How does our observation of a subject change it's state?

Question

I'm totally new to the study of quantum physics, and I need an explanation of one of the basic principles. I've read many references to the fact that our observation of an item changes it's state, such as the scenario proposed in "Schroedinger's Cat". How exactly does observation of a subject change it's state?

Ok, this is what I'm taking from the current explanations. Our observation of an object changes it's state from essentially an uncertain state to a certain state. So if we observe the object it obtains a state of certainty. How do we know that it had an uncertain state in the first place? If we observe it in any way it's state becomes certain. So we have no scientific basis to claim that it's state was ever uncertain. If we try to observe the item to gain said evidence, it's state becomes certain. At the end of the day, quantum mechanics is impossible to prove and has no practical application in physical reality.

Answer 1

Perhaps the best way to think about this is in terms of the uncertainty principle. A wave has a definite momentum, described by its wavelength (imagine an infinite train of waves, like a sine graph). If you were to ask "where is it?" The answer would have to be "spread over all of space."

But suppose you tried to detect the wave. You might find it here, or there, or somewhere else, and if the wave corresponds to just one particle--something irreducible, unsplittable--then sudenly the wave can't be anywhere else but where you've found it.

So your measurement has "collapsed the wave" from a thing spread over all space, to something localized. But you could have found the wave anywhere. So the wave must have been a sort of combination, called a superposition, of all of these possible results. This is the model for how measurement changes the state of a quantum system.

Edit: The point of this is that the initial state, which is a well-defined momentum state (also a well-defined energy state) is not--and CANNOT be--a well-defined position state. And such a state can be prepared quite easily.

There are some quantities which cannot be simultaneously precisely known--there is a trade-off between them. After the measurement you know the position precisely but the momentum could be anything.

This is the sense in which a pure and perfectly well-defined state can still be a superposition of other states. The other states have to be defined in terms of incompatible (jargon: "noncommuting," or "canonically conjugate") quantities (jargon: "operators").

I disagree that quantum mechanics has no basis in reality. For example, it explains many experiments; it explains why electrons can't live in the atomic nucleus; it explains how that machine you typed your message on works.

That said, there is a bit of philosophical discomfort in the idea that we "determine" reality--whatever that is--by our measurements, and there is and has been much debate about what is really going on. Are there "multiple universes" of wherein all possibilities are realized? And how, given that WE are also goverened by quantum rules, can we really make any choices whatsoever? But to date all measurements--and there have been lots of them--support the idea that the way in which the wavefunction collapses is totally random.

Answer 2

The whole Schroedinger's Cat thing sounds to me like the tree falling in the forest with nobody there to hear it. A silly philosophical question.

Quantum mechanics may be strange, but it is useful. Transistors would never have been invented without it, since it is only quantum theory that explains how transistors work.

The Heisenberg uncertainty principal says it is impossible to know both the position and momentum of a particle to better than a certain precision. The question is, does it HAVE an exact position and momentum, even if we can't know it. The answer is, no, it doesn't, because it isn't really a particle. It is a wave function. The only way to observe the wave is to "touch" it somehow. Either by bouncing another wave (light wave) off of it or by touching it directly. The touching disturbs it from what its state was before you observed it.

Answer 3

I'm not so sure that observation changes the state to certain. You only measure something with certainty if you factor in ALL of the effects of observation.

We know the initial state is uncertain, because YOU don't know what the measured outcome will be, that is why you are observing it.

At the end of the day, quantum mechanics are a means to an end, help us explain our universe, and further advancements in lasers, chemical bond processes, CAT scans, etc. etc. etc.




Check these wikis

http://en.wikipedia.org/wiki/Uncertainty_principle
http://en.wikipedia.org/wiki/Observer_effect
http://en.wikipedia.org/wiki/Quantum_mechanics

Answer 4

Newton's laws may help.

For every action there is an opposite and equal reaction.

In quantum work this is NO LONGER totally a law.

To put it simply, for every action there is some type of reaction.

There are basically two kinds of observation. Passive and active.

Passive takes a measurement of what is there and consums some of it.

You put enough thermomenters into a patient at the same time and the temperature is going to drop as each new thermoment absorbes some heat.

This is why the pack you in ice when you have 106 temperature. So the ice absorbe some of that heat.

A series of probes outside an excellerator capture radiation of a given particle, while those areas without the probes pass the radiation or reflect all of it back.

So, the observation robbes some of the energy to take the readings.

If you use active readings they you are sending out radiation that interacts with the particle.

It would seem logical that in the universe of MATTER Newton's law holds. Send a billiard ball into deep space at 10,000 MPH and if you photograph it or use radar on it none of that will have a major reactions. The speed might eventually drop to 9,999.999999 and you can also attribute that to "aether" or free hydrogen.

If you take an electron travelling at the speed of light and just look at it, you will absorbe some of it's radition, you will be a gravity well that alters it's direction minutely (your mass is far greater than its) and if you measure it you prevent radation from being evenly distributed in space. You cause a "hole" in the radiationi patten.

Put up enough sensors at the airport to measure sound waves from jet planes and eveutally they absorbe and reflect so much of this sound that they aren't needed anymore, because the sound level is now much lower than without these probes.

If you observe the light from a star though a telescope you don't exeert much force on the star itself, but you are disturbing the distribution of light waves from the star to a small degree.

You are also causing some to be reflected back into space at different angles due to the mirror or lens shape.

Again, this is small compared to the star, but when it comes to a SINGLE photon of light, it becomes immense.

It is said only about 80 to 90% of light passes through a telescope lens, the rest is reflected in all directions by all the lenses.

Thus observing the light of a star causes a box of PHOTONS to fall into a tube and 90% of them focus into your eye and 10% of them scatter like rubber balls hitting a curved surface.

Observing one of the photons thus changes it state. It may lose 5% or 10% of it's photonic power to your passive device or get excited due to your active device.

The amount of state change is probably proprotional to the relative mass and energy of the observer and the observee.

If too many humans go outside tomorrow and start watching the SUN all day long from now on in, the sun could burn out maybe 1 second quicker than if no one looked up at at.

Remember an Atomic Clock in space orbiting around the EArth only loses a faction of second from the duplicate clock on earth at "rest."

That's enough to prove a point of realtivity.

If you have two mesuring probes and shoot and electron at a given speed and let no one observe it, then shoot another at a given speed and let 500 people observe it throught a window, if there is even a small measurement change it can indicated that those 500 people watching the electron caused the change.

Finally, think of what we tend to call the side effects of an "observation" which requries tangible present of mass and energy. We call the primary side effect:

Interference.

It's what happens to your broadcast TV set when your neighbor picks up his wireless phone and walks near your walls.

You might hear a buzz or get a ghost image.

A car outside might break up the picture and sound for a second.

A jet plane 2 miles away might cause a color shift in the picture.

This is because these tangible objects are interfering with the propgations of waves.

If a jet can alter your TV picture from 3 miles away, you can alter an electron being only 100 feet away.

If the theory of the "aether" is correct, then one pebble thrown into the aether of near space could displace enough of the aether to eventually alter the fabric of space 5 light years away by a tiny fraction in a few millions real years.

Of course I never took this in college, I just like to read and tinker with "what ifs"

The problem is no one can explain easily. Even Einstein can't explain relativity in simple enough terms for dear old Granny to understand!

Answer 5

"DonSoze" the first responder tried to introduce, to you the "cat in the box" scenario.

The experiment is called Schordinger's Cat, after the professer who advanced this thought experement to illustrate the principle.

What the experiment sugges is the opposite of what "DonSoze" indicates. The cat's state, in the unopened box is that it is either alive or dead. In other words. it is "both" alive and dead until the box is opened, allowing the observer to determine it's state. The observer neither kills it nor brings it to life, but the observation settles the matter.

I usually like to introduce another analogy. In "Blackjack," or "21," the Ace has the value of either one, or eleven. In fact, until the hand is done, the Ace is BOTH one or eleven. In commons strategy, the player does not make that determination until, at least, the third card is dealt. It is only when the player announce that he will take no more that the value of the card is determined. If the play decides, even in his mind, the value too early, he eliminates many possibilities, even winning ones. If he assings the value too early, he make decisions, later in the hand, based on the perceived value.

That is NOT a good strategy if the card's OTHER value is not considered. The Ace has BOTH values.

The cat is alive AND dead.

The position of any particular electron is not anyting until the observation is made.

Light is BOTH a particle AND a wave until you take its measure.

Answer 6

Quantum mechanics assumes unknowns states in order to facilitate it's mathmatical formulas. It's a shortcut because we can't yet measure on the level that Quantum mechanics works on. So instead of being able to find the "right answer" with the "right tool", the scientiest assume all the possible states and then work from there. It's not really true that things changes when we observe them, just our knowledge of them changes in the formulas.

Answer 7

That's right , it has no application to physical reality. It has an application to Quantum reality! Quantum reality is not provable on the physical level because it can't be measured. Q is no less real, just different, on a higher energy level. When we observe , our quantum energy is focused....the subject is made real. The subject goes from non-existent to existent...its state has changed. Reality is subjective on the Quantum level. A clock will objectively measure time, but subjectively it seems to go faster when we're having fun.

Answer 8

It's one of the unknowns of quantum physics. The act of observing the result seems to force it into one state or the other if it has a potential to be in 2 different states.
Generally observing does require some kind of interaction with the object like bouncing some kind of energy off of it, but in many cases we are simply intercepting energy that is coming off of the object or interaction and this is one of those cases.

From : http://en.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat
Today, the mainstream view is that the thermodynamically irreversible triggering of the device acts as if it were an observation, in that the triggering apparatus generates decoherence that appears to "collapse" the wave function.

Answer 9

I believe the term is the Heisenberg uncertainty principle. The term became part of social vernacular when Jurassic Park came out and Jeff Goldblum described it, given his girlfriend was part of it.

The whole idea, form what I gather as a layman, is that a situation inately is one thing, but by observation we impose a new set of rules on it - our beliefs if you will. NOT really our beliefs, but more our understanding of how things work.

For instance say we see a tree grow a certain way, but WHAT if there are other organisms that would become part of the tree if there was nothing watching it. Does that make sense? Simply by being there we change what MIGHT happen.

TFTP

Answer 10

Well, if remember discussions about observation
notwithstanding Heisenberg's uncertainty principle,
when one sees an object the receiving light on the object
is reflected back to our eyes, light hitting an object gives
an object a bit of a push, we cannot see this effect, light
hitting the electrons of an object has an effect as light
bounces off the electrons, it changes the speed of the
objects electrons ...
Of course it's more complex than that, but there is my
take on an answer for you ..
Good luck, you will surely have fun and excitement with
experiencing the study of natures ways ....
cheers !!!!!!
gwiz