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The Double-Slit Experiment and the Delayed Choice Quantum Eraser

NeoIkaruGAF

Gold Member
I first heard about this in Michael Crichton's novel, Timeline.

It's very fascinating, but I won't pretend I have even the faintest grasp on this. I barely had serious physics lessons in school.
 

Coolwhhip

Neophyte
I've always loved this experiment and the potential implications of it's various outcomes. It's almost like a bug in the matrix revealed. A clue into the landscape of understanding reality beyond our current comprehension.
To me my intuition tells me our concousness is somehow part of the equation of reality, that we are an essential component in the manifestation of what we experience around us, that there is not the separation between us, and the outside, it is causally intertwined.

animals too?
 

VN1X

Banned
Sabine Hossenfelder DESTROYS Quantum Eraser!
Keanu Reeves Reaction GIF
 

jason10mm

Gold Member
If this is true FTL communication must be possible?
My understanding of entangled particles is that yes, it would be theoretically possible to 'transmit' information by observing entangled particles but first you would have to get them to their destinations, via non-FTL means (since FTL physical travel is theoretically impossible).

Same with wormholes. I can theoretically make a wormhole portal that connects to another wormhole portal, but I gotta fly the second one to the destination at non-FTL, THEN I can hop through and travel FTL.

I'm sure 2150 elementary students will read this thread and have a good chuckle, like we would of 1870 discussions on what the surface of Mars is like.
 

jason10mm

Gold Member
This seems plausible in that the act of observing or using sensors somehow interferes, like even the presence of light is enough to change particle behaviour. I've thought long on this experiment and only this makes sense.
This is my view as well. Kinda like having to use spotlights to 'observe' animals at night. The very act of illuminating them is interfering with their behavior. I fully grok that there are most certainly underlying laws we have no real grasp of, the very nature of stuff like Dark Matter suggests that, but the idea that we are somehow altering the fundamental nature of particles by our meat brains observing it seems ludicrous.
 

bitbydeath

Member
but the idea that we are somehow altering the fundamental nature of particles by our meat brains observing it seems ludicrous.
I wouldn’t rule it out, our brains are incredibly powerful, but keep in mind we are only altering our own realities and not everyone else’s.

Think as though we are each wrapped in our own time bubbles. Depending on states of consciousness, we can speed it up, or even slow it down.

Even whilst awake, some events feel like they really drag, while others can fly by. We never mastered time and these experiences are all personal to you.

Imagine if we could unlock our dreams ability of lasting mere seconds while feeling like hours or days, that would massively change how we operate in the waking world.

We’re quite accustomed at moving time forwards at different speeds, we just need to figure out the back button.
 
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A lot of people have been exposed to Young's famous Double Slit Experiment, which introduces students to the wave/particle duality of light. But this experiment takes us fully into the strange realm of quantum mechanics, too.

h9Qm5mP.png


I've included a sequence of videos below that gradually increase in mindfuckery, but should be fun for curious gaffers. Apologies for any errors in my summaries and explanations.

In the experiment, a photon or electron is shot toward two slits and there is a sheet on the other end that reacts to where it ends up. An interference pattern occurs when an initial wave sent to the two slits splits into two waves and these waves collide with each other on the other side, per the figure above. We can tell when something is behaving as a wave if it produces that interference pattern on the sheet at the other end. If it's behaving as a particle, the interference pattern won't form and we'll just see two lines on the sheet.

The Double Slit experiment works for some atoms as well (a little differently). Shoot a stream of atoms at the two slits, and they'll behave like waves, forming an interference pattern where they end up on the other side. Drop the frequency down to one atom at a time and you'll detect one atom at a time on the other end, behaving like a particle, but eventually, by continuing to shoot one atom at a time toward the two slits, a wave-like interference pattern will form on the other side, even though each atom is not being split between the two slits and multiple atoms are not interacting with each other en route.

Well, let's try observing which slit each atom passes through to get more information. When you do measure which slit each atom passes through via a detector, the interference pattern on the far end disappears, instead showing a straightforward two-slit pattern with the atoms returning to behaving like particles, or bullets passing through two holes in a steel plate. Watch the first video for a clearer picture:



The second video gets more into the quantum mechanics involved in our attempts to explain what is going on in the double-slit experiment. We could think of the trajectory from the starting point to the endpoint as a superposition of all the possible outcomes, a probability wave function moving through both slits that collapses into a particle when directly measured/observed.



The third video, on the Quantum Eraser, is where it gets really mental. Physicists devised a method to split a photon into two identical half-energy photons after passing through the two slits, sending one of the photons in the new pair to the interference screen as before, but the other photon to a detector that tells us which of the two slits the pre-split photon passed through, as a method of indirect observation of the first photon.

Even though they're separate photons at that point and we're not observing the one sent to the interference screen -- observing it changes its behavior from a wave to a particle -- if we observe the OTHER photon in the pair, the interference pattern produced by the photon sent to the screen disappears as if we observed it and not its buddy, an example of quantum entanglement between the photons.



734KYgo.png


But there's more: the experiment was designed so that the observation of the second photon from the pair is farther away than the distance between the first photon traverses to the screen, and therefore takes place AFTER the first photon has ALREADY HIT the interference screen. If the first photon or its entangled buddy aren't observed en route, it should behave as a wave and produce an interference pattern on the screen. Yet the observation of the second photon that tells us which of the two slits it passed through takes place after the first photon has already hit the screen...and that observation reverts the first photon's outcome to particle behavior (ignoring causality, since it has already hit the screen...) and prevents the interference pattern from forming.

76nGV8y.gif


Not done yet. Now for the Quantum Eraser, where the experiment above is done, but some of the second split-photons don't go to the A/B slit detector, and instead continue along and pass through a beam splitter in such a way where we can no longer know which of the two slits the photon originally passed through when it's measured by another set of detectors (i.e. the information about which slit it traversed is destroyed, called the which-path information). And in that case, the interference pattern from the first photon re-appears.

And, let's not forget that the first photon had already reached its destination when these various events with the second photon transpire, so this is changing the past.











Thoughts? :lollipop_hushed:

This kind of behavior is so interesting. The observer effect where particles will behave a certain way depending on your observation of it is mind-boggling. What does it all mean? Does our conscious mind actually affect the universe in some way?
 

sono

Member
Sabine Hossenfelder DESTROYS Quantum Eraser!
And along the way calls out Matt from PBS space time, John Lincoln of Femilab and Joe Scott, she calmly respects them and then states "I think they are all wrong" and then calmly and professionally explains her opinion of why. Matt from PBS then acknowledges she is correct in the video comments. It is an incredible video. Groundbreaking in its own way.
 

sono

Member
Explain it to me like I'm retarded.
Peter is 46 years old and he is captain of container ship. He ships goods between two places that are 100km apart, lets call them A and B.

He starts his round trip at A with the ship only half full.

Three quarters of the way to B he adds more containers to fill the ship which slows him down by a factor of two. On the return trip his ship is empty.

How old is the captain ?


Peter is 46 years old as per the first sentence. Nothing is changed by all the other stuff it is just irrelevant information to the q. Thank you Sabine for letting me borrow this.
 
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mopspear

Member
Correct me if I'm wrong but they've done this with buckyballs which are 30ish carbon atoms I think. So it affects slightly larger stuff too.
 

6502

Member
This kind of behavior is so interesting. The observer effect where particles will behave a certain way depending on your observation of it is mind-boggling. What does it all mean? Does our conscious mind actually affect the universe in some way?
I think their use of wording is important. By observation they mean taking a measurement. If I dipped a thermometer into water it too would at some level affect the wave pattern in the water.

Most of us need a much better understanding of the measurement devices, underlying theories etc before we could really add or comment on what the scientists are doing. Bearing in mind by their nature these videos have to dumb down / cut out a lot to get these digestable for the few who are interested in watching them.

Edit - having watched the debunk video she makes it a lot easier to get. Again, the videos are not giving the detail and implying incorrect conclusions. So what chance do ordinary people have of grasping the subject in this case?
 
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BlackTron

Member
I think their use of wording is important. By observation they mean taking a measurement. If I dipped a thermometer into water it too would at some level affect the wave pattern in the water.

Most of us need a much better understanding of the measurement devices, underlying theories etc before we could really add or comment on what the scientists are doing. Bearing in mind by their nature these videos have to dumb down / cut out a lot to get these digestable for the few who are interested in watching them.

Edit - having watched the debunk video she makes it a lot easier to get. Again, the videos are not giving the detail and implying incorrect conclusions. So what chance do ordinary people have of grasping the subject in this case?

You really cut to the chase here. Without understanding the medium of the "observing mechanism" you can't even begin to have a take on it. Wouldn't it be funny if we found out they were just shy?
 

Crayon

Member
Reality has some jank. You'd think they'd have patched that one by now but they already got our money.
 

sono

Member
Sabine mentions that the quantum experiment relies on two effects: interference and entanglement.

Generally out in the interweb there is a lot of sensational claims about specifically entanglement (instantaneous communication over great distances, faster than the speed of light etc, etc, etc, which I always thought were just plain nonsense)

Thankfully, Sabine does another great video on the subject of entanglement and relating it Einstein's famous statement about "spooky action at a distance" and, somewhat, in defence of Einstein of the context of that statement, describes there isn't anything spooky going on



The wave function is just a human math statement of probability of what you know at that point in time, nothing more. When you measure something, .. you just now have more information and so the probability goes to 100.
You have two socks in black bag, one red, one blue. Before you reach in without looking to pull one out the probability is 50:50 you will pull out a red or a blue sock. After you pull out one sock you know the colour of the sock still in the bag without looking at with 100% certainty. Nothing changed, its colour didnt instantaneously materialise because you picked the other one.
 
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NinjaBoiX

Member
I’ve always found this stuff fascinating, but I’m nowhere near smart enough to fully understand most of it!
 
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