Can you travel forward in time?

You have to go fast enough.

According to general relativity, it actually is possible to travel to the future. If you were to stand in a strong enough gravitational field, time around you would accelerate, while your own clock would remain unaffected, and your own body would continue aging at a normal rate.

In theory, given you can somewhat survive (but also exit) this gravitational field, you could travel to the future by stepping into that strong gravitational field, watch outside time accelerate, and then exit to the desired time in the future.

However, there are a few catches.

The machine must continue to exist in the future, and in-between. So it's not like in Doctor Who where a TARDIS appears out of thin air. The time machine will actually be sitting there for all that time.

When you are traveling in time, and waiting in your machine with it's strong gravitational field, outside observers (assuming they can see you) will see you as if you're frozen in time. Well, at least, moving in super slow-motion.

The future you'll get will be similar to the one you'd be getting if you'd fall into a super-long coma or got cryogenically frozen. You won't travel to any "alternate realities" or anything of the sort. As far as the world is concerned, you are stuck in your machine, like an insect stuck in amber, and history simply carries on, with this variable in consideration.

Since the machine is simply sitting there for all the duration of your "travel" to the future, with you inside, there's the possibility that you or your machine might be vulnerable to outside events such as wars, catastrophes, supernovas, etc. You are basically a sitting duck to the outside world. The faster you move towards the future, the less time you'll get to foresee and mitigate unexpected threats, and you would probably encounter serious troubles mere seconds after stepping into your machine. That's assuming that you are using a machine, of course.

Good news though, you'll still look like your dashing self if you make it to the future, and to those of the future it'll really look as if you've just stepped right from the past. So, meeting your grand-grand-daughter, even if she is then older than you, is amongst the realm of the possible, assuming that you have had children before entering the time machine. To her, it'll just look as if her ancestor just spent all this time frozen in a time machine.

Which sounds kinda pathetic, I have to admit.