Many physicists accept the notion that wormholes to the past exist in quantum foam. This implies that the past exists, independent of what the observer views to be the present.
Unfortunately much time and thought is wasted on the question of physical time travel to the past via artificially enlarged wormholes and subsequently even more time and thought is wasted on the question of temporal paradox, stemming from the hypothetical supposition that a chronologically linear series of events (a.k.a. a "timeline") is universally immutable and irreversible. The impossibility of a temporal paradox, we are told, proves that time travel itself is impossible.
The only real temporal paradox here is that grown, intelligent human beings can believe in the existence of wormholes to the past (however tiny) and at the same time dismiss the notion that the universe can only contain one immutable timeline.
To elaborate: the classic hypothetical temporal paradox is the question, 'What happens if a traveller to the past kills his or her own grandfather, thus preventing his or her own birth?'
'Impossible!' theorists declare, 'because then the time traveller would never have been born in the first place, and subsequently would not be around to perform such an act, a fact which in itself would paradoxically guarantee that the time traveller would indeed be born blah blah blah.'
These theorists seem to forget that a small enough particle (using a wormhole in the quantum foam) could do something similar to our disturbingly murderous time-travelling grandchild on a smaller yet nevertheless equally real scale. In fact it's probably happening all the time.
The question of temporal paradox is easily resolved: when the past becomes the present a new and independent timeline is established. Any actions taken to alter history can only effect the newly established timeline. The murderous grandchild may slay his grandfather in the past, but once he has returned to the 'present' in his original timeline (assuming he is able to) this odious act will simply never have taken place. In other words, the immutability and irreversibility of a chronologically linear sequence of events is relative and not universal.
That is why nobody turned up to that party Stephen Hawkings threw for time travellers from the future: they did go, just not to the one in our timeline.
But these questions of physical time travel are, in my opinion, the least worthy of our consideration when we entertain the notion that the past exists, as this has far more important and far-reaching implications regarding the nature of the universe and our own existence within it as conscious individuals.
People whose spatiotemporal parameters are currently converged with our own.
People whose spatiotemporal parameters have diverged from our own.
People whose spatiotemporal parameters have yet to converge with our own.
A convergence of spatiotemporal parameters.
A divergence of spatiotemporal parameters.
*All these terms are relative.
The Big Time Loop
Eternal return (also known as "eternal recurrence") is a concept that the universe has been recurring, and will continue to recur, in a self-similar form an infinite number of times across infinite time or space. The concept is found in Indian philosophy and in ancient Egypt and was subsequently taken up by the Pythagoreans and Stoics. With the decline of antiquity and the spread of Christianity, the concept fell into disuse in the Western world, with the exception of Friedrich Nietzsche, who connected the thought to many of his other concepts, including amor fati.
In addition, the philosophical concept of eternal recurrence was addressed by Arthur Schopenhauer. It is a purely physical concept, involving no supernatural reincarnation, but the return of beings in the same bodies. Time is viewed as being not linear but cyclical.
The basic premise proceeds from the assumption that the probability of a world coming into existence exactly like our own is greater than zero. If either time or space is infinite, then mathematics tells us that our existence will recur an infinite number of times.
If we accept the position that the past exists and add to it the notion that the irreversibility of any chronologically linear series of events is relative rather than universal (ie that parallel timelines can exist) then there is no necessity for either time or space to be infinite in order for "eternal recurrence" to occur. In effect, the universe could recur an infinite number of times simultaneously within its own finite spatiotemporal parameters.
In this scenario, if we were to grant immortality to a hypothetical human observer, he or she would only be consciously aware of one recurrence at a time, because memories are physically stored in our brains and therefore possess their own spatiotemporal parameters (which is why most of us don't remember what we did tomorrow). Were we however to also grant our hypothetical observer the ability to remember each recurrence, he or she might erroneously form the opinion that there was a sequential order to those recurrences, when in fact they were concurrent.
Omnia mutantur, nihil interit
In the film Groundhog Day, Phil Connors (Bill Murray) experiences the same day over and over again. The most implausible aspect of the film is not that the character is caught in a time loop - the universe (as described in this post) is itself one huge time loop - but that the character is able to remember previous recurrences. If he wakes up in the same identical physical state every morning, then the parts of his brain that store memory should too. Having said that, Groundhog Day is still by far and away my favourite example of time travel in film, as it is one of the few movies involving time travel that manage to avoid the "universal timeline" paradigm that is employed all too often by Hollywood.