When particles are not being observed do they still exist?

Main Question or Discussion Point

I was hoping someone could explain, in terms I can understand, the movement of particles. Clearly I am not a scientist, but I just want to fully understand the idea that when particles are not being observed they either no longer exist or just do as they please. The example I was given is that when you are looking at your bedroom door it's tiny particles are aware they are being observed and appear as your bedroom door. However, when you look away the door might not exist at all. If this is true how do they know they are being observed? And if they are responding by either becoming positively or negatively charged based on the awareness of the others charges, how do they know they are being observed in order to respond correctly? I hope I have phrased this is an accurate enough way. I am sure it is apparent that I don't have a real understanding of the subject.
 

Answers and Replies

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When they aren't being observed their wavefunction is a superposition of possible states, this doesn't mean they don't exist.
 
tom.stoer
Science Advisor
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"Particle" is a concept that can be described in many different ways.

If one looks at quantum field theory one could say that a particle is something that carries a certain amount of momentum and energy, has a well defined invariant mass m² = E² - p² and transformes therefore in a proper way under Lorentz transformations. If this is sufficient, than such a "particle" does always exist, even w/o looking at it. But the concept is rather abstract and stricly speaking one is still not able to describe such a particle accureately by modern physics - only approximations are known.

If a "particle" is something that is localized then I would say that a quantum object (that may exists always in the above mentioned sense) is revealed as a particle only via a local (in space and time) interaction.In that sense a particle does exist only when looking at.
 
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You should read about the http://en.wikipedia.org/wiki/Double_slit_experiment" [Broken], which classic demonstration of the effect of observation in quantum mechanics. I thought for a while about how to explain this phenomenon of observing a particle affecting its behavior, but I personally don't know a good way to talk about that outside of the math. I suppose that just means my understanding is not as good as it should be.

the idea that when particles are not being observed they either no longer exist or just do as they please.
Such statements are sort of justified by a possible method of calculating in quantum mechanics that goes something like this: Say I have a particle that I have measured to be at position A, which is somewhere. I want to predict where is will be five seconds from now. In fact, I can't predict this, according to the math. I can predict the /probability/ that I will find it at a given location. How do I find the probability that it will be at some given position B? Well, I draw /all possible paths/ the particle could take from A to B. For each path I calculate a number. Then I add up all the numbers to get the probability of finding the particle at B five seconds from now. Of course, I could do the same thing to find the probability of the particle ending up at any other position.

Since we can think about the particle as travelling along all possible paths, in some sense the particle is doing "whatever it pleases" during the intermediate period. Why then is the universe not total chaos? Well, the probabilities to end up at various points are more or less what you'd expect. If I have an electron in my lab moving left at one meter per second, the math tells me I'm unlikely to find it halfway around the world five seconds from now. I'll almost certainly find it five meters to the left of where it was (well, except gravity will pull it down and stuff).
 
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Thanks so much for your explanation. I feel a like I have a little better grasp of the subject now. It really is interesting stuff! Also, thanks for the link and pointing me in the right direction so I can learn more about it. I will read up on the double-slit experiment as you suggested.
 
Pythagorean
Gold Member
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Observer doesn't necissarily mean living being does it? Isn't any ineraction technically an observation? Ie two particles colliding can be said to be observing each other?
 
tom.stoer
Science Advisor
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If you say that observation forces a particle to localize in a pointlike manner then two colliding particles do not "observe each other" b/c the qm formalism explicitly says that afterwards you still have a quantum superposition, not a pointlike particle.
 
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Thanks so much for your explanation. I feel a like I have a little better grasp of the subject now. It really is interesting stuff! Also, thanks for the link and pointing me in the right direction so I can learn more about it. I will read up on the double-slit experiment as you suggested.
A good place to start is http://research.microsoft.com/apps/tools/tuva/index.html" of quantum mechanics. Watch Lecture 6.
 
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Observer doesn't necissarily mean living being does it? Isn't any ineraction technically an observation? Ie two particles colliding can be said to be observing each other?


Yes, but you have to believe in instantaneous influencing across the universe. Which goes against SR among some other deeper philosophical objections. Moreover, a whole class of such non-local hidden variable theories was removed by Zeilinger & co. recently and this nonlocal hidden variable(reality) is a teleological argument.
 
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