And that is only important if the past is traversable. (IMHO, it is not.)
I’m in the camp that the only time that exists is now. The past exists only in our memories, and the future only in our dreams.
You might as well just say that the past is the only time that ever existed, and the future is the only time that ever will exist. It’s a meaningless platitude.
But how do you deal with the fact that the present is only an infinitesimal point? You don’t know what your computer screen looks like right now, for instance, only how it looked a few nanoseconds ago.
Any time we venture outside of what is actually well accepted as foundations of science, anything at all is somewhere between sophomoric philosophy and meaningless platitudes. Time travel is well within that catagory.
I deal with that by having a brain that processes macroscopic information in a way that ends up creating a narrative of the illusion of passage of time, hiding the nitty gritty that any individual particle can only experience what it currently experiences.
That the computer monitor is close enough to me that the time it takes for photons to get to my eyes is far, far less than it takes for the nerve signals to get to my brain and be processed gives me the illusion that pretty much anything on this planet shares the same “Now”, even though, if we get down into experiments with individual particles and very accurate clocks, we see behind this illusion, to a world that defies intuitive explanations.
I don’t experience time as a series of plank units, I experience time as a series of much, much, much longer moments that are variable in length as my brain interprets them.
Or every time they try to do it something happens to break the mechanism before it can be used. Note these need not be accidents, it could be that the universe resisting your time machine would put scaling forces on whatever your mechanism is that scale to infinity. Like trying to reach the speed of light with solid matter. This would make it impossible to actually make a time machine work, and it need not involve slapstick hijinks like one old movie I vaguely remember. In this movie, the time travelers are trying to kill hitler but something always happens to get in the way.
That sounds like the universe we live in. You can write down some equations that show that time goes both ways, but once you actually start to put numbers into those equations, you find that they tend to give you meaningless results in terms of necessary punts.
It sounds like the universe is resisting our time machine by not allowing it by the laws of physics.
Any hijinks, no matter how slapsticky, would be happening at the quantum level.
Everything is the quantum level.
Ouch, deep.
I don’t see it as problematic. Czarcasm suggests that travel to the future means the future is predetermined. I suggest that if you can travel to the future of your own universe then to some extent that must be true, at least for the part of the universe you can observe.
On another note, it’s interesting that the nature of the paradoxes change if you can travel into the future instead of the past. If you can travel to the past and meet your younger self that is a paradox if you don’t recall it happening. If you travel to the future and meet yourself that is not a paradox, it is the expected result.
Travel to the future means nothing, as we travel to the future all the time. We have ways of increasing that rate of travel, and it may even be possible to come up with clever ways that don’t involve orbiting black holes or traveling at stupid speeds.
Travel to the past changes everything.
It is only if you are planning on returning from the future that causality starts taking notice of your shenanigans.
Except that in order to do that, then you have to have traveled back into the past at some point in order for you to catch up to the point where you meet yourself.
But it is not, it is a boundless 3-space (or multispacetime, if time has more than one dimension). And it cannot be described with surgical precision: this exact moment right here, right now, does not correlate with a similar exact moment on Tau Ceti e.
In other words, the past is shoeful of improbabilities overlapping to the spacetime horizon, the future is a dynamic map of probabilities, and the present is the variable constant that lies somewhere between them, depending on where and when you are.
Time travel into the future is easy. Sit quietly inside a box that has no windows, with no cell phone or anything like that, and just wait while the universe keeps spinning outside your box. Step out of your box when you reach the target time. Voila, you have traveled forward in time. What happened to the universe while you were sequestered? It’s just a bunch of stuff that happens. Was it predetermined? I don’t know, but the fact that you put yourself into a box certainly doesn’t affect the answer to that question. This box I described is a time machine that moves forward at 1 second per second. Trade it in for a box that moves forward at some other speed and you get the exact same result.
There are no paradoxes from moving into the future. None. The only way to imagine a paradox that involves moving to the future is if part of the story also involves moving into the past. That’s where you get the paradoxes. Please note that we already live in a world where everyone is moving into the future all the time and we are doing it at different speeds. Gravity and velocity affect the passage of time. Different spots on Earth have different local gravity and travel and different velocities. The differences are microscopic but they are there. If this created paradoxes then we’d be seeing paradoxes all around us right now.
Do you know how many unsuccessful attempts there were on Hitler?
And now that I think about it, I learn about new ones all the time, always being presented as history.
Nobody can know all the history. When you learn a new bit, was it always history, or did someone time travel back, and now you have more history to learn?
If it isn’t mutable, then we live in a clockwork universe, where all actions past/present/future are locked into a single path.
I travel back in time to wish myself a happy birthday. If that’s immutable, then I always did it, always remembered it, and prior to doing it, it’s certain that I will do it, exactly as remembered.
There was the one successful time travel effort to kill Hitler. People from the war-ravaged year of 2018 traveled back to prevent Hitler from surviving WWII, escaping justice (there were no Nuremberg trials in the original timeline), forming the fourth Reich, and instigating WWIII which lead to global thermonuclear conflict between New Germany, the US and the USSR. The northern hemisphere of 2018 used to be a nuclear ash heap, with pockets of survivors scattered all over. Australia, largely spared from the holocaust, became the new center of civilisation. With the development of the time machine, the decision was made to attempt to alter history. Probability studies (not proven, but believed) showed that killing Hitler before the war had too many uncertainties. Chances are, the world would have been worse.
So, agents of Her Majesty’s Time Corps traveled to April 30, 1945 and killed Hitler and made it look like suicide. Thus stabilizing the time line and creating the utopia we now live in.
They’re called ‘helicopters’ - and yes…yes, I saw that XKCD comic, too. 
I was so with you right up until this. 
I was looking at it from the perspective of those living in the original 2018 timeline. They were only caring about stopping world wars. Having their probability simulations show a timeline that did not have a hostile nuclear detonation from WWII up until their current date seemed like a winner. Or at least, the best they could do.
Obviously, there are bugs to be worked out in temporal simulations. If they’d have had access to our timeline’s super computers, they most likely would have made a different choice. The simulations they were capable of running were only good at predicting large-scale changes. Details like trump, global warming, lack of a moonbase or whether flying cars existed were beyond their capabilities.
Not sure what you are saying here. Obviously we can travel to the future, and at near light speeds it can seem to be faster than one second per second. And the travel to the past creates obvious paradoxes if it’s possible.
Czarcasm’s point about a fixed future distracted me from the core issue I was interested in. Is traveling in time to a different universe the question we are really interested in in terms of time travel? If I can jump to another universe at a seemingly different time then what difference does it make in the universe I started in? I can go meet my future self, but not the one I’d be in this universe in the future. The stock he tells me to buy when I return to my own universe and timeline may tank here even though it made him rich in his. I can kill my own grandfather in another universe but I would still exist, it’s my current counterpart in the other universe who will have a problem. Is there even a way to correlate time across universes? There can be a universe exactly like this one except that everything happens one day later according to the calendar.
No, you can’t visit your future self by traveling into the future, because you stepped into a time machine in 2018 and vanished. There is no future you in the world, because you stopped existing for X years.
Alternatively, you stepped onto a NAFAL spaceship and traveled to Tau Ceti and back. You were gone for 22 years according to clocks on Earth, but according to your watch it only took you 22 minutes. You wouldn’t expect to meet your future self on Earth in this case, would you? Because you know what happened to you. You got on that space ship and were gone for 22 years.
Same thing if you had a time machine to take you 22 years in the future via whatever wibbly-wobbly-timey-wimey mechanism time machines work on. You could only meet your future self if after your visit to the future you got back in your time machine and traveled back to the past, so you could live the 22 years to meet your past self in the future. But this time machine only travels into the future, never the past, so you can’t do that. There’s no temporal paradox or multiple time lines here, because your machine only goes into the future and therefore can never create a paradox. You travel 22 years into the future, and when you get there you find that you vanished 22 years ago and have just reappeared now, just as if you’d been away on a trip to Tau Ceti.
What if your time machine clones yourself, and one of the clones stays behind thinking the time machine didn’t work? Then the clone that DID travel through time meets up with the other clone 22 years into the future.
(Family guy had a similar premise dealing with teleportation I think).