I get that there are 3 dimensions of space and only one dimension of time, but it still seems like you should be able to go both forward and backward. It’s like if you had a 1-dimensional space (which I assume would be an infinitely thin line, correct me if I’m wrong) you could still go both left and right.
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Mass is the property of an object that determines the speed that it travels through time. An object with 0 mass, such as a photon, does not travel through time. From the perspective of a photon, its existence lasts exactly 0 seconds, regardless of how far it travelled.
Something with positive mass, such as everything we know, travels in the same temporal direction that we do. So if you want to imagine time as being a line where left is “our past” and right is “our future” – something with positive mass will always have momentum towards the “our future” direction of that line. The more mass you have, the faster you travel in that direction.
The flipside this is that anything with negative mass would travel in the opposite direction. That direction is towards our past, until you hit the big bang, at which point it becomes “the future” for anything that had negative mass at the start of the universe.
If you could somehow flip your mass from being positive to negative then you would begin travelling back towards the big bang, pass through it, and eventually emerge in a universe in which everything had negative mass – although in that universe, you would perceive yourself as having positive mass. This is because positive is a relative term – we simply assign it to things that move forward and in that “negative” mass universe you would be moving forward through time, relative to everything else in that universe.
There is no way that we know of to create negative mass. If objects with negative mass were present at the big bang, then they all would have immediately begun travelling in the opposite temporal direction from us. If time was a line where the big bang was 0, we would be on one side of the zero while all of the negative mass would be on the other. In other words, we could never interact with or perceive and of that negative mass because its somewhere that we are not and cannot ever get to. But, strictly speaking, there’s no fundamental limit in physics that prevents you from travelling backwards through time.
The situation is more similar to an astronaut who is stranded in space. To alter the direction in which they’re travelling, the astronaut needs some method of propelling themselves. If they don’t have that, then they’re stuck travelling forward. We’re the stranded astronaut that has a positive mass rocket strapped to our back, continuously moving us forward through time. Without a negative mass rocket to change direction, we’re basically stuck going forward for the rest of eternity.
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