I remember asking this before, but I don’t remember where and in any event I must not have understood the answer because I’m still confused. If I may try again…
I understand* about the principle of quantum superposition of states, where a particle can be in all possible states until an observation is made on the state of the particle. This principle is said to extend to radioactive decay, such as the decay of an unstable isotope; this is famously part of the Schrodinger’s Cat thought experiment. What I don’t understand is how such a system can be considered to be “unobserved”: doesn’t the emission of radioactive decay particles “force” an observation? If I have an unstable nucleus in a box surrounded by radiation detectors, and one day they suddenly register that they’ve been struck by a gamma-ray photon from within the box, isn’t that a spontaneous collapse of the wavefunction? How could I have “refused” to take a measurement of the system, and thus preserved the state of superposition? If I close my eyes, stick my fingers in my ears and say “I’m-not-observing-you! LA-LA, LA-LA, LA-LA!”, how is it that nonetheless I get pelted with decay products?
*with the caveat that if anyone claims they understand quantum mechanics, it means that they don’t know what they’re talking about.
You’re not stating the exact conditions of the box correctly, which may be part of the problem.
Put the cat in the box. Use a radioactive decay mechanism consisting of a single atom. The emitted particle will release poison when it hits the trigger. Any atom has a 50/50 chance of decaying within its half life.
Now go to the end of the half life.
You have not made an observation, you’ve merely waited a specified period of time. You don’t know whether decay took place or not. There is no outside observer, nothing outside the box that looks inside. The only observation method is to determine whether the cat is alive or dead. The cat is in a sealed box so you don’t have this information.
The issue of who has information and what it means to be observed is at the heart of the problem. (And it can be expressed mathematically rather than melodramatically for those who dislike analogies.) As it stands, in the absence of observation the math states that the cat is neither alive nor dead, or, alternately, 50% and 50% dead.
Now, is the cat an observer? If not, what is the meaning of observed? That’s the problem in a very tiny nutshell.
In other words, is the geiger counter an observer? If the geiger counter is attached to a printer which records the time and date of each decay, is that an observer?
My question on all the above is this:
Suppose the geiger counter is not inside a closed box, but it is out in the open, observable and observed by anyone and everyone who cares to look. Suppose further that it is an infinitely accurate geiger counter, which will annouce each decay immediately as it happens. If so, then it seems to me that all uncertainty has been eliminated, and there is never a time when the odds are 50/50; rather we know exactly when the decay didn’t occur, and when it did occur, and with 100% certainty. Is there something about the quanta which prohibits this scenario?
I think the point is that what you’re describing is a classical scenario. Everybody agrees that classical scenarios proceed as our expectations suggest.
What bothers people about the quantum world is that it is possible to create scenarios which are explicitly not classical. It is those that engender the arguments.
Perhaps an analogy would be in relativity. There are many things which it does describe perfectly. But it cannot describe a singularity and so it breaks down inside a black hole.
It’s the exception, therefore, that’s of interest here, not the norm.
Well…I think the issue is what counts as an observer. The detector? The cat? A human peeking in the box? The wall of the box? The atom itself? An air molecule?
If there is no observer then how does most of the Universe exist in any state? Picture a planet somewhere many light years from any conscious being to observer it. How can the planet exist on its own terms unless the other atoms that comprise it count as “observers”? Or is the planet there and not there at the same time till someone points a telescope at it and causes the probabilities to collapse on either planet exists or planet does not exist?
Good news, everyone! With my new “Probability Collapser” I can create planets just by observing the space that I expect them to inhabit.
If you need me, I’ll be in the Angry Dome.
But seriously, isn’t indirect observation a method used to circumvent the uncertainty factor? Without looking at the item of interest, you bounce particles off of it and then examine the particles. Or am I completely off base?
To further clarify(?) my question, what I don’t understand is how an unmeasured, unstable nucleus can be said to be in a superposition of having decayed and not having decayed. If decay products are observed, we haven’t probed the system in any way- it’s “volunteered” it’s status as decayed.
Note that Schrodinger made up his cat story to show that the idea of superposition of states was nonsense. He thought the cat would be either dead or not dead, it could never be half-dead.
And “what counts as an observer?” is an interesting question that as far as I know can’t be answered. We know that the macroscopic universe doesn’t behave as a superposition of states–at least as we observe it. But that’s just when we’re looking. How do we know it isn’t in a superposition of states when we don’t look? We don’t.
But there’s nothing particularly special about human beings compared to other animals that somehow we’re the only entities in the universe that can collapse a waveform. So it seems pretty likely that the macroscopic universe still exists when we close our eyes.
Again, all I can say is that the problem comes into play before the observation is made. Presumably something has happened in that internal, even if that something is a non-decay. What has happened, though, cannot be determined. The math doesn’t give details. It’s only after the “observation” that the outcome is known. Not being able to predict the entire course of events pre-observation is what troubles physicists. After the observation everything is fine. Nobody cares about that part.
There are probably numerous prior discussions of these topics on the dope, but this one was probably the first thread I started that really took off, so I have a very soft spot for it.
One comment needs to be made about that older thread.
Too many of the posters keep trying to play games with the setup as if by using a different story all the problems would go away.
The story emerged out of the math. QM inherently deals with uncertainty and observation. The story of the cat is a way to illustrate the math in macroscopic terms. Asking whether an observation is made if your eyes are closed misses the point entirely. The issue is the collapse of the waveform, what that means, when it takes place (and even if it takes place), and what happens between the quantum and macro world. Your analogy has to be the equivalent of waveform collapse, not a random story about people looking at stuff. Most alternatives by non-physicists don’t conform to this and so they only confuse the issue.
Which sort of speaks to my point. It all goes back to the radioactive atom either having decayed or not decayed. The whole situation arises because the decay of nuclei doesn’t follow classical rules and can only be described by quantum physics; but I still think that unless you subscribe to the “many worlds” interpretation and regard the present as the superposition of two possible futures, I know of no situation which can be regarded as the superposition of a decayed and not-decayed nucleus, which seems to me as ridiculous as a dead or not dead cat. Unlike things like spin states or entangled particles, which can effectively be regarded as an extraordinarily fragile but real condition, radioactive decay forces the issue: there seems to be no way to preserve the superposition. How could you NOT perform the observation of detecting decay particles?
It seems to me that the only answer to Schrodinger’s thought experiement is that the wavefunction collapse has somehow already occurred, causing the nucleus to decay. I couldn’t tell you who or what in that system was the observer- the background production of particle pairs?- but I think the measurement has taken place long before the geiger counter detects the decay.
Why is it that the observation of decay products isn’t an observation?
The whole point of the box is that you cannot tell anything about what is in the box. If you could shake the box and listen for a pissed off cat, or sniff the box for the scent of a dead cat, those would be observations equivalent to opening the box and looking in.
Observation in the analogy means “anything that permits you to determine the status inside the box.” Observing (or being pelted with) decay products is such an observation.
Perhaps it is better stated how you get from the microscopic to the macroscopic.
We are nothing but a collection of a lot of microscopic stuff that behaves oddly in a quantum mechanical fashion. So how does it all add up to macro stuff that behaves more normally as we expect?
Try this:
Milliseconds post Big Bang there were no living observers. There were just a helluva lot of particles zipping about. Eventually things cooled off and formed planets and stars and such. Without anyone around to “observe” all this how did any particle ever collapse its waveform and settle down to build actual things? How can a star form out of the primordial gunk if every atom is akin to, “I’m here but not here!”?
I think the standard answer to the conundrum is that when an observer “collapses” the waveform, the observer itself enters a superimposed state. Though I have a personal distaste for the Many Worlds interpretation, it’s perhaps the easiest one to use to explain this: In the universe where the atom decayed, I observe the cat to be dead, and the universe contains (among other things) a decayed atom, a crushed bottle of poison, a dead cat, and a sad researcher. In the universe where the atom did not decay, I observe the cat to be alive, and the universe contains an intact atom, an intact bottle, a live cat, and a happy researcher. Not only does the cat exist in different states in the different universes, but so does the researcher. My mom outside the laboratory doesn’t yet know about the results of the experiment in either universe, so she’s still non-superimposed (she has the same state in all universes), but after I call her up to tell her how the experiment went, now she’s in a superposed state, too: In one universe she’s happy, and in the other she’s sad.
It’s been a while since I studied this, but if I recall, uncertainty goes down as mass goes up, and it was explained to me that it was essentially the result of statistical average.
For example, if I have a gram of isotope and I wait one half-life, I now have half a gram of the original isotope and half a gram of other stuff. If I wait two, I get one quarter and so on. Acting in such immensely large quantities, we can be quite confident about what happens to the average atom. However, if I label an atom in that gram of other atoms and want to know whether that particular atom has decayed, all I can do is guess until I measure it.
Or, if I throw a baseball at you, I can be quite confident of the average velocity and position of the baseball as a whole. It moves in a very predictable parabola. But no single molecule (let alone electron) within that ball has exactly the same velocity as the average within the ball (due to spin, vibration, heat motion, etc.)
Or, if I roll a pair of dice a million times, I’ll get far more 7s than other numbers. It’s so predictable that I can build a casino and make a billion dollars on that. But all I can say about your next roll is that you’ve got a 1/6 chance of a 7.
I understand that when we look at the whole things average out.
However, an electron in the quantum realm is maybe here, maybe there, maybe somewhere else and indeed seems to be all those places at once. E.g. An electron will pass through both slits as long as no one looks at it.
So then how does that electron get captured by an atom in the right slit? Or is that atom in both slits? If so how does an atom so separated attach with other atoms to form large structures? Its components are all over the place.
You just have to think of an electron as neither particle nor wave, but as a multi-dimensional probability distribution, that’s all.
After all, an electron isn’t just anywhere or in just any state. It is random, but both state and location are in a defined distribution, just like a roll of the dice is a random event within a defined curve. If I’m playing craps, I only have so many outcomes (integers 2-12), and I know that some outcomes are impossible (1, 6.5 and 13, for example). Likewise, an electron is more likely to do some things than others, and some things are considered impossible (or at least virtually impossible).
When an electron interacts with a proton to form hydrogen, it adopts a different probability distribution as a result of the interaction. Now certain conditions have been made more likely than others. For example, you can describe that first level electron shell and show how it is different from a second level electron shell. Both are still random distributions, but they’re different random distributions - different both both each other and from the distribution of a lone electron.
Two hydrogen atoms can interact with each other to form a molecule that changes the probability distributions of all four subatomic particles.
There are also theories (known collectively as ‘objective collapse theories’) that view the collapse of the wave function as well as the wave function as ‘real’, in contrast to the Copenhagen interpretation, which states that a collapse takes place, but doesn’t view the wave function as fundamental (only the square of its absolute has physical significance), or for instance the many-worlds interpretation, where the wave function doesn’t collapse, but instead all possible states persist decoherent from each other. These theories aren’t really different interpretations of QM, but bona fide distinct theories, since they generally need to include modifications to the Schrödinger equation (for instance) in order to introduce an objective state reduction, or add some entirely new mechanism to bring about the collapse. Generally, this effectively leads to a probability for a system to spontaneously reduce to a classical state that grows bigger the more the system assumes classical dimensions; the systems, in some way, ‘measure themselves’ and loose their quantum nature.
This solves the problem in the OP in such a way that once decay products of some atom interact with ‘the environment’, it becomes in effect part of this environment and is thus near instantaneously reduced to a classical state of being decayed (provided the environment is a sufficiently macroscopic system), observer be damned.
However, those theories come with their own problems, most notably the apparently superluminal and non-local character of the collapses.