Can future events affect the past?

[Jmeagle]

In a new experiment done,an Australian team of physicists have said they have said they had a particle in the future affect the past.
https://cosmosmagazine.com/physical-sciences/time-travel-and-single-atom

 

[phinds]

In a new experiment done,an Australian team of physicists have said they have said they had a particle in the future affect the past.

This forum requires specific citations in order to discuss an experiment.

 

[Jmeagle]

Sorry,just posted a link.

 

[phinds]

I'm out of my depth trying to explain that one but it sounds very interesting. That link is not included in the list of specifically acceptable sources for this forum and I have no idea if it's ok, but the experiment seems legit so I think it should be. We have many members who will be able to shed more light on it, I'm sure.

 

[Jmeagle]

So the team sent a particle back in time or prove retrocausality?

 

[phinds]

So the team sent a particle back in time or prove retrocausality?

No, that is not what it says. Did you read the link?

 

[e.bar.goum]

This is the recent implementation of the Wheeler's delayed choice experiment with cold atoms that has been talked about quite a lot in this forum. The paper: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3343.html

General reading. https://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment

 

[Jmeagle]

Its says they affected the past from the future.im a bit confused.or am I misreading the article.

 

[StevieTNZ]

The past you refer to was undetermined (the system was in a superposition of acting as a wave and a particle at the same time). So a decision in the future caused the system to behave as a particle or wave.

I think however that explanation is very vague and up to interpretation. May I suggest 'Quantum Enigma' page 153:

The "delayed-choice experiment" suggested by quantum cosmologist John Wheeler, ..., comes closest to testing the backward-in-time aspect of quantum theory. It confirmed the prediction of quantum theory that an observation creates the relevant history."

to clarify what I mean.

 

[Jmeagle]

So an event in the future did not affect an event in the present or past in reality?

 

[Drakkith]

So an event in the future did not affect an event in the present or past in reality?

I don't think there's a 100% agreed upon answer to your question.

 

[Jmeagle]

Im a bit confused,what did the experiment prove?

 

[ShayanJ]

Im a bit confused,what did the experiment prove?

How much QM do you know?
I don't think you can get an answer, if you don't know enough about QM.(Which it seems to me is the case.)

 

[Jmeagle]

I Don't know really anything about QM I am just curious.

 

[ShayanJ]

I Don't know really anything about QM I am just curious.

I want to bring into your attention what StevieTNZ quoted a few posts ago:

The "delayed-choice experiment" suggested by quantum cosmologist John Wheeler, ..., comes closest to testing the backward-in-time aspect of quantum theory. It confirmed the prediction of quantum theory that an observation creates the relevant history."

This is what happened. This is what the experiment immediately implies and we shouldn't think further than this. This is a typical situation in QM.

Let me see if I can give you an analogy. Imagine I tell you that I want to take you somewhere, an island or a desert. For going to the island, we should use a boat. For the desert, we should use a car.
I don't tell you which one we're going to do. Then I close your eyes and do all things necessary to make sure you can't know we're on a boat or in a car. But after reaching there, I open your eyes and you see you're in an island and you immediately find out you were on a boat all along the way. This isn't strange because your history was there, you just didn't know it.
But this experiment is different. There was no specific history to the atom, but after the measurement, the relevant history is created. Its not a popular view to interpret it as "future affecting past". In fact I myself don't like "future affecting the past", its just over-thinking it.

 

[Jmeagle]

So retrocausality or time travel to the past is still impossible?

 

[ShayanJ]

So retrocausality or time travel to the past is still impossible?

I can't speak that generally. But about this experiment, yes, it doesn't mean you can affect the past or travel back in time.

 

[Drakkith]

This sounds similar to the idea of using quantum entanglement for FTL communication. I assume the problems inherent in that area are also applicable here.

 

[ShayanJ]

This sounds similar to the idea of using quantum entanglement for FTL communication. I assume the problems inherent in that area are also applicable here.

Actually I was thinking the same thing!

 

[bhobba]

So retrocausality or time travel to the past is still impossible?

Its a subtle issue and we have interpretations where it does:
https://en.wikipedia.org/wiki/Transactional_interpretation

Generally you can't send information into the past - that would violate causality.

But there are all sorts of notions about with parallel universes and what not that if true allow such problems to be circumvented.

It also needs to be said that experiment is just a variation of the delayed choice experiment and what's going on is no mystery:
http://quantum.phys.cmu.edu/CQT/chaps/cqt20.pdf

Thanks
Bill

 

[Jmeagle]

Ok,so its wrong to say the future can affect the past or present?because if that were to happen causality would be violated.

 

[Jmeagle]

So the experiment is nothing new?

 

[e.bar.goum]

So the experiment is nothing new?

It was a thought experiment when it was initially proposed in 1978. It has been previously done with single photons. (Jacques, V. et al. Experimental realization of Wheeler’s delayed-choice gedanken experiment. Science 315, 966–968 (2007).) This is the first time it has been done with single atoms.

I think the most popular interpretation of the experiment isn't retrocausality, but that it's better to abandon the idea that objects behave definitely as particles or definitely as waves.

Any explanation of what goes on in a specific individual observation of one photon has to take into account the whole experimental apparatus of the complete quantum state consisting of both photons, and it can only make sense after all information concerning complementary variables has been recorded. Our results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, we believe that such a viewpoint should be given up entirely

http://www.pnas.org/content/110/4/1221

 

[Jmeagle]

Ok so no back to the future type deal,even for particles or atoms.

 

[e.bar.goum]

Ok so no back to the future type deal,even for particles or atoms.

It depends on your interpretation of the experiment. It's all a bit subtle.

 

[Jmeagle]

What about events in the future?can they affect the present or past or what that require faster than light methods?

 

[Jmeagle]

Which interpretation is the widely considered as the favorite?

 

[StevieTNZ]

I really think it is in your best interests to study a bit more about QM, rather than all these questions which in my mind have been addressed and we are simply getting a reiteration of the same question in different forms.

 

[bhobba]

Ok,so its wrong to say the future can affect the past or present?because if that were to happen causality would be violated.

No. Its an issue if information can be sent into the past. And even if you could there are some way out ideas that if true would mean its not an issue.

Thanks
Bill

 

[bhobba]

Which interpretation is the widely considered as the favorite?

Probably good old Copenhagen:
http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

Thanks
Bill

 

[Jmeagle]

So you are saying the future can affect the past as long as no information is sent?

 

[bhobba]

So you are saying the future can affect the past as long as no information is sent?

It's possible - we have interpretations of QM where such is the case.

Thanks
Bill

 

[Jmeagle]

Is there an interpretation where there is nothing retrocausal?

 

[Jmeagle]

Can particles affect the past without violating causality?

 

[bhobba]

Is there an interpretation where there is nothing retrocausal?

Indeed there is - nearly all are including Copenhagen.

Thanks
Bill

 

[bhobba]

Can particles affect the past without violating causality?

Yes.

The first theory of that type was one Feynman came up with during his Phd days:
https://en.wikipedia.org/wiki/Wheeler–Feynman_absorber_theory

Thanks
Bill

 

[Gaz1982]

I want to bring into your attention what StevieTNZ quoted a few posts ago:

This is what happened. This is what the experiment immediately implies and we shouldn't think further than this. This is a typical situation in QM.

Let me see if I can give you an analogy. Imagine I tell you that I want to take you somewhere, an island or a desert. For going to the island, we should use a boat. For the desert, we should use a car.
I don't tell you which one we're going to do. Then I close your eyes and do all things necessary to make sure you can't know we're on a boat or in a car. But after reaching there, I open your eyes and you see you're in an island and you immediately find out you were on a boat all along the way. This isn't strange because your history was there, you just didn't know it.
.

This sounds like a variation of the "hidden variables" idea to me

 

[Jmeagle]

Has anything ever been proven to affect the past without violating causality?

 

[bhobba]

Has anything ever been proven to affect the past without violating causality?

No. Its just theory.

Thanks
Bill

 

[Gaz1982]

Has anything ever been proven to affect the past without violating causality?

I'll jump in and say no.

But like you I'm interested in this phenomena. I have a theory about this but it's too unproven for here and would likely see me burned at the stake

 

[Jmeagle]

So the experiment is like quantum entanglement?

 

[bhobba]

So the experiment is like quantum entanglement?

No - but having observations it involves entanglement:
http://www.ipod.org.uk/reality/reality_decoherence.asp

Without going into the details what's going on is in simple cases decoherence can be undone.

Thanks
Bill

 

[Jmeagle]

Im a bit confused when you said events in the future can affect the past but as long as no information can.has this ever happened or is this what you meant by it being theory?

 

[bhobba]

Im a bit confused when you said events in the future can affect the past but as long as no information can.has this ever happened or is this what you meant by it being theory?

There are theories that say its how it works but no one has ever been able to figure out how to test it one way or the other.

Thanks
Bill

 

[jerromyjon]

Can particles affect the past without violating causality

That defeats the definition causality, that what occurs right now cannot change what occurred in the past.

 

[ShayanJ]

This sounds like a variation of the "hidden variables" idea to me

It was only an analogy, and if you continue reading my post, you'll see that I explained how this analogy isn't perfect.

 

[bahamagreen]

Could someone clear up a few things?

Doing the experiment with an atom rather than photons seems important because the atom that makes it to the finish line is supposed to be the same atom that took off at the start, unlike photons... the mean free path for photons in air is a couple of centimeters and each interaction with a mirror is a new emission. The delayed choice experiment using photons is employing a serial sequence of absorbed and freshly emitted new photons along the paths, so the presence or absence of the final mirror seems irrelevant except for the first and subsequent photons created after the choice is made manifest.

The retro-causality appearance seems based on the assumption that the initial photon is the same one that finishes the course... that its state was established at the start and is then mysteriously appropriate for the shifty measurement situation at the end. But seen as a chain of emissions, each new photon gets a "fresh state" based on the measurement situation, right? When the final mirror is placed either in or out, can't the next made photon and all subsequent ones take states as if all the past of the experiment didn't matter and all that was being done was to send a photon to some measurement condition?

So it looks like the trial with the atom is different and important, but the description is not clear. It does seem clear that only one atom was used and this same atom made the whole trip, but the part about the paths is unclear...

"The team then allowed the atom to fall towards crisscrossing laser beams. The lasers split the atom’s trajectory into two possible paths. After the atom passed the crossroads, the equipment randomly switched to a set-up that either recombined the two possible paths, or did not."

What does it mean that the atom's trajectory was split? How was this done?
What does it mean that the two possible paths were either recombined or not? How was this done?

"The atom behaved in the same way as the photon. If the paths were recombined this produced an interference pattern typical of a wave, showing the atom traveled (sic) down two paths at once. If the paths were not recombined, the atom banged into one of the detectors at the end of each track, in the same way a pebble would."

If one atom travels down two paths at once is there twice as much mass present as when it travels down one path?

 

[e.bar.goum]

Could someone clear up a few things?

Doing the experiment with an atom rather than photons seems important because the atom that makes it to the finish line is supposed to be the same atom that took off at the start, unlike photons... the mean free path for photons in air is a couple of centimeters and each interaction with a mirror is a new emission. The delayed choice experiment using photons is employing a serial sequence of absorbed and freshly emitted new photons along the paths, so the presence or absence of the final mirror seems irrelevant except for the first and subsequent photons created after the choice is made manifest.

The retro-causality appearance seems based on the assumption that the initial photon is the same one that finishes the course... that its state was established at the start and is then mysteriously appropriate for the shifty measurement situation at the end. But seen as a chain of emissions, each new photon gets a "fresh state" based on the measurement situation, right? When the final mirror is placed either in or out, can't the next made photon and all subsequent ones take states as if all the past of the experiment didn't matter and all that was being done was to send a photon to some measurement condition?

So it looks like the trial with the atom is different and important, but the description is not clear. It does seem clear that only one atom was used and this same atom made the whole trip, but the part about the paths is unclear...

"The team then allowed the atom to fall towards crisscrossing laser beams. The lasers split the atom’s trajectory into two possible paths. After the atom passed the crossroads, the equipment randomly switched to a set-up that either recombined the two possible paths, or did not."

What does it mean that the atom's trajectory was split? How was this done?
What does it mean that the two possible paths were either recombined or not? How was this done?

"The atom behaved in the same way as the photon. If the paths were recombined this produced an interference pattern typical of a wave, showing the atom traveled (sic) down two paths at once. If the paths were not recombined, the atom banged into one of the detectors at the end of each track, in the same way a pebble would."

If one atom travels down two paths at once is there twice as much mass present as when it travels down one path?

You're thinking a bit too classically here. This is straight-up quantum mechanics, but nothing exotic. You need to remember that atoms behave like waves, just like photons. Consider the wave-like behaviour of atoms - atom interferometry works just like photon interferometry, except you switch the role of the laser. You can construct beam-splitters from appropriate laser fields, and just like in the case of photons, the wave-function of the atom is such that there is a 50/50 chance of measuring the photon having traveled down one arm or the other, or you can construct an interference pattern (hence "choice").
The mass isn't double. There's still only one atom, but in a superposition of states.

Perhaps it would help to read some stuff about atom-interferometry in general? It's a very cool (pun intended) field, and they see widespread use, not just for fundamental physics research. The best measurements of gravity are done with cold atoms - make it robust enough to put on a plane, and suddenly, you're using cold atoms for mining exploration! There are a few descriptions for how you make atom interferometers here:

http://physics.aps.org/articles/v8/22
http://arxiv.org/abs/0712.3703
https://en.wikipedia.org/wiki/Atom_interferometer
http://physics.okstate.edu/summy/publications/ContemporaryPhysics_42_77.pdf

 

[harrylin]

I want to bring into your attention what StevieTNZ quoted a few posts ago:

This is what happened. This is what the experiment immediately implies and we shouldn't think further than this. This is a typical situation in QM.

Let me see if I can give you an analogy. Imagine I tell you that I want to take you somewhere, an island or a desert. For going to the island, we should use a boat. For the desert, we should use a car.
I don't tell you which one we're going to do. Then I close your eyes and do all things necessary to make sure you can't know we're on a boat or in a car. But after reaching there, I open your eyes and you see you're in an island and you immediately find out you were on a boat all along the way. This isn't strange because your history was there, you just didn't know it.
But this experiment is different. There was no specific history to the atom, but after the measurement, the relevant history is created. Its not a popular view to interpret it as "future affecting past". In fact I myself don't like "future affecting the past", its just over-thinking it.

OK...Just in order to get the idea, is there a significant difference of principle with opening the box containing Schrödinger's cat, and discovering that the cat has been killed a long time ago?

 

[ShayanJ]

OK...Just in order to get the idea, is there a significant difference of principle with opening the box containing Schrödinger's cat, and discovering that the cat has been killed a long time ago?

In the Schrodinger's cat experiment, decoherence suppresses quantum effects long before you open the box.