## Preamble:

I have tried my best to describe some scientific conjectures and concepts, and there’s a high probability that I’m wrong. So, I ask readers for their help in correcting me. The record of my conversation with GPT4 is in the latter part of this post. This article was written and proofread by GPT4 and me.

Particles in two entangled states are distributed in two infinitely distant parts of the universe. If we measure one of them, it randomly confirms its state, and the other one inevitably gets the corresponding result (if one is a right-hand glove, the other one is necessarily a left-hand glove; they are a pair).

What exactly is randomness, why is it random, we don’t understand, and scientists don’t understand either. They are just incredibly skilled at describing these processes using mathematics, and time and time again they verify that these equations are reliable in the lab and in actual engineering. Many modern communication, computer, and internet technologies are heavily influenced by quantum mechanics, especially those involving microelectronics, semiconductors, and communications - such as common home fiber optics, or data center fiber optics.

But people can’t help but be curious: one glove is random, but how to explain the immediate corresponding change in the other glove?

The first intuitive idea is, isn’t this easy to understand? They were one left and one right before they split, and we only know the result when we open the pair. This is the most common-sense idea. But this idea was later proven wrong by experiments related to Bell’s Inequality. One glove always randomly determines whether it is left or right when it is observed (let’s just listen to the scientists’ conclusions). Before being observed, their state is uncertain, not just unknown. These two have compelling differences and evidence in mathematics, experiments, and repeated experiments.

The second idea is that there is an unknown mechanism that transcends spatial dimensions. Normally, if one glove is confirmed to be a left-hand glove, it must send a messenger across the vast universe to inform the other glove, so the other one knows that it should “immediately” become a right-hand glove. But the problem lies in this “immediately”. If they are very far apart, even at the fastest speed— the speed of light, it may take hundreds of thousands of years. But scientists continue to tell us through experiments that this “immediately” is really “immediately” in the semantic sense: there is no time difference.

This means that there are phenomena that scientists cannot explain: if the two entangled gloves want to perfectly coordinate their pair state, in previous explanations, they had to wait for something like a photon to deliver the signal to the other. But the experiment points out that there is no waiting process for their state confirmation, which contradicts all known physical knowledge. We can continue to imagine that there is a superdimension that we cannot describe, cannot observe, where these two particles or gloves are actually in the same place in a higher dimension; but this imagination cannot be observed, described, proven, and verified, and currently, it can only be said to be at the same level as science fiction.

Finally, there is a hot hypothesis, the recently discussed “Retrocausality”.

Imagine two gloves physically entangled together. They are not entangled in space (after all, we can separate them in space and distribute them to two different places in the universe that are far apart), but they are entangled in time. This means that when we observe one of the gloves, its state is determined; at the same time, their states were also confirmed at the beginning of entanglement. This leads to the grand play: the other glove, as if by magic, confirms its relative state in a way that transcends space.

As far as we know, this seriously defies intuition. We believe in the sun rising and setting, we often say that what’s done cannot be undone, the wood is already made into a boat, black hair turns white. In the past two million years of life experience, in the laboratory and on the construction site, it has always been so. For over a hundred years, scientists have been educating us: time moves in the direction of increasing entropy, and this has been written in all textbooks. Mathematics and physics have not made a clear direction for time, for example, if you reverse time, the laws of physics are still valid.

The last hypothesis proposes a new, dazzling conjecture, no less than “multiple universes”: what’s happening now affects what we thought was the past, and then re-affects our present. Please note that this further means - what happens in the future may be affecting our past and present. Here, the boundaries of time are blurred, it is no longer a clear direction or arrow, but a large blurred surface. Between the past, present, and future, there is you in me, and me in you, interwoven together.

In this imagination, all the language and vocabulary we used to describe the past, present, and future will fail, because the concept of time will be replaced by some new physical explanation and concept. We no longer say “the wood has become a boat”, but “the boat is formed and there is wood, there are seeds germinating”. “What’s done cannot be undone” is actually “the water on the ground determines there is water in the basin”. Doesn’t this also make sense? How the textbooks of the future will write this is probably beyond our imagination.

Meanwhile, I remember an experiment ten years ago: only within quantum entanglement, observers see the change of time.

The experiment included a simulated universe with a pair of entangled photons, and observers could measure their state in two ways. The results show that for observers inside the universe, time is an emergent phenomenon; for observers outside the universe, there is no time.

In other words, an observer akin to a “god” outside our universe (who uses a clock completely independent of our universe), to him, these two entangled particles would never seem to change—under such circumstances, what we call time, seemingly doesn’t exist.

On the other hand, if this observer enters our universe and compares the evolution of the particles with other parts of the universe, the observer would then see change; this change can be viewed as a measurement of time.

Under this framework, time only exists for the observer within the universe, and it’s closely related to quantum entanglement.

Time From Quantum Entanglement: An Experimental Illustration
https://arxiv.org/abs/1310.4691

## Converstation

Based on these concepts, I suddenly became very curious and had a brief chat with GPT4. Here are the details.

Me: Combining your understanding and knowledge, can you explain in detail what retro-causality means through metaphors, examples, etc.? What does it mean? Why is that? What’s the impact?

GPT4: Retro-causality is a complex but very interesting concept. We are very familiar with the normal cause-and-effect relationship, just like throwing a stone into a lake, the throw of the stone (cause) precedes the ripples on the lake (effect) in time. But when we talk about “retro-causality,” it’s as if the ripples on the lake precede the throw of the stone in time, which feels very counterintuitive.

In quantum physics, retro-causality is used to explain some very strange phenomena, such as entangled particles. Two entangled particles, no matter how far apart they are, a measurement on one particle will immediately affect the state of the other particle. This is a kind of nonlocal phenomenon. It seems to violate our causal intuition because in classical physics, the transmission of information needs a certain amount of time, it cannot be completed instantaneously.

Retro-causality provides a possible explanation: it may be that our choice of particle measurement (in the future) affected the state of the particles in the source (in the past). This view overturns our traditional understanding of cause and effect, leading people to ponder whether future events can affect past events.

If retro-causality is widely accepted, it will have profound impacts on our scientific and philosophical concepts. First, it will change our understanding of time and cause-and-effect relationships, and may even have an impact on philosophical issues such as free will. Secondly, in terms of practical applications, if future actions can affect past events, then we may be able to use this law to design new technologies and tools, such as time communication. These are very challenging and forward-looking issues.

Me: So, forward causality means that the entangled state of the particles determines their state after being observed, and retro-causality means that the event of being observed determines its entangled state before it was observed (according to the human timeline), right?

GPT4: Yes, you have a very accurate understanding. In the normal cause-and-effect relationship, the entangled state of the particles (in the past) determines their state after measurement (in the future). But in retro-causality, it is our choice of particle measurement (in the future) that affects the previous entangled state of the particles (in the past).

This way of explanation, to some extent, solves the nonlocality problem in quantum entanglement. Because if the measurement can affect the past state of the particles, then even if the two particles are very far apart, our measurement of one particle can immediately affect the other particle, because this influence is carried out through time, not space.

However, retro-causality is still a controversial concept because it goes against our intuition and experience. To accept this concept, we need to have a profound change in our understanding of time and cause-and-effect relationships.

Me: My goodness, it’s really mind-boggling. Can we also understand that, although in our physical experience, events happen one after another, from when we were young until we get old. But in physical structure, all events have already happened? Or say, the events may have already happened in reverse, and then already affected the past state, but in our memory system, it’s interpreted as a certain time order (such as the direction of entropy increase)?

GPT4: Your understanding is correct to a certain extent, especially in the context of quantum physics. In physics, some theories suggest that all time (past, present, and future) exists simultaneously to some extent, a concept known as “block universe theory.”

As for the directionality of time (what we refer to as the “arrow of time”), it’s often associated with the increase of entropy. The principle of increasing entropy is the second law of thermodynamics, which states that the overall level of disorder (or entropy) in a closed system will continue to increase. Therefore, the flow and directionality of time that we experience in our daily lives is due to the continuous increase of entropy in the universe we inhabit.

As for the theory of retro-causality, which suggests that our actions now can affect past events, it obviously contradicts our common understanding of cause and effect. But in the world of quantum physics, such phenomena could indeed occur. This not only challenges our traditional understanding of time and cause-and-effect relationships but also brings us a deeper understanding of the universe and physical phenomena.

Me: This is perhaps like ripples, one after another, influencing each other. It’s hard to say which single ripple caused a crest on a splash. One wave goes out, merging into another group of waves, and this group thus forms new shapes and images. The direction of the waves is in all directions, but the image needs to be observed from a specific angle to be summarized or imagined: this is a mountain, this is a cow, this is a fish. Sometimes this judgment can even make quite accurate predictions within a wave cycle.

GPT-4: Your analogy is very apt. As you mentioned, the entangled particles in quantum phenomena are like ripples, these ripples influence and intertwine with each other, continuously propagating and changing the shape of the entire lake surface.

The concept of the “observer” you mentioned is also an important element. In quantum mechanics, the role of the “observer” or “measurement” is central because the act of measurement affects the state of the quantum system. This is strikingly consistent with your analogy where the image of ripples needs to be observed from a specific angle to conclude its shape.

Finally, the prediction you mentioned is a core part of science. Science is about predicting and explaining natural phenomena by observing and understanding these fluctuations, patterns, and laws. And quantum mechanics, despite being filled with hard-to-understand phenomena, can still provide some very accurate predictions.

Your analogy helps us to better understand and feel this complex and magical quantum world.

Me: You seem very serious, haha. We’ll catch up later!

(END)