The concept of "travelling backwards in time" is very odd.
To "travel in time" I guess first one would have to define what exactly is this "time" one is about to travel backwards in.
So... what exactly is "time"?
And does "time" even exist as something concrete which can be traversed? Or, is "flow of time" just something created within the brain as a way to keep track of changes in things we experience?
Or is it something else, something totally different?
(I don't know the answers, I'm asking based on what "time travel" could be possible in the first place. Hoping John Titor chimes in, too.)
I don't know the answers to all of your questions, but I can say with confidence that the "flow of time" is not an invention of the brain. We can (and frequently do) make measurements that demonstrate time is an objective dimension of the universe that interacts with space and matter.
There have been really awesome experiments around this. For example, it has been demonstrated that moving a particle at relativistic speeds increases its half-life, which can only be explained by time dilation.
I'm only a layman who likes to do the occasional thought experiment, but does time really have to be an objective (concrete and elastic and traversable) dimension to explain time dilation? Isn't there any alternative explanation?
For example, to illustrate what I mean, let's think about time dilation:
Particles moving at relativistic speeds must by necessity travel larger distances (space) as they move: something moving at ~ 299 km/s moves obviously 299 km in a second, whereas something moving at 1 m/s would have moved only 1 meter. Presumably things like fields and other force carriers, which make matter interact with matter etc. will also have to travel larger distances to interact. They have to cover the distance between the particle(s) moving together as well as the parts of the universe which aren't moving as fast.
This way, in the outside observer's frame (=the one with the clock), these fast-moving particles seem to "slow down" because the things facilitating interaction are travelling larger distances, which takes a longer "time" (=more ticks of the observer's clock). And thus the half-life becomes longer.
Now, one can follow-up this thought experiment: what if there was an Ant-Man in a tiny space ship looking at his wall clock and moving at relativistic speed? The Ant-Man would not spot differences in himself, in his frame, since his clock and bodily processes and matter-matter interactions etc. would run slower too. However, the Ant-Man would see the outside world slowing down (I think), since travelling larger distances takes a longer time. When Ant-Man brakes, his experience would be for the outside world to accelerate (I think) possibly in some kind of a jumbled manner towards the end, and then "lock" into place.
And as Ant-Man's chemical reactions and matter interactions slowed down, moving at relativistic speeds resulted in him having "aged" less compared with people who were stationary.
IANAPhysicist, but traveling backwards in time would violate at least the law of conservation of matter. Traveling forward in time, on the other hand, feels like something actually possible. You're already traveling forward in time at the rate of 1, so nothing would break if you somehow created a bubble of space around you where time ticks at a different (positive) rate.
I thought of people who have deep frozen themselves because of medical reasons. That's kind of forward time-travel, no? (Assuming they ever can be successfully resuscitated)
To "travel in time" I guess first one would have to define what exactly is this "time" one is about to travel backwards in.
So... what exactly is "time"?
And does "time" even exist as something concrete which can be traversed? Or, is "flow of time" just something created within the brain as a way to keep track of changes in things we experience?
Or is it something else, something totally different?
(I don't know the answers, I'm asking based on what "time travel" could be possible in the first place. Hoping John Titor chimes in, too.)