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Quantum Physics - The double slit experiment * 1
    #8167778 -



Light can be a defined as a wave or a particle but it changes its properties when it is observed.

Mind bending! How can this be?

Why does having an observer effect the physical properties?

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Re: Quantum Physics - The double slit experiment [Re: MDMC]
    #8167807 -

makes me think of if a tree fell in the woods did it make a noise


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Re: Quantum Physics - The double slit experiment [Re: IAMenlightened]
    #8168439 -

Seems the noise may choose to sound different when it knows you are listening.

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Re: Quantum Physics - The double slit experiment [Re: MDMC]
    #8168449 -

Not just light but particles (mass) also! Why? Who knows why the universe is the way it is...

Look into the Heisenberg uncertainty principle to gain a little conceptual understanding of why you cannot observe something without effecting it. In short the only way to observe anything is to bounce shit off of it. With sight we bounce light off of things we see. But of course when you bounce light off of something, you effect it.

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Re: Quantum Physics - The double slit experiment [Re: DieCommie]
    #8169386 -

I thought we absorbed light with our eyes, I didn't know we shot light out of them at stuff.

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Re: Quantum Physics - The double slit experiment [Re: Newbie]
    #8169442 -

:rolleyes: We use light bulbs to shoot light at stuff... but you already knew thats what I meant :wink:

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Re: Quantum Physics - The double slit experiment [Re: DieCommie]
    #8169624 -

Not me, I capture ambient light with my forehead and focus it in the direction of my gaze.



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Re: Quantum Physics - The double slit experiment [Re: MDMC]
    #8171242 -

Why does having an observer effect the physical properties?

This is a fallacy, probably based on the fact that quantum physicists use a different meaning for "observer" than is normally assumed.

When you do an experiment with light, like the double-slit experiment, what changes is the setup of the experiment...not the actual light. The light is exactly the same, whether we measure it as being a particle or as a wave.


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Once, men turned their thinking over to machines in the hope that this would set them free.
But that only permitted other men with machines to enslave them.

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Re: Quantum Physics - The double slit experiment [Re: trendal]
    #8171606 -

What do you mean what changes is the setup of the experiment? The light does change if it is observed or not. You cannot measure light as a wave, when you measure it it collapses to a particle. If you run the double slit experiment (with light or mass) and dont look to see what slit it goes through, you get an interference pattern. If you do look to se what slit it goes through, you do not get an interference pattern. That is a clear indication that observation does have an effect.

John Bell showed this is the case in 1964. He proved that there are no hidden variables, and the fact that we dont know where the particle is before observation is not due to our ignorance but is a fact of nature. This eliminates the so called 'realist' interpretation, and leaves only the copenhagen or agnostic interpretations.

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Re: Quantum Physics - The double slit experiment [Re: DieCommie]
    #8171668 -

What do you mean what changes is the setup of the experiment?

Very simple...the experiment changes based on whether you have 1 or 2 slits open. A change in the experiment provides a change in observation.

The light does change if it is observed or not.

"Observed" is such a loaded word...

Rather, we should say that light is affected by whatever it is we use to observe it. The idea of "observed" meaning an "intelligent observer" is wrong to use here.

You cannot measure light as a wave

Really? Then what is the double slit experiment performed with two open slits doing?

when you measure it it collapses to a particle

The notion of a "particle" is wrong and out-dated. Light is not a small ball of anything.

If you run the double slit experiment (with light or mass) and dont look to see what slit it goes through, you get an interference pattern. If you do look to se what slit it goes through, you do not get an interference pattern. That is a clear indication that observation does have an effect.

Again, observation (although commonly used) is not the right word to use. If you set anything in the path of the light beam to detect it, you are changing the experiment. It isn't the same as running the experiment without a detector.


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Once, men turned their thinking over to machines in the hope that this would set them free.
But that only permitted other men with machines to enslave them.

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Re: Quantum Physics - The double slit experiment [Re: trendal]
    #8171679 -

The point I'm trying to get across is that light is only "weird" when you think of it in a classical physics approach. If you cast aside any and all notions of "particles" or "waves" and just look at what experimenting tells us, it isn't strange at all.


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Once, men turned their thinking over to machines in the hope that this would set them free.
But that only permitted other men with machines to enslave them.

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Re: Quantum Physics - The double slit experiment [Re: trendal]
    #8171776 -

Quote:
The notion of a "particle" is wrong and out-dated. Light is not a small ball of anything.



So you are saying that wavefunctions dont collapse to particles when observed? (yes, I know observed does not imply consciousness)

Quote:
Really? Then what is the double slit experiment performed with two open slits doing?

You can infer that it was a wave going through the slits, because the particles that impact the screen form an interference pattern. So, what I mean was if you make a observation it will collapse to a particle. If you wait for it to impact the screen to make you observation, it will be a particle, but the particles form an interference pattern.

Quote:
If you cast aside any and all notions of "particles" or "waves" and just look at what experimenting tells us, it isn't strange at all.

And what is it that it tells us? It tells us that it behaves as a particle sometimes, and a wave sometimes. You have to take both into account to describe experiments. Why would you cast aside waves and particles? The schrodinger equation is a wave equation, and the collapsed wave function is the delta function which is a particle.

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Re: Quantum Physics - The double slit experiment [Re: DieCommie]
    #8172164 -

The collapsed wave function is merely the highest probability location of the eigenstate of the linear superposition. Just think of the hydrogen atom, if you try and measure where the electron is it will take the highest probability location, I forget what that turns out to be. You just use the position operator. It is true though, that if you place a detector at a slit it will mess up the results, there a lot of theories why that is. It goes deeper than just the 'we cant observe it' phenomenon, you can explain it much better mathematically.

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Re: Quantum Physics - The double slit experiment [Re: trendal] * 1
    #8172924 -

> and just look at what experimenting tells us, it isn't strange at all.

Ah, so please explain the mechanism behind this non-strange event.  :wink:


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Just another spore in the wind.

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Re: Quantum Physics - The double slit experiment [Re: trendal]
    #8172990 -

Your saying that when a device observes which hole, it has to alter the particle/wave in the very act of observing it?

Isn't there a method of detection that doesn't alter it? I suppose not.


What about the particle interfering with itself? That is strange!

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Re: Quantum Physics - The double slit experiment [Re: trendal]
    #8172998 -

Can you explain entanglement?

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Re: Quantum Physics - The double slit experiment [Re: DieCommie]
    #8174977 -

So you are saying that wavefunctions dont collapse to particles when observed? (yes, I know observed does not imply consciousness)

I'm saying the notion of "particles" doesn't work for light.

You can infer that it was a wave going through the slits, because the particles that impact the screen form an interference pattern. So, what I mean was if you make a observation it will collapse to a particle. If you wait for it to impact the screen to make you observation, it will be a particle, but the particles form an interference pattern.

Then don't you see my point? Light is not made of particles. The fact that we see two different things in the double slit experiment is proof of that. Likewise, light is not made of waves, either.

Your last sentence didn't really make sense...how can a "particle" create an interference pattern with itself?

And what is it that it tells us? It tells us that it behaves as a particle sometimes, and a wave sometimes. You have to take both into account to describe experiments. Why would you cast aside waves and particles? The schrodinger equation is a wave equation, and the collapsed wave function is the delta function which is a particle.

Our observations of light under some circumstances appears as a wave...in others it appears as a particle. That doesn't mean that light is a wave or particle...just that it appears as such. You're right, we do have to take both into account when describing light.

Why would you cast aside waves and particles?

Why? Because they cannot both be an accurate description of light.


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Once, men turned their thinking over to machines in the hope that this would set them free.
But that only permitted other men with machines to enslave them.

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Re: Quantum Physics - The double slit experiment [Re: trendal]
    #8175890 -

In the end, we talk about whatever model makes more sense for what we are doing. There are many physical models that talk about light as a particle, and some that refer to it as a wave, and some that you have to take both into account. I think you are arguing a semantical point here Trendal.

For instance, when doing nonlinear optics we more or less take both into account to get out our non linear susceptibilities. You take the Schrodinger equation and for the potential you use the dipole moment caused between the incoming electric field and the electron.

However you can also simply use a Lorentzian model where for instance the electron is simply a particle. In this model the electrons are attached to springs that move around with the electric field.

Also theres a model that you can use where the photons behave as particles and the susceptibility is based off of momentum transfer and other things.

The results are all similar, though the SE gives you the entire picture, the other models give you a fair amount of information and if all you wanted was certain bits of info those models work just fine. I think you have the impression that physicists always work with the most accurate models or keep their equations as physicaly realistic as possible. The truth is that scientists dont try and make things more complicated than it needs to be. There are so many expansions out to the first order, for instance you will find first order Taylor expansions several times in some derivation, or you will find this expansion: (1+x)^1/2 = 1+x/2 Things like that, and these equations still work.

It all depends what level of accuracy you are going for. Sure as I said there are cases where youd want to take the particle/wave into account, but there are a huge amount where you can choose which one you want to model.

Another example would be mode-locked lasers. In the laser cavity you will have a large number of modes of light, these will build with constructive interference and get released in pulses. This is a wave model.

And other things will use a particle model.

Basically, it does make sense to model light as a certain thing, it just depends what you are trying to get accomplished.

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Re: Quantum Physics - The double slit experiment [Re: TheCow]
    #8176019 -

damn, this shit is fascinating. you know, i don't really understand any of this, but it's fascinating as shit to me. what the hell is quantum physics? what is antimatter and how did they make an matter antimatter explosion?


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:kingcrankey: I did not say to edit my signature soulidarity! Now forever I will never remember what I said about understanding the secrets of the universe by paying attention to subtleties!

:facepalm: I'm never giving you the password again. Jerk

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Re: Quantum Physics - The double slit experiment [Re: imachavel] * 1
    #8176221 -

Quantum physics basically is the laws that govern things on a small scale. The way things work on small scales is very different then the way we intuitively think things work. One popular example that turns your intuition upside down is quantum tunneling, basically there is a finite chance that any particle or collection of particles can move through potential barriers (like a wall) without destroying the barrier or wall. You can calculate the probabliity you would be able to go right through a door without breaking it. (its a very low probability as you would guess...) Interesting mind bending stuff!

Anti-particles are not as interesting as they sound (I think). For every particle, there is an anti particle. The anti particle has the same spin, mass but the opposite charge. For example the anti particle of an electron is a positron. Instead of being negative it is positive. If a particle collides with an anti particle they annihilate each other and turn into pure energy (gamma rays). Hence we dont have anti matter around us, it would annihilate with the 'regular' matter. The anti matter explosion you speak of is the process of annihilation.

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