DIscreteness of light

[Deepak Kapur]

Science says that light is not continuous but discrete, as it comes in small packets called 'quanta'

 

I want to ask....

 

Is there any space between the quanta of light? If not, isn't this a continuity in its own right?

[swansont]

You can detect single photons and have systems where at most one photon is present. I don't see how continuity applies.

[studiot]

It's a fair question about space between, if you are going with the little balls or photons model.

 

The best way to answer this is to think of a gas.

The molecules in the gas are buzzing about, but there is lots of empty space around them.

The greater the number of molcules in a givne space the less the free space of course, but there really is normally much more free space than molecule, even in a gas plasma.

 

Now there is one big difference between gas molecules and photons.

Gas molecules have a range of speeds and we discuss the average speed, and the spread.

 

Photons all travel at the same speed. But we can still talk of phton density, just as gas density.

 

So no, continuity is not an issue with this model.

[Deepak Kapur]

You can detect single photons and have systems where at most one photon is present. I don't see how continuity applies.

I want to say that in a ray of light there is no space between the photons that make up the ray.....so, isn't light ray continuous?

[John Cuthber]

I want to say that in a ray of light there is no space between the photons that make up the ray.....so, isn't light ray continuous?

You might want to say that but it isn't always true so you can't honestly say it.

[timo]

Most of the discreteness that is often attributed to light comes from the discreteness of the process that created the light. Not from the nature of light, which is the nature of the electromagnetic field.

 

The electromagnetic field has very few discreteness constraints, especially non that are as trivial as "light only comes in packets" or "you can only have certain energies" or "you can only have certain frequencies". The most famous/important constraint on light itself would be "if you enforce a given frequency (which usually would be an effective condition you have from having a certain creation process in mind), you can only have certain energies". And strictly speaking I think even this is only true when restricting yourself to a certain regime, the so-called free-field solutions (a very important regime for practical work, though).

 

Separating between the discreteness of light and the discreteness of the creation processes may seem like hairsplitting. It is pointless to talk about light that is not created, after all. But the discreteness conditions of different creation processes are different, and I think making this destinction helps in understanding physics.

[studiot]

Interesting distinction, Timo. +1

[swansont]

I want to say that in a ray of light there is no space between the photons that make up the ray.....so, isn't light ray continuous?

 

Light rays are a model for doing ray tracing. It's not a description of the actual mechanics.

[Enthalpy]

Discrete photons means that light is absorbed and created in packets. It does not imply that photons have separated positions.

 

Under very special and difficult circumstances, one can distinguish the emission of two photons, their absorption, and tell that the photon absorbed there was the one emitted then and here. Usually, it isn't the case: too many photons make light, are emitted and absorbed too close to an other to tell.

 

An other, more fundamental difficulty is that, as bosons, arbitrarily many photons can be in the same state exactly. Then, telling "I saw the photon 123456" just makes no sense, because they're strictly identical. They just get detected at random times. How to check a distance then?

 

Multi-photon absorption happens as well, and to my understanding several photons disappear exactly at the same time.

[swansont]

Discrete photons means that light is absorbed and created in packets. It does not imply that photons have separated positions.

 

 

It doesn't mean they overlap, either.

[JohnSSM]

Here is a somewhat related question to this topic.

If light were particles, as they were emitted from a source, wouldnt they begin to spread out, leaving empty space between them? And if you were very far away, and moving laterally to the source, the source would "blink" on and off as you "detected" one particle and then the next. I mean, either the particles spread out as they travel away from a source, or a distance must form between them,. Or the source emits a photon at every possible angle, which seems to be an infinite number, and that doesnt seem right...

No matter how many straight lines you draw, radiating from a source, at some distance, there will still be an inch between those lines...so how do we see a continuous stream of photons from very distant objects as we move?

[studiot]

Perhaps it has something to do with the way our education system singles out wave/particle duality as something special and unusual that makes students stumble so much over the idea.

 

But the situation is not unusual or special at all.

There are many many situations in physics where no one model that we have is good enough for all situations but we just breeze through these without difficulty.

 

Is there such a thing as a light inextensible string or do all stringa possess some weight and some stretch?

Is water compressible or incompressible?

Is air compressible or incompressible?

Are the field lines between the paltes of a capacitor straight or curved?

 

All the above and many more may be modelled either way - and we do so quite naturally depending upon the situation of application.

So it should be with waves v particles.

 

~We should chose either waves or particles or some combination we call wavelets that best suits the situation.

Edited August 15, 2015 by studiot

[Delta1212]

Here is a somewhat related question to this topic.

 

If light were particles, as they were emitted from a source, wouldnt they begin to spread out, leaving empty space between them? And if you were very far away, and moving laterally to the source, the source would "blink" on and off as you "detected" one particle and then the next. I mean, either the particles spread out as they travel away from a source, or a distance must form between them,. Or the source emits a photon at every possible angle, which seems to be an infinite number, and that doesnt seem right...

 

No matter how many straight lines you draw, radiating from a source, at some distance, there will still be an inch between those lines...so how do we see a continuous stream of photons from very distant objects as we move?

They do spread out, but while they don't fire off at every possible angle at once, they can fire off at any possible angle. So it's not a bunch of streams of photons firing off single file. Any point in space you pick that has an unobstructed line of sight to the light source with eventually receive a photon from it. How long it takes is just a mater of how bright and how far you are.

 

So the simple answer to your question is that this does happen. It's just that for anything you can see with the naked eye, you aren't far enough away for the gap between photons to register. We do have to account for this for really, really distant objects, though, which require long exposures to gather enough light to get a good image of them.

[swansont]

Here is a somewhat related question to this topic.

 

If light were particles, as they were emitted from a source, wouldnt they begin to spread out, leaving empty space between them? And if you were very far away, and moving laterally to the source, the source would "blink" on and off as you "detected" one particle and then the next. I mean, either the particles spread out as they travel away from a source, or a distance must form between them,. Or the source emits a photon at every possible angle, which seems to be an infinite number, and that doesnt seem right...

 

No matter how many straight lines you draw, radiating from a source, at some distance, there will still be an inch between those lines...so how do we see a continuous stream of photons from very distant objects as we move?

 

On the one hand, yes. As Delta1212 has mentioned. Most systems do not detect single photons, so we don't notice this.

 

OTOH, photons are not BBs. They show particle behavior when we detect them (localization and energy quantization) but that doesn't tell you what is happening as they travel.

[JohnSSM]

Good answers, all

[Enthalpy]

 

It doesn't mean [the photons] overlap, either.

 

They can overlap, and have to overlap in multi-photon processes like air ionization by laser pulses or frequency doubling and tripling.

One should not distinguish a photon "size" nor "duration" from the wave packet.

If light were particles, as they were emitted from a source, wouldnt they begin to spread out, leaving empty space between them?

 

You shouldn't imagine photons as small objects, and especially you shouldn't imagine them as smaller than the wave packet. That would mislead you.

 

While the photon - just like an electron too - can concentrate into a small pixel if needed, there is no means to tell before that the photon was that small. Any attempt to measure the position or size of the photon during its flight would change the effect at the detector. For instance, for a telescope mirror to work, you need the photon to be reflected over the whole surface. A small photon would give a blurred image.

 

A better working representation os that the particle has no other property than the wave (that's a basic idea of QM) but can keep some attributes when the wave adapts during an interaction. If a photon propagates from light-years away, it is light-years wide. If it interacts with the mirror, there it has the size of the mirror. If it is detected by an electron in a camera pixel, there it has the size of the electron, that is a few thousand atoms in a semiconductor or sometimes more.

 

This resembles much a wave. What we really need from the particle is that the energy packet is still defined by the frequency, plus a few more attributes, like the charge in the case of an electron.

[Sensei]

They can overlap, and have to overlap in multi-photon processes like air ionization by laser pulses or frequency doubling and tripling.

I fail to see how air ionization by laser,

or doubling/tripling frequency are examples of overlapping photons..

 

After absorbing the first photon, excitation of electron in atom, takes some time.

If yet another second photon excite it even further, before it go back to ground state, electron will reach higher energy level, and release even more energetic photon.

 

Send beam of photons with E=10.2 eV (n=1 -> n=2) to Hydrogen,

then another beam of photons with E=1.8889 eV (n=2 -> n=3),

and you will get as a result photons with E=12.09 eV (n=3 -> n=1)..

 

Laser used to ionization is generating exclusively polarized photons.. ?

If two or more photons have different polarizations, then IMHO they don't overlap.

Edited August 18, 2015 by Sensei

[swansont]

can overlap ≠ must overlap, and

don't overlap ≠ can't overlap

[Theoretical]

I would highly recommend you read my previous thread. The atomic world by its very nature produces pulses of light which you could call packets, but this is not a requirement or universal law. I've done numerous experiments at the visible light region which shows intense light shined far away to the point of where quantum mechanics predicts it should be quantized "packets" is not true. Light is classical. Atomic world is quantized in nature, but the very nature of light itself is not quantized. Also I've done extensive so-called "single photon" experiments at radio frequencies below 50 MHz. The results show light is not quantized. In due time all of this and a whole lot more will be documented in PDFs and video.

[Phi for All]

I would highly recommend you read my previous thread. The atomic world by its very nature produces pulses of light which you could call packets, but this is not a requirement or universal law. I've done numerous experiments at the visible light region which shows intense light shined far away to the point of where quantum mechanics predicts it should be quantized "packets" is not true. Light is classical. Atomic world is quantized in nature, but the very nature of light itself is not quantized. Also I've done extensive so-called "single photon" experiments at radio frequencies below 50 MHz. The results show light is not quantized. In due time all of this and a whole lot more will be documented in PDFs and video.

 

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[Phi for All]

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