Re: Why can't waves cause the photoelectric effect

From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 03/04/05 

Date: Fri, 04 Mar 2005 11:28:54 +0100

franklinhu@yahoo.com wrote:
> Bjoern Feuerbacher wrote:
>
>>franklinhu@yahoo.com wrote:
>>

[snip]

>>Please define what exactly you mean with "intensity" and "energy
>>level" here.
>>
>
>
> I would say that this is indeed a major problem with describing this
> experiment. The explanations I've seen leave this ambiguous and seem to
> correspond to the idea that if it is brighter to our eyes, it has more
> energy and more intensity.

Well, what explanations *have* you read? Must I remind you again that
you shouldn't rely on popular science sources for learning something
about physics, but try to get a real education?

> However, a more precise description would be
> that the intensity could correspond to both a combination of the wave
> amplitude and the number of wave cycles hitting an object per unit
> time.

I don't think that any physics text would actually define it that
way.

A much more sensible assumption would be, IMO, that they used the
power consumption of the light source for determining the light
intensity, i.e. "intensity" refers to the energy content of the
light (and according to Maxwell's equations, therefore to the
square of its amplitude).

>>>so we would expect
>>>that higher intensity would increase the speed of the electrons
>>>from
>>>the surface. We don't see this experimentally, so we must conclude
>>>that light is a particle.
>>
>>That's only part of the reasoning. You conveniently ignore the other
>>part: that the speed of the electrons indeed increases when the
>>frequency is increased.
>>
>
>
> No, I did explain in a commonsense way, that the faster you shake
> something (increase frequency) If there are parts which are loosely
> bound (like nuts in a tree), these parts will be ejected faster with
> an increase in frequency.

I must have missed that part of your explanation.

Anyway, this conclusion does not follow in general. For starters,
electrons bound in a metal have little to do with nuts in a tree.

[snip]

>>>As such, the
>>>duration of the wave generated is limited - probably to only a
>>>single wave
>>
>>"single wave" makes little sense. Do you mean "single period"? If
>>yes,
>>you are utterly wrong. Light frequencies are around 10^15 Hz, light
>>emission takes about 10^(-10) s. Hence there are 10^5 periods in
>>every wave train emitted.
>>
>
>
> That is very interesting that it takes that long to eject light from a
> single electron transition. How on earth did they determine such a tiny
> time period?

There are various possibilities, I don't remember all the details.
For the 100th time: open a book on atomic and molecular physics,
where all these things are explained.

IIRC, one method is to measure the "intrinsic line width" Gamma, which
is connected to the time of light emission by
   t = h / Gamma,
where h is Planck's constant.

> Do we have any theories on how it produces this 10^5 wave
> packet from an electron moving from one energy level to another?

Yes. QM.

> Has this length ever been experimentally measured?

What one can measure is e.g. the so-called "coherence length", AFAIK.
That gives a good estimate for the length of the light trains.

For the 101th time: open a book on atomic and molecular physics,
where all these things are explained.

> In any case, my argument did not rely on it being a single period, but
> rather that it have a definite and limited duration. This makes each of
> the "wave packets" nearly identical to each other as they are
> generated. It is this identical nature which explains why when it hits
> the metal surface, it ejects electrons with a consistent speed.

As already mentioned, the idea of "photons = wave packets" is
anything but new.

>>>and its amplitude is also limited since the range of motion from
>>>energy level to energy level is limited.
>>
>>The amplitude of the emitted wave has precisely nil to do with
>>the "range of motion from energy level to energy level".
>>
>
>
> A more precise description would be that the change of electron energy
> from level to level is fairly fixed, which determines the frequency and
> amplitude of the wave generated to be a fixed value.

Yes, that would be better. And even better would be if you talked
about "wave packet" here instead of "wave".

[snip]

>>>It would not be a continuous wave of variable amplitude. The
>>>intensity
>>>would not be due to the amplitude of the individual waves, but
>>>rather the density of waves hitting an object.
>>
>>I can't comment on that as long as you don't tell me how exactly
>>you define "intensity".
>>
>
>
> I think the idea of intensity is the key issue here. The usual theory
> says that intensity corresponds to amplitude of the wave.

That theory would be called "Maxwell's equation". You did not bother
to study them in the meantime, did you?

> The wave
> itself is thought to be continuous like a radio carrier wave.

No, that's not necessary for associating intensity with the amplitude.

[snip]

>>>A bright object is simply
>>>emitting more of these "single" waves per unit time than a dim
>>>object.
>>
>>Care to tell me what the big difference (conceptually) between your
>>concept of "single waves" and the concept of photons is? You *do*
>>know that photons are often described as *wave packets*, don't you?
>>
>
>
> As you describe it, there is no difference. What I am describing is
> exactly a wave packet - which is a wave phenomenon, not a particle
> phenomenon.

Err, ever heard of "wave-particle duality"? That is exactly the point
- that photons have partly wave properties and partly particle properties!

You are free to try explaining the Compton effect also with your idea
of wave packets.

> A photon described as a wave packet makes perfect sense in
> the photolectric effect.

As a qualitative picture, yes. But as usual, as soon as you
try to get quantitative, this won't work out.

> A photon does not need to be a corpuscular
> particle with a particular volume, diameter, etc. and occupies a
> particular quanta of space.

Err, no particle physicist ever pictured a photon as having a volume,
diameter etc.

And what exactly is "a quanta of space" supposed to mean? First,
"quanta" is plural; the singular would be "quantum". Secondly, we
don't know if space is quantized.

>>>This seems terribly obvious based upon what we know about light
>>>generation, so why is this argument not brought up?
>>
>>Essentially, it is. Photon = wave packet is an idea which has
>>been around for 70-80 years now. It works for many applications
>>- but ultimately, it fails if you look closer at the idea.
>>
>
>
> Then you would agree that the photon = wave packet is a reasonable
> explanation for the photoelectric effect

A *qualitative* explanation, yes. Feel free to try to *quantify* this
and see if it still works.

> and that the photoelectric
> effect by itself is not a great piece of evidence for the particle
> nature of light since a wave explanation cannot be ruled out?

Again, there is not such a big difference between "wave packet"
and "particle".

> Why did Einstein get a Nobel prize for the photoelectric effect when it
> had a fairly simple counterargument?

Because what you presented is not a counterargument to the particle
interpretation. You simply assumed that physicists mean "something
with a definite volume, diameter etc." when they say "particle". But
they don't.

The point of Einstein's interpretation was *not* that "light consists
of things which have a definite volume, diameter etc.". It was
"light consists of discrete things with definite energy, the energy
is not distributed continuously over the wave". And nothing you said
contradicts that.

> What are these other closer ideas which make the wave picture of
> photons fail?

1) Doing quantitative analysis instead of just handwavings for the
photo effect.
2) Compton effect.
3) Essentially every single effect predicted by QED.

Bye,
Bjoern

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