How can you have a cat that is both alive and dead at the same time? Such was the question quantum physicist Erwin Schrodinger posed in rebuttal to the weirdness of the Copenhagen Interpretation of quantum physics, an interpretation that he in fact through his theoretical research contributed to. He ultimately rebelled!
When debating the nature of quantum physics, you question what does it all really mean? One of the central points requiring pondering features a thought experiment by physicist Erwin Schrodinger. He, along with Albert Einstein, didn’t agree with the idea that probabilities rule the quantum universe, and that observations or measurements were central to turning a probability into a certainty. By linking a quantum uncertainty event, with a macro outcome, Schrodinger hoped to show the absurdity of the former.
Schrodinger’s Cat has got to be one of the strangest thought experiments ever conceived, but it was conceived with the idea of putting the boot into the Copenhagen Interpretation of all things quantum. The Copenhagen Interpretation basically means that everything is in a state of probability until, and only until, an actual observation or measurement is made; then, and only then, probability morphs into reality and certainty. Prior to that observation or measurement, the various possibilities are said to be in a state of superposition. Translated, if you throw a dice and it rolls under the sofa out of sight, the top value of the dice is in a state of six superpositions. The top of the dice is at the same time simultaneously one, two, three, four, five and six. That superposition of state, that combination of all possibilities is called the wave function of, in this case, the dice. Only when you remove the sofa and look will the six probability superpositions collapse (the collapse of the wave function) into one actual value. The point is, according to the Copenhagen Interpretation, prior to looking, the top face of the dice actually, in reality, has a value of one, two, three, four, five and six - simultaneously.
Okay, now back to the cat. The idea is that you have some unstable (radioactive) atom, and there’s a 50/50 chance that it will go ‘poof’ and give off a decay particle within one hour. That’s the quantum or micro bit. Now you have a box that contains a Geiger counter or some radioactive decay particle detector (that’s part of the macro part). You also have a hammer in the box poised over a glass vial of poison gas (also part of the macro part). If the Geiger counter detects a decay particle, it triggers a switch which releases the hammer which smashes the vial, releasing the poison gas. Oh, there’s also a cat in the box (the really essential macro part). After one hour, there’s a 50/50 chance that the cat is either alive or dead. That’s what rational people would say. Some, those of the Copenhagen Interpretation School, would argue that the cat exists in a dual state of both 50% aliveness and 50% deadness until such time as an observer looks into the box and measures the cat’s 100% aliveness or 100% deadness. Then, and only then, does nature make up her mind (in quantum theory, the wave function – a measurement of probability – collapses to an exact value) and you find either a dead cat or an alive cat, which tells you whether or not the radioactive substance did, or did not, emit a radioactive decay particle. In a way, the cat itself serves as a sort of Geiger counter!
This thought experiment was to illustrate the apparent absurdity that in quantum theory some ultimate outcome can have before-the-fact equal but mutually exclusive possibilities (something can both be and not be at the same time – the upper dice face can be all six values at the same time) or that in quantum physics, there’s no definite state of existence until there is a measurement or observation (same difference).
The idea is that if in the micro or quantum world something can have equal but mutually exclusive possibilities (again, an outcome can both be, and not be at the same time - wave-particle duality comes to immediate mind), yet the macro or classical world is made up of micro or quantum bits, then that suggests that macro objects (like a cat) can simultaneously exist in two mutually exclusive states or possibilities (the cat can both be, and not be, alive at the same time). In this case, the cat is both alive and dead until such time as someone looks!
Perhaps a better analogy is in showing how probability remains probability until an observation is made is in a hand of cards. All possibilities are equally probable, all possibilities are realised in actuality, but you don’t know the specific outcome, your precise hand, until you look and the probability wave function, that superposition of all possible outcomes, collapses to one, and only one certainty. The observer is the be-all-and-end-all.
On that point, does it have to be a human that does the measuring or observing if all it takes is an observer to collapse the wave function in order for Mother Nature to decide either this or that? Could any observer do? I mean the cat itself is an observer! So if after only one minute a decay particle is given off, the cat will observe the results (hammer falling; vial breaking) just prior to dying, and there will be a dead cat in the box for the next 59 minutes. What if an insect crawled into the box and observed the cat. What about a bacterium in the box. Would nature, via the bacterium then decide that the cat is to be declared really dead and act accordingly? What if a computer, or some form of artificial intelligence or a robot did the observing? Of course it doesn’t have to be a visual observation. I mean if you hear the cat meow, the cat is alive. If you smell the rotting corpse (or the poison gas), then obviously the cat is dead. If you feel the cat and it’s moving, then it’s obviously alive, and so on.
However, back in the macro world of the relatively very large, to me it’s obvious that there’s no bloody way from a human perspective of knowing after one hour if the cat is alive or dead without observing (via one sense or another). One thing the cat most certainly isn’t is both alive and dead at the same time and I think it’s absurd to suggest otherwise – yet that remains one valid interpretation of quantum physics. Is there a way of knowing, without peeking, whether or not that unstable (radioactive) atom emitted a decay particle?
I suggest replacing the vial of poison gas with nitro-glycerine, or for even greater effect, say a thermonuclear bomb (and leave the cat out of it). After the one hour is up (if not before), there will be no doubt, no need for debate, no need to even look, about whether there was, or was not, a radioactive decay particle emitted. There cannot be simultaneously both an intact and unexploded, and an exploded vial of nitro (or a thermonuclear bomb). It’s either/or time.
What this thought experiment actually tells us about quantum physics remains a bit of a philosophical puzzlement to me I’m afraid, and the fact that it’s discussed in nearly every book on quantum physics suggests that it has lost none of its strangeness.
There’s another aspect to this that’s equally as strange. Both the radioactive atom and the cat are entangled. What that means is that if you know the state of one, you know the state of the other. Say you observe the radioactive atom and note that it hasn’t decayed; it hasn’t gone ‘poof’, then you know, instantaneously, that the cat must be alive. If you note that the atom has gone ‘poof’, you know the cat is dead – instantaneously. Ditto, if you observe that the cat is alive, the atom didn’t go ‘poof’; if you find a dead cat, the atom did decay. Again, if you know one outcome, you know the other outcome – instantaneously. That’s true even if the cat and the radioactive atom were on opposite sides of the observable universe. You have received a bit of information faster than the speed of light! When you think about it, information usually has to travel from a source (say from a computer screen or a flash of lightning or the sound of a gun going off) through to your senses hence to our brain. That takes a finite amount of time. It’s not instantaneous. Because in an entangled situation you can receive information instantaneously – faster than light speed – Einstein was not at all amused. He’s quoted entanglement as being a case of ‘spooky action at a distance’. The more usual thought experiment is the creation of a matter-antimatter pair of particles that go their separate ways across the cosmos. Millions of years later, they are on opposite sides of the Universe. If someone ultimately observes one of the pair, then that observer instantaneously knows the state of the other particle even though that particle is so far away that it normally would take millions of years for that state-of-the-particle information to reach the observer: spooky action indeed.
Still, when it comes to the nitty-gritty of trying to pin down the specifics of quantum activities, all is probability, and things can both be and not be at the same time with equal probability, only becoming either/or when the observer struts her stuff and observes. [The observer can be an instrument, but ultimately that instrument transmits the observation to the human that operates the instrument.] But what if there is no observer? Would the cat remain in a limbo state for all eternity? Clearly that’s not, and can not be, the case. The cat is either alive, or it is dead, and the observer be damned! The observer is irrelevant. There seems to be a philosophical if not an actual physical contradiction here. That was Schrodinger’s point.
There is one other fly in this ointment. You have the cat-in-the-box experiment. After one hour an observer enters the room and looks into the box. The wave function collapses and as far as that observer is concerned, the cat is now dead, or alive. But now what about the state of the cat to people outside the room? As far as they are concerned, everything is still in a superposition of state. The cat is still in limbo. Extend that to people in another building, in another suburb, in another city, state, or country. Even if everybody on Earth knew the state of the cat after one hour, what about an astronaut on the Moon? Is the cat still in limbo because an extraterrestrial light years away hasn’t received the news? As far as that extraterrestrial is concerned, the answer has to be ‘yes’, even though all Earthlings know that the state of the cat is no longer the subject of speculation.
Perhaps when all is said and done, quantum paradoxes, well weirdness anyway, explains the most popular interpretation of quantum physics. It’s called the ‘shut up and calculate’ interpretation. In other words, just do the experiments; just crunch the numbers; just apply the technology, and don’t worry about what it all means!
Addendum: Consider the following real and verified experiment – send one photon at a time, say one per minute, at a beam splitter – a half-silvered mirror. The important bit is that one photon starts and ends its journey between the device that emits the photon and the device that detects the photon, before the next photon is released. The photon has a 50-50 chance of going straight through the half-silvered mirror towards a fully reflective mirror, which then reflects off that and heads back towards the half-silvered mirror where it will be further reflected towards a photon detector. Or, the photon when originally emitted will be reflected off the half-silvered mirror. If the photon is reflected, the photon would then again meet a fully-silvered mirror and be reflected, heading back towards the beam-splitter (the half-silvered mirror) to be directed to the photon detector. Thus there are two possible pathways – call them the ‘alive’ path and the ‘dead’ path. The two pathways are identical in length. What happens is, the upshot of the experiment is, at the photon detector, the detector detects a classic wave interference pattern, against all possible expectations. That means the lone photon interferes with itself, which means the photon took both pathways, and thus was in two places at the same time, and thus was both ‘alive’ and ‘dead’ at the same time! Now if you peek, the lone photon’s wave function, the superposition of states, will collapse and you’ll find the lone photon either on the ‘alive’ path or on the ‘dead’ path – either/or, not both. No wonder Schrodinger invented his hypothetical dead-alive cat to illustrate the absurdity of it all. Alas for Schrodinger, the photon experiment has been done, and the dual state of alive-dead verified.