The idea of traveling back in time to meet a younger version of yourself is fascinating. However, what happens if we attempt to change the past to achieve a different future? Does a self-consistent universe allow such alterations, or does it inevitably lead to paradoxes? If we are able to change the past, does that not break causality? Is retrocausality a real phenomenon?
If we travel back and alter the past, we encounter the famous grandfather paradox. This paradox arises when a time traveler goes back and prevents their grandfather from meeting their grandmother. Either by direct intervention or indirectly. If this event prevents the traveler’s existence, then they could never have traveled back in time in the first place. This contradiction suggests that altering the past in a way that disrupts causality isn’t possible. The general consensus among physicists is that causality violations are not possible.
However, solutions to Einstein’s general relativity equations suggest the existence of closed timelike curves (CTCs), theoretical pathways that allow travel back to the past. Even in such cases, events inside the CTC must remain self-consistent, ensuring that no paradoxes arise. This principle is sometimes referred to as the Novikov self-consistency principle. This states that any event occurring inside a time loop must be consistent with the overall timeline. And it should prevent violations of causality.
Wheeler-Feynman Absorber Theory
While classical physics resists causality violations, quantum mechanics introduces intriguing possibilities, including retrocausality—the idea that future events can influence the past. One example of this is in the Wheeler-Feynman absorber theory, which provides an alternative interpretation of electromagnetic radiation emission and absorption.
In classical electromagnetism, when an accelerating electron emits radiation, the emitted electromagnetic waves travel outward and other particles absorb them. Conventionally, we think of this as two separate events: emission by the source and absorption by the receiver. However, Wheeler and Feynman proposed a different approach. Instead of treating emission and absorption as independent events, they suggested that both are part of a single interaction involving both the emitter and absorber.
Maxwell’s equations allow two kinds of solutions for electromagnetic waves:
- Retarded waves: These travel forward in time, as expected (cause precedes effect). This is how we usually perceive radiation propagation.
- Advanced waves: These travel backward in time, appearing as though they originate from the future and influence the present.
Wheeler-Feynman Absorber Theory and Retrocausality
According to Wheeler-Feynman theory, when a charged particle accelerates and emits radiation, it sends an initial disturbance outward. Absorber particles in the future react to this disturbance and generate their own electromagnetic waves: retarded waves traveling forward in time and advanced waves traveling backward. These advanced waves travel back to the emitter at the moment of emission, exerting a force that mimics the resistance (radiative damping) experienced by the emitter.
This concept implies a kind of retrocausality, where future events influence the past. Imagine throwing a stone into a pond, creating ripples that move outward. When these ripples reach the shore, the shore generates two new sets of waves—one traveling forward in time as expected and another moving backward in time, reaching the stone just as it began disturbing the water. This analogy provides an intuitive way to understand retrocausality in the Wheeler-Feynman interpretation.
While these ideas do not imply that we can freely alter the past, they challenge our conventional understanding of causality and suggest that under specific conditions, the future and past may be more interconnected than previously thought.
Quantum Entanglement
Quantum entanglement is one of the most fascinating and counterintuitive aspects of quantum mechanics. When two particles are entangled, an observation or measurement of one particle instantly influences the state of the other particle, regardless of the distance between them. Even if they are light-years apart, the act of measuring one particle’s state seems to determine the corresponding state of its entangled partner instantaneously. This phenomenon challenges our classical understanding of causality and locality. It appears as information transmission faster than the speed of light, which contradicts Einstein’s theory of relativity.
To resolve this apparent paradox, Bell’s theorem tells us that reality must be non-local. In other words, nature cannot simultaneously be both “local” and “realistic.” Locality means that objects are only influenced by their immediate surroundings, while realism suggests that physical properties exist independently of observation. Since experiments have repeatedly confirmed Bell’s theorem violations, the reality we perceive does not adhere to both principles at the same time.
Quantum Entanglement and Retrocausality
But what if there is another explanation? What if quantum mechanics appears non-local because it is retrocausal—that is, future events influence the past? This perspective could provide an alternative interpretation of entanglement without requiring faster-than-light communication.
To illustrate this, imagine two people, each possessing a box with a cricket ball inside. Each ball is either red or blue, but the individuals have no prior knowledge of their colors. If they open their boxes, they will find random colors that have no correlation. However, this randomness is not the case with entangled particles. Suppose two people each possess an entangled particle, and one of them observes their particle. The instant they measure it, the other particle “chooses” a corresponding state, no matter how far apart they are.
How It works
Under a retrocausal explanation, the process would unfold as follows:
- At t0, the two particles are created in an entangled state. At this moment, they do not yet have definite properties, such as spin direction. The universe “waits” to determine their states based on future measurements.
- At t1, one person measures their particle. Instead of this action instantaneously affecting the other particle, information about the measurement is sent backward in time to t0.
- At t0, this retrocausal effect assigns the properties of both particles based on the future measurement outcomes. Now, when the second person measures their particle at t1, they find that it has the corresponding state, not because of faster-than-light signaling but because the measurement choice influenced the past.
This interpretation preserves the consistency of quantum mechanics while avoiding superluminal communication, which would violate relativity.
The Delayed-Choice Experiment and Retrocausality
Another experiment that hints at retrocausality is the delayed-choice experiment, particularly Wheeler’s version involving a double-slit setup. This experiment examines whether a photon behaves as a wave or a particle, depending on whether we measure which slit it passes through.
The Double-Slit Experiment
The experiment consists of a photon emitter firing individual photons at a barrier with two slits. The behavior of each photon depends on whether or not a measurement is made:
- No measurement (Wave-like behavior):
- The photon passes through both slits simultaneously as a wave.
- The wave components interfere with each other, producing an interference pattern on a detection screen.
- Measurement (Particle-like behavior):
- If detectors are placed at the slits to determine which slit each photon passes through, the interference pattern disappears.
- The photon behaves as a particle and passes through only one slit.
Modified version: Delayed Choice Experiment
Now, a modified version of this experiment introduces a lens and telescopes:
- A lens is placed beyond the slits to slightly diverge the photon paths.
- A detection screen is positioned where the interference pattern would appear.
- Two telescopes are set up to observe the photons after they pass through the slits.
This setup leads to two scenarios:
- Detection screen is in place (Wave behavior):
- The lens keeps the wavefunctions of the photons in superposition.
- The detection screen records an interference pattern, suggesting that each photon passed through both slits simultaneously, behaving like a wave.
- Detection screen is removed, and telescopes are used (Particle behavior):
- The wavefunctions spread out without the screen.
- Each telescope aligns with one of the diverging paths, meaning if a photon is detected in Telescope 1, it must have passed through one slit, and if detected in Telescope 2, it must have passed through the other slit.
- Since we now have which-path information, the interference pattern disappears, and the photons behave like particles.
Retrocausality
The crucial aspect of this experiment is that the choice to observe the photon’s path is made after the photon has already passed through the slits. If we remove the detection screen and use telescopes, the photon appears to have “gone back in time” to behave as a particle rather than a wave.
This suggests that the future decision—whether to measure which slit the photon passed through—determines the past behavior of the photon. This apparent backward influence of measurement on past events is a strong argument for retrocausality.
Electron-positron annihilation
We commonly understand electron-positron annihilation as an electron (e−) and a positron (e+) colliding and converting into two photons. However, Richard Feynman proposed an alternative interpretation: a positron is an electron traveling backward in time.
In this view, an electron moves forward in time until it emits a photon. At that moment, it begins traveling backward in time, appearing to us as a positron moving forward. The positron then reaches a point before the photon emission occurred, making it seem as if an electron and a positron have emerged spontaneously. In reality, the positron is the same electron traveling back in time. When the positron annihilates, it emits a photon and flips back to moving forward in time, continuing the cycle. This suggests that annihilation is not the destruction of two separate particles but a single electron looping through time.
Positron: Electron Travelling backward In Time
A useful analogy is to imagine a future version of yourself suddenly appearing before you and instructing you to move backward in time. As you obey, you eventually reach the same moment when you initially appeared to your past self, directing them to do the same. This loop continues indefinitely. To an external observer, it would appear that two versions of you exist momentarily, meeting and then disappearing, releasing energy in the process. However, from a different perspective, it is just one individual zigzagging through time. To the external observer, time only moves forward.
These retrocausal interpretations align with broader discussions on time travel in physics. When physicists explore time travel in both quantum mechanics and general relativity, they do not ask whether it is outright possible but whether it can occur without paradoxes.
1. Quantum Mechanics and Self-Consistency
Retrocausality in quantum mechanics, such as viewing the positron as an electron moving backward in time, does not lead to contradictions because it never allows sending signals that would alter the past inconsistently. Wheeler and Feynman’s absorber theory for example, rely on self-consistency, where the past and future influence each other in a way that always maintains logical coherence.
Even in models involving closed timelike curves (CTCs), such as those proposed by Seth Lloyd, quantum states evolve in a manner that preserves consistency. This means that while strange behaviors may occur, they never create logical contradictions.
2. General Relativity and Time Travel
General relativity also provides theoretical frameworks where time loops could exist. Solutions to Einstein’s equations, such as Gödel’s rotating universe or the geometry of Kerr black holes, suggest that closed timelike curves may be possible. However, if they do exist, the Novikov self-consistency principle ensures that events occurring due to time travel must always be internally consistent—meaning no paradoxes like the famous grandfather paradox can arise.
This principle suggests that even if a person were to travel back in time, they could not change the past in a way that contradicts established history. Instead, their actions in the past would have always been part of history, reinforcing the idea that time loops must be logically consistent.
3. The Pattern in Physics
The fundamental theme in physics is not that time travel is impossible but that it must obey self-consistency. Nature does not outright forbid retrocausality or closed timelike curves, but it demands that whatever happens does not contradict itself. Whether considering quantum mechanics or general relativity, physics permits time to behave in non-intuitive ways, as long as logical consistency is intact.
This stands in contrast to the popularized, fictional notion of time travel, where characters freely change the past, creating paradoxes. In real physics, any change to the past must integrate seamlessly into history, ensuring that the timeline remains logically sound.
Thus, in both quantum mechanics and relativity, time loops and retrocausal interpretations do not violate fundamental principles. But instead offer a fascinating glimpse into the deeper, self-consistent nature of time itself.
sources:
https://en.wikipedia.org/wiki/Wheeler%E2%80%93Feynman_absorber_theory
https://en.wikipedia.org/wiki/Wheeler%27s_delayed-choice_experiment