws the other a basketball when the other catches it he is repelled away from the first. This isn't the greatest example since the photon has zero mass but it's a good concepual example.

Hawking radiation is kinda explained earlier. Virtual particles are those that can exist due to uncertanty principal. It is possible to create energy and teherefore mass, however this is very short lived. The mass created is called a virtual particle.

Universe expansion is everything is expanding. The gaps between nuclii and their electrons, the gaps between atoms in your body, the gaps between points on Earth and so on. Space is expanding equally.

 

[SckizoBoy]

One option might be to deprotonate with t-butyl lithium, then methylate with methyl iodide.

Nnnnn, likely side reaction: LDA deprotonates at carbon alpha to the carbonyl, enolises, O[sup]-[/sup] gets methylated. Though the zinc cat is looking interesting. And seeing as how the LDA is too strong to be used in mild conditions, -78 will have indicated carbon as the likely deprotonation site.

EDIT: Woops, just realised it's not propargyl... one too many carbons.

 

[cookyy2k]

What's the title of your dissertation going to be? And what made you want to research galactical dynamics?
On a completely different note, h or h-bar?

My dissertation is going to be about how spiral galaxies form like they do since this isn't well understood. I'm using a supercomputer to simulate how a lot of particles behave interacting through gravity and in certain set conditions to see if I can recreate a spiral and why that does so when others fail to.

I've always had an interest in astrphysics from very young and I had the opportunity to work with a world-leading professor in the field for my masters project. I decided galactic dynamics was for me during that since I had great fun with it and did well.

Also hbar=h/2pi so it really depends which anwer you want. e.g E=hv (E is energy, v is frequency) or E=hbar*w (w is the angular frequency)

I have a question, not sure if you deal with this sort of thing.
Is it possible for matter to exist in 0 K?
Since its the 'ultimate zero', the electrons couldn't have any motion, but that's not possible. If you could try and explain this to me I'll be very thankful.

At 0K the energy of the electron is not 0, their is a thing called zero point energy where you can remove all the energy you want from the system and it will always have this energy. for example in einstein electronic specific heat E=hbar(n+0.5) where n is the number of quanta of energy. As you can see if you remove all energy you're still left with E=0.5(hbar) so this is a non-zero finite energy.

 

[Popadoo]

I'm not sure if this is true, but I heard the center of a Black Hole is infinitely dense. If this is true, which it might not be, this must mean it has infinite gravitational energy. And if there is infinite gravity, everything from the very far reaches of the universe would be pulled in.
This question is only relevant if the density of the center of a Black Hole really is infinite.

 

[cookyy2k]

I'm not sure if this is true, but I heard the center of a Black Hole is infinitely dense. If this is true, which it might not be, this must mean it has infinite gravitational energy. And if there is infinite gravity, everything from the very far reaches of the universe would be pulled in.
This question is only relevant if the density of the center of a Black Hole really is infinite.

This is where their is a BIG problem. relativety (massive things) can't deal with small things and quantum mechanics (small things) can't deal with massive thiungs. So the centre of a blackhole which is very small and very massive we don't have a theory that works. If you do try either of those theories you end up with a 1/0 which tends to tell you you have something wrong.

 

[similar.squirrel]

What's your stance on the whole morphogenetic fields theory?

 

[IvoryTowerGamer]

Plus since Steven Hawking himself now disagrees with the big bang theory (even though it's his theory) so who am I to argue?

What theory does he support now then? I thought he was for M-theory, which includes the big bang as a part of it.

Thanks for the answers to my earlier questions, by the way! You really helped clear up a lot! You are quite good at giving easy to understand answers!

 

[Bajinga]

Would the "speedy thing goes in. Speedy thing comes out" theory in Portal work in real life?
(Ignore the fact that wormholes will never be made on command.)

 

[DasDestroyer]

I've heard that if you move at relativistic speeds, time will slow down for you allowing you to travel forward in time. Is this true? And if so, how does it work?

Also, is it possible to travel backward in time?

 

[nerd51075]

I'm just out of high school, and I graduated with no peers in any of the hard sciences offered at my school (Mathematics, Physics, and Chemistry), and I would just like to say that it is inspiring to not know what you are talking about at all. It makes me feel like there is a ton of knowledge out there waiting for me to find it. I just thought I would take this opportunity to thank you for this thread.

 

[Quaxar]

Alright, string theory or loop quantum gravity?
3-2-1-FIGHT!

I'm confused as to how Hawking Radiation works.

I will admit this is laziness on my part as I have not researched into it. But I do not understand how a black hole can emit energy and eventually evaporate. Once matter crosses the event horizon of a black hole it can not escape. Since matter is a form of energy, shouldn't it be impossible for black holes to emit energy?

This can be through a process called quantum tunneling, this is where a particle can travel through an unpassable potential aslong as that potential is thin enough.

A matter/antimatter pair can be produced just inside thet even horizon of the blackhole from some of the vaccuum energy within. One of these particles will fall into the blackhole but the other will tunnel outside the event horizon and escape. Since both masses were produced from the blackhole's energy this loss of a particle translates to a loss of energy and therefor mass for the blackhole. The particle that is emitted can then be detected as a wave (due to wave particle duality) of radiation which can be detected.

Quantum tunneling, you sure?
Sure, I didn't study it (not yet anyway) but I've written a paper about black holes and I can't remember reading about a particle tunneling out of the Schwarzschild horizon. Do you happen to have a source for that, out of pure scientifical interest?