We've established that current physics says time travel to the past is impossible. The very idea is considered hokum, magic and witchcraft.
It can't be possible because of causality, paradoxes. And if we're honest, that feels logical and just a little bit safer.
Stephen Hawking famously conducted a time travel experiment with a simple party invitation.
It was the most exclusive party invitation ever issued, an invitation for time travellers only. An invitation that was sent after the party had occurred.
Professor Hawking had the venue decorated with balloons. There was a large buffet and the Champagne was chilling on ice. But no-one came, prompting Hawking to state that he'd just proven time travel to the past was not possible.
But maybe they didn’t like egg sandwiches, maybe they don’t breathe air in the future they’ve from or maybe they're just shy.
Any proposals and conjectures from the world of science concerning travelling to the past are encased in physical impossibility. But in theory, if you disregard being able to build it or survive it, things like the Tipler Cylinder could work.
Astronomer Frank Tipler proposed that time travel backwards could be possible if you take a huge lump of matter, 10 times the mass of the sun. Roll it up into a long cylinder. Spin that cylinder at a few billion revolutions per minute. Then, in your nearby spaceship, you could follow a very precise spiral around it and get caught in a loop, what scientists call a "closed time-like curve."
The laws of causality would be breached and time itself would become a single entity, all happening at the same time. There would be no forwards or backwards in time, just time.
Aside from the obvious construction headaches of such an endeavour, the cylinder would also need to be of infinite length.
Another theory involves cosmic strings.
Cosmic strings are left over from the early cosmos. They are narrow tubes of energy stretched out across the length of the universe. They are supposed to contain huge amounts of mass and in principle, you could warp space-time around them.
String theorists say that strings can be infinitely long strings or string loops. If two such strings got close enough to each other, space-time would bend so strongly, and in such a configuration that time travel might be possible.
An exotic form of matter with so-called "negative energy density" has the most bizarre properties, such as moving in the opposite direction of normal matter when pushed. In theory, negative energy density could exist but not in sufficient quantities to enable the construction of a time machine.
But some time-travel research proposes negative energy density is not needed.
A doughnut-shaped hole is enveloped within a sphere of normal matter. Inside this doughnut-shaped vacuum, space-time could get bent upon itself using focused gravitational fields and this would form the closed time-like curve.
To go back in time, a traveller would race around inside the doughnut, going further back into the past with each lap. Again, the strength of the gravitational fields required would be almost incalculably strong.
General relativity provides scenarios that could allow travellers to go back in time and advances in quantum theories might also provide some understanding of how to overcome paradoxes.
But theory is one thing. The only unifying feature of where we are at the moment is that either the energy would be impossibly large, the mass would be impossibly large, constructing it would be impossible and us surviving the trip would be impossible.
So, when we ask ourselves, "Is it possible?" The odds don't look good.
But that's today. Tomorrow is another day.
A thousand years ago, a human travelling at 600mph through the air would have been impossible. It would have been hokum, magic and witchcraft. A few thousand years before that, it would have been gods.
So, who's to say the laws of physics might not become similarly overhauled in the future? Who's to say there won't be something that can overcome causality?
It would have to be a very long way into the future, the sort of people Stephen Hawking was hoping would come to his party.
And who's to say someone in the distant future hasn't changed physics as we know it and already achieved time travel to the real past.
Because we wouldn't know it if they did. How could we?
The future of everyone and everything would change including us, so we would be unaware of any of those changes, things would always be just as they'd ever been.
But the time ripples created by these changes could mean that at any moment, someone in our ancestral line might not be required. We might suddenly never have existed, wiped from the timeline for all eternity.
Is the feeling of never having existed the same as not being alive or does it even qualify for a feeling?
In the sci-fi novel, Machine Sense (2019) by Dominic Schunker, the protagonists start seeing the changes.
They find people they know don't know them anymore, a building appears where it didn't stand yesterday. They uncover the people who are moulding the world to their own way at the expense of entire familial timelines, an undetectable temporal genocide.
So, like most everything else, time travel to the past can bring with it the potential for absolute abuse of power, absolute evil, and the singular horror of never having existed.
Stay tuned for Episode 3.
Teaser for Episode 3 – Should we do it?
The prevailing wisdom in the science community dictates that causality and paradoxes are the reasons why we could never travel back in time but a more fundamental question arises.
Even if we could do it, should we?