Time travel

Yeah probably. He would be better to check the prices before turning the machine on, so he would arrive at the time when the prices were what he thought they were.

Arriving in the past ignorant to [what was] the present. Bit questionable what benefit that would bring though I'd guess, as there's no information to base a decision.

Then again, if the double leaves a marker, say a particularly odd price etc, against a stock, the original would know to trade that particular one

Guess that relies on the marker being spotted each time and would collapse the second the original fails to spot it...

Time travel is not possible with out physical location travel. It would take all 4 dimensions to even think about going backwards. Look it this way, if all you did was travel in one dimension, ie time, the Earth would move out from under you. It is always in motion in our solar system and in the galaxy and in the universe.

As far as to changing events, I guess that depends on how the time line actually works. If there are multiple time lines then when you go back in time, you create a new time line, while your original one keeps on going. It there is only a single time line then you could alter the past and change your future. But also that brings into question the Grandfather paradox.

I tend to feel that if we did travel to the past, a new time line would be created.

I find the multiple time-line idea difficult to accept.
If it is so, there must be billions of new time-lines being created every second, or if there isn't, WHAT determines their creation?
Even allowing that maybe only humans create them, that's still six billion lives doing so right now, and many billions more before us, and that's entire Universes springing into existence at every turn, a hard thing to explain or accept, for me.
The moving planet/solar system/galaxy problem doesn't seem un-solvable, if you can "fix" the space element of the "space/time" locus to enable a change to the time element. That would not seem unreasonable to me.

Time travel into the past is not physically impossible, it's just logically incoherent. Theoretically an object could travel through a wormhole to inhabit a remote spatio-temporal locus. How to create such a wormhole is unclear; perhaps they occur naturally.

The thing that confuses me is that people seem to keep talking about the consequences of going back in time and interfering with history. If people have free will, then you needn't interfere with anything for history to change - you could simply wind the clock back and everyone could potentially make different choices in the situations they were in before, leading to a different history. You would only have to directly interfere in their situations if they had no free will and their choices depended on their actual circumstances. And we all know that people have free will, right?

Totally agree. Besides, I don't believe in time as something that can be changed. Unless you change the hands on a clock. It's a measurement invented by humans.