That is understandable -and why I really don't believe time travel is possible.
If I consider only myself at two different points in time -let's say an hour apart -I am at least mostly the same stuff as I was. Our stuff gets replaced over time, but not actually with new stuff -just stuff which was once not part of us.
My past self does not exist to be visited.
The same is true on an all-inclusive level.
The past, as a whole, simply does not exist to be visited/traveled to.
It became the present.
That is not to say it is impossible for interactions to be slowed or sped up in relation to each other within the whole. Time can be thought of as a sequence of events -but also an attribute of the whole which may not remain constant throughout the whole. One part may be caused to LAG behind somewhat, as it were, but can never be LEFT behind in the way the "movie version" of time travel would require.
Some say that time -spacetime, whatever -began with the big bang. "Universe" time certainly did. Some believe the universe may also collapse.
If it did, things would essentially return to a pre-universe state.
The attribute of time/spacetime within it would no longer apply.
Though it may return to a previous state, the fact that it once existed -that there was indeed a sequence of events -could never be negated -it would just no longer apply.
Scale is relative to the passage of time by observation.
Imagine being stuck in the center of a galactic void. In the void there is nothing for hundreds or thousands of light years. To the observer in such a location, time might appear to almost stand still.
Imagine being at the collision point inside a particle accelerator. To the observer at such a scale at that location there would be a multitude of particles moving around you at incredible speeds. Time might appear to nbe happening very fast.
For both observations and scales, time is the same but the relativity of how it is sensed is different.
The Universe as a whole (if its even possible to imagine the Universe as a whole-we lack the information), exists at its on definition of time. This definition of time might be said to be the baseline time.
Within this Universe, at the different scales, speeds and observations, time moves at different rates according to those scales within.
It can be a difficult concept to fathom but our science proves it by the measurements we make.
For instance, we observe other stars similar to ours and understand star formation and lifecycles. We understand that Sol is in main sequence. We realize for it to be in main sequence it is about half-way thru its normal lifecycle. Roughly 4.5 billion years with an expected lifecycle of roughly 12 billion years. The Earth is about 4.3 billion years old according to our measurements and observations.
There are red giants twice the age of the Sun. There are blue giants half the age of the Sun. If the Sun is the observer, its relative time rate would be one and our relative time rate would be lightning fast. However compared to the Universe, the Sun's relative time rate may be fast. Ours might be a flash in the pan. But its all the same time rate.
Simply put, we measure time from a scale which we can understand. Time which occurs beyond our ability to fathom can't be measured. To us, it can't be measured so it can't exist.
Scientists Have Measured The Smallest Fragment of Time Ever
http://www.sciencealert.com/scientists-measure-the-smallest-fragment-of-time-ever-witness-an-electron-escaping-an-atom
the team was able to narrow events down to a time frame of 850 zeptoseconds.
Zeptoseconds is proof we don't have an complete way to measure time.
All human measurements of time is based on seconds or years.
This is because seconds and years are measurements within our scale of understanding.
They then used a near-infrared laser pulse, which lasted 4 femtoseconds (10^15 seconds). Overall, they calculated that the ejection of an electron took between 7 and 20 attoseconds, depending on how the electron interacted with the nucleus and the other electron.
Farthest Known Galaxy in the Universe Discovered
http://www.space.com/18502-farthest-galaxy-discovery-hubble-photos.html
The new record holder is the galaxy MACS0647-JD, which is about 13.3 billion light-years away.
A measurement of distance based on how far light travels during one
year.
One
year Earth time.
If the same measurement were made using another planet, say Neptune, the distance would be different.
If seasons were the measurement we used for determining a year, the distance measurement would be different still.
Our reference of time passage is based on our relative observational scale.
Using a different observational scales change the reference but the time is the same. Time itself doesn't speed up or slow down, our observational measurements only make it appear to do so.
My past self does not exist to be visited.
Yes and no.
In the sense that your past self existed at one time, the reality happened and therefore existed. People who believe time travel could be possible believe that existence in reality is registered in a great series of snapshots. They believe to transport someone to that snapshot is how time travel must work. Logically, it does make sense, if ya tilt your head just so.
What they don't understand is the reality in which your past self existed no longer exists because time is the change of static states. Static states too complex and encompassing to duplicate. A feat which would require remaking the entire Universe to a previous static state, precisely.
http://en.wikipedia.org/wiki/Unit_of_time#List
The term Planck scale refers to quantities of space, time, energy and other units that are similar in magnitude to corresponding Planck units. This region may be characterized by energies of around -10^19 GeV, time intervals of around -10^43 s and lengths of around -10^35 m
If you imagine time as a series of photographs taken at intervals of a yottaplex fraction of plank time shutter speed of the entire Universe you might pick out one photograph as a destination. The photos on either side will appear to be the same but they are not.
If you could duplicate the entire Universe to that of a single photo, you could theoretically travel in time. If you had a way to do it.
But...as soon as you arrive at that moment, all photos past the destination would evaporate because you would be inserting new data into an old photo.
The old photo would no longer exist and all subsequent photos would reflect the change creating a different reality.
Time travel is nothing more than a fantasy of science. Like all fiction it is entertaining but not accurate.
The movie
Tenet plays with the idea of time in a different dimension. In the movie they tried to demonstrate a dimension where time runs backwards. Scientifically, it fails but it is entertaining to watch them try.
The gateway/doorway they use to switch dimensions would cause the occupant to never be able to leave the gate. They would be stuck between the two dimensions, forever. This is because within that gate, the reality of the two time dimensions would center on the null. The occupant would be directly influenced by both at the same time equally.
It also stands to reason, if time runs backwards in a different dimension, time still exists.
The movie
Parallels attempts to play with the idea of parallel dimensions. Its obvious their display was extremely limited to the concept but it does touch base in some entertaining ways. They failed to account for changes in local physics. It would seriously end the film if they traveled thru the portal to a reality in which the Earth never formed or the Earth exploded long ago. So many possible variations a movie couldn't possibly explore but the idea of multiple instances is entertainingly portrayed. Plus, they were never experiencing a parallel dimension at all. A parallel dimension is an exact duplicate (parallel) which runs beside its duplicate. Think parallel lines "=". The moment they suggested a change between the realities those dimensions became alternate, not parallel.