Does it make sense to speak about the Grandfather paradox in QM?

In summary, the conversation discusses the concept of time travel in the context of quantum mechanics. It is suggested that since quantum mechanics is not deterministic, the idea of moving back in time should also be probabilistic. This is in contrast to classical mechanics where the future state is determined by the previous state. The conversation also mentions the concept of closed timelike curves, which could potentially allow for time travel. However, most physicists do not believe that such curves can exist due to their physically impossible properties. Additionally, it is noted that measurement in quantum mechanics is not time symmetric, which raises questions about the possibility of going back in time.
  • #1
jordi
197
14
Since QM is not deterministic, the future state B is not determined by the previous state A (at time A, B was only a possibility, not a certainty).

Then, when we are at time B, and assuming we could move back in time (of course, we cannot do that, but let us make a Gedankenexperiment), it just makes sense that moving back in time should also be probabilistic, not deterministic.

So, with a high probability, if we could move back in time, we would not end up in state A, but in state C (whatever it is).

Only a big coincidence could result in C having the grandfather alive (most possible states would be with no grandfather whatsoever).

So, if we use QM as a framework, the Grandfather paradox does not exist (or it could exist, but with an exceedingly small probability).

In fact, here one could ask: what does going back in time means in QM, if we do not end up in the "original" A state? Wouldn't this evolution towards the past analogous to (another) dynamics into the future? Which experiments could be done to really be sure we had gone back in time?
 
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  • #3
Moderator's note: Thread moved to QM forum.
 
  • #4
jordi said:
when we are at time B, and assuming we could move back in time (of course, we cannot do that, but let us make a Gedankenexperiment)
You can't make a valid thought experiment that violates the laws of physics. So you can't just wave your hands and assume "we could move back in time". You have to figure out if the laws of physics allow such a thing. @PeroK referred to one way that the laws of physics might allow it, namely closed timelike curves; but most physicists do not believe closed timelike curves can actually exist, since the mathematical solutions in relativity that include them have properties that most physicists think are physically impossible.

jordi said:
it just makes sense that moving back in time should also be probabilistic, not deterministic.
No, it doesn't, because measurement in QM, at least as it is handled in the basic math, is not time symmetric. (What various QM interpretations say about this is another question, discussion of which belongs in the QM interpretations forum.) You have multiple possibilities before a measurement, but only one of them is observed to happen. There is no "backward in time" analogue in QM where you have multiple possibilities after a measurement but only one before.
 

FAQ: Does it make sense to speak about the Grandfather paradox in QM?

What is the Grandfather paradox in QM?

The Grandfather paradox in QM is a thought experiment that explores the implications of time travel and the concept of changing the past. It involves a hypothetical scenario where a person travels back in time and kills their own grandfather, thereby preventing their own existence.

Does the Grandfather paradox apply to quantum mechanics?

The Grandfather paradox does not have a clear answer in quantum mechanics. Some interpretations of quantum mechanics suggest that changing the past is not possible, while others propose that multiple timelines or parallel universes could exist, allowing for the possibility of the Grandfather paradox.

How does the uncertainty principle affect the Grandfather paradox in QM?

The uncertainty principle states that the exact position and momentum of a particle cannot be known simultaneously. This means that the outcome of any action taken in the past, such as killing one's grandfather, cannot be predicted with certainty. Therefore, the Grandfather paradox may not be possible due to the inherent uncertainty in quantum mechanics.

Can the Grandfather paradox be resolved in quantum mechanics?

There is currently no consensus on how to resolve the Grandfather paradox in quantum mechanics. Some theories propose that the paradox could be avoided if time travel is only possible within a closed loop or if the past is protected by some sort of "cosmic censorship." However, these ideas are still highly debated and have not been proven.

What are the implications of the Grandfather paradox in QM for the concept of causality?

The Grandfather paradox raises questions about the fundamental concept of causality, which states that an event must have a cause. If time travel and changing the past were possible, causality would be violated, leading to logical inconsistencies. This is one of the main reasons why the Grandfather paradox is still a topic of much debate and speculation in the scientific community.

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