Using similes/metaphors that don't exist yet?

I was just wondering what your take was on using a simile/metaphor that wouldn't make sense in the time period you are writing. For example (I know this is a terrible example, but I am tired and my fingers are sore):

Ancient Greece:
The bird flew past her face like a laser beam

Lasers obviously weren't around in Ancient Greece (don't correct me if I'm wrong, I just want to get the idea across) and so, although it may be the perfect simile, it wouldn't make sense it the character were to think it.

What do you think? Is it wrong to do this? Or is it okay if the narrator is speaking, pretty much to the reading audience?

:-) Thanks and good luck finishing your Nanos!

Yeah, don't use those kind of similes, but it would be cool if you used sort of 'historical' similes.

The awkward thing is using phrases that originally were used to describe things that don't yet exist - steam engines, say - but which have picked up a metaphorical usage that does describe something that could happen in your time. For example "Going off the rails", or "letting off steam". Before gunpowder, the word 'fire!' would not have been used to describe the loosing of a weapon of war, even though it trips off the tongue more readily than "Shoot!"

Talking about someone being galvanised into action, or calling someone a sadist, ought also to be avoided, and are less obvious traps than your simile.

Definately not. As a reader I get angry whenever I stumble upon such an anachronistic phrase. Even, if I don't believe an author should try to imitate ancient speech, because that sounds artificial. He/She neither should be more modern, than the time the novel is settled. So nothing as "he got red like an angry turkey" in medieval european settings as well as no "running out of steam" before the steam engine was invented.
What I like instead are creative new phrases, that fit in time. So to take the example with the bird, it might fly like shot by a ballista.

It would mean you weren't in the character's POV but in the voice of an omniscient and futuristic narrator!

Depends on the POV, to an extent. Avoid in first person, to a degree in involved third person, fine in omniscient narrator.

The fact is the entire language you are using is anachronistic - I occasionally do my geek thing and argue with every word in a faux 17th C world that has an Enlightenment coinage, because it amuses me. Mostly I just let it go. Even if you do your best, your grammar and diction will be modern, so my logic is that even if you are writing first person, what you write is a translation. So, while you should try to stay as consistent with your MC's worldview as possible, don't kill yourself over it. Provided it's not too overt or jarring, it should be okay.

I'm terribly pedantic about this in my writing; I check the etymology of everything I even think might be anachronistic.

I think an anachronistic style can work if it's clear that that's what you're going for. But when I run into a too-modern phrase in a novel that's otherwise accurate, it does annoy me. I can't help feeling that it's authorial laziness; either not doing the research or not being bothered to think of a historically-correct alternative.

I wouldn't use it, even as an omniscient narrator, unless the narrator is, for some reason, a modern person.

I don't worry about exact word usage, especially if my characters aren't actually speaking English, but lasers in Ancient Greece is just going to seem weird and misplaced.

I definately would not use such similes or metaphors if they would not have existed in that era.

Think of what things were similar of that timeslot and use that instead or go with nature ie A bird flew past her face like a bolt of lightening, - just an example.

Nature rarely changes over the centuries

If you're doing comedy it might be a funny insert though. Cue Blackadder II (set in Tudor England): "Dead as a ... dead Dodo."
Otherwise I wouldn't do it.

Unless you're doing something like Blazing Saddles, do your best to keep your expressions in with the times. That is what readers expect. And you have to keep the readers happy.

Blazing Saddles used the anachronisms for a satirical parody effect. The movie is making fun of western movies and had the people acting like 20th century people in a 19th century western setting. (If I ever watch it again you will know something has gone wrong with my head.)

You might be able to get away with an expression like "busier than a dog scratching fleas" even if the expression wasn't actually used back then. There will have been dogs with fleas in those days, and it wouldn't bother those who see that. But if there is reader that knows for a fact that that expression wasn't used by the Greeks until two hundred years later, if at all, that reader will be sending a letter to the author.

You might be able to get away with it if you have contemporary narrator telling the story:

Yes, Mike, that was how they did it a long time back. That bird flew my your ancestor faster than a laser beam, scared her pretty good, too. Made it home in record time it scared her so much.

Now, let's get back to the story...

That might work for a frame around the story, but not for the whole story. It would frustrate the reader.

And you don not want to do that.

The only possible way I would ever use metaphors that wouldn't exactly make sense was if my characters were time travellers, or if the narrator is somehow acquainted with the metaphor. Say, a person finding old journals in her attic and then imagining the life of the person writing the journals or something like that :)