
A: The problem is that you are looking at quantum physics with the eyes of classical physics – the physics of Newton. Things are there as tiny little balls put together. And that certainly works, on the macroscopic level. But as soon as you look at things small enough, the little balls appears to not be balls at all. They can sometimes behave like balls, and sometimes behave like waves on water.
Let me take one practical example: get a laser pointer, one which you can make shine all the time. Also, get something which you can make a narrow slit out of and which you can make more narrow, and something to shine the laser dot on. The tricky bit is actually the narrow slit thing – unless you’re handy, you may have to steal one from a physics lab at school. Even better, set up the experiment there, so that the kids there can marvel at the weirdness of quantum physics.

In physics never anything weird will happen, as a result of any experiment.

So what happens when you make the slit more narrow? Well, for starters, the dot will be more narrow. But then at one point something strange and unintuitive happens:

?

What happened there? Well, with a very narrow slit, you have defined the position of the ingoing photons on the slit so precisely that you can’t know the vector of the outgoing photons from the slit.

?

This is part of a strange bit called the uncertainty principle: there are pairs of properties (like position and vector) which are intrinsically linked so that you cannot know both at the same time.

In order to understand physics it is never important to know numerical values.
For example it is important to understand that water can boil but it is not important that this happens at 100 degrees.

If you know one of them, you cannot know the other. Or you can have an imperfect knowledge of both. By making the slit more and more narrow, you know exactly where the photons going through the slit are, so you cannot know where they are going.

Tricky sentence. It is even more complicated, you cannot measure the speed of any individual photon.

And that shows as that pattern on the screen. There’s the wavelength of the laser involved too, which is why you get a diffraction pattern rather than a line.
So what you do is that you describe the outgoing vector as a waveform of probabilities, like so:
If you shoot photons one at a time, you still get the same pattern: you cannot know where it lands until you observe it on the screen.
“Okay, but that actually doesn’t explain it how things can be not there unless it’s observed” I hear you say. “You’re just describing that we don’t know where the light went, not that it is everywhere.”
Well, you have a point. Not a correct point, but still a point. So let’s get serious. Lesson two: make two slits instead of one, like so:
What do you think will happen at the screen?
Well, you get this:
How do we explain this? You can easily explain it by treating the photons going through the double slit as waves. Then you get waves amplifying and nullifuing one another, like so:
This is an interference pattern. You could explain it as a wave of one photon in the left slit interfering with another photon in the right slit. Where waves ovelap, you get a brighter spot, and where they cancel out one another, you get a darker spot.
But it gets weirder: even if you shoot your photons one at a time, you get the same pattern.
It is as if when you shoot a single photon, the photon interferes with itself. And the only way that would happen is if the photon were at both slits at the same time. But that’s ridiculous, right?

No!

Well, here’s the even weirder thing: if you set up an instrument to observe what happens at one of the slits, the interference pattern goes away. Note that you’re not covering one slit, you’re just looking at one slit. And just looking at the slit is enough for the photon to not go through both slits at once and thus not causing the interference pattern.
<
So it actually seems as if the photon actually is everywhere until you observe it. Only then is its position defined. What’s worse: observing it seems to retroactively define what it was up to until you observed it. The event of the photon hitting the screen seems to determine which way it took and how it interfered with itself in that unknown gap between the doubleslit and the screen.
And this has some serious philosophical implications: you use looking at a chocolate cake, but the common example is the old classical one with the tree falling in the forest when nobody is looking – did it actually happen? There are even quantum physicist philosophers who seriously ponders the idea that only the bits of the universe which are observed actually exist. They call it the strong anthropic principle: it is the act of observation which makes the universe be.
And, of course, this is where we have to get Schrödinger’s cat out of the box, or rather Schrödinger’s equation. It’s this one, named after Erwing Schrödinger who came up with it:

The Schrödinger’s equation, for what ever it means, is not very good to understand physical processes.
The only way to understand the physical behaviour of photons is to perform different experiments and to describe the results carefully.
My understanding is that every process involves a different Schrödinger’s equation and each requires it own experiments to calculate the parameters required. This step does not make much sense.

You don’t have to understand it – I certainly don’t – but you have to understand its implications. It describes the waves mentioned above. You can see it as the quantum mechanical equivalent of Newton’s second law, F=ma. You put in what you know about the particle, and then you know where it is going. The difference is that the Schrödinger equation does account for the wavelike nature of quantum particles, and not just the particle nature. So instead of the absolute certainty of the particle’s state in Newton’s second law, Schrödinger’s equation gives you a waveform which describes the probability of the different outcome states of the particle.

The comparison between the physical nature of photons and Newton's law F=ma is far fetched.

But because of that wavelike nature and because of the doubleslit experiment, some of Schrödinger’s colleagues, like Niels Bohr in Copenhagen, Denmark, came up with the interpretation that it was actually what happened: the photon is actually everywhere to a different degree as described by Schrödinger’s equation, until it is observed – just like it seems to be in the experiments above.

To claim that a photon is every where is tricky.
Comparing photons with water waves, the water waves are every before the two holes (they propagate in parallel before the two holes, all with the same wave length). After the holes, two sets of waves propagetes after the holes, each set of waves centered around a hole. These waves interfer which each other.
When photons are considered more or less the same should happen assuming that an interference pattern is observed.
When that is the case there are both photons before as after the holes (in 3D)

Then the waveform collapses into an observation. It also happens retroactively: we know what the particle was up to until it was observed. This is called the Copenhagen interpretation, by the way.

To understand the behaviour of photons, in some way or another the photon has to be observed (or measured) i.e. to hit a screen. This is always the case. To call that a collapse of the wave function has no physical meaning.
After the photon hits the screen it will create a black dot on a white screen. This black dot can always be observed, but the original photon, which created this dot, is not involved.
From a physical point the fact if an interference pattern is observed, that means there is a screen, does not make any difference as if there is no screen. That means the original photon, which created a black dot, still exists.

And if you think that is absurd, that is what Schrödinger thought too, and came up with the famous cat thought experiment.

From a thought experiment you can learn 'nothing'. Only real experiments make sense.

Take one cat and put it in a box (and by “box”, physicists usually mean “something completely isolated from the rest of the universe”). Also put in something which is described by the Schrödinger equation, like for instance radioactive decay, and a detector.

Why is the concept of Schrödinger equation used? How do you know that radioactive decay can be described by a Schrödinger equation? The only thing that we know is that in general the outcome of each radioactive decay process is unpredictable. What you at least needs to know is the halflife time of the radio active element. See: https://en.wikipedia.org/wiki/Halflife

The detector is then tied to a hammer, which is dropped on a cyanide bottle if the radioactive sample triggers it. Tune the detector and the sample so that in a given time, there is a 50% chance that it will trigger. Then close the box.
The outcome is then pretty obvious: after a given time, you open the box, and in 50% of the cases, you have a dead cat, and in the other 50% of the cases, you have a bloody furious live cat tearing your face off.
But the interesting thing is not the outcome, but what happens before it, before we look. According to Niels Bohr’s interpretation, the radioactive decay both is there and is not there until we look.

The opinion of Niels Bohr is 'strange', because the event when there is a decay and the event of observation are both important.

Since the quantum event of radioactive decay is a wave function, the rest of the contents of the box is basically an amplifier of that wave function to the macroscopic level. And since nothing is looking (except the detector and the cat, but ignore that for now – this is a thought experiment after all), and if Niels Bohr is right, the cat is alive and dead at the same time.
For the cat, it is slightly different: it can be in a nice cozy box and then suddenly it dies. Or it can be in a nice cozy box until someone opens the box and let the light in. It is part of the same quantum system as the radioactive sample, so its observation is valid from its point of view.
But for us, outside the box, we can’t have a clue. We’re not part of the cat’s quantum system, so from our point of view, all outcomes seems to happen at the same time, and the act of looking retroactively decides which fate happened to the cat.


This is what Schrödinger found to be completely absurd. But still, Bohr had a point: the doubleslit experiment shows that it actually seems to be what is happening.
There are some other interpretations, like the pilot wave interpretation, decoherence, and the eversooften mentioned Many Worlds interpretation, which by the way led to science fiction author Michael Moorcock inventing the term “multiverse”. But no matter which one we favour, we still don’t actually know what happens to the cat.
So what about the chocolate cake?


Well, there are lots of possible outcomes of the experiment of a chocolate cake on the table, including that it spontaneously disappears when you’re not looking.


But in reality, the cake is a part of the same quantum system as you are. Air bounces against it, it interacts with the plate it is sitting on, and so on. So that system is observing the cake even if you are not actually looking. You are basically inside the box with the cat and cyanide and radioactive sample.
Also, the chances that the entire cake system will do something radical like catching fire or disappear is so infinitely small that you would have to wait until the heat death of the universe a gazillion times over for it to happen once. The chance is definitely nonzero, but so unlikely that we can treat it as zero.
One more thing: do not attempt the chocolate cake experiments when there are kids in the house. Chances are that it will disappear, although not as spontaneously as in quantum physics.
Unless you want them to have cake, of course.





























