Does light only behave like a particle when observed?

[JustinB]

My question is - is all light a wave until observed, then wave function collapses and behaves as a particle? This is far fetched (and i don't subscribe to it!), but theoretically could light from a distant object passing through a double slit experiment and exhibiting a particle pattern be indicative of aliens?

 

[PeterDonis]

is all light a wave until observed, then wave function collapses and behaves as a particle?

No.

theoretically could light from a distant object passing through a double slit experiment and exhibiting a particle pattern be indicative of aliens?

No. Whether or not there is a "particle pattern" at the screen (by which I assume you mean a pattern that does not show interference) depends on whether or not there are detectors at the slits to show which slit the light went through. It has nothing whatever to do with whether somebody observed the light far away and long before it got here.

 

[JustinB]

No.
No. Whether or not there is a "particle pattern" at the screen (by which I assume you mean a pattern that does not show interference) depends on whether or not there are detectors at the slits to show which slit the light went through. It has nothing whatever to do with whether somebody observed the light far away and long before it got here.

I meant observed in the sense that you were explaining - where we are trying to find where it is located. Sorry I did not make that clear.

Another question you may be able to answer since it got me thinking - what happens when you observe which slit it goes through (causing it not to show interference), then have that light pass through a polarized lens? I had read that polarization blocks X or Y wave axes...but if it's not acting as a wave at that point, what happens?

 

[jfizzix]

My question is - is all light a wave until observed, then wave function collapses and behaves as a particle? This is far fetched (and i don't subscribe to it!), but theoretically could light from a distant object passing through a double slit experiment and exhibiting a particle pattern be indicative of aliens?

A complete description of the double-slit experiment must include the interaction of the light with any and all measurement devices; not just the screen in the back, but whatever is done at the slits to gain information about which path the light went through. In this fuller description, you can actually show that the more the light interacts with the slit to give which-slit information, the less visible the interference pattern will be. In the limit that there is enough info to show exactly which slit the light went through, the visibility goes to zero and the interference pattern disappears completely.

 

[Jehannum]

Ignore any explanation that singles out "observation" and implies that a conscious observer has a special role in the process.

It's better to think of a light quantum existing as a wave of possibility until its energy is absorbed by an absorber; then there is a complete, localised transfer of energy to the absorber. No observer is needed.

This will allow you to move on with your understanding of quantum physics and leave the "pop-sci" stuff behind.

 

[PeterDonis]

what happens when you observe which slit it goes through (causing it not to show interference), then have that light pass through a polarized lens? I had read that polarization blocks X or Y wave axes...but if it's not acting as a wave at that point, what happens?

Observing which slit the light goes through does not stop it from "acting as a wave" after the slits.

It's best to stop thinking of "wave" vs. "particle" altogether when talking about quantum systems. A polarization filter has a certain probability of letting light pass through; the probability depends on how the light is polarized in relation to how the filter is oriented.