Help, my brain hurts. Does anything exist?

freddiy

Hi there. I have read that quantum particles change when we observe them, Does this mean, nothing in the universe is as it is unless we observe it? If so then how can the planets that give life exist before we observe them? I am sure I sound like an idiot, but I hope you can understand my questions. Thank you.

DennisN

Hi freddiy! If your brain hurts, you may have been thinking too much.

First a note on terminology; "observation" in science usually means measurement, detection etc. by a human and/or machine. Often this means preparing an experiment, manipulating some things and measuring what happens. That things change (in some respect) when we measure them is perfectly normal; there's no free lunch (see e.g. Observer effect).
Sort of; but I wouldn't say change, their basic properties do not change (e.g. mass, charge). They can however behave differently depending on how we experiment with them. The double-slit experiment is a common example;

• no particle detectors at the slits -> light (photons) will form an interference pattern
• particle detectors at the slits -> the photons will not form an interference pattern

This does not depend on if we observe the experiment or not, it depends on how the experiment is set up, i.e. with or without particle detectors. It might seem strange, but it's not paranormal.
So my answer is obviously no, certainly not. Yesterday I observed the Moon in the evening. Today I observed it again. My conclusion is that the Moon exists in exactly the same way when I'm not observing it.

lostprophets

if i observed the slits with my eyes close up, would that change them also?
the saying "i can feel his/her eyes piercing through me would suggest we give off a wave of energy through our eyes... i know its a saying but do we?
if so would this energy wave be enough to change things at a sub atomic level.
we could not do this with our eyes to the moon its to massive but with teeny weenies maybe so.

DrChinese

It would make more sense to say that the particles themselves exist all of the time, but that some specific properties of those particles do not exist unless observed. Most properties of a system are conserved, such as total energy. So it is not like things are popping into and out of existence.

Mark M

Hi Freddiy!

First of all, it isn't observation that causes particles to "collapse" to one state, it's interfering with them. It appears as if observation causes this, because to observe something, you must shine light on it it, or set up a detector. This is called decoherance.

Second, when the particles are left by themselves it's not that they don't exist. Instead they are described by what is called a wavefunction. Think of it like a blur between different positions, speeds, charges, etc. so that you cannot tell what it's properties are. Then, when you interfere with it, you cause it to collapse to a much more definite state, an egienstate. But, it still has some uncertainty, you can never fully determine a particle's properties, only make very accurate estimates.

BTW, welcome to the forum!
It's not the act of observing the experiment that causes decoherance. It's interfering with the experiment. In the case of the DSE, it's the fact that you are bombarding the particles with your detectors. Human consciousness or observation has no effect on this.

Naty1

I disagree...keep on thinking until you reach some conclusions.


See my post #8 for some ideas on wavefunction:

http://www.physicsforums.com/showthread.php?t=579640

All that of post relates to quantum theory.


In fact everything in the universe IS as we observe it. But what we observed is not fixed:
Here are a few examples of how OBSERVATIONS change in relativity:

[1] Unruh effect: an accelerating observer will measure a different local temperature than an inertial [fixed velocity] observer,

[2] Clocks tick at different rates according to observers with different velocities and those in different gravitational potentials...so GPS satellite clocks, for example, tick at different rates than do earth bound clcoks [Wikipedia has a good article on this.].

[3] While the speed of light appears the same to all observers locally, different observers moving at different velocities relative to the source will observe different colors [different wavelengths]...this is called the Doppler effect.

None of these are quantum effects..all are relativistic effects.

So in our two greatest theoretical constructs of all time, GR and QM, we learn that things are not so simple as they appear. That's likely because evolution so far has not prepared us, not developed senses, for them as it has, say, to detect a predator heading right at us

Naty1

Another 'expert' here, has said something quite different, like.....'Particles are rare and only exist when we observe them'.....suggesting most things are in a wave form most of the time....the views are complementary....not necessarily mutally exclusive...

For example, what does an electron 'look' like??
It looks one way when confined in an atomic orbital,in a cloud around a nucleus; it is stuck in different size and shape clouds in different nucleii; it looks bigger when confined in a box....it changes wavefunction depending on how it is 'confined'....which means how it's degrees of freedom and energy are constrained. And shoot electrons [or photons, quanta of light] through a pair of slits, the 'double slit' experiment and 'unexpected' patterns emerge [unexpected from our usual experience, not un expected to scientists].
Yet as Dr Chinese says, electrons follow energy conservation laws too...they are not free to do anything they wish!!

"Don't doubt me." as Limbaugh says[ lol] ...look here for neat illustrations of what 'electrons look like' in different orbitals in THE SAME ATOM!!!:

http://en.wikipedia.org/wiki/Atomic_...Orbitals_table

Naty1

one more idea freddiy:

" Who do you think you are?"

Do you realize you cells replace themselves completely every so often.....I forget about how long it takes, but unlike a rock, for example, you actively replace every cell in your body periodically whether you or anyone else observes it or not....talk about change!!!

jeffmoor

So, tell me if I understand... A particle, when left alone, does exist, and has a specific position, charge, and momentum at each instantaneous moment in time. Because of uncertainty (our inability to measure it without interfering with it) we can't know these all at once, so describe the particle's properties with a wave function. It's not that, in all reality, the particle doesn't have specific values for these properties, it's just that we can't measure them. The wave function is our probabalistic descriptor of the particle. Is this true?

Demystifier

It's not really that observation per se changes the world, it's the interaction with the environment that changes it.

bugatti79

But why do we need a wave function to describe that the properties are so called 'blurred' when we know that the particles have distinct properties even when we are not 'measuring/interacting with them with our machines?

In other words, why do we describe something as 'blurred' or the 'particle is in many positions at once' just because we cant measure it with 100% certainty?

DrChinese

Welcome to PhysicsForums, jeffmoor!

No, your assessment is not quite accurate from the perspective of the standard model. The particle exists, yes. It has elements such as mass and charge and momentum, which are conserved. However, these do not all have specific values at all times. Some, such as momentum, take on discrete values only when observed. And when they take on a discrete value, some other properties (those called non-commuting) take on an uncertain character. This has nothing to do with our inability to measure it without changing it. Note that commuting properties do not have this restriction. I can measure spin without changing momentum, yet any spin measurement effectively erases all prior spin values. Go figure!

Your last statement (wave function being probabilistic descriptor) is correct. However, we don't really know where the line is as to the source of the apparently random variations that are seen.

JamesOrland

From what I understand, that's not quite it. Instead, it's more like the particles are interfering with us. Decoherence isn't that the environment has leaked into the system, but rather the system has leaked into the environment.

What I mean? I mean that decoherence is what happens when the environment suddenly depends on the system being observed. That is, when you measure the particle, there is suddenly a LOT of cells in your brain that change to reflect what you measured, so now your state of being depends on the state of the system being observed.

That is somehow what is explained at many textbooks, but it's not quite true. Uncertainty isn't on the map, it's in the territory; that is, it's a part of the world, it's not a part of how we understand it. It has been explained above that there are properties which cannot be known simultaneously. That has nothing to do with our ability to measure it, we can measure either property (momentum or position) with a high degree of accuracy without affecting it too much. But talking for instance about momentum and position, once the wavefunction describing a single electron's position has been confined to a narrow area (that is, its position is known with some accuracy), the momentum becomes indefinite, and it can take any value.

As I said, it's not about measuring. It really is blurred. An electron really is in many places at the same time, and furthermore, those places can even interact! (That is, one electron can interact with itself.) The particles have some distinct properties that are fixed, like mass, but there are others that are not fixed.

The idea that a wavefunction describes a probability is called the non-realistic view of the wavefunction. There's another idea that claims that the wavefunction is an actual part of the universe, something physically real, instead of just a description of "randomness".

DrChinese

That tends to be my view, and the following paper will likely make this a more accepted viewpoint:

The quantum state cannot be interpreted statistically
http://arxiv.org/abs/1111.3328

bugatti79

Forgive my ignorance, but how do we know the electron is in many places at one time. Is this verified from the double slit experiment?

Mark M

Actually, no. The uncertainty is a built in characteristic of particles, not an experimental limit. A particle at any time occupies several positions, and literally exists as a wavefunction. The view that you are expressing was that of Albert Einstein, but today we know (from John Bell) that wavefunctions are very much real, not just probabilistic descriptions.

Like I said at the beginning of this post, particles do not have distinct properties. It's not just an experimental limit, it is a fact of quantum mechanics that the best you can do is predict the probability of a specific outcome.

Thanks for posting a more in depth definition! I was just trying to keep it simple for the OP.

JamesOrland

Yeah, it tends to be mine, too :) I just got home, but I'll read the article you mentioned.

Yes, among other stuff. The effect observed in the double slit experiment is exactly what one would expect if suddenly the electron became two electrons, one coming from each slit, and then interacting with itself, to a T. I mean, truly, if you make your calculations assuming there's actually two waves, one coming from each slit, and that they interact, you will get the same interference pattern observed. That is not the only experiment that confirms this fact, but it's the most well known and the simplest to picture and explain.