# The wave/particle nature of light

## Main Question or Discussion Point

Hi, i was just wondering if the experts here can fill me in on the current understanding of the issue. My teacher told me something about the particle model can explain diffraction due to photons having a gravitational attraction to the nearest slit??

It would be good if someone could make me a list of the contrasting differences of the particle/wave model and where we're likely to be headed in the future in terms of understanding, thanks in advance.

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mathman
The two classical experiments that led to the idea of wave-particle duality are the double slit experiment (diffraction pattern) and the photo-electric effect (Einstein's Nobel Prize). A good (but not too technical) detailed description is a little book by Feynman called QED.

ZapperZ
Staff Emeritus
2018 Award
SeReNiTy said:
My teacher told me something about the particle model can explain diffraction due to photons having a gravitational attraction to the nearest slit??
Pardon me, but where do you go to school? Someone needs to pull your teacher out of a physics class and teach him/her some basic physics. Something as nonsensical such as this should not be tolerated. I shudder at the thought at other possible errors you are being taught.

Zz.

Kane O'Donnell
He might not have meant gravitational attraction in the technical sense, but more that the particle 'gravitates' towards one or the other of the slits. However, this kind of imprecision is *very* dodgy practice in Physics education.

In any case, the teacher is wrong, at least in the sense that modern QM dictates that in a two-slit (or however-many-slit) experiment the particle trajectory has no meaning - the experiment doesn't measure particle trajectory, it measures where a particle hits a detector after traversing the system.

It turns out that if you send a heap of particles through, the statistical distribution starts to look like the distribution you'd get if it were waves hitting the slits, not particles.

In that sense, there is some crossover between what we consider 'wave' and 'particle' behaviour, but it is just plain silly to wonder whether an electron, for example, is a particle or a wave. An electron is an electron. We have very good theories that describe the behaviour and properties of electrons. Some of these properties (such as 'wavelength') happen to have similarities with classical wave properties. On the other hand, when you perform experiments on single electrons (or photons, same deal), they always look like single electrons - a single electron going through a two-slit system will be detected in *one* spot on the detector screen, not the whole screen at once. It takes a heap of electrons before the 'interference pattern' looks like an interference pattern.

There are a lot of physicists (and people who read popular science books) who have decided that particles are particles and waves are waves, and even that electrons are waves or photons are particles or whatever. The problem is, human beings were the ones who decided what waves and particles are defined to be, and the universe doesn't have to be that way.

It has been my practice, much to the chagrin of my girlfriend (also a physicist) to refer to any object described by QM as a 'quob' - a QUantum OBject, because it removes the notion that QM describes waves/particles. It describes quobs.

Ahh, now back to the thing I was intending to write about. Feynman was fairly unequivocal about whether electrons, say, are particles or waves - he calls them particles, but what he means is quobs ( ). If you're looking for a *discussion* about wave/particle duality, you might not find it in his writings.

Kane O'Donnell

I am attending year 12 which is the last year before university in australia. What my teacher was saying is Newton tried to explain diffraction with the gravitational attraction of photon's towards the slit, and he is saying some scientists very recently (last 3 months) proposed the same thing except with more complication, i was just wondering what is the "current" stance on the issue.

Btw, thanks very much to those who helped out!

Kane O'Donnell
Well that explains what he meant by gravitational attraction

By the way, give me a buzz if you need any help/advice about HSC Physics, I guess you'll be going into exams soon? I did the new HSC Physics syllabus/exam in the first year it came out. Hope you're enjoying it.

Cheers,

Kane

mathman
On the other hand, when you perform experiments on single electrons (or photons, same deal), they always look like single electrons - a single electron going through a two-slit system will be detected in *one* spot on the detector screen, not the whole screen at once. It takes a heap of electrons before the 'interference pattern' looks like an interference pattern.
The important point is that, even though the electrons or photons go through the slit one at a time, a diffraction pattern builds up. Thus the individual particles appear to act like waves.

Kane O'Donnell said:
Well that explains what he meant by gravitational attraction

By the way, give me a buzz if you need any help/advice about HSC Physics, I guess you'll be going into exams soon? I did the new HSC Physics syllabus/exam in the first year it came out. Hope you're enjoying it.

Cheers,

Kane
Yes i am approaching exams very soon, it would be very generous of you if you could give me a few pointers.

The topics are:
Motion (40%)
Gravity (15%)
Sturctures and Materials (30%)
Light and Matter (15%)

russ_watters
Mentor
SeReNiTy said:
I am attending year 12 which is the last year before university in australia. What my teacher was saying is Newton tried to explain diffraction with the gravitational attraction of photon's towards the slit, and he is saying some scientists very recently (last 3 months) proposed the same thing except with more complication, i was just wondering what is the "current" stance on the issue.
A gravity-based model for diffraction just plain wouldn't work.

Several problems just off the top of my head: a piece of steel with two slits has a significantly higher mass than a piece of aluminum with two slits. So the diffraction pattern should be different for each. Is it....?

Second, a gravity-based diffraction pattern would be a parabolic distribution and would look almost exactly the same were you to have one slit, two slits, 3 slits, etc (provided the screen was far enough from the slits). Does it...?

Third, the earth has a gravity field quite a bit stronger than any plate with two slits in it (massive understatement). Flipping the experiment on its side should vastly affect the outcome. Does it...?

Quite frankly, your teacher should be teaching what he's supposed to be teaching.

Chronos
Gold Member
Bohr is attributed to having said "If you aren't confused by quantum physics then you haven't really understood it".

russ_watters said:
A gravity-based model for diffraction just plain wouldn't work.

Several problems just off the top of my head: a piece of steel with two slits has a significantly higher mass than a piece of aluminum with two slits. So the diffraction pattern should be different for each. Is it....?

Second, a gravity-based diffraction pattern would be a parabolic distribution and would look almost exactly the same were you to have one slit, two slits, 3 slits, etc (provided the screen was far enough from the slits). Does it...?

Third, the earth has a gravity field quite a bit stronger than any plate with two slits in it (massive understatement). Flipping the experiment on its side should vastly affect the outcome. Does it...?

Quite frankly, your teacher should be teaching what he's supposed to be teaching.
Yes is see exactly what you mean now, thanks for clearing that up for me, once school holidays are finished my teacher would be confronted...LoL

russ_watters
Mentor
SeReNiTy said:
Yes is see exactly what you mean now, thanks for clearing that up for me, once school holidays are finished my teacher would be confronted...LoL
Just don't be too confrontational - at some point, this teacher will need to grade you...

The way I view elementary "particles" like photons, conceptually speaking, is quite simple. A quantum "particle" is a wave that counts as one unit. It moves like a wave, but since it's still only one unit it can only interact at one point, and for this reason it looks like a particle. The quantum is a wave that passes through both slits, forms a diffraction pattern, and subsequently interacts at a point on the other side of the slits. The intensity of the wave provides that the quantum is more likely to be absorbed in certain areas than in others.

russ_watters
Mentor
If that works for you, great, but I still don't understand how a particle or even a clip of a wave can be in two places at once (or perhaps, nowhere except when its detected). Its bizzare....but I know its true.

Quantum mechanics is weird