144 replies to this topic

[Talus]

I got yet another question

Dead simple this time

If photons don't have mass, which of course they don't (discounting energy value as mass), how do they have momentum when p = mv? :S

I'm guessing the classic definition of momentum doesn't work in the quantum world. If this is the case then what does momentum mean in quantum physics?

[Dauth]

E^2 = (pc)^2 + (m^4)c^2

Lorentz law for invariant mass.

Momentum means the same thing in quantum physics, just in some cases there is an inherent quantum error (uncertainty principle)

[Libains]

That makes my brain want to hurt.

[Talus]

So momentum is proportional to energy? I know the full relativistic (bad spelling methinks) equation but i never thought of rearranging it, somtimes the simple things work best.

Thanks a lot for the help btw.

I got yet another one for you if you don't mind It's nice to have an answer to the questions that are never explained to me (and that my teachers just don't know the answers too)

Right, another one on quantum physics

I know there is the controversial issue over the exsistance of gravity's gauge boson: the Graviton.

Surly if it exists then the effect of gravity will be one half of the speed of light (as the boson, to my knowlege, has to travel to the object back from the source). This could be tested relativly easily.

If this was true then it would have been done. But it hasn't, so I have my facts wrong again

Any idea where?

Thanks

Btw, out of interest, what did you do at university and where did you go?

You seemed to have picked up the technical side of what I read about flawlessly (unless your self taught, massive kudos to you if true) so whatever course it is I liked to have a look at it as a choise for when I go.

Thanks again, hope you don't mind me asking.

[Chyro-edit: posts merged. Please refrain from double posting.]

Edited by Chyros, 03 July 2008 - 19:55.

[Dauth]

Graviton doesn't need to travel back, just the single distance would be sufficient, thus it moves at c, in this manner people are using very heavy elements in long straight rods to detect a graviton passing through the material. I prefer the idea of mass warping spacetime and the graviton moving in space and thus could move faster than c, but this is hairy physics.

I completed a degree in Physics with Theoretical Physics at The University of Manchester and I gained the title MPhys (hons) along with the 2:1

[Talus]

I didn't know that, thanks.

And that's the exact degree i went for an open day for the other day, looks great

Nice job with the 2:1

I'm sure i'll have more questions, just not quite atm

Talus

[NanSolo]

Here's a question: how many things are actually infinite, as opposed to things that are actually finite but we call infinite because the number is so large it would take an infinite amount of time to calculate?

[Dauth]

Not a huge amount, number theorists have lots of tricks for dealing with huge numbers, I very rarely run into infinities.

[Shirou]

There that fact that many physical constants, such as the movement of atoms (their temperature) cannot reach 0 because in order for that, some other value would have to reach infinity. Therefore, just like mass can't reach the speed of light, mass can neither reach beyond Zero Point Energy.

Now onto the question.. why, can't an atom reach the absolute Zero? What is in the way of that? I only know what I said before in this reply.

Edited by Aftershock, 10 July 2008 - 13:09.

[Dauth]

An atom can reach absolute zero but it takes an infinite number of steps to do so.

Without delving into a huge amount of thermodynamics, the entropy of the system requires that each step in cooling be smaller than the previous one, thus you need infinite number of repeats of a process to get to absolute zero.

[Talus]

I'm back

Another question:

Cosmic Microwave Background Radiation (CMB) is thermal blackbody microwave radiation, which apears to be almost isotropic, from the big bang and has a net movement away from the center universe, to my knowledge.

First question: Why is it microwave frequency when it's from the big bang, surly it would be high gamma frequencies? I've been told red shift is the cause and i've been told the temperature is the cause. But the red shift idea is a little counter intuative, why should an object have lower relative energy just because it's moving toward an observer?

Second question: If CMB has a net movent away from the center of the universe how is it everywere at once? Surly it will be a lot less prominent in it's origion and not isotropic.

Cheers for any help.

Talus.

[Dauth]

Red shift is moving away from something, Red photons have lower energy than blue ones.

Space has expanded, there is no centre or edge for the radiation to be sourced at or congregate at.

[Rich19]

I've got a question or two on Heisenberg's uncertainty principle and quantum cryptography. As far as I know so far, information is transmitted in quantum states using photons. Look at the sources below for a basic overview of the process. The main but I'm looking at is in the first source, starting with "The foundation of quantum cryptography lies in the Heisenberg uncertainty principle, which states that" and ending with "Bob will either receive no message or a garbled one, and in either case will be able to deduce Eve's presence."

Spoiler

E.V.E. is spying on Bob

I get the basics of how it works, but I'm not so confident as to how it is guaranteed security by quantum mechanics. Also, I keep seeing the equation

when looking at stuff about the uncertainty principle, but that's momentum and position. Do you know of an equation like that relating to polarisation states?

Sorry if it's a little hard to understand what I'm asking...


Some sources I'm looking at at the moment:
http://www.csa.com/d...overview.php#n6
http://en.wikipedia....um_cryptography

[Dauth]

Right, the equation mentioned is the uncertainty principle in the x direction. The uncertainty in the polarisation states would not work from an equation like this and because they only use 4 states.

The equation there states if you know where a particle is to 10^-15m, it will have hbar/(2*10^-15) units of momentum. No polariser is perfect therefore a deviation slightly away from vertical or horizontal won't show up.

Polariser is like =, ideally only flat lines get through, but as you can see small angles can be accepted (left of bottom bar to right of top bar and vice versa).

I should point out, Alice, Bob and Eve are names used by physicists everywhere and does not relate to the people here .

After Alice and Bob have their key, if Eve tries to intercept and send on then the message that reaches Bob will not be the same as the one sent by Alice, and thus it will be garbled. If Eve intercepts before the key is decided upon then when it is used the computability with Bob will not be there and the message will be garbled again.

Hope this helps a bit.

[Rich19]

It does indeed, and thanks for the fast reply as well. I'm still a little confused though - you said that the polarisation states uncertainty would not work from the equation, but then explained the equation and talked about polarisation in the same paragraph (second one).

I'm looking into this as part of my physics coursework, because I've done some stuff with cryptography before and find this practical application fascinating. However, the coursework needs to include me doing stuff with equations, and most of the stuff I've found so far just says "quantum theory states that..." without explaining how it does that.

[Dauth]

Because the measuring states are very discrete, out by 45degrees the uncertainty is not going to affect the polarisation directly. Most polarisers are good to about 1 or 2 degrees from centre (IIRC) so something 45 degrees different will always give a different response.

There are options to study quantum cryptography, at second or third year degree level Physics/Computing. The equations require the understanding from doing the earlier years and they are also probably linked to a journal atm and not publicly available. I suggest that you look at the volume of data that could be transferred in this encryption.

[Rich19]

Alright, thanks.

[Shirou]

Now especially at the one with the glass over it.

how does this happen? How can a grape + microwaves equal plasma balls?

[Dauth]

Background stuff first
Microwaves work by oscillating water molecules, the friction in turn generates heat.
The waves follow the same path at all times, so you have to rotate the object to get an even cook.

Now to the grape,
It wasn't moving this was not a huge issue here so can be ignored for the moment.
The microwaves heat the water molecules which evaporate some contents of the grape.
These evaporated gases form the plasma you see.
When the glass is on top, the plasma cannot dissipate as quickly so it stays around longer.

Hope this makes sense.

[Shirou]

I went that far pretty much already, I know the functioning of a microwave. but I just didn't get how it becomes plasma. to your explanation, isn't that just evaporation and doesn't explain why it becomes plasma? What is in grapes that makes them so special and why doesn't a bottle of water generate plasma if it's that easy..

[Dauth]

I don't know the makeup of the grape and thus the plasma. There are some components in the grape that obviously are not present in water, for instance the carbon in the sugars of the grape is not present in water.

A plasma of carbon and oxygen is normally called a flame. So this is my instinct as too the reason.

[Dr. Strangelove]

This isn't exactly science, and forgive me for being a little rusty, but if lim1/0=undefined, what is lim1/1/infinity?
______________________________________________________n->n__________________n->n________

Edited by Dr. Strangelove, 30 November 2008 - 10:41.

[BeefJeRKy]

lim 1/0 --> infinity
therefore lim 1/1/inf = lim inf --> inf
the limit does not exist but still tends towards infinity.

[Dauth]

lim 1/x approaches infinity as x approaches 0

1/(1/x) = (1/1)*x = x

[Dr. Strangelove]

I feel so stupid now.