Can future events affect the past?

[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.

 

[harrylin]

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

Ah yes, good point, nobody believes anymore that it depends on a human observer. Thanks

 

[DrChinese]

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

As already said, it is a matter of interpretation. It would be safe to say: the experimental results are compatible with a form of retrocausality, without absolutely demonstrating such.

Keep in mind that if there were retrocausality, the form of it would not be compatible with deterministically affecting the past. You could only affect the past in a random manner and could not send a signal. Consistent in some ways with how you can affect the future at the quantum level, for that matter.

 

[Jmeagle]

Is a interpretation of retrocsusality a common view?

 

[zonde]

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

In science we simply assume that past affects future and only that way.
If you want to include possibility that future affects past you would have to develop completely new philosophical framework with some sort of metatime (think how past affects future then future changes the past so it leads to new future and so on and on as metatime goes on). But this obviously is not science.

 

[Xertese]

Observation is what affects the future as all possibilities exist until one is observed.

Quantum 101

 

[Nugatory]

Observation is what affects the future as all possibilities exist until one is observed.

This statement is true, but only if by "observation" you mean something very different than what the word means in ordinary English usage. In particular, it does not mean that some observer makes an observation and becomes aware of the event.

 

[Nugatory]

E.bar.goum has already provided a pointer to the previous discussion of the experiment.
Jmeagle we've already suggested several times that you will have to learn more about quantum mechanics; your initial question has been answered about as well as it can be before you learn some more about QM, so this thread is closed.

 

[jerromyjon]

so this thread is closed.

Antiretrocausality at work?