Spoiler alert: If you have not watched the ‘Avengers: Endgame’ yet. I suggest that you do not read this post until you do.

Hi Everyone,

Today, I want to discuss something a little bit different from my regular economics topics. I want to discuss time travel. I watched the movie ‘Avengers: Endgame’ a few days ago. The movie involved time travel and quite a bit of it. This movie probably involved the most complex approach to time travel I have ever seen. While watching the movie I was scratching my head wondering if all of the events described actually made sense. After about a day, I concluded that the logic was mostly sound and even completely sound if I used my imagination a little.

Multiverse

The time travel ideas expressed in the Avengers movie only makes sense if we operate under the assumption that we live in a multiverse; that is, we live a universe with multiple alternate realities. Travelling through time involves travelling between these realities.

Travelling into the future should be less complex and the theory, which I am aware of, suggests we can travel into the future in the same reality. Travelling back in time is a far more complex issue. Travelling back in time in the same reality does not appear possible as our past effects our future. This puts us in a causal loop where many things cannot logically happen. For example, I could not kill my younger myself, as I would not exist to kill my younger self. Nebula, a character in the movie kills her younger self but no such problem occurred. The most logical explanation is alternate realities on alternate timelines. The Nebula who was killed was from another reality, therefore did not affect Nebula from this reality.

So just for fun, I would like to discuss time travel theory in the context of the multiverse to get some idea if the events of ‘Avengers Endgame’ are logical.

Travelling into the future

Travelling into the future appears possible in the same reality. It also appears possible to travel into another reality as well. I am not sure how we would ensure we stay in the same reality or for that matter ensure we did not.

Travelling back to the present could be more problematic. If we have travelled into the future in the same reality, it should not be possible to travel back in that reality as events have already played out in our absence. Travelling back to the present would be like travelling back in time. The future we travelled to would become our new present.

If we travelled into the future to another reality, travelling back should not be a theoretical problem, as we would have had no impact on our own reality timeline. I further elaborate my point using the following diagrams.

Figure 1 contains an imaginary timeline. The green line marks the present, the red lines are in the past and blue lines are in the future.

Figure 1: Timeline

Travelling into the future and back again can be explained using the following diagrams.

Figure 2: Travelling into the future in the same reality

Figure 3: Travelling back to an alternate reality

Figure 4: Travelling into the future to an alternate reality and then back again

‘Avengers Endgame’ did not use time travel into the future. Therefore, there is no reference for comparison.

Travelling into the past

As I mentioned earlier, travelling into the past within the same reality appears illogical and it does not fit the ‘Avengers Endgame’ logic either. Travelling into another reality should solve the time travel problems caused from altering the past. Travelling into the past in an alternate timeline allows us to do anything without altering anything in the original timeline. I could kill myself but that is myself in the alternate reality and not myself from my own reality. The more things we do in the alternate reality, the more we are likely to ruin that reality but it still does not affect our original reality.

If travelling back in time can only be done by travelling to alternate realities, we are also unable to change any of the bad things in our reality’s past. For example, if a bus hit my best friend three years ago and I went back in time and prevented that happening, I would have saved my friend in the alternate reality but my friend in my own reality would remain dead. However, it could be possible to bring my friend from the alternate reality back with me to my reality. ‘Avengers Endgame’ had a similar situation where Gamora from an alternate reality took the place of the original Gamora who died. The alternate reality Gamora is not the same Gamora that died.

Multiple trips backwards and forwards

Travelling backwards and forwards in time involves crossing backwards and forwards between different realities. We can only return to a reality later on the timeline than we left it. For example, if we left our original reality in 2019, travelled to Alt reality 1 to the year 2010, travelled to alt reality 2 to the year 2050 and then travelled to Alt reality 3 to the year 1970. We have occupied 4 different realities. We can travel to any of these realities to the year 2060 and none of them to the year 1960. Travelling to 1960 is not possible because our actions could affect our past actions, which occurred in the future of that reality. If we wanted to travel to 1960, we would end up in another alternate reality. Figure 5 diagrammatically explains the above logic.

Figure 5: Travelling backwards and forwards in different realities

Other people travelling through time

If it is possible for us to travel through time, we should also assume that other people can as well. This also includes people from alternate realities. We established that we are unable to return to a reality at an earlier date. Travelling to these realities is also not possible for other people either. These rules apply to everyone from all realities. For example, if Jack from Alt reality 1 travelled to our reality, we could not travel to the Alt reality 1 prior to Jack’s departure. It is not possible to prevent Jack from entering our reality. We could travel to Jack’s reality in the future, which would prevent Jack travelling back to his own time. This would occur, as Jack’s reality would have played out in his absence, therefore no longer accessible for him. See figures 6 and 7 below.

Figure 6: Jack travels into the future to our reality

Figure 7: Someone travels into the future in Jack’s reality

The present for Jack’s reality has shifted into the future because of someone else’s time travelling.

In ‘Avengers Endgame’, the Infinity Stones needed to be returned to their respective realities to prevent these universes progressing without the stones. This needed to occur before any other events, which would make these realities no longer accessible in the desired time. For example, someone else could travel into one of those alternate realities at a later point on the timeline. This would have prevented the avengers from returning the Infinity Stones at the desired times. The escaped Loki from one of the alternate realities could have travelled to an alternate reality before the Infinity Stones were returned.

In ‘Avengers Endgame’, Captain American returned the Infinity Stones to their respective realities at a point in time just after they were taken. This was done so as not to plunge these other realities into chaos and diverging from the original reality. Captain America did not return to his reality as planned (5 seconds later). Instead, he returned as an old man. In the movie, it was unclear when he returned. If we follow our logic so far, he should have returned a few seconds later than the original planned 5-second interval or just slightly before allowing time to naturally elapse to the point he could no longer return. Considering his old age, he must have spent a very long time in at least one other reality.

Figure 8: Travelling back in time, allowing time to elapse and then travelling back in time again

It is possible that the alternative reality has progressed far beyond the timeline of Captain America’s own reality. This would also mean that the reality that Captain American occupied could not be revisited at an earlier time.

Time travel in other movies

Other movies that I have watch that have involved time travel did not use alternate realities but instead attempted to use just one reality. Changes in the past appeared to rewrite the original timeline. For example, ‘Back to the Future’ put Marty (the time traveller) in danger of never being born because of his inadvertent interference in his parent’s first encounter. If, as the movie suggests, time travel is in the same reality and Marty is not born because of his interference, he will also not be able to interfere in the first place to prevent his own birth. We are stuck in a causal loop.

The movie/book ‘The Time Machine’ also appears to have time travel within the same reality but offers an interesting solution to the causal loop. Alexander (the time traveller) builds a time machine to go back in time to save his fiancée. Every time he saves her, she dies again almost immediately. He is unable to save her and bring her naturally into his present. Alexander goes on a quest to find the answer to his question. He goes thousands of years into the future. An underground-dwelling intellectually evolved human explains that she cannot survive. If she survived, he would not have built the time machine to save her.

Another approach to time travel, which can be used to avoid the causal loop, is assuming all actions are preordained. This means everything in life is predetermined and we have no control over any actions at any point in time. Therefore, if we go back in time and change anything, nothing in the present time will change as time has already accounted for these changes before they have occurred. The movie ‘The Time Traveller’s Wife’ appears to have adopted this approach to time travel. Henry (the time traveller) involuntarily travels through time meeting and engaging with people in his life. On an occasion when he travelled back in time, his wife met him for the first time when she was only a young girl. The events of travelling back in time appeared accounted for in the future without changing anything. He even travelled to a time after his death and met his daughter. His death was predetermined; all actions he took would lead to that death.

Conclusion

Time travel is a very interesting topic. It has appeared many times in movies and books. The effect of travelling back in time on the future has always been a difficult one to explain. The multiverse explanation appears the most logically and does not have the problem of the illogical causal loop. The multiverse explanation can get a little complicated when many time jumps are made by many different people and possibly from many different realities. The ‘Avengers Endgame’ approach to the multiverse appears logical based on what I observed in the movie. Captain American’s appearance at the end of movie may have a few flaws depending on how he reached the present day. I cannot fault the movie, as no explanation was actually provided.

There is also the possibility that time travel may not be possible, well at least not into the past. The multiverse may not exist and the past might be impossible to change in any way. Travelling into the future could be more likely if we are able to experience time differently. Einstein’s relativity theory considers a connection between space and time. People can experience time differently depending on how fast they move. I do not really understand the full science behind it but at least there is some science suggesting time travel is possible.

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