Can a Uber driver start the trip without the passenger in the car? ...and take an imaginary passenger for a ride to make money?

Yes, this is technically possible.  Once we are at the location of the request, i.e. the pin, we can swipe "Start a Trip."

No, the drivers wouldn't do this as it will guarantee a 1-star rating.  Also, the customer will complain and reverse the charge anyways.  Not worth it.

A driver did this to me today. Except he didn't even get to my location, he got as close as 1 km away and then the trip started according to my app. I could see him driving to my destination without me. I cancelled the trip and was charged the full fare but immediately raised a case about it - the app has a "driver went without me" option, so this must happen frequently - and was refunded the full amount a minute later. I have no idea why anyone would do this though, obviously anyone to whom this happens is going to claim a refund.

Unfortunately the drivers can do that...I just had an issue last Saturday with a 'gentleman', who couldn't find us at Gatwick Airport and my phone run out of battery whilst waiting for him approximately 15-20 minutes. Not being able to track him anymore, we used my partner's phone and called another driver (using her account), however the first driver after arriving to the scene decided not to loose a £62 trip and did the whole trip with no passengers on board...

I gave the first 'gentleman' the massive one star rating and contacted Uber straight away, but they don't want to refund me, saying that the trip was totally legitimate although I think neither they nor their driver would be able to prove any details about this trip like number of passengers etc.

Now I lost my money and have no idea how to claim it back...will never use Uber again...they don't protect their customers...very dissapointed:(