How do we know the idea that particles don’t exist in one definitive spot until we measure them, isn’t just our lack of knowledge due to not measuring them yet?
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In developing modern science we essentially come up with models that are pure math. Lots of equations from which you can plug in numbers and get out predictions. And these mathematical equations are the most accurate predictive tools humans have ever created. If they were inaccurate to any significant degree, things like computers and GPS wouldn’t exist.
But this creates an issue: what does it all *mean*. And this is where we have a difference between the math and the interpretation of the math. One way of interpreting the math is that the values for location or speed or what have you don’t exist at all until we measure them. Another is that all possible values for location and speed are real, but manifest in infinite multiple worlds. And there are many more different kinds of interpretations beyond those.
In addition, there is the question as to whether it even matters. You see, the math works the same regardless. So some say that what’s “actually” happening doesn’t matter as long as the math works. Since the different interpretations don’t change the underlying math, they are completely irrelevant, or at least completely academic.
To get to your specific question, this is the topic in Quantum Mechanics known as “hidden variables.” It asks the very question you’re asking. And the ultimate, and unsatisfying, answer is: we don’t know. We have different interpretations – none of which affect the underlying math. Some say: no, there aren’t any hidden variables and the universe is fundamentally random*. Other say: yes, there are, and the universe is fundamentally deterministic as a result.
^(* – I say “random” because that’s the ELI5 term. A more accurate term would be “stochastic” which is random in the sense it’s not strictly predictable and doesn’t adhere to a pattern, but also isn’t necessarily uniform (which is a property most people assume when you just say “random”).)
Sometimes we don’t. But that’s okay! It doesn’t mean that the answers we have now are bad. Science largely breaks down to a few simple concepts:
-Get curious and find a question you want an answer to (hypothesis)
-Spend time watching the thing you are curious about in order to make observations and learn more about it
-Check how the observed results compare to your hypothesis
-Repeat until you have an answer AND when new observations add more information
Sometimes scientists ask a good question that we don’t have the tools to make an observation about, so other scientists and engineers have to make new ways for us to “see” that thing. Part of the beauty of science is that it is always growing and allowing us to learn more with new inventions.
This is a misunderstanding. Particles have positions, and we can measure them accurately. Particles are constantly moving around though, and there’s a limit to how accurately we can measure both their position and how they’re moving at the same. We can measure one or the other accurate, but not both at the same time.
When things are REALLY tiny, they behave more like waves than marbles. So, imagine the particle as a ripple on a pond instead of a marble. You look at the pond and see lots of peaks and troughs in the ripples. If you want to measure the speed of the wave, you look at a single spot and count the ripples as they pass the spot, but if you count how fast the ripples move you can’t tell the height of all the waves in the ripple if you only look in one spot. You could take a snapshot of all the waves and figure out their heights at that exact moment, but you’d not be able to tell how fast they’re moving because it’s not a movie.
It turns out when you are looking at something so small that it behaves more like a wave than a marble, there’s the more you focus on measuring one property (position, or movement) at a specific time, the less accurately you measure the other. A guy name Heisenberg actually worked this out based purely on math from a bunch of physics experiments that were trying to describe the movements of electrons in atoms.
I recommend looking up the double slit experiment. It’s usually most newcomer’s first exposure to just how weird quantum physics can be. The full implications of the results of this experiment are beyond the scope of ELI5, but the general principle and observation are.
Basic facts in classical physics: if you shoot balls randomly at a board with a single slit big enough for balls to pass through, and measure where they land on another backboard (imagine paintballs leaving a splat). You will see a single bar matching the slit in the first board. Sure, some may hit the corner and fly off at an angle meaning you will get specklings elsewhere, but there will be a big, clear bar. Also, if you increase the number of slits to 2, you will get 2 bars on the backboard. For waves, pretend you lower the boards into water so the slit is half submerged. Then drop a pebble in the water, watch the waves hit the board. The wave on the other side of the board looks like you dropped the pebble right at the slit, moving radially outward. To measure it is a bit trickier, but imagine the backboard is color changing when it gets wet. The higher the wave laps against it, the higher the second color will show. You would expect to see the tallest spot in the middle and it tapers off on either side since the wave had to travel further due to hypotenuse stuff). If you increase the slits to 2, you end up with two different waves as if you’d dropped two pebbles at the slits. These waves collide. If a peak happens to line up with a peak, they add to each other, making the splash on the backboard higher. If a peak and a trough line up, they cancel out and there is no change in the height of the color on the backboard. This ends up with some wavy patterns showing up called “interference patterns” as the two waves interfere with each other.
Ok, now for quantum stuff. Instead of paintballs, let’s fire electrons at the slits. (And the slits are a lot smaller now). We think of electrons as little particles. Little spheres that we can shoot like paintballs out of an electron gun. And if we fire them at one slit, we see what we would expect from particles like paintballs out the other side: a single bar. However, if we shoot them through 2 slits, we don’t see 2 bars like we did for paintballs, we see an interference pattern like we did with waves.
“Well” think the physicists, “maybe we sent them through too quickly and they bumped into each other causing the patterns.” So they send them through one electron at a time. Still, they see wave-like patterns. Almost like the electron splits in two, goes through both slits, then interferes with itself. “No way!” Shout the physicists, “Let’s put a special detector next to one of the slits to see which one the electron went through, surely, it can’t be both.” But when they measure which slit it went through, the interference pattern goes away and out come the two bars like the paintballs.
You see, when we say an electron is both a particle and a wave, one simplified interpretation is that it’s a particle, but we don’t know where it is exactly. We can only predict a wave of probabilities where it could be, sort of like the individual molecules of water in the wave of classical mechanics… Sort of. Except it’s all of the molecules at once in all possible positions. See, when we set the electrons through one slit, any marks on the backboard has to come through that one slit. If you were to freeze the image with an electron on the other side of the slit and measure the probability wave (well, that’s impossible, but…) you would see a 100% chance that it came through the one slit (because all the ones that didn’t have already beed cut out) and it would look like a superposition of every possible trajectory a particle could have taken.
If you did the same freeze frame with the double slit, you would see that there’s a 50% chance it came from either slit. It would start out as a superposition of every possible trajectory of a particle through both slits, but when those probability waves meet each other, the trajectories can interfere and collide with each other. So the electron is in a superposition of having gone through both slits at the same time and interfering with itself. Until we measure it, and the superposition collapses.
First there is one important thing to state about physics: The goal of physics is to develope tools/theories to predict what will happen in certain scenarios. For example gravity is a theory that can be used to predict what will happen in certain scenarios, like dropping a pen.
You could also develope a theory that says the pen should fall up, or you could develope a theory that says some guy outside of our universe decides what will happen. But these two theories are not usefull, the first one contradicts our reality and the second one gives us no way of predicting what will happen.
Now coming back to your question:
The “normal” theory of particles in definitive spots is really good at predicting how our world works at big scale, for example in predicting how a tree falling on a house might be bad for the house.
But some time ago we noticed that our theories failed to predict the outcomes of certain experiments, like the double-slit experiment. That meant we needed a new theory, as our old theory contradicted reality. This is where some genius people came up with quantum mechanics, a new and more correct theory. This new theory only works, because we don’t require particles to have a definitive position. We had to “give up” that very intuitive idea, because we could not create a working theory that allowed particles to have a definitive spot.
If you want to know more about the double-slit experiment, you can google that or ask here again.
This is the best video analysis I’ve seen on the subject:
[https://www.youtube.com/watch?v=3CyN8rYdX6g&t=5m56s](https://www.youtube.com/watch?v=3CyN8rYdX6g&t=5m56s)
Not sure an actual 5 year old would understand, but the visuals do help a lot.
There actually isn’t any information we don’t have access to yet. Bell’s Theorem says that there are no hidden local variables. Local here means that there are no influences that determine the “true” location of the particle within the distance light can travel in the amount of time considered. So for instance, if you were to suggest that X event which happened T time units before the measurement contributed to the measured location, X would need to be farther than cT length units away from the particle at the instant of the measurement. Bell’s Theorem does not say these events that can influence measurements beyond light travel exist or don’t exist, only that events that are at or closer than light distance definitely don’t exist. Quantum entanglement is probably an example of a beyond-light-travel event, but as far as I know we can’t explain it yet, or at least can’t ELI5 it yet!
Ok, you are getting very long and complex answers, let me try my hand here.
What you propose is a model in which particles have pre-existing conditions that we just don’t know. This is a very appealing model as it feels intuitive and like something large scale physics would allow.
However, experiments seem to disagree with this model.
The double slit experiment showed the idea, and three polarising lenses can be used to show some very fun properties of light that mathmatically suggest that pre-existing qualities aren’t the answer. Light can get through filter arrangements that it could not if quantum qualities were pre-set. Radiation is also a very interesting case study, physically it doesn’t make sense for the particles to leave, but from the perspective of quantum mechanics, that particle just had a chance to be in that position even if it didn’t have any means to get there.
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