Right–but be very careful to not touch the earlier version of yourself.
And serious people have proposed that antiparticles can be understood to be normal particles moving back in time - so (for example) an interaction between an electron and a positron could be interpreted as a self-interaction between the electron and that same electron moving backwards in time at a later point along its worldline.
If I went back in time, how easy would it be to touch a particular electron in my body to one in my alter ego’s ?
All electrons are interchangeable. If you were in the process of going back in time, that might potentially make your electrons positrons (and your protons and neutrons antiprotons and antineutrons), which would make touching anything, including air, extremely ill-advised. But once you “arrived” and started moving forward in time again as normal, that shouldn’t be an issue any more, no more than it’s an issue for your particles to touch other particles in our normal timeline world.
But it’s not like I spontaneously created all the atoms and molecules in my body. They all came from somewhere else. I just watched Hamilton on Broadway today. Think of one single iron atom in Hamilton’s blood during the final years of the 18th century. In fact , let’s give the atom a name. Call it OFIA, as in “our favorite iron atom.”
When Hamilton was shot and killed in that famous duel, some of his blood spilled on that field in New Jersey, and let’s assume that OFIA was in that pool of blood. I know this is a stretch, but now let’s say that, through a series of natural events, OFIA made it into the spinach that I ate last week. My body absorbed OFIA and now it’s in my blood (along with millions of other iron atoms).
Now let’s say that next week, I build a time machine and go back to the time of Hamilton to the days before he was shot. I’m there, in 18th century New York, walking around in horse shit, with OFIA still in my blood. But OFIA is also in Hamilton’s blood, since he’s still alive walking around. How can OFIA be in my blood at the same time it’s in Hamilton’s blood? That would mean that matter was created spontaneously, and now more iron exists than was actually present at that time.
It wasn’t spontaneously created, it arrived through the time machine. And those extra atoms would vanish again when the back-in-time you themselves went back in the time machine.
And if you went back and killed yourself? I don’t know if we actually know what would happen in such a paradoxical situation. The ways of using super fast speeds to travel through time at different rates given above don’t give the option of being around at the same time as a different version of you, so the paradox doesn’t apply.
Any iron atom has extent in duration - so you’re using one portion of its lifespan while Hamilton is using another portion of its lifespan. It’s been seriously proposed that there’s only one electron in the universe - weaving back and forth through time, and appearing to us as the billions of electrons and positrons that make up the world One-electron universe - Wikipedia
And, boy, are its arms tired!
“Time is an illusion. Lunchtime doubly so.” - HHG
I love smartass answers that have the added benefit of being the truth. 
Of course, you won’t have to worry about the germs of the year 1319, if that was your destination, because you’d land in the vacuum of space since Earth has moved quite a bit in 700 years.
Intentional, and planned or targeted time travel is impossible. To understand this one must first understand that the universe’s time isn’t the same as earthling’s chronological time. Time is not a year, or month, day or hour to the universe. To the universe, it is the movement of all that is contained within it. If all that is within the universe ceased to move, the universe would cease to exist. Also, this movement is in one direction, so it is said time moves forward. However, the truth of the matter is the movement of what is contained in the universe moves forward.
To travel back in time one would have to reverse all that is contained in the universe. It all would have to backtrack the exact steps (if you will) it took to move forward until the universe is configured front to back, top to bottom exactly the same as it was in that moment in the past. However, the supposed traveler is part of that as well and would be constrained to be where he/she/it was at that same instant as well.
One cannot travel forward in time as that hasn’t happened yet. All that is contained in the universe has not moved there, where ever there turns out to be. Since this location is an unknown, the future is literally an unknown, and to travel one must have a destination.
It is easy to look at a watch as you spin its stem making the hands rotate backward and imagine that’s what is being spoken about regarding traveling back in time. Rotate the stem in the other direction, it seems there’s an easy illustration of traveling forward in time. This is anthropomorphic reasoning regarding time. As such it is highly fantastical and highly impossible.
Though, if you’re as quick-witted as most wish to be then you’ll recall I prefaced this with “Intentional, and planned or targeted time travel…” This wasn’t just to string words together to seem smart. (That’s the purpose of all that followed from there. :rolleyes:) It has been theorized or postulated that there could be things called wormholes, and these wormholes could actually connect two different phases of time.
Traveling through a wormhole could (theoretically) deliver someone to a random place in time. It then followed (by sci fi writers) that given the right technology a wormhole could be fabricated. This theory, even though Einstein liked it, does not take into account that the past and future no longer exist, so the likelihood of a destination existing at the end of a theoretical wormhole is slim to none.
But, don’t let a thing like the actual factual stop you sci fi writers out there. I’m a big H.G. Wells fan, myself. 
What is your source for these statements?
Some examples of science fiction are more fictiony than others. That’s why TV Tropes invented the Mohs scale of science fiction hardness. The three snippets of dialog they use to illustrate soft, hard, and really hard s-f involve how a time machine works.
Common knowledge at this point, my man. Or, common sense if you will.
Or assumptions without evidence, if you will.
Cool! So you have a worked-out theory that explains why closed timelike loops are impossible even though they seem to be allowed by General Relativity? Have you published it?
As long as matter and energy summed are conserved, no problem. Perhaps in order to time travel the "mass’s equivalent (as energy) must be removed from the universe at the time where/when you are deposited. Since there’s no truly good working theory to accomplish this, my guess is as good as any.
Conservation of energy doesn’t work the way that most people think it does. What it actually says is, basically, if you have a box, then the change in the energy in the box is equal to the energy that flows through the walls of the box. If the energy flows into the box in the form of a time-traveling spaceship flying into it, that’s just fine (at least, as far as conservation of energy is concerned).
It’s “impossible” because the implication is that 1980 and indeed every other moment in time must “exist” somewhere such that we can travel to it. That is the part I can’t wrap my head around.
Or, another way to put it is that if all those other moments in time exist as possible time travel destinations, what makes this particular moment in time in the “present” any more or less significant than any of those others moments in time. Like 7/24/1980 9:00pm EDT doesn’t know it happened 40 years ago from my perspective.
You mean the concept of space-time? That basically means you can specify an event by giving its coordinates of where and when it happened, not such a stretch to wrap one’s had around IMO. People do it all the time. It’s true that the “present” moment will depend on the observer and therefore où sont les larmes d’hier soir but spacetime is just a mathematical model of the universe and does not deal (directly) with questions such as the psychological perception of time, arrow of time, and the like, or even weird ways of contriving timelike loops