Science questions not worth a thread I: I'm a moron!

[Arakhor]

Cop-out answer: No. Amusing answer: If you think you understand quantum physics, you really don't understand quantum physics.
(Disclaimer: All I know of quantum physics comes from Professor Brian Cox or Professor Jim al-Khalili via the BBC.)

Three of the more famous concepts connected with quantum physics are Schrödinger's Cat (where the cat is either alive or dead and you cannot know until you look), Heisenberg's Uncertainty Principle (the more precisely you know either the location or speed or a particle, the less precisely you know the other) and the light box experiment (where light appears to act as both a wave and a particle at the same time).

 

[hobbsyoyo]

Can someone explain the basics and/or mystery behind quantum physics?

I really wish I could, but I hardly understand it myself. The thing about quantum physics is that unlike normal physics, it is entirely unintuitive. The things that happen at the quantum scale are very different from every day experience - nothing from your life prepares you to expect the kinds of stuff that goes on. Unfortunately for Average Joe's, quantum physics is incredibly useful (for things like computer chips, GPS, etc) and at the same time completely mystifying. What this means is that everyone accepts quantum physics as a real thing without having a clue what it's about so it's easy for quacks to sell them on things like perpetual motion machines and other impossible 'inventions' in order to bilk them out of their money.

I fall into the category of Average Joe myself when it comes to quantum physics and I'm sorry I can't really answer your question.

 

[emzie]

Can someone explain the basics and/or mystery behind quantum physics?

Disclaimer, I studied biology, not physics; this is a very basic summary of the weirdness.

A very simple summary would be this:

A particle is defined as a probability rather than an absolute. Because of this, a particle, a particle can actually have seemingly conflicting properties that are unintuitive: a photon can cancel itself out, which is something macro objects don't do.

What do I mean by cancel itself out?

Imagine a wave crashing into a wall with two narrow openings. So it'll look something like this

[wall][][wall][][wall] where the [] is a small slit cut in the wall.

When a wave hits this, most of the wave will bounce off the wall, but two small segments of the wave will continue through the slits. This will create, on the other side of the wall, two new waves radiating out from those narrow openings.

Waves are a series of troughs and crests -- the low part and the high part of a wave. So this: ~~~~~~~~~~

When the two waves meet one another, sometimes they will meet crest-to-crest and the power actually amplifies and you get a larger crest. Sometimes they will meet trough-to-trough and you will get a lower trough. And sometimes they will meet trough-to-crest and they cancel each other out.

So when you look at the pattern that emerges on the wall behind the two slits, you get an interference pattern that looks something like this:

[wave hitting] [nothing] [wave hitting] [nothing] [wave hitting]

When you fire a photon at a double slit like this, even when you fire a single photon at a time, you end up with this interference pattern which means the photon has interfered with itself or in classical terms, we might say it was at two places at once.

As a picture:

Quantum mechanics goes much deeper than this, and it only gets more weird.

 

[dutchfire]

The main weirdness is that the classical picture of physics doesn't work anymore. You usually think of things that exist and have properties (ie. my chair has a location and a colour). These properties can be observed, but they exist regardless of observation. Even if I´m not looking, my stair still has a location.

In Quantum Mechanics, you can no longer necessarily talk about how things are, just about what you can observe.

 

[uppi]

Can someone explain the basics and/or mystery behind quantum physics?

The basics are just a lot of math. Because quantum mechanics is so different from our daily experience and thus our intuition, the only thing you can trust is the math. If you work with quantum mechanics a lot, some things do become intuitive once you get used to them, but new things might always baffle you.

As for the mysteries: There are a lot of those. For example the Quantum Zeno effect: If something is in an unstable state that is supposed to evolve into another state, I can prevent this by measuring that state infinitely often. One could say, that something that is observed all the time cannot move. It has to be unobserved for some time to be able to change its state. I once stumbled over that effect in the lab when I was measuring the state of the atom to often.

 

[Muhammed]

Thanks for the very interesting replies! I've been reading a lot about it around the web, and my head did end up exploding. Fascinating stuff!

@Contre: I get the wave analogy. Thanks. Does this mean that we know for a fact that a subject(neutron, atom, partical whatever) cannot be at the same time and place, and that this is just a matter of perception?

@Uppi: I might understand a tiny bit of that.. help me out here:
Say we throw a ball from point A to point B.
The ball would have to go through a surdent point to get from A to B. This point would be X.
But to get from point A to X we might say there's even another point..
Etc. Etc. Etc.
Infinity!
So we might say we have "meassured" the ball to a point where it(theoretically) is not able to move at all.
Is this in any way the same thing?

 

[uppi]

@Uppi: I might understand a tiny bit of that.. help me out here:
Say we throw a ball from point A to point B.
The ball would have to go through a surdent point to get from A to B. This point would be X.
But to get from point A to X we might say there's even another point..
Etc. Etc. Etc.
Infinity!
So we might say we have "meassured" the ball to a point where it(theoretically) is not able to move at all.
Is this in any way the same thing?

There's a bit more to it: As you say, we have infinitely many states between A and B, but we measure in such a way, that the measurement result will always A or B. So the outcome will never be X. The principles of quantum mechanics state that if we measure A, the state will be in A, even if it was in state X before.

If X is halfway between A and B, then there is a 50% probability that you end up in either A or B in half of the cases. so on average you are halfway there but in every single case you either have made it to B or fall back to A.

Now consider the states an infinitely small amount away from A: The probability for them to end up in B is 1/infinity, so it is essentially zero. So once you measure them, the state will fall back to A. To get to B, these states would have to be passed to get to states with higher probability of being B. But if you are constantly measuring, you cannot pass those states, because every time you try, it's back to A.

 

[hobbsyoyo]

When you fire a photon at a double slit like this, even when you fire a single photon at a time, you end up with this interference pattern which means the photon has interfered with itself or in classical terms, we might say it was at two places at once.

I thought when you sent a single photon, it still wound up making a point, however where it landed on the other side was governed by the wave behavior. In other words, when you send one photon through, it won't come out as a wave but instead will land in a place that aligns with the crests and troughs of the wave pattern (because it travels in a waveform), it's only after you've sent a bunch through though that the pattern is obvious as more and more points register. Crank the number of photons sent really high (like shining a continuous light as opposed to just one photon) and you see the full wave interference pattern not individual dots from individual photons (as there are so many that they overlap).

I'm not sure though because we never sent a single photon through a slit, it was always a beam such as from a light or a laser, which will give you the interference wave pattern as you are sending through billions continuously.
Edit: not sure if I'm being clear

 

[emzie]

I thought when you sent a single photon, it still wound up making a point, however where it landed on the other side was governed by the wave behavior. In other words, when you send one photon through, it won't come out as a wave but instead will land in a place that aligns with the crests and troughs of the wave pattern (because it travels in a waveform), it's only after you've sent a bunch through though that the pattern is obvious as more and more points register. Crank the number of photons sent really high (like shining a continuous light as opposed to just one photon) and you see the full wave interference pattern not individual dots from individual photons (as there are so many that they overlap).

I'm not sure though because we never sent a single photon through a slit, it was always a beam such as from a light or a laser, which will give you the interference wave pattern as you are sending through billions continuously.
Edit: not sure if I'm being clear

Even though each photon hits in a specific place, the interference pattern still manifests over repetitions -- which is to say, I guess, that each photon hits the detecting plate at one point, but the point is different each time and the distribution is the same as firing many photons.

 

[Mise]

I'm not sure if hobbsyoyo is talking about single slit experiments or double slit experiments. In double slit experiments, a single photon will "interfere with itself". Though each photon will still land in one single place on the screen (as a particle would), if you fired e.g. 1 photon every second through the slits (so that there is only ever 1 photon going through the slits at any one time), the distribution pattern on the screen over time is identical to the interference pattern you see when you shine a continuous beam of light through the double slits.

In single slit experiments, where the slit width is on the same order of magnitude as the wavelength of the incident photon, the photon will diffract, just as a wave would, and the distribution pattern over time is identical to if you shine a continuous beam of light again. You can see this, if you prefer, as a manifestation of the Heisenberg Uncertainty Principle, where the slit limits the uncertainty in position (in the direction perpendicular to travel), and thus increases the uncertainty in momentum (in the direction perpendicular to travel).

 

[hobbsyoyo]

Even though each photon hits in a specific place, the interference pattern still manifests over repetitions -- which is to say, I guess, that each photon hits the detecting plate at one point, but the point is different each time and the distribution is the same as firing many photons.

Yes this is exactly what I thought. Thanks.

 

[uppi]

I'm not sure though because we never sent a single photon through a slit, it was always a beam such as from a light or a laser, which will give you the interference wave pattern as you are sending through billions continuously.

Creating true single photons certainly isn't easy, but it is far from impossible. So sending a single photon on a double slit can be done and has been done.

Photons are fun though. You can see quite a lot of the weirdness of quantum mechanics with them.

For example, there are 50:50 beamsplitters that will reflect 50% of the light and let 50% of the light through. So each individual photon has a 50% chance of leaving at each of the two output ports. But if two photons in the two input ports are exactly the same and they arrive exactly at the same time, they will always leave at the same output port, never at two different output ports.

 

[Mise]

Is that because they're indistinguishable? (And if you could distinguish between them by using a beamsplitter, it would be a contradiction.)

 

[Cheezy the Wiz]

@Contre: I get the wave analogy. Thanks. Does this mean that we know for a fact that a subject(neutron, atom, partical whatever) cannot be at the same time and place, and that this is just a matter of perception?

You can't know both a particle's speed and exact location at a given time at the same time, so if you know the particle's speed (the speed of light, in the case of this photon), then by definition you cannot know its exact location. This certainly factors into trying to "solve" the two-slit experiment by Newtownian (i.e. classical) methods, since that's how we would think about it if it were a bunch of balls being thrown at a wall with two holes in it.

As a layman (which I am also, I have a liberal arts degree), the best way to deal with Quantum Physics is to simply not ask questions and accept what the people with big brains say, because to get to the level where it actually makes sense requires postgraduate work in the field, and most of what they know isn't intuitive, it's observed and accepted for what it is while being observed. Things don't "make sense" in the Twilight Zone of particle physics, they obey different rules than the "big world" that we observe with classical mechanics does, so our brain, coming directly from "real life," doesn't understand things correctly.

 

[Bluemofia]

Is that because they're indistinguishable? (And if you could distinguish between them by using a beamsplitter, it would be a contradiction.)

It's because the math works out that they can only go through the same one to constructively interfere with itself (they're bosons, so they can occupy identical quantum states) to have a non-zero probability. If it were to leave through different ones, it would cancel itself out, leaving 0 probability.

Basically, you take the wave analogy an apply it to the beam splitter. The photon takes all paths at once, and interferes with itself at some of them to make those particular paths with a zero probability, leaving the ones with non-zero probability to be observed.

(I am purposely avoiding this topic on QM except for when I am certain, despite being a physics major and have taken 2 semesters of QM because I don't feel like my knowledge level is adequate to convey it with the implied credibility I have. But I agree with Uppi, QM is very straightforwards if you can understand the math. But once you understand the math, you understand 95% of QM.)

 

[hobbsyoyo]

You can't know both a particle's speed and exact location at a given time at the same time, so if you know the particle's speed (the speed of light, in the case of this photon), then by definition you cannot know its exact location. This certainly factors into trying to "solve" the two-slit experiment by Newtownian (i.e. classical) methods, since that's how we would think about it if it were a bunch of balls being thrown at a wall with two holes in it.

As a layman (which I am also, I have a liberal arts degree), the best way to deal with Quantum Physics is to simply not ask questions and accept what the people with big brains say, because to get to the level where it actually makes sense requires postgraduate work in the field, and most of what they know isn't intuitive, it's observed and accepted for what it is while being observed. Things don't "make sense" in the Twilight Zone of particle physics, they obey different rules than the "big world" that we observe with classical mechanics does, so our brain, coming directly from "real life," doesn't understand things correctly.

Watch The Fabric of the Universe (A Nova documentary from PBS). It is very enlightening and it shows you don't have to be a PhD to understand the stuff. To master and apply it, sure, but not to grasp the basics. I think the main problem is that PhD's do such a terrible job of explaining this stuff (QM) to pleibes out of disinterest or sheer lack of ability to do so.

 

[Mise]

It's because the math works out that they can only go through the same one to constructively interfere with itself (they're bosons, so they can occupy identical quantum states) to have a non-zero probability. If it were to leave through different ones, it would cancel itself out, leaving 0 probability.

Basically, you take the wave analogy an apply it to the beam splitter. The photon takes all paths at once, and interferes with itself at some of them to make those particular paths with a zero probability, leaving the ones with non-zero probability to be observed.

(I am purposely avoiding this topic on QM except for when I am certain, despite being a physics major and have taken 2 semesters of QM because I don't feel like my knowledge level is adequate to convey it with the implied credibility I have. But I agree with Uppi, QM is very straightforwards if you can understand the math. But once you understand the math, you understand 95% of QM.)

Ahh yeah, I get you. Thanks!

P.S. I know exactly what you mean about the "avoid QM / implied credibility" thing. I have a BSc in physics but I'm somewhat famous for getting things completely wrong!

 

[uppi]

Is that because they're indistinguishable? (And if you could distinguish between them by using a beamsplitter, it would be a contradiction.)

Yes, the effect only works as described when the photons are fully indistinguishable. If they're only partly indistinguishable, you see photons leaving the beamsplitter at different ports, but still less than expected (and more than expected leaving at the same port).

But photons that are not orthogonally polarized and are detected at exactly the same time will always interfere, even if they're quite different. Although they might have been distinguishable by frequency or envelope, the measurement did not distinguish between them. Of course, if they're different enough, your detector and/or your patience will not be good enough to detect the effect.

 

[emzie]

Watch The Fabric of the Universe (A Nova documentary from PBS). It is very enlightening and it shows you don't have to be a PhD to understand the stuff. To master and apply it, sure, but not to grasp the basics. I think the main problem is that PhD's do such a terrible job of explaining this stuff (QM) to pleibes out of disinterest or sheer lack of ability to do so.

Richard Muller (the guy who made headlines for doubting global warming and then flipping his position when he examined the evidence) teaches a really awesome course called, "physics for future presidents" and UCBerkley has the whole thing on youtube. That was my launching point a while ago for concepts in physics.

 

[SS-18 ICBM]

How did the Curie unit come about? 3.7x10^10 seems like a strange number. At least Avogadro's number has a reason for it.

EDIT: Never mind. Should have read further. Apparently it's the activity of 1 g of radium. Still stranger than the becquerel though.