Novie questions about the double-slit experiment

[sophiecentaur]

Let's forget about the 'particle-like' nature once for all then.

I am not sure how one can take this example as an equivalent one to the double-slit experiment. For it to be so, whether a given antenna emits or not would depend on the receiver. Depending on where the receiver would be, an interference pattern would be received or just a single-wave one. And if one receiver able to receive only a single-wave pattern does receive it, then a second observer in principle able to see an interference pattern wouldn't have a chance to see it.

Am I right?

There is a direct similarity between light emerging from a narrow slit and being diffracted into a wide (180degree) beam and the signal from a radio antenna (vertical dipole / omnidirectional). The detection of the illumination pattern of the screen by the light depends as much on the sensitivity of the light measuring equipment (your eye) as it does on the right tuning of the radio receiver. In the one case we have two slits with coherent light hitting them from behind (essential) and in the other case we have two antennae fed with the same RF signal (the two transmitted signals are also coherent). The pattern with one experiment can be an exact scale model of the pattern with the other if you get the ratios of dimensions and wavelengths right. I we assume that the receive antenna has a wide directivity - just the same assumption as for the projector screen.
Does that address your problem? (I think that must be your point.)

Don't forget, this is the basic principle of all directional radio antenna arrays.

 

[lostprophets]

greetings
like this subject..
has anyone made the plate which has the slits init out of the same stuff as the plate at the back...
by this i mean .if i fire one electron/photon at a plate of two slits , can we see or measure if any of that electron/photon hit the plate and did not go through the slit..
this i imagine would be a very teeny weeny bit..
one photon particle must surely have its own baby photons/particles coming off it.
so
if we had a plate with one slit would we still see a spread on the plate with the slit.
whilst the particle goes through the slit,the baby particles coming off IT would surly hit the plate and not go through the slit,just as some would also go through the slit.

 

[sophiecentaur]

greetings
like this subject..
has anyone made the plate which has the slits init out of the same stuff as the plate at the back...
by this i mean .if i fire one electron/photon at a plate of two slits , can we see or measure if any of that electron/photon hit the plate and did not go through the slit..
this i imagine would be a very teeny weeny bit..
one photon particle must surely have its own baby photons/particles coming off it.
so
if we had a plate with one slit would we still see a spread on the plate with the slit.
whilst the particle goes through the slit,the baby particles coming off IT would surly hit the plate and not go through the slit,just as some would also go through the slit.

If I were you, I'd avoid some of the pictures of this that you clearly have in your head. They are seriously flawed and won't help you - believe me. Deriving the formulae for interference using the particle model is waaay beyond you . Stick to waves if you want to understand what's going on because Quantum Mechanics is not needed at this point.
A home brewed model is pretty much doomed to fail you - seriously.

 

[lostprophets]

i hear you and i understand how the wave works.
i also see the single photon as a wave in its self.and not just a singular.
it has its own wave in my mind.this maybe wrong i don't know.
videos that use words such as ..a single photon/particle,to me are miss leading to the ordinary person not in the know.so why are they used?
thats not a question directed at you or anyone ,its just an observation.

so i know how the photon/wave works in the split experiment but not the why when observed. but I am sure i will. but to do so i need to ask questions as i don't have the tools for practical.
i have no idea what measuring tool is used to measure/watch the wave.or what effect it would have on the wave other than making it act different.
i take it the measuring eye has been put in various places to measure and not just at the front by the slit...ie behind the slit?

 

[sophiecentaur]

i hear you and i understand how the wave works.
i also see the single photon as a wave in its self.and not just a singular.
it has its own wave in my mind.this maybe wrong i don't know.
videos that use words such as ..a single photon/particle,to me are miss leading to the ordinary person not in the know.so why are they used?
thats not a question directed at you or anyone ,its just an observation.

so i know how the photon/wave works in the split experiment but not the why when observed. but I am sure i will. but to do so i need to ask questions as i don't have the tools for practical.
i have no idea what measuring tool is used to measure/watch the wave.or what effect it would have on the wave other than making it act different.
i take it the measuring eye has been put in various places to measure and not just at the front by the slit...ie behind the slit?

That is not in accordance with the accepted model of a photon. A photon has no size, is massless and travels at c. It is nothing like a pebble, little bullet, 'wavelet' or any of the particles which have mass. You cannot include waves in a description of Photons any more than you can describe a wave as 'made up of photons'. The two concepts are far more separate from each other to allow such cosy descriptions.
There are many misleading animations of photons, but animations are very often produced more with the artistic effect in mind that accurate Science.

I don't think you can possible "know" as your model is not correct. You can 'watch' a wave with your eyes, a radio receiver or an IR detector. You can observe the way that waves interfere with each other in those ways too. You can observe the arrival of an individual photon but, once observed, it will cease to behave according to the wave of which its energy was a part. Hence, in the two slits experiment, you will either record waves as they hit the place where each slit happens to be OR you will observe photons arriving at different rates in the light and dark stripes of the interference pattern which forms on the screen. You cannot know (the concept is meaningless in fact) 'which slit' an individual photon passed through. They say that the wave function 'collapses' once you observe the photon.
If you really want to sort this out in your mind then you need not do any experiment. Just read about the topic and don't try to make up your own model for Electromagnetic Radiation - many people, cleverer than you (I may retract this in twenty years when your theory is accepted!) have evolved a very good theory to explain it all (at this particular level).
P.S. To have a cat's chance in hell of coming up with a valid theory of your own, it is absolutely necessary to understand exactly what the present theory is telling you.

 

[lostprophets]

there is more chance of this... P.S. To have a cat's chance in hell of coming up with a valid theory of your own
than me doing an experiment as i wouldn't no where to start and as I've said the tools are not at my disposal ...
but many thanks for your words. i will take them on board in my mind as it is my only tool on this.. abit like Newtons head with the apple...

as I've said i can only ask questions . the answers i receive i process's into pictures which lead to experiments in mind which lead to more question.this is where you come in.
so
can i ask again please.
the two slit experiment with the " spying eye".
has it been done with the "spy" behind the slit?
so the wave does not know its being looked at until its gone through the slit?
ive looked on line but can't find anything to say this as been done.
im sure it must have and just wanted to know if the outcome was the same as if the "spy" was at the front.

peace

 

[sophiecentaur]

I have said exactly what happens with the "spying eye." You can use any detector you want and place it where you want. If you like, you can use a very low level of light and a photomultiplier, which will react to every individual photon. However, the only way to be sure of having detected all of the photons 'going through' one slit is to put a detector over it. It will react to every photon that lands on it. Unfortunately, as the only source of photons to land on the projection screen is then the 'other slit' you will get no interference pattern. There is no way of determining which of the photons that form the interference pattern went through which slit. As I said, the question is, in fact, meaningless.

This form of 'question and answer learning' is very inefficient. You need to read a lot and assemble the facts and ideas in your own head first. You can then post individual questions on forums like this one and you are more likely to get good and useful responses.

 

[lostprophets]

thanks again sophiecentaur.
but you have left me as frustrated as a man in a room full of naked lesbians.
i do like your reply but I am still none the wiser of the outcome of the spying eye behind the slit.
are you a politician ?
is the outcome the same as when the spy is at the front or the same as when there is no spy at all,or a totally different outcome?
or you don't know if it has been done?
i can't do experiments ...no tools .
the sun is shining its a photontastic day .i do feel bad putting these question onto you.
ive only been into this subject for about 6days

8)

 

[sophiecentaur]

This "spy" of yours is a Detector?
You put it in the places I have described. As I said, it could be a photomultiplier, to observe individual photons. BUT, once detected, the photon is no longer part of the experiment - wherever you happen to detect it.
You really must read around this - it's described in about a million different web pages.

I have to go now for a day or two.

 

[lostprophets]

ok.
i,d like to ask a simple question please.
did or has anyone every checked the first screen with the slit in it (the single slit screen not the double one and not the screen the photon hits at the back) to see if there is any sign either side of the slit, that a photon has past through it?

many thanks
peace

 

[Drakkith]

ok.
i,d like to ask a simple question please.
did or has anyone every checked the first screen with the slit in it (the single slit screen not the double one and not the screen the photon hits at the back) to see if there is any sign either side of the slit, that a photon has past through it?

many thanks
peace

If you detect the photon BEFORE the slit, then the photon never passed through it. If you detect the photon AFTER the slit, then the photon went through it. A photon cannot be detected before the slit AND pass through it, as it has already been detected.

 

[lostprophets]

If you detect the photon BEFORE the slit, then the photon never passed through it. If you detect the photon AFTER the slit, then the photon went through it. A photon cannot be detected before the slit AND pass through it, as it has already been detected.

sorry i think you miss understood me.
once the photon has gone through the slit and hit the back screen. can we check for any hint in the first screen that a photon has passed by/through the slit...does it leave any evidence/trail that it went through the slit..?
i guess anything coming off it would be very small indeed so maybe after a period of time firing one at a time would be the best time to check for a trail.
when i say either side of the slit i do mean either side on the front face of the screen or the internal edges of the slit

the other question i have is
when i see the results on the back screen i notice the line starts off as a dot then grows into a vertical line.
so
does the position of the "gun" "laser" that fires the photon get changed?

if not then does this not show that the photon was a wave to begin with ,or at least when it came out of the laser/gun thingy?
i don't mind being know where near here, but I am just trying to make sense of why

 

[Drakkith]

sorry i think you miss understood me.
once the photon has gone through the slit and hit the back screen. can we check for any hint in the first screen that a photon has passed by/through the slit...does it leave any evidence/trail that it went by..?

To my knowledge there is no "trail" left by a photon and no way to detect the photon without interacting with it and absorbing its energy.

the other question i have is
when i see the results on the back screen i notice the line starts off as a dot then grows into a vertical line.
so
does the position of the "gun" "laser" that fires the photon get changed?

No, the slit itself is rectangular, so the resulting pattern is also a rectangular.

if not then does this not show that the photon was a wave to begin with ,or at least when it came out of the laser/gun thingy?
i don't mind being know where near here, but I am just trying to make sense of why

I don't know what you mean. If you fire actual bullets through a rectangular slit, the pattern on a target behind the slit will be rectangular. If the slit is circular, then the pattern will be circular. It is the same with a wave.

 

[lostprophets]

To my knowledge there is no "trail" left by a photon and no way to detect the photon without interacting with it and absorbing its energy.



No, the slit itself is rectangular, so the resulting pattern is also a rectangular.



I don't know what you mean. If you fire actual bullets through a rectangular slit, the pattern on a target behind the slit will be rectangular. If the slit is circular, then the pattern will be circular. It is the same with a wave.

oh my.my head is wobbling now.
so
why does a bullet not hit the back screen in the same place every time without moving the gun up or down?

 

[Drakkith]

oh my.my head is wobbling now.
so
why does a bullet not hit the back screen in the same place every time without moving the gun up or down?

It's just an analogy. Imagine it's a machine gun that isn't stable enough and each shot is off slightly due to the kick.
The random pattern of the bullets will look similar to the shape of the slit.

 

[lostprophets]

ok. is there any video anywhere that shows the actual experiment inits entirey and not some cartoon hero.
there seems to be very many videos that are misleading as to what actually is what..
i don't know the dimensions of the slit or anything and have many questions because of this.
its like someone telling me that a football pitch is green rectangle that humans kick a ball on into a goal,when in fact i need to how big the pitch is,what is made of ,the size of the ball,what the ball is made of and so on and on and on...
if what you say is true about the movement of the machine then i imagine these slits are very very small in length, it must be a wobbly machine ,as the size of the dots to the size of the lines are indeed very way out.so much so that if i were to compare it to scale with something else i imagine i,d be very surprised...i can imagine a beam of photons doing this but i have to say I am surprised one at a time they move so much.

im sure technology will improve with time

 

[Drakkith]

if what you say is true about the movement of the machine then i imagine these slits are very very small in length, it must be a wobbly machine ,as the size of the dots to the size of the lines are indeed very way out.so much so that if i were to compare it to scale with something else i imagine i,d be very surprised...i can imagine a beam of photons doing this but i have to say I am surprised one at a time they move so much.

im sure technology will improve with time

Whatever the source for photons is, a light or laser or whatever, it is not designed to shoot them through a specific slit, nor does it move around during the experiment like a machine gun would. Again, that was just an analogy. Doing so would defeat the whole purpose of the experiment. Instead the size of the slits and their spacing is designed to show that photons interfere and create an interference pattern at the detector.

See this page: http://en.wikipedia.org/wiki/Double_slit_experiment
There are plenty of references in the article that should explain most of what you are asking about the setup of the experiments.

 

[sophiecentaur]

If the light is only allowed to pass through one slit then all you will get is the broad diffraction for a single slit.

The whole point of QM is to tell us that, once you have detected the presence of a photon or electron in one slit, it can no longer contribute to any diffraction pattern beyond.
This is a NEW IDEA which you need to take on board before going a single step further.

 

[VCortex]

No one seems to want to talk to me, so ill just wade in here..

SophieCentaur, do you know what would happen if you had two slits that were being monitored, but then you had another set of unmonitored slits behind the first set (a double double slit set up) that were for instance 1.61803399 times further from the emitter than the first set?

 

[sophiecentaur]

No one seems to want to talk to me, so ill just wade in here..

SophieCentaur, do you know what would happen if you had two slits that were being monitored, but then you had another set of unmonitored slits behind the first set (a double double slit set up) that were for instance 1.61803399 times further from the emitter than the first set?

You don't seem to understand that the act of monitoring will destroy any photon it detects. That photon can no longer be a part of your experiment. It's not like cars going past on the road. Until you 'monitor ' any photon, its energy is part of the wave and could be anywhere. Sheer chance allowed you to find it where you found it - same as sheer chance made it part of the fringe pattern if you didn't find it earlier.
You have to start thinking differently. It's QM.

 

[lostprophets]

good mornings
hi sophiecentaur
i have another question.
does the spyin eye spy the slit from top to bottom?
if so
what would happen if the spying eye only measured from the middle down wards or middle to top or from a third from the bottom to a third from the top?
if taken it that they have used the "spy" on a single slit experiment also yes?
and if so did the back screen give the same pattern as the one without the "spy"

 

[VCortex]

But if you can't detect a photon without destroying it, surely then particle-wave duality is moot? How do you even get any change in the interference pattern from observation?
Wouldn't you get an interference pattern build up from the unmonitored photons through the slits, and a blank from the monitored slits, as sure as if the intervening obstruction had no slits at all? Your reply (without a statistical qualifier on detection rate for the monitored slits) seems contrary to every demonstration & explanation of the test that I have seen (& the law of energy conservation?!), unless I'm completely misunderstanding something obvious (hence the questions :)

 

[Meta14]

I've been trying to get answers from Yahoo! Answers, but I haven't really been getting them. Can the double slit experiment be done with bigger objects, say grains of sand, or more? Would there be any difference in the results? I'm already aware that C60 was used, but I don't know if the results were identical to that of a photon or electron, or if there was any differentiation.

 

[sophiecentaur]

good mornings
hi sophiecentaur
i have another question.
does the spyin eye spy the slit from top to bottom?
if so
what would happen if the spying eye only measured from the middle down wards or middle to top or from a third from the bottom to a third from the top?
if taken it that they have used the "spy" on a single slit experiment also yes?
and if so did the back screen give the same pattern as the one without the "spy"

You would have two different experiments - one for the top half and one for the bottom half. The unmonitored half would give a good pattern with a big contrast between light and dark fringes. The half with monitoring would show weak or no fringes

 

[sophiecentaur]

But if you can't detect a photon without destroying it, surely then particle-wave duality is moot? How do you even get any change in the interference pattern from observation?
Wouldn't you get an interference pattern build up from the unmonitored photons through the slits, and a blank from the monitored slits, as sure as if the intervening obstruction had no slits at all? Your reply (without a statistical qualifier on detection rate for the monitored slits) seems contrary to every demonstration & explanation of the test that I have seen (& the law of energy conservation?!), unless I'm completely misunderstanding something obvious (hence the questions :)

All the demonstrations behave as expected from the theory.

 

[VCortex]

You would have two different experiments - one for the top half and one for the bottom half. The unmonitored half would give a good pattern with a big contrast between light and dark fringes. The half with monitoring would show weak or no fringes

Ok, so from this can we say that both halves are exhibiting a wave pattern, with the monitored half showing decreased intensity due to interaction with an 'observation field' of the same intensity & 'material' as the measured unit, that has a statistical detection/destruction(/uncertainty?) threshold which precludes the destruction of all photons through the slit? Is that a sensible way to interpret the result?

 

[Joncon]

But if you can't detect a photon without destroying it, surely then particle-wave duality is moot? How do you even get any change in the interference pattern from observation?
Wouldn't you get an interference pattern build up from the unmonitored photons through the slits, and a blank from the monitored slits, as sure as if the intervening obstruction had no slits at all? Your reply (without a statistical qualifier on detection rate for the monitored slits) seems contrary to every demonstration & explanation of the test that I have seen (& the law of energy conservation?!), unless I'm completely misunderstanding something obvious (hence the questions :)

You can place polarizers behind the slits. A polarizer won't destroy the photon, but any photon which passes through it is now polarized in a certain direction. So, in theory, you could later inspect the polarization of the photon to determine which slit it passed through. The fact that this information is available in principle can be enough to destroy the interference pattern.

I've been trying to get answers from Yahoo! Answers, but I haven't really been getting them. Can the double slit experiment be done with bigger objects, say grains of sand, or more? Would there be any difference in the results? I'm already aware that C60 was used, but I don't know if the results were identical to that of a photon or electron, or if there was any differentiation.

I believe the Zeilinger group have demonstrated diffraction by adding 48 fluorine atoms to a C60 molecule, I can't find a reference for this though. As for a grain of sand - I doubt it. The problem is that, the more massive an object gets, the larger it's momentum is and therefore smaller the wavelength - meaning less chance of seeing any wave effects.

Some drivers (usually drunk) have apparently attempted to diffract their cars around lamp posts, but none have been successful ...

 

[lostprophets]

Some drivers (usually drunk) have apparently attempted to diffract their cars around lamp posts, but none have been successful ...

this is true.but the difference there is the car is hitting the tree and not either side of the tree.
if it were to go by the side of the tree it wouldn't hit the tree .
but
everything traveling with it would hit the tree and go around the other/both sides of the tree.
so if it was emitting a wave the wave would go with the car ,hit the tree and go around the other side of the tree opposite to the side the car went.
so if the car was a photon the same thing would happen yes?

 

[sophiecentaur]

Ok, so from this can we say that both halves are exhibiting a wave pattern, with the monitored half showing decreased intensity due to interaction with an 'observation field' of the same intensity & 'material' as the measured unit, that has a statistical detection/destruction(/uncertainty?) threshold which precludes the destruction of all photons through the slit? Is that a sensible way to interpret the result?

that sums it up, I think. The pattern for the monitored path would be built up with 'resolved' photons from the other slit (no interference) which dilutes the interference pattern due to unresolved photons from both slits.

 

[Meta14]

I believe the Zeilinger group have demonstrated diffraction by adding 48 fluorine atoms to a C60 molecule,

I'm not too proficient in technical terms, but by diffraction, are you referring to the interference pattern?