Albert Einstein once said: “The separation between past, present and future is only an illusion, although a convincing one.”

Whether he was a consultant on bangs-for-bucks Terminator Genisys isn’t clear but his flexible notion of “now and then” appears to be a guiding principle.

Various familiar characters – Kyle Reese, John Connor and mum Sarah – along with various incarnations of the Terminator hop about the temporal spectrum like this was Back To The Future 2 with a stone in its shoe.

The original made by James Cameron in the 1980s is re-imagined and re-booted from a new perspective, as is Judgment Day and all that follows, or doesn’t follow depending on your timeline.

Indeed, the presence of Matt Smith (of Dr Who fame) as a physical incarnation of malevolent Skynet is a reminder that time travel does tend to inspire the most ridiculous hokum and the most stodgy of exposition (usually littered with the word “quantum”) to explain away its U-turns and impossibilities.

There is much mirth that Arnie Schwarzenegger, taciturn in robotic form, is given the task of tackling this tongue-twisting tumble of indecipherable jargon.

Arnie, of course, is the only constant as other characters have new faces. Game Of Thrones gem Emilia Clarke is stoic and kick-ass as Sarah, Jason Clarke is the latest embodiment of John Connor while Kyle is an unremarkable Jai Courtney.

Arnie, in the 30 years (or blink of an eye depending on your time machine) has gone from fearsome, mindless 80s hunk to large, lovable clown called “Pops”. (“Old but not obsolete,” is his new catchphrase although he does assure us he’ll be back.

The story centres on the final defeat of Skynet at which point the robots send back “bad” Arnie to kill Sarah. But time travel means that anyone can be anywhere, anytime so what greets the naked young Arnie is not what greeted him in the 1984 original.

Arnie in Terminator Genisys

Meanwhile the rebooted trio of Kyle, Sarah and Pops have to fight a fearsome new machine and have to prevent a different Judgment Day, this one in 2018.

With time travel, of course, there is no peril, no consequences and anything done can be undone so nothing really matters. With that as the mindset, the movie’s mindless splendour can be enjoyed to the full – large-scale set pieces, a thrilling scene on the Golden Gate bridge and some metal-crunching one-on-ones, all with Arnie’s red eye in Wink mode.

Aside from that, there is nostalgia for the franchise iconography which is much easier considering that the fings that ain’t wot they used to be, literally aren’t, weren’t or possibly never will be and vice versa.