Have you ever wondered if the moon or an electron is present if you are not looking at it? Or any object for that matter?
But what does “looking at” mean? How do you check if moon is there when you aren’t watching? What would it even mean to check, without looking?
In general we use light to see things, or sound to hear, or touch to feel. All the above methods to “check” if a thing exists bottom out in the same thing: photons bouncing off the moon into your eye, or your fingertip’s atoms getting close enough to repel your phone’s atoms.
So checking always means something has to touch the object, however slightly. So if you want to see an electron you’d fire a photon and observe its position or momentum. Same way you can see the moon by photons bouncing off of it.
For the moon, this touch changes nothing. A photon bouncing off something the size of a moon is unnoticeable. But for an electron, the moment you bounce a photon off of it — its path changes, and thereby its position.
So now you’ve got a problem: how do you look at an electron? Maybe fire a gentler photon? That doesn’t disturb the electron’s position at all.
A photon’s nudge isn’t fixed, it depends on the photon’s wavelength. Shorter wavelength = you can pin down position more precisely, but it hits harder (more momentum, bigger nudge). Longer wavelength means gentler nudge, but more vague, you can’t tell exactly where the electron was, only roughly. The Heisenberg’s uncertainty principle in action.
So if you use a photon of the longest wavelength to least disturb the electron then we would be most unsure about the position of the electron.
If you can recall the famous Young’s Double Slit experiment
Fire electrons, one at a time, at a wall with two slits cut into it. Behind the wall, a screen catches wherever each electron lands. If electrons are just tiny particles, you’d expect two piles building up on the screen, one behind each slit. That’s not what happens. Instead, a striped pattern builds up, like ripples on water passing through two gaps and interfering with each other.
So which slit did it actually go through?
To check this, Fire a real photon at the electron as it passes through the slits — just enough to probe which slit it took.
The moment you know “slit A”, the stripes vanish. Two piles instead, like it was a particle all along. You touched it, you nudged it, of course it changed.
But what if I use a “magical” photon which doesn’t affect the position of the electron at all?
It still passes through the slits as a particle, leaving two shades.
So if disturbance/momentum isn’t the real cause… what else changed the moment you got “slit A” as an answer, even with zero physical nudge?
Suppose the magic photon tells “slit A” to a rock sitting in the lab, nobody ever reads it, no human finds out, ever. Does the pattern still die? Yes, the same experiment, with an additional setup called the quantum eraser, has been done.
Here. The magical photon knowing matters. It was never about consciousness or observers. It’s about whether which-path information exists anywhere in the universe, correlated with the electron.
What actually matters is entanglement: the photon and the electron become correlated, a record exists and that record alone, is enough to collapse the possibilities down to one path and kill the interference.
What does “record” mean? Who keeps the record? - The universe. Yes.
The “which-path” information about the electron leaks out into the environment. The interference pattern needs the paths to stay mixed together, undecided and a record, once it exists anywhere, un-mixes them permanently.
More accurately: before the photon interacts with it, there’s no fact about which slit the electron goes through. The interaction is what creates the correlation. After the photon bounces off it, the combined system (electron + photon) is now entangled. Photon-went-left pairs with electron-took-slit-A, photon-went-right pairs with electron-took-slit-B. Neither one “has” the fact alone, the fact only exists in the correlation between them.
That’s called decoherence : that spreading of the record into the photon, then wherever the photon goes into the universe.
Einstein assumed the moon has one definite position always, “checked” or not - that’s local realism.
Locality means the belief that nothing can influence something else faster than light. Realism means the belief that things have definite properties whether or not anyone measures them. John Bell proved “local realism” fails.
The moon isn’t in some special “checked forever” state because we looked at it once. It’s that the moon is constantly decohering — sunlight, cosmic rays, stray particles are bouncing off it every nanosecond, each interaction leaving a “record” in some photon that flies off into space. Billions of records, continuously, whether or not any human eye is involved.
The moon exists whether you look or not — because the universe is always looking.
“Realism” starts when decoherence starts.
So is the electron there when nobody looks? Not until someone “looks” at it. Is the moon there when nobody looks? Always, because something is always looking.
The moon is real because it cannot stop touching the universe. The electron is real only when something finally does.