# Galactic "delayed-choice experiment"??? **URL:** \ **Category:** Great Debates\ **Created:** [November 23, 2002, 9:52pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396 "2002-11-23T21:52:33Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![buzzz\_kill](https://avatars.discourse-cdn.com/v4/letter/b/439d5e/32.png) [@buzzz\_kill](https://boards.straightdope.com/u/buzzz_kill)\ **Post date:** [November 23, 2002, 9:52pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/1 "2002-11-23T21:52:33Z")
**Has a _“delayed-choice experiment”_ been performed from a light source outside our galaxy?** Not a simnply a thought experiment but, the real thing? ---
**Author:** ![smiling\_bandit](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@smiling\_bandit](https://boards.straightdope.com/u/smiling_bandit)\ **Post date:** [November 23, 2002, 9:57pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/2 "2002-11-23T21:57:07Z")
Is this a joke? ---
**Author:** ![Duck\_Duck\_Goose](https://avatars.discourse-cdn.com/v4/letter/d/50afbb/32.png) [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)\ **Post date:** [November 23, 2002, 10:11pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/3 "2002-11-23T22:11:01Z")
Worse–it’s quantum physics. But from an extraterrestrial perspective. My head hurts. ---
**Author:** ![buzzz\_kill](https://avatars.discourse-cdn.com/v4/letter/b/439d5e/32.png) [@buzzz\_kill](https://boards.straightdope.com/u/buzzz_kill)\ **Post date:** [November 23, 2002, 10:39pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/4 "2002-11-23T22:39:41Z")
Ahhhh come on this stuff rocks! It was obviously a jest, but wouldn’t this be a significant experiment (if it could ever actually be performed that is!) This would seem to show that the photons would have to have known whether we would detect them billions of years (depending on it’s source) before our solar system was even formed. If this particular delayed-choice experiment is ever carried out, it would show, incredible as it may seem, that the past of the quantum world is influenced by factors associated with how we choose to conduct our observations now!!! Am I on the right path? or should I have taken a right at Albuquerque Help!!!My head is hurting too!! ---
**Author:** ![kambuckta](https://avatars.discourse-cdn.com/v4/letter/k/53a042/32.png) [@kambuckta](https://boards.straightdope.com/u/kambuckta)\ **Post date:** [November 23, 2002, 10:47pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/5 "2002-11-23T22:47:22Z")
Now, I _sort of_ understand the principle that proposes that our observations _now_ can impact upon the state of the world in the _past_, however illogical it seems. But I have no idea whatsoever what _you_ are on about ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [November 24, 2002, 7:19pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/6 "2002-11-24T19:19:17Z")
Is it possible to have some kind of **back story** explaining the significance of the question and why it’s being asked? Or, perhaps, a _ **link** _ or two. I’d be glad to comment on whether I see significance in the endemic infection by Rickettsia of _Hydrochoerus senegalensis_, but I’d expect that somebody asking such a question have the common decency to explain his terms and his reason for asking such a question in the first place. Same applies here. ---
**Author:** ![blowero](https://avatars.discourse-cdn.com/v4/letter/b/41988e/32.png) [@blowero](https://boards.straightdope.com/u/blowero)\ **Post date:** [November 24, 2002, 10:01pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/7 "2002-11-24T22:01:25Z")
> [@](#): > > \*Originally posted by Polycarp \* > Or, perhaps, a _ **link** _ or two. Here ya go: [http://physicalworld.org/restless\_universe/html/ru\_5\_15.html](http://physicalworld.org/restless_universe/html/ru_5_15.html) > [@](#): > > Measurement causes potentiality to become actuality. > […] > The Copenhagen interpretation calls simple realism into question. If the most that you can say about a position measurement you are about to perform is that various values may be obtained, with various probabilities, then it may well mean that the object has no position until it is measured. Note that this is quite different from saying that the object has a position which you don’t happen to know - it is as if the object had not made up its mind where to appear until the position measurement has been made. I think this is what he is talking about. Experiments have been done which demonstrate that the outcome of events at the quantum level remain in some sort of indeterminate state UNTIL they are observed. I have no idea how they actually perform these experiments, so I can’t answer the OP’s question. But I believe such experiments measure the _deflection_ of particles, so I suspect that even if one could do so with photons emanating from a distant star, one would still be measuring an event (deflection) here on Earth, and whatever the photon did before that would be irrelevant. I dunno, is that right? I guess we’ll have to wait until someone who knows about quantum physics sees this thread. ---
**Author:** ![Aspidistra](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/aspidistra/32/3894_2.png) [@Aspidistra](https://boards.straightdope.com/u/Aspidistra)\ **Post date:** [November 25, 2002, 12:03am UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/8 "2002-11-25T00:03:59Z")
Are we talking about the [Einstein Podolsky Rosen](http://members.shaw.ca/quadibloc/science/eprint.htm) paradox? Very VERY briefly: 1. Experimenters produce pair of particles at point X. They fly off in different directions 2. Some time later, a measurement is performed on one of the particles at point Y (as far away as you like from X). Then a measurement is performed on the other particle at point Z. _even if_ points Y and Z are far enough away from each other that there is no time for a signal to travel between them between the two measurement times, the results of the “Z” measurement are different, depending on what you choose to measure at “Y”. (it’s ‘delayed choice’ because the choice about ‘what to measure’ is delayed until there appears to be no physical way that it could have an effect on the other measurement.) This is a very freaky result of QM, because it appears to violate the locality principle - it seems that what you do at one point in the universe can affect another point even if they are light years away from each other. However, it doesn’t imply cause-and-effect working backwards in time. The point is that the two measurements are too far removed from each other for cause-and-effect to (theoretically) work at all - and yet it does! I believe the experiment has actually been done a number of times (it’s not that tricky, except philosophically) and , yes, it ‘works’. So … ummm… what was the point of the question again?? ---
**Author:** ![Greenpiece](https://avatars.discourse-cdn.com/v4/letter/g/57b2e6/32.png) [@Greenpiece](https://boards.straightdope.com/u/Greenpiece)\ **Post date:** [November 25, 2002, 12:09am UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/9 "2002-11-25T00:09:31Z")
I know… let’s have a deliberately exclusive thread which makes most ordinary posters here feel inadequate!! Great idea! ---
**Author:** ![smiling\_bandit](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@smiling\_bandit](https://boards.straightdope.com/u/smiling_bandit)\ **Post date:** [November 25, 2002, 3:55am UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/10 "2002-11-25T03:55:31Z")
Just to make sure: The OP is ludicrously specific, unprovable, and above all vague question. Not having studied Quantum Physics in college yet (or ever), I am left in the dark. ---
**Author:** ![Duck\_Duck\_Goose](https://avatars.discourse-cdn.com/v4/letter/d/50afbb/32.png) [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)\ **Post date:** [November 25, 2002, 4:48am UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/11 "2002-11-25T04:48:58Z")
Well, I didn’t study quantum physics, either, but all I did was look up “delayed choice experiment” on Google and read enough of the first couple of hits to know that (a) the OP was about quantum physics or something, (b) I didn’t get it, and © my head hurt. The OP sez he was joking, anyway, so as there doesn’t seem to be a debate here, maybe some kind mod could move this puppy to the forum For Jokes About Pointless Extraterrestrial Silliness? ---
**Author:** ![smiling\_bandit](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@smiling\_bandit](https://boards.straightdope.com/u/smiling_bandit)\ **Post date:** [November 25, 2002, 12:31pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/12 "2002-11-25T12:31:31Z")
Yeah, but I don’t even understand _how_ he’s joking. 🙂 ---
**Author:** ![FriendRob](https://avatars.discourse-cdn.com/v4/letter/f/ecc23a/32.png) [@FriendRob](https://boards.straightdope.com/u/FriendRob)\ **Post date:** [November 25, 2002, 3:12pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/13 "2002-11-25T15:12:12Z")
No, it’s not a joke, physicists have thought about this kind of experiment. I’m not sure if it’s been done or not. An EPR ( Einstein Podolsky Rosen) experiment has two parts. 1. Splitting a photon beam 2. Recombining it. QM says that if you measure which path the photon took then you will not see interference when you recombine the beams. If you don’t measure, you will see interference. The “delayed choice” comes in when you decide whether or not to “measure” _after_ the splitting has already occurred. Since experiments show you still get the same result, you have to conclude that the photon “knows” ahead of time whether to take both paths or just one. (Well, you don’t really, because there are other ways of looking at it. But that’s the popular gee-whiz way of putting it.) **buzz** ’s OP asks “What if the photon needs to know billions of years ahead of time whether to go both ways or only one?” The problem is, even if you use photons from a distant galaxy, you need to split the beam after they arrive. No reason to expect “old” photons to behave any differently. Now, if the beam were split by some natural process at or near its source, then the OP would make sense. A possible process is a [gravitational lens](http://www.iam.ubc.ca/~newbury/lenses/research.html). ---
**Author:** ![FriendRob](https://avatars.discourse-cdn.com/v4/letter/f/ecc23a/32.png) [@FriendRob](https://boards.straightdope.com/u/FriendRob)\ **Post date:** [November 25, 2002, 3:13pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/14 "2002-11-25T15:13:50Z")
BTW - Welcome to the SDMB, **buzzz\_kill**! ---
**Author:** ![Jonathan\_Chance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jonathan_chance/32/701_2.png) [@Jonathan\_Chance](https://boards.straightdope.com/u/Jonathan_Chance)\ **Post date:** [November 25, 2002, 3:46pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/15 "2002-11-25T15:46:28Z")
Yeah, that’s a helluva question. Not really a GD, though. But keep 'em coming. I’d rather debate QM than another damn SUV thread. ---
**Author:** ![Duck\_Duck\_Goose](https://avatars.discourse-cdn.com/v4/letter/d/50afbb/32.png) [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)\ **Post date:** [November 25, 2002, 4:03pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/16 "2002-11-25T16:03:28Z")
“Are SUVs proof that aliens are experimenting on us?” ---
**Author:** ![Revtim](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/revtim/32/1042_2.png) [@Revtim](https://boards.straightdope.com/u/Revtim)\ **Post date:** [November 25, 2002, 4:35pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/17 "2002-11-25T16:35:14Z")
It seems like a perfectly valid GQ question to me. I think that answer is that it happens all the time, but there’s no way to analyze the results. Everytime an astronomer fires up his instrument, there is some probability that he will view an image of an extra-galactic object. This probability is 100 percent when he or she actually views another galaxy. If the photons that make up this image have the possibility of taking multiple paths (like being gravitationally lensed around some other massive object, whereby it could take one path around the object or another path around it) then the experiment is basically executed. QM, as I understand it, says that the photons did not take any single path until a detection system (in this case the astronomer) “sees” the photon. So, this experiment happens all the time. But how one would actually get measurable data I don’t know. I just thought, technically the extra-galactic photons of the image could be diverted into an earth-bound two-slit apperatus. Still, I don’t see why these photons should act any diffferently from an earth-bound source. ---
**Author:** ![dalovindj](https://avatars.discourse-cdn.com/v4/letter/d/65b543/32.png) [@dalovindj](https://boards.straightdope.com/u/dalovindj)\ **Post date:** [November 25, 2002, 4:40pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/18 "2002-11-25T16:40:36Z")
It’s been done. It’s something I have noticed and wondered about. The scientific term I’ve heard is “Spooky action at a distance.” Heh heh. I suppose that’s more of a layman’s term but it sums it up nicely. [Here](http://www.csmonitor.com/2001/1004/p15s1-stss.html) is an article that describes the whole thing (theory and experiments) in pretty plain english. > [@](#): > > In 1997, a team at the University of Geneva conducted a particularly dramatic demonstration by entangling packets of light called photons, then sending them in opposite directions down fiber-optic lines to detectors nearly seven miles away. > > When they measured properties of one photon, it had an instantaneous effect on the other. If the interaction behaved in a classical way, a measurable amount of time would have passed between measurement of one and the effect on the other. Entaglement is a key word here. I’ve never heard of this having the potential to effect the past though . . . DaLovin’ Dj ---
**Author:** ![Revtim](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/revtim/32/1042_2.png) [@Revtim](https://boards.straightdope.com/u/Revtim)\ **Post date:** [November 25, 2002, 6:09pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/19 "2002-11-25T18:09:02Z")
I think that entanglement is different from delayed-choice. ---
**Author:** ![dalovindj](https://avatars.discourse-cdn.com/v4/letter/d/65b543/32.png) [@dalovindj](https://boards.straightdope.com/u/dalovindj)\ **Post date:** [November 25, 2002, 6:19pm UTC](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396/20 "2002-11-25T18:19:43Z")
So the theory is that nothing will exist (except as probability) as anything unless we observe it, and since we observe the universe, and it took billions of years for everything to get the way it is, our observing it now translated through time and locked in all of those particle interactions? So until awareness came into the picture nothing had been decided and awareness effected the past to allow for its own existence? Pass the aspirin . . . [Next page](https://boards.straightdope.com/t/galactic-delayed-choice-experiment/138396.md?page=2)