Ask a physicist anything. (7)

[Chalnoth]

I'm somewhere else at the time.

I do try to set the volume down to 0 before turning it off.

I've remember to this do just once though.

It takes about 6 seconds for the TV to fire up when I press the power button on the remote which gives me plenty of time to run for it.

Does the whole driver update thingy sound plausible?

I didn't even know it was possible to update the drivers on modern TV's. I didn't think they had any way to upload the data. But as long as this is a relatively recent phenomenon, my bet is that it's a piece of the electronics that is failing. So that means repair or replace. But if the problem is intermittent (i.e. doesn't happen that often), then it may be extremely hard for the repair person to find the problem, meaning you might get a big bill with no solution. Personally, I'd just cough up the cash and replace it.

 

[Zippy the Wonderslug]

^ Okay. Thank you.

So one other question I would like some help with...

I posted a question on how to get a certain TV station in this thread quite some time ago.

It's WILX, which is NBC.

I live very close to the Capitol in the state of Michigan so it seems like I should be able to receive this signal.

I can get about 16 stations with just a simple RCA indoor antenna that looks something like this.

Anyway, after looking at this site, I guess NBC is on a VHF air wave.
TV Fool

VHF is the rabbit ear thingies, yes?

Anyway, my neighbor gave me this broken piece of junk today.
http://img3.imageshack.us/img3/8001/dsc04569gl.jpg

What can I do in getting this to capture the VHF signal?

I don't mind breaking it apart. Trust me. I keep giving him about $20 a day like this whole month because he's been hard up lately so I could use the stress relief.

The only thing I really have is a wire clothes hanger.

Any theories at all and I would be most willing to test them.

Thanks!

 

[Zippy the Wonderslug]

Oh, and sorry Chalnoth, I didn't really respond to your question.

This only happens during the winter, probably since my TV sits directly in front of a loose window.

 

[Chalnoth]

Sorry, I have no idea how to help with this.

 

[Chalnoth]

Oh, and sorry Chalnoth, I didn't really respond to your question.

This only happens during the winter, probably since my TV sits directly in front of a loose window.

Yeah, it being temperature-related makes good sense to me. That also coincides with the fact that you say that it goes away after the TV is on for a bit, meaning that it works properly when the electronic components are warm, but there's a problem when they're cold.

 

[Zippy the Wonderslug]

^ Maybe I'll just take a blow dryer and bake the TV for a few minutes before turning it on.

I remember having a friend who used to do this with his car in order to start it.

Ohmygosh, I remember watching him and thinking how he was so pathetic. *lol*

 

[Zippy the Wonderslug]

Oh, and to answer your other concern, I'm pretty sure that the TV has a USB slot for whatever reason.

Maybe it's to upload updated drivers.

I'll try and look at the back of the television and find it's model number tomorrow.

As for now, I'd rather not touch this insane beast.

 

[Upisoft]

Momentum is conserved in mirrors. The photons impart momentum to the mirror when they bounce off of it. But here we're talking about a transparent medium where the photons don't bounce.

Indeed, the momentum is conserved if you look at the whole system. I was pointing out that a quantum state of a phonon was not conserved (its momentum) when looking only at the photon. Thus interactions always transfer something (energy, momentum, etc.) except in the special case of transparency, when nothing is transferred to the glass. Or am I mistaken? What transfers to the glass when an electron is slowed down?

Well, that's just because the photons don't interact frequently enough for their interactions to add up to an electron jumping an energy gap: the photons are emitted too rapidly, or conversely the electrons aren't able to "borrow" energy for long enough.

I already explained that frequency of interactions (if they are absorption/reemittion events) must be very high, or otherwise the speed of light would vary with the thickness of the glass and also it will disperse. Some of the photons would get less intearctions than others making the light blur(in space-time) after passing the glass. Thus a packet of photons will not be a packet of of photons after passing the glass. That effect was not observed.

 

[Chalnoth]

Indeed, the momentum is conserved if you look at the whole system. I was pointing out that a quantum state of a phonon was not conserved (its momentum) when looking only at the photon. Thus interactions always transfer something (energy, momentum, etc.) except in the special case of transparency, when nothing is transferred to the glass. Or am I mistaken? What transfers to the glass when an electron is slowed down?

You mean what transfers to the glass when the photon slows down? Because that isn't a change in momentum of the photon: that would imply a change in frequency. And the frequency of the photon doesn't change as it passes through a transparent medium.

I already explained that frequency of interactions (if they are absorption/reemittion events) must be very high, or otherwise the speed of light would vary with the thickness of the glass and also it will disperse. Some of the photons would get less intearctions than others making the light blur(in space-time) after passing the glass. Thus a packet of photons will not be a packet of of photons after passing the glass. That effect was not observed.

All this means is that the timescale for re-emission is even shorter. Oh, and packets tend to spread out over time anyway. There is also some amount of scattering of the photons in any medium (though the amount of scatter is small in highly-transparent materials).

 

[Upisoft]

You mean what transfers to the glass when the photon slows down? Because that isn't a change in momentum of the photon: that would imply a change in frequency. And the frequency of the photon doesn't change as it passes through a transparent medium.

The absolute value of the momentum does not change, thus the frequency does not change. Only the direction changes, thus now the photon is reflected and travels in different direction. As far as I know, there is no slowing down of the photon, just instantaneous transformation of its energy in another kind of energy.

All this means is that the timescale for re-emission is even shorter. Oh, and packets tend to spread out over time anyway.

Doesn't seem to be the case when we watch distant galaxies. Studies of distant supernovae show they behave the same way as closer supernovae. If the packets tend to spread over time, then the effect would be observable when we watch distant supernovae.

There is also some amount of scattering of the photons in any medium (though the amount of scatter is small in highly-transparent materials).

Yes, but scattering is well understood process that has nothing to do with transparency.

 

[Chalnoth]

The absolute value of the momentum does not change, thus the frequency does not change. Only the direction changes, thus now the photon is reflected and travels in different direction. As far as I know, there is no slowing down of the photon, just instantaneous transformation of its energy in another kind of energy.

Ah, yeah, that's true. So clearly there is some small transfer of momentum. But it's clear that this transfer of momentum only occurs at the boundary, not within the bulk of the medium. This transfer, therefore, has nothing to do with the absorption/re-emission process, but instead with what happens at the boundary.

Doesn't seem to be the case when we watch distant galaxies. Studies of distant supernovae show they behave the same way as closer supernovae. If the packets tend to spread over time, then the effect would be observable when we watch distant supernovae.

As the intervening medium is a near-perfect vacuum, the dispersion relation keeps the light in sync. That isn't, in general, going to be the case within a refracting medium.

 

[Upisoft]

Ah, yeah, that's true. So clearly there is some small transfer of momentum. But it's clear that this transfer of momentum only occurs at the boundary, not within the bulk of the medium. This transfer, therefore, has nothing to do with the absorption/re-emission process, but instead with what happens at the boundary.

Yes, this should happen pretty close to the boundary or some photons could be trapped in infinite loop somewhere in the deep regions. Also it is obvious that it is not an effect of single absorption/re-emmition, because the new direction of the photon depends on how the atoms and electrons are arranged. Changing the angle of the mirror while keeping the point of the reflection at the same place (i.e. rotating the mirror along axis that goes through the reflection point and belongs to the plane of the mirror surface) will change the direction of the reflection, thus showing that not only the point of reflection is important but also what is near it. Thus the electron will interact with the whole thing, not only with a single electron... That's what probably happens with transparency I guess... But how... that's beyond me.

As the intervening medium is a near-perfect vacuum, the dispersion relation keeps the light in sync. That isn't, in general, going to be the case within a refracting medium.

Yeah, waves tend to travel with different speeds given their frequency, but is that what happens with LASER light, that happens to have many photons(bosons) occupying the same energy level?

 

[Chalnoth]

Yes, this should happen pretty close to the boundary or some photons could be trapped in infinite loop somewhere in the deep regions. Also it is obvious that it is not an effect of single absorption/re-emmition, because the new direction of the photon depends on how the atoms and electrons are arranged. Changing the angle of the mirror while keeping the point of the reflection at the same place (i.e. rotating the mirror along axis that goes through the reflection point and belongs to the plane of the mirror surface) will change the direction of the reflection, thus showing that not only the point of reflection is important but also what is near it. Thus the electron will interact with the whole thing, not only with a single electron... That's what probably happens with transparency I guess... But how... that's beyond me.

Well, I really think you're trying to look at it at too low of a level without going into the math. To understand the quantum behavior, you really have to go through the quantum mechanical calculations. Suffice it to say, the quantum behavior reduces to the classical behavior in the limit of large numbers of photons.

Yeah, waves tend to travel with different speeds given their frequency, but is that what happens with LASER light, that happens to have many photons(bosons) occupying the same energy level?

Well, you were talking about a wave packet. Wave packets occupy a limited frequency range. They have to, if you think about it, because if they had exactly one frequency, they would necessarily be spread across all of space. In order to be localized in space, the wave packets have to be spread across frequency.

 

[Upisoft]

Well, I really think you're trying to look at it at too low of a level without going into the math. To understand the quantum behavior, you really have to go through the quantum mechanical calculations. Suffice it to say, the quantum behavior reduces to the classical behavior in the limit of large numbers of photons.

I think they use mirrors successfully in experiments that involve single photons. That means they are pretty sure about the angle of reflection even in a single photon example.

Well, you were talking about a wave packet. Wave packets occupy a limited frequency range. They have to, if you think about it, because if they had exactly one frequency, they would necessarily be spread across all of space. In order to be localized in space, the wave packets have to be spread across frequency.

Why do you think they are not spread across all of the space? From viewpoint of a photon the start point and the end point are the same (unlimited contraction of space) and it requires no time at all to go from the starting point to the end point.

 

[Chalnoth]

I think they use mirrors successfully in experiments that involve single photons. That means they are pretty sure about the angle of reflection even in a single photon example.

Yes, and the behavior isn't identical to the behavior of an entire laser. It has to be pretty similar, because each individual photon has to behave in a way to add up to the behavior of the whole laser. But it is pretty similar.

And in this case it's also worth mentioning that there's another large number: the number of atoms in the material.

Why do you think they are not spread across all of the space? From viewpoint of a photon the start point and the end point are the same (unlimited contraction of space) and it requires no time at all to go from the starting point to the end point.

It's pretty trivial to show that a laser beam is quite localized. Look at the spot it makes. That spot is small.

 

[acropolis]

Well, I really think you're trying to look at it at too low of a level without going into the math. To understand the quantum behavior, you really have to go through the quantum mechanical calculations.

Quantum mechanics, and the physics of particles on the quantum scale, is really only accessible via the math. Thought experiments at those scales are pretty much useless since the results at those scales are not at all intuitive.

 

[Upisoft]

Yes, and the behavior isn't identical to the behavior of an entire laser. It has to be pretty similar, because each individual photon has to behave in a way to add up to the behavior of the whole laser. But it is pretty similar.

And in this case it's also worth mentioning that there's another large number: the number of atoms in the material.

Indeed, but absorption is a process that transfers the energy of a photon into another particle, usually electron. The number of atoms and their arrangement would only matter if no such process as absorption takes place. The photon should be able to interact with many atoms in the same time. I don't consider that impossible. What I consider impossible is that a photon interacting with one particle and many atoms in the same time.

For example I could accept in double slit experiment that the photon goes through the both slits in the same time. But saying the photon goes through the both slits and through only one slit at the same time is.. well BS.

It's pretty trivial to show that a laser beam is quite localized. Look at the spot it makes. That spot is small.

I was talking about the direction along the photon path.

 

[Chalnoth]

Indeed, but absorption is a process that transfers the energy of a photon into another particle, usually electron. The number of atoms and their arrangement would only matter if no such process as absorption takes place. The photon should be able to interact with many atoms in the same time. I don't consider that impossible. What I consider impossible is that a photon interacting with one particle and many atoms in the same time.

For example I could accept in double slit experiment that the photon goes through the both slits in the same time. But saying the photon goes through the both slits and through only one slit at the same time is.. well BS.

Well, the reason why I used the "large number of atoms" statement is to point out there there are likely to be a great many sequential interactions, such that quantum effects may play a role very near the surface of the material (as in, just a few atoms in), but are likely to average to the classical result very rapidly.

I was talking about the direction along the photon path.

That doesn't really matter. If the photons had a single wavelength, they couldn't be localized in any direction. They also are demonstrably not localized along the path of the laser, as you can turn a laser on and off.

 

[Upisoft]

Well, the reason why I used the "large number of atoms" statement is to point out there there are likely to be a great many sequential interactions, such that quantum effects may play a role very near the surface of the material (as in, just a few atoms in), but are likely to average to the classical result very rapidly.

So, you say reflection is not a single event if you look at it from quantum point of view, Did I got it right?

That doesn't really matter. If the photons had a single wavelength, they couldn't be localized in any direction. They also are demonstrably not localized along the path of the laser, as you can turn a laser on and off.

Why not? The wavelength is function of the energy of the photons. As they are bosons they tend to produce other bosons with the same energy, unlike fermions. That's what makes laser possible.
But I now see whet you are getting at, you just had to mention principle of uncertainty. You can't know position and momentum with unlimited precision, so if you make a laser that gives out pretty much monochromatic light, you necessary need a beam of photons with the same momentum. It can't be only a packet.
Still that does not exclude the possibility that large number of photons, much larger than the number of affected atoms say hundreds or even billions times larger, are somewhere in the transparent material.

 

[Chalnoth]

So, you say reflection is not a single event if you look at it from quantum point of view, Did I got it right?

I don't know. I was thinking more in terms of refraction. Naively, it seems likely to me that reflection can be traced to a single event, but it doesn't always happen at the exact same depth in the material.

Why not? The wavelength is function of the energy of the photons. As they are bosons they tend to produce other bosons with the same energy, unlike fermions. That's what makes laser possible.

Well, this is just basic wave mechanics. It is fundamentally impossible to physically localize a wave which only consists of one wavelength. This is, in fact, the origin of the Heisenberg Uncertainty Principle: the more localized a wave is in space, the less localized it is in frequency (which is related to momentum).

Still that does not exclude the possibility that large number of photons, much larger than the number of affected atoms say hundreds or even billions times larger, are somewhere in the transparent material.

I don't get what you're saying here.