Quantum mechanics is really, really weird

TheOriginalHappyGoat

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One of the most famous experiments in physics is the double-slit experiment, in which a beam of light is shot through two slits to produce an interference pattern on a surface or detector on the other side. What's strange about it is, even if the beam of light is reduced to send only one photon at a time, the pattern still emerges, as though each individual photon is passing through both slits at once and interfering with itself.

About two decades ago, scientists further explored this phenomenon with the delayed choice experiment. Basically, a photon is sent through a beam splitter, off mirrors, and toward a (potential) second beam splitter. If the second beam splitter is there, the individual photons behave like waves that were split and recombined, and show an interference pattern. If it isn't, they behave like particles that chose only one of the two paths, and display no interference. It as if the photons know whether or not they should act as particles or waves when they hit the first splitter. Even more strange, it appears if the "choice" of whether or not to use the second splitter can be delayed until after the photon hits the first splitter, and reach back in time to determine what choice the photon made.

Now, scientists have tested this on a grand scale. Photons were split on earth, bounced off a satellite, and returned to earth to either be recombined or not. The choice in each test run was made after the photon had already reached the satellite in space, and yet the photons still behaved (i.e., showed an interference pattern or not) as though they already knew what the choice would be when the hit the first splitter, before ever heading off into space.

It's really difficult to wrap your head around what this really means. Much like entanglement, in which a paired particle reacts instantaneously to a measurement made on its partner at a distance, it seems to potentially violate our basic assumptions about relativity and causality. So, do we simply not quite understand the makeup of reality, or does reality somehow shift itself to match our measurements of it, even reaching back in time if necessary to do so? Creepy.

Edited for clarity
 
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Marvin the Martian

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One of the most famous experiments in physics is the double-slit experiment, in which a beam of light is shot through two slits to produce an interference pattern on a surface or detector on the other side. What's strange about it is, even if the beam of light is reduced to send only one photon at a time, the pattern still emerges, as though each individual photon is passing through both slits at once and interfering with itself.

About two decades ago, scientists further explored this phenomenon with the delayed choice experiment. Basically, a photon is sent through a beam splitter, off mirrors, and toward a (potential) second beam splitter. If the second beam splitter is there, the individual photons behave like waves that were split and recombined, and show an interference pattern. If it isn't, they behave like particles that chose only one of the two paths, and display no interference. It as if the photons know whether or not they should act as particles or waves when they hit the first splitter. Even more strange, it appears if the "choice" of whether or not to use the second splitter can be delayed until after the photon hits the first splitter, and reach back in time to determine what choice the photon made.

Now, scientists have tested this on a grand scale. Photons were split on earth, bounced off a satellite, and returned to earth to either be recombined or not. The choice in each test run was made after the photon had already reached the satellite in space, and yet the photons still behaved (i.e., showed an interference pattern or not) as though they already knew what the choice would be when the hit the first splitter, before ever heading off into space.

It's really difficult to wrap your head around what this really means. Much like entanglement, in which a paired particle reacts instantaneously to a measurement made on its partner at a distance, it seems to potentially violate our basic assumptions about relativity and causality. So, do we simply not quite understand the makeup of reality, or does reality somehow shift itself to match our measurements of it, even reaching back in time if necessary to do so? Creepy.

Edited for clarity
My guess is our understanding of time is the issue. I was just reading the concept that quantum entanglements may create the direction of time. I am not sure how that plays in here, but it makes sense that entanglements themselves might reside outside time.
 

TheOriginalHappyGoat

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My guess is our understanding of time is the issue. I was just reading the concept that quantum entanglements may create the direction of time. I am not sure how that plays in here, but it makes sense that entanglements themselves might reside outside time.
That's interesting, but beyond my understanding. For example, I keep asking, if an accumulation of quantum entanglements pushes the flow of time, why do the entanglements accumulate in one direction if time isn't already fundamentally moving in that direction? Seems like it's a chicken-and-egg problem to me.
 

Marvin the Martian

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That's interesting, but beyond my understanding. For example, I keep asking, if an accumulation of quantum entanglements pushes the flow of time, why do the entanglements accumulate in one direction if time isn't already fundamentally moving in that direction? Seems like it's a chicken-and-egg problem to me.
I have the same fundamental lack of understanding. There are a lot of other pages on the subject, but they provided little clarity. The best I can come up with is that time does not flow in one direction for everyone. At least that would explain Merlin. A bad attempt at Arthurian humor.

I had been reading the idea time flows in one direction because it follows entropy. Time leads from less to more anthropic. But that has the same problem. But in my reading that lead to quantum associations which led to the link above. Which means entropy is a filling of quantun associations.

I imagine you read about the space rock from outside the solar system? Or at least, that is what they want you to think.

Edit for image below

 

TheOriginalHappyGoat

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I have the same fundamental lack of understanding. There are a lot of other pages on the subject, but they provided little clarity. The best I can come up with is that time does not flow in one direction for everyone. At least that would explain Merlin. A bad attempt at Arthurian humor.

I had been reading the idea time flows in one direction because it follows entropy. Time leads from less to more anthropic. But that has the same problem. But in my reading that lead to quantum associations which led to the link above. Which means entropy is a filling of quantun associations.

I imagine you read about the space rock from outside the solar system? Or at least, that is what they want you to think.

Edit for image below

I have been following the news about the extrasolar asteroid. Fun stuff.

To build off your aliens joke, though, when it comes to this time thing, there's also the fact that we experience time in only one direction. This interplay with consciousness means it is also a metaphysical problem, which means it may remain unresolved in physics for quite some time, no matter how many cool QM theories people come up with.
 

Marvin the Martian

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That's interesting, but beyond my understanding. For example, I keep asking, if an accumulation of quantum entanglements pushes the flow of time, why do the entanglements accumulate in one direction if time isn't already fundamentally moving in that direction? Seems like it's a chicken-and-egg problem to me.
On the direction of time, Hawking had theorized in Brief History time flowed one way during expansion and the other during contraction. But then went on to say contraction could not happen due to missing mass.

It seems quantum physicists are returning to contraction, the Big Bang was really the Big Bounce. Quantum gravity seems to be the reason it is believed expansion will stop.

I also read an interesting idea that some black holes in our universe are remenents of the previous universe. They did not get caught up in the great contraction.

If it were not a known scientific principle, if someone wrote quantum mechanics into a script they would be laughed out of the business.
 
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On the direction of time, Hawking had theorized in Brief History time flowed one way during expansion and the other during contraction. But then went on to say contraction could not happen due to missing mass.

It seems quantum physicists are returning to contraction, .

IIRC, I listened to an interview a month or so ago with a "Ted" quantum physicist and he did not believe in contraction.

I've long wondered if black holes "explode" into new universes once they get so much stuff into them.
 

Bill4411

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IIRC, I listened to an interview a month or so ago with a "Ted" quantum physicist and he did not believe in contraction.

I've long wondered if black holes "explode" into new universes once they get so much stuff into them.
Maybe part of the missing mass in the Universe has gone into a new one? I know one theory our Universe and others would look like a big bunch of soap bubbles if you could see them from a distance. Or my favorite theory, “it’s turtles all the way down”:
https://en.m.wikipedia.org/wiki/Turtles_all_the_way_down
 

TheOriginalHappyGoat

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IIRC, I listened to an interview a month or so ago with a "Ted" quantum physicist and he did not believe in contraction.

I've long wondered if black holes "explode" into new universes once they get so much stuff into them.
The current most popular theory seems to be that expansion will not slow down, and black holes will eventually dissipate with Hawking radiation. If a new universe arises from our cold, dead one, it will probably be from a quantum vacuum fluctuation.
 

TheOriginalHappyGoat

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On the direction of time, Hawking had theorized in Brief History time flowed one way during expansion and the other during contraction. But then went on to say contraction could not happen due to missing mass.
I don't see how time can flow two different directions in the same universe. If that happens you end up with a dead-end problem (i.e., people on both sides of the expansion/contraction border work toward that border, and then they are stuck). Luckily, contraction probably won't happen, so I don't have to worry about trying to figure how that one works.

It seems quantum physicists are returning to contraction, the Big Bang was really the Big Bounce. Quantum gravity seems to be the reason it is believed expansion will stop.
I like the Big Bounce theory, because it's elegant, but I haven't seen any talk of scientists moving back toward it. It seems to be a fringe theory at the moment.

I also read an interesting idea that some black holes in our universe are remenents of the previous universe. They did not get caught up in the great contraction.
I remember reading something like that, too, but if contraction is the opposite of expansion, that shouldn't be possible, since the contraction would happen at every point in space simultaneously until it reached a singularity. There could be nothing left behind, because there would be no place for there to be anything. I wish I could remember how they explained their way out of that one, because I know the theory you're referencing, but I can't remember where I read it.

If it were not a known scientific principle, if someone wrote quantum mechanics into a script they would be laughed out of the business.
I think Rock got it right above: what's cool about physics is that relativity works, and QM works, but they are still somehow mutually incompatible. So even though we have a pretty good grasp on how the universe works at all scales, we're still missing something that pulls it all together.
 

Marvin the Martian

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The current most popular theory seems to be that expansion will not slow down, and black holes will eventually dissipate with Hawking radiation. If a new universe arises from our cold, dead one, it will probably be from a quantum vacuum fluctuation.
Inflation is clearly the theory, but if you Google big bounce you will see more acceptance than 20 years ago. Hawking is decidedly not a bounce supporter.
 

TheOriginalHappyGoat

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Inflation is clearly the theory, but if you Google big bounce you will see more acceptance than 20 years ago. Hawking is decidedly not a bounce supporter.
It's funny how Hawking is sort of the international arbiter for these things.:p

Point of fact: expansion, not inflation. Inflation refers specifically to the short-lived extremely fast expansion in the very early universe. It would be compatible with either an open or a closed universe, as it's meant to solve the horizon problem, and doesn't really speak to current expansion.
 

Marvin the Martian

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It's funny how Hawking is sort of the international arbiter for these things.:p

Point of fact: expansion, not inflation. Inflation refers specifically to the short-lived extremely fast expansion in the very early universe. It would be compatible with either an open or a closed universe, as it's meant to solve the horizon problem, and doesn't really speak to current expansion.

You are right, just multi tasking.
 

iuwclurker1

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Please, by all means, contribute.
I did: What is the physicists' definition of space? That's the starting point. I don't follow modern physics the way you do so I don't know how they currently define it. This is no definition:

Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum known as spacetime. The concept of space is considered to be of fundamental importance to an understanding of the physical universe.​
 

CO. Hoosier

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One of the most famous experiments in physics is the double-slit experiment, in which a beam of light is shot through two slits to produce an interference pattern on a surface or detector on the other side. What's strange about it is, even if the beam of light is reduced to send only one photon at a time, the pattern still emerges, as though each individual photon is passing through both slits at once and interfering with itself.

About two decades ago, scientists further explored this phenomenon with the delayed choice experiment. Basically, a photon is sent through a beam splitter, off mirrors, and toward a (potential) second beam splitter. If the second beam splitter is there, the individual photons behave like waves that were split and recombined, and show an interference pattern. If it isn't, they behave like particles that chose only one of the two paths, and display no interference. It as if the photons know whether or not they should act as particles or waves when they hit the first splitter. Even more strange, it appears if the "choice" of whether or not to use the second splitter can be delayed until after the photon hits the first splitter, and reach back in time to determine what choice the photon made.

Now, scientists have tested this on a grand scale. Photons were split on earth, bounced off a satellite, and returned to earth to either be recombined or not. The choice in each test run was made after the photon had already reached the satellite in space, and yet the photons still behaved (i.e., showed an interference pattern or not) as though they already knew what the choice would be when the hit the first splitter, before ever heading off into space.

It's really difficult to wrap your head around what this really means. Much like entanglement, in which a paired particle reacts instantaneously to a measurement made on its partner at a distance, it seems to potentially violate our basic assumptions about relativity and causality. So, do we simply not quite understand the makeup of reality, or does reality somehow shift itself to match our measurements of it, even reaching back in time if necessary to do so? Creepy.

Edited for clarity
This is really a fascinating thread. Thanks for starting it.

If anybody hasn't read The Grand Design, I highly recommend it. In fact this thread provokes me to read it again. Hawking and his co-author attempt to bring together relativity, quantum theory, and gravity to explain "why is there something instead of nothing". They don't succeed. But they do succeed in causing one to think about what they think about.

One of my take aways from the book is that what we believe we know about the history of the universe, what it is, and where it is going, is all based on human observation, understanding, and reason. In that sense, all the theories about everything are correct. The theories, including what we determine to be reality, are limited by our collective reason.

That discussion leads me to the equally fascinating subject of the human brain, its history, what it is, and where it is going. In many ways, the deepest reaches of the human mind are as mysterious as the deepest reaches of space.



I
 

TheOriginalHappyGoat

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This is really a fascinating thread. Thanks for starting it.

If anybody hasn't read The Grand Design, I highly recommend it. In fact this thread provokes me to read it again. Hawking and his co-author attempt to bring together relativity, quantum theory, and gravity to explain "why is there something instead of nothing". They don't succeed. But they do succeed in causing one to think about what they think about.

One of my take aways from the book is that what we believe we know about the history of the universe, what it is, and where it is going, is all based on human observation, understanding, and reason. In that sense, all the theories about everything are correct. The theories, including what we determine to be reality, are limited by our collective reason.

That discussion leads me to the equally fascinating subject of the human brain, its history, what it is, and where it is going. In many ways, the deepest reaches of the human mind are as mysterious as the deepest reaches of space.



I
Thanks. I know you like to bring up this brain thing a lot, which I always fine interesting, although it does sometimes feel like a bit of an aside. But in this case, I think you're spot on to raise it. We can study the nature of time from a purely physical point of view, in terms of entropy and causality (or perhaps, as Marvin points out, entanglement), but this is one case where I don't think it's really meaningful to ignore the consciousness aspect. Aside from the physics, time is really only meaningful because we experience it, and that can't really be explained from just a mathematical standpoint, because we don't have a good grasp on what exactly the mind is (if anything). Einstein once called the passage of time "a persistent illusion," because, like most physicists, he imagined spacetime as essentially static, with all points in time basically equal. And both relativity and QM currently require us to view the universe in this way. But so long as we stubbornly move from one point in time to the next, together, that's a model of reality that is simply unsatisfying. After all, if we're going to explain the universe, we need to include our own mode of experiencing it, or the model will be incomplete.
 

TheOriginalHappyGoat

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I did: What is the physicists' definition of space? That's the starting point. I don't follow modern physics the way you do so I don't know how they currently define it. This is no definition:

Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum known as spacetime. The concept of space is considered to be of fundamental importance to an understanding of the physical universe.​
You should have backed up one sentence: "the boundless three-dimensional extent in which objects and events have relative position and direction." I know, still not a very good definition, but slightly more definite than the part you quoted. :)

That said, in physics, space (and spacetime) is more of a mathematical construct than anything else. Relativity imagines the universe existing in "Minkowski space," which is basically what we think of as spacetime, but it's really just a mathematical model, and it doesn't speak to what the fundamental nature of physical space (or time, for that matter), actually is. Similarly, QM imagines both space and time as nothing more than symmetric dimensions in which things exist and happen. Again, really just a mathematical model. The fact that reality doesn't seem be symmetric (at least with respect to time) is one reason why this is so problematic. The actual fundamental nature of physical space and time is still an unresolved problem in physics (and may always be).

So, long story short, there isn't really an answer to your question beyond "We're not really sure, to be honest."
 

iu_a_att

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have been enjoying watching World Science Festival videos on you tube. Also PBS Space Time has good you tube videos on double slit experiment and quantum eraser.