
#1
Nov2103, 01:49 PM

Sci Advisor
P: 1,341

Greetings !
Here's an example by Steven Weinberg I read sometime ago (in SciAm I think): Imagine a light beam coming from a quasar billions of light years away. It encounters galaxy superclusters on the way and is split so that gravitational lensing is observed when we look at the quasar from the Earth. Now, let's take the uncollapsed WFs of photons in the beam. We now have the choice of detecting the photons in a 1 or 2 slit (as in the doubleslit experiment) configuration. If we choose to use a single slit and aim at one of the images we'll collapse the WF of the photon (let's say we have just 2 images). If we use two slits we'll collapse it and get the ussual difraction pattern. If we try to interpret these results it appears abvious that we've decided upon the path  if any, that the photon took during all these billions of years. The above example is of course intresting in its own right, and was used by Weinberg for emphasis of some of the "conscious mind" stuff in various interpretations of QM. However, I'll be using it in this post in a somewhat different way, and want to inquire about physics rather than its interpretations (though separating the two is not that easy at this point). First let's suppose that our 2 WF splits are both trapped for a while in an orbit around two different black holes, before they reach us. Let's further assume that the light beam is extremely intense. So much that such an amount of light at a close orbit would be able to affect the BHs. Now let's play the "what if" game: The BH's are affected by the light (the WFs of some of the photons in the beam will have to collapse for that to happen, I guess) and we detect the shifts in its radiation due to these gravitational distortions before the quasar's beam reaches us. Now, let's say that some WFs did not collapse. I assume we'll agree that these will not affect the BHs initially ? And now, we run one of the above versions of the experiment. What will happen in each case ? Thanks ! Live long and prosper. 



#2
Nov2203, 01:39 AM

Sci Advisor
P: 1,341

Well, anybody ?
Anyway, here's a simpler way to look at it that I cooked up: There exists a variation of the double slit experiment when there's a mirror with equal chances to reflect the photon in 2 different directions, eventually the split WF converges after passing through a single slit  sepearate for each part and far enough not to allow them any difraction. Now you again have the choice of measuring in different ways. I'll add something to this experiment  at each separate path of the split WF we'll position something that will be affected by the passage of the photon. I'll be able to check that something (in case the WF actually collapses in one of the paths) before I detect the photon. Now, I run the experiment and detect the photon at the end. The same dellema as in the enitial case follows : If I measure the difraction pattern then would it seem like the photon never passes anywhere ? Or if I measure one of the cases and collapse the WF "deciding" upon a certain path  would I see my "past" changing  or maybe I won't even know it happenned ? Thanks ! Live long and prosper. 



#3
Nov2303, 02:13 PM

P: 513

drag,
if I understand you correctly, you ask if it's possible do detect which path the photon took, and still observe the interference pattern. AFAIK, this is not possible. The standard answer is: If you make such a measurement, you alter the initial state so that the interference is destroyed. I guess your idea is that gravity lenses/black holes may be a special kind of detector that does not obey this rule. I don't know anything about quantum gravity, so I really can't help here. 



#4
Nov2403, 07:41 AM

Sci Advisor
P: 1,341

What's up in this case
Greetings !
I've not explained my Q sufficiently in the second post. Anyway, I figured a few things here as I considered it some more, so, if I may, I'd like to ask a different question: Does a particle's WF, while it is uncollapsed, cause external effects (like spacetime curvature = gravity, electric fiedls and so on) ? Thanks ! Live long and prosper. 


Register to reply 
Related Discussions  
Learn the general case first, or the special case first?  General Math  6  
ABS Case  Computing & Technology  1  
In case of a tie.  Current Events  10  
So what is it, in that case?  Calculus  7 