# What do photons look like?

1. Nov 10, 2007

### wawens

A particle is easy because it is made up of an addition of wavelengths that, (Schrodinger like), localize the particle into a kind of 'packet'. And we cannot say exactly where the particle is because, well, its a collection of waves so of course we cannot say its at a certain 'point' Fine, no problems. But what about light of constant frequency? A constant frequency wave (eg red light) has a fixed momentum and as such we know its momentum exactly, therefore we can know nothing about its loaction. Imagine a sine wave of fixed wavelength going somewhere. This is the uncertainty principle. We cannot say, that the photon is 'a little bit of the wave' because we would need a length of the wave to take.
If we know its momentum exactly we cannot know its location. Fine, so when a 'photon' gets absorbed by a screen we can say nothing about its path? ALso, the photon time stands still, so from its point of view it took no time to get from slit to screen. Indeed a journey of a billion years would be no instant for the photon.
So, how can we visualize a photon 'packet' (rememeber its made of one frequency only)

No bluffing now......

2. Nov 10, 2007

### jdogg0075

I think you are mixing up both waves that make "particles", and physical waves that particles make.
Im not sure if im understanding you here. But i think i do. Your referring to say a specific wave frequency of like for example 510 nm. Modern science can now measure things with these sort's of lengths they are assumed to be "known". I could be compleatly misunderstanding you though. : )

I also wouldn't say that a photon has an exact momentum. Photons are massless, thus no momentum. They do have KE though that can stop things with momentum.

And finally how we "visualize" photons is through a neat retina and just entire eye that converts things into what we see. As far as im concerned particles are not real. In our 3D eyes they might be. But really everything exists as a wrapped up energy "kink".

-Justin

3. Nov 10, 2007

### Vazier

before i give my opinion on the question i think the statement you gave is false cause what i think is the wave packet is a probability wave and when in maths we write it as a collection of waves it is only because that is how the waves mechanics is worked out and it isnt a collection of probability waves.

anyways i am assuming you are asking what the probability wave of a photon looks like and gave the answer likewise but if you are asking what a photon actually looks like, well we cannot say cause we need to see the photon using something that is smaller than a photon and i dont think there is anything we can use right now.... i dont even think we know the size of a photon :D

anyways i think the wave of photon is a gaussian curve. well to check this you would have to solve the schrodinger eq with the eq for a gaussian curve and try but this the best i got, how i got to this is because we cannot know the frequency and wavelength of the photon with complete accuracy and this gives us a uncertainty in the velocity and hence the position of the photon and since the photon cannot reach the a detector early or later (cause we know the velocity of the photon to some extent) the wave has to die out on either side of the probability wave.

Last edited: Nov 10, 2007
4. Nov 11, 2007

### wawens

Length of a Photon

Thanks for great replies.
Lets say one photon starts off from a single
aperture and arrives at a screen some time later.
The photon is 510 nm. Maybe from a laser gun.
Now, what length is that photon? Is it in some sort
of packet with a beginning and an end? Is it a magnetic/
electric oscillation of say, 123 beats? Is it a sine wave
of 12.9 meters long that is abruptly cut off at both ends?
we know what happens when it gets to the screen (absorbed
etc) but what is it on its way to the screen.

Incidentally, I was lead into this line of questioning because of
the 'delayed choice experiment'.
http://www.ronsit.co.uk/SimpleDelayedChoiceExperiment.asp

5. Nov 11, 2007

### wawens

Length of a Photon

Thanks for great replies.
Lets say one photon starts off from a single
aperture and arrives at a screen some time later.
The photon is 510 nm. Maybe from a laser gun.
Now, what length is that photon? Is it in some sort
of packet with a beginning and an end? Is it a magnetic/
electric oscillation of say, 123 beats? Is it a sine wave
of 12.9 meters long that is abruptly cut off at both ends?
we know what happens when it gets to the screen (absorbed
etc) but what is it on its way to the screen.

Incidentally, I was lead into this line of questioning because of
the 'delayed choice experiment'.
http://www.ronsit.co.uk/SimpleDelayedChoiceExperiment.asp

6. Nov 11, 2007

### lightarrow

Let's talk about the photon wavefunction and the corresponding wavepacket (which is not the photon): it can be almost any lenght, from ~ 10^(-8) m to many kilometers; if the photon were *exactly* monocromatic, as in your initial question, that lenght would be infinite. I don't know how much long can be made with the present technology.

About what exactly is a photon...welcome in the world of "no one knows".

Last edited: Nov 11, 2007
7. Nov 11, 2007

### Gazouille

What if there is simply no "photon on its way to the screen"?
Photons that are there for one observer don't exist to some other observers.
An example is the question "Does a charge in free fall (accelerating) emit photons?" : depends for whom.
To me, a photon is a link between two 4D points. It doesn't need to have an "appearance" of its own.

Last edited: Nov 11, 2007
8. Nov 12, 2007

### Vazier

if you go on to read the John Wheeler's philosophy link given below and read through that you will find a very interesting point. Wheeler says that the photon is in a state where it is the intermediate between a particle and and a wave and that its behaviour changes according to the experiment being done. this the the concept of duality of matter and particle which is one of the most important statements in the world of quantum mechanics. to answer the question of what is a photon, it a paricle or a wave depending on the situation but not both. i know it is hard to undertsand that if your just started learning it but ...... well welcome to quantum mechanics...... :tongue2: