Basic question regarding the Double Slit Experiment

[josephgerth]

I have a question regarding the double slit experiment. I have been doing a little reading (The Grand Design, to be specific) and I was left with a burning question.

First of all, I am not a physics major. I have taken only one college physics class, and although it is something that really, really interests me, I have really no formal background on anything but classical physics. So stepping into the realm of quantum physics may not be the smartest endeavor of mine, but you have to start somewhere right?

Anyways, according to what I understand of The Grand Design (and please correct me at any point!), the double slit experiment is the basis for many of the quantum theories out there. It was the first experiment that forced scientists into searching for an alternative method for describing their observations, since classical laws did not fully explain everything that was happening at the atomic - sub atomic level. So it's an incredibly important experiment!

As I understand the experiment (and again I have never actually performed or viewed this experiment), you shoot single photons through a slit that is just wide enough to allow single photons through. These single photons should form a double band on the paper behind the two slits, but instead they form an interference pattern, meaning that the single photons somehow went from acting like particles to acting like waves.

Furthermore, if you try to "observe" the slits to see what is happening with the individual photons, you suddenly get two bands on the paper behind the slits. This implies (according to the book, anyway) that the act of observing somehow changes what the particles choose to do (stay as a particle, or change into a wave). (And again, correct me where I have went wrong please).

My first question: How do we know that these single photons are not bouncing off the side of the slits? Even though the side each slit is very small, a single photon is vastly smaller. Metaphorically, if I kick soccer balls through an opening just wide enough for one soccer ball to fit, I wouldn't expect it to go perfectly through that slit and travel at a straight line, would I? Surely, some of the soccer balls would ricochet off the walls of the opening just a little to steer them off their course. And I realize that some soccer balls will miss the opening entirely. I mean that most of the soccer ball will miss the walls of the opening, but it grazes the side just enough to steer it off course.

My next question, and this is the big one: The author of The Grand Design asserts that the act of observing is what causes changes in the experiment. This assertion comes right after he mentions something about how shining light (so that we can see what is happening) on any atomic (or sub atomic) particle changes how those particles act and react. Yet, they state that it the observation alone that changes the system. To me, that says that observing doesn't change a thing; the particles don't say to themselves, "Hey, we're being watched, let's act differently!", they act differently because the light we are using to observe them affects their course. It would be like having fans blow a strong wind as we try to kick them through the ball openings. That wind, while maybe not incredibly impactful, will change the system. Therefore, it isn't the act of observing, but what we do to try and observe, that causes the change.

Sorry if these questions have been asked before. Also, on account of the fact that I use my Kindle App to read my books, I don't have any real page numbers to quote from (although I will gather some quotes if anyone would like). This is my first post; I hope that there are some people out there that can give me some answers.

Thanks for the help!

 

[DrChinese]

My next question, and this is the big one: The author of The Grand Design asserts that the act of observing is what causes changes in the experiment. This assertion comes right after he mentions something about how shining light (so that we can see what is happening) on any atomic (or sub atomic) particle changes how those particles act and react. Yet, they state that it the observation alone that changes the system. To me, that says that observing doesn't change a thing; the particles don't say to themselves, "Hey, we're being watched, let's act differently!", they act differently because the light we are using to observe them affects their course. It would be like having fans blow a strong wind as we try to kick them through the ball openings. That wind, while maybe not incredibly impactful, will change the system. Therefore, it isn't the act of observing, but what we do to try and observe, that causes the change.

Welcome to PhysicsForums, josephgerth!

These are good questions. The answer was not always entirely clear, but we can now say that the answer is NO: the observation itself does NOT change the system in the way you describe. Somehow, when the knowledge itself is gained, that is what changes things. There are many experiments that prove this, but here is one.

Place a polarizer in front of slit A and another in front of slit B. You probably know that a polarizer filters light and that certain orientations of pairs of these will filter light according to Malus' cos^2 theta rule, where theta is the difference in the angle orientation.

When the A and B polarizers are parallel (at the same angle), there is a full normal interference pattern. When the A and B polarizers are crossed (90 degrees apart), there is NO interference pattern. The difference between these is that in the second, it is possible to determine which slit the photon goes through. So no interference, as your book says. In the first case, the which slit information is not available. The point being that in both cases, the photon traverses one polarizer. So obviously the polarizer itself is not the issue.

 

[Astralos]

I cannot believe this. I am actually also reading The Grand Design on my Kindle App on my PC as well, and I just finished reading all of Chapter Four. Quantum physics is a new endeavor to myself as well, and I had the same questions that you have. According to the author, it is the act of observing, not the method of observation, that causes the particles/waves to act differently. Like he said, it is the act of observation in the present that changes the history in the past. The particle took all possible paths before getting to its destination. The interference comes from its ability to gain knowledge from its own superposition.

 

[DrChinese]

According to the author, it is the act of observing, not the method of observation, that causes the particles/waves to act differently. Like he said, it is the act of observation in the present that changes the history in the past. The particle took all possible paths before getting to its destination. The interference comes from its ability to gain knowledge from its own superposition.

This is accurate. And to top off the point: it is possible to "learn" the which path information (usually with a beam splitter) and later "erase" that knowledge. When this is done, by recombining output streams (no easy task because of the precision required) so that the actual path taken can no longer be determined, the interference pattern is present. Variations on this experiment have been performed: DCQE (delayed choice quantum eraser) and tests of Hardy's paradox with an interferometer, for example. While these demonstrate somewhat different effects, the idea is there.

 

[naima]

In your first question you suggested that the photon bounced again the sides of the slits.
In fact you have to consider (to sum over) all the possible paths through the slits.
Do not forget those which are not linear and those which do not interact with the matter around the slits.

 

[BruceW]

The first question: Its likely that any reflection from the side of the slit is negligible, and its likely to just be absorbed anyway. (Also the interference pattern doesn't fit with the idea that the photons had bounced off the sides of the slit).
Second question: when the observer finds out which slit the photon went through, it changes the wavefunction, regardless of how the observer found that out. This is in fact one of the postulates of quantum mechanics, that can't be proved, but they seem to be true since all the experiments done so far have agreed with quantum mechanics.

 

[San K]

I have a question regarding the double slit experiment. I have been doing a little reading (The Grand Design, to be specific) and I was left with a burning question.

My next question, and this is the big one: The author of The Grand Design asserts that the act of observing is what causes changes in the experiment. This assertion comes right after he mentions something about how shining light (so that we can see what is happening) on any atomic (or sub atomic) particle changes how those particles act and react. Yet, they state that it the observation alone that changes the system. To me, that says that observing doesn't change a thing; the particles don't say to themselves, "Hey, we're being watched, let's act differently!", they act differently because the light we are using to observe them affects their course. It would be like having fans blow a strong wind as we try to kick them through the ball openings. That wind, while maybe not incredibly impactful, will change the system. Therefore, it isn't the act of observing, but what we do to try and observe, that causes the change.


Thanks for the help!

the act of observing has nothing to do with it...observing or not will not change the outcome...but the setup will...

its the setup/instruments/configuration/orientation...that changes the (results of) experiment

you can say:

the setup interacts (entangles) with the photons
or
the photons interact with the setup
or
both interact (entangle) with each other

and they interact non-locally...faster than the speed of light...perhaps "beyond/outside" space-time