Grandfather Paradox

Can you travel back in time and assassinate an ancestor without generating a logical paradox? Astonishingly, the answer could be yes!

The grandfather paradox is a hypothetical logical problem that could arise if a being travels back to a particular time in the past. The term comes from the concept that a person travels back to a time before their grandfather had children and assassinates him. This could make their own birth impossible. Grandfather paradox dives into this contradiction and problem.

What Is Time?

In the past, most cultures assumed time as a line. The present was thought to be the midpoint, the past extends in one direction, and the future spreads out in the other direction. Physicist Isaac Newton proposed the concept of a global time. In this idea, time is the same everywhere in the universe. On the flipside, especially eastern philosophers thought of time as a non-linear entity. They believed that time has numerous variations depending on the state of physicality one stays in. In line with this abstract philosophical idea, modern physics has been pointing to something similar. Imaginations have been piling up with both fictional and hypothetical models.

Physicist Albert Einstein formulated the theories of relativity which is the most comprehensive and foolproof concept of time to date. Einstein showed that time is not linear and universal. It varies depending on other physical quantities like speed, gravity, mass, etc. Relativity allows time and space to fold and twist themselves in structures that have been considered close time-like curves. If these time tunnels and loops exist, they could enable time travel. The practicality of such activity is still under investigation.

What Is the Grandfather Paradox?

The grandfather paradox or consistency paradox happens when the past is manipulated in any way, thus resulting in a contradiction. If a time traveller went back in time and assassinated their grandfather in his childhood, it could result in a situation where the time traveller’s parent is not being born at all. In that weird case, the time traveller also wouldn’t be born at all. So, who killed the grandfather in the first place? If this scenario is plausible, the traveller would not be able to kill the grandfather. There is no predicted outcome to this. Consistency paradoxes happen whenever changing the past is viable.

A probable solution is that the time travellers can do or indulge in any event that did happen but cannot be involved in anything that did not happen. When we do or involve something that did not occur results in a contradiction, the other possible solution is that each alteration of the past results in an alternative past with its own individual different outcomes. Quantum mechanics do point to a Multiverse with infinite histories. New histories or timelines emanate out from time altered junctions. It goes like this: when the time traveller kills the grandfather, at that point, a new timeline or history sprouts where the grandfather dies young, and all his offspring do not ever exist. On the other hand, the history or timeline of the time traveller continues to exist independently on another paradigm. Both timelines or histories exist as complete separate paradigms. In this concept, each time a person manipulates the past, a new independent history or timeline is created. Time loops are the other hypothetical outcomes of such worlds.

Implications of the Grandfather Paradox

The logical volatility of time travel is a familiar concept in most fictional interpretations, as well as the philosophy associated with it. In the early versions of the grandfather paradox, many imposed that it’s logically impossible to time travel. They argued on the basis of classical physical laws and conventional rational parameters. They thought that time has only one direction, like that of a pointing arrow. They were not aware of the complex consequences of probability and relativity. Now we know the possibilities that the uncertainty principle and relativity opened up. Contradictions such as the grandfather paradox do not point out that explicit time travel is not possible. The logical viability of time travel predominantly relies on the concept of time. As of now, nobody has a concrete conceptualisation of time apart from relativity. The idea of time as a fundamental physical dimension has opened some possibilities. But it still lacks a solid observational ground, especially in the case of travelling back to a past event. Most of the logical issues can be solved if physical laws are considered probabilistic rather than strictly deterministic. It opens up the possibility of numerous outcomes from a journey back in time, few of which may not be contradictory.

Practicality of the Grandfather Paradox

The most accepted version of time travel is by using a wormhole. Wormholes are space-time holes. Among the most bizarre consequences of general relativity, wormholes are typically represented as shortcuts between two points in space. The theoretical physicist Stephen Hawkings proposed that wormholes could probably loop back to a point in the past. This could be very much possible as time and space are inseparable according to the special theory of relativity. Anything that can influence space should influence time also. Such a coupling is called the closed time-like curve.

Even if the laws of physics allow backward time travel, there are many obstacles that might hinder the possibility of the grandfather paradox.

Hawking proposed two possible workarounds to deal with this paradox. According to the “consistent histories” model, the entire spectrum of time (future, present, and past) is strictly predetermined. In this model, we can only travel back to a particular point in the past only if we had already been to the exact point in our own history. In this model, we can interact with the past, but we cannot alter it. So, the grandfather paradox has zero chance to arise. The second approach is based on quantum mechanics, where an event has several probable outcomes with varied likelihoods of occurring. There is a ‘many worlds’ interpretation of reality in the quantum realm, where all the various outcomes occur in different and independent parallel timelines. In this scenario, the grandfather paradox can be resolved if the time traveller begins out in a timeline where the grandfather survived long enough to have offspring, and then after travelling back and killing the grandfather. This will continue along a parallel timeline in which time traveller will never be born.

The video explains the basics of the measurement of distances in space.

Other Major Time Travel Paradoxes

Predestination paradox arises when the actions of persons travelling back in time become part of the past events and may eventually cause the event they are trying to block from happening. This creates a ‘temporal causality loop’ through which Event A in the past impacts Event B in the future. In turn, it causes Event A to happen, with this circular event loop ensuring that the time traveller does not change history. Any attempt to block something from occurring in the past will only lead to causing itself instead of blocking it. This paradox tries to show that events are always predestined to happen in a particular way, and whatever has occurred must occur.

Bootstrap paradox is a kind of paradox in which information or a person or an object sent back in time creates an infinite loop where the person or the object has no observable origin. In other words, it behaves as if it has no origin. It is also called the Ontological Paradox. Ontology is a section of philosophy that deals with the existence or nature of being.

Polchinski’s paradox is another bizarre time travel paradox. It is named after the physicist Joseph Polchinski who proposed this paradox. In this scenario, a billiard ball goes into a wormhole and comes out of the other end in the past. It emerges just in time to crash with its younger form and stop it from travelling into the wormhole in the first place. Polchinski’s paradox is being taken seriously by many theoretical physicists as there are no contradictions in general relativity to hinder the possibility of closed time-like curves, space-time tunnels, or interdimensional time travel. In fact, it has the merit of being in line with the laws of motion.