It is like something out of sci-fi, only a little less believable.
It also happens to be true. You see, scientists have made an invisibility cloak so perfect that, not only does it hide a person or thing, but an actual event.
A robber goes to steal from a bank. He walks in, cracks the safe, and takes all the money. Later, the police examine the CCTV and see… nothing. No robber. No safe door opening. Just a typical scene.
“Aha! It must be some clever computer person doing fancy things!” would be most people’s resolution to this paradox. But a closer look at the footage would show that it had not been looped or tampered with or anything like that… the reason that the camera saw nothing was that there was nothing it could see.
Of course this could never happen, to make something invisible you would have to imbed it in a special type of crystal (the perfect version of which is yet to be invented) and that is less than salubrious for any self-respecting thief.
But we can do it! By bending light in a very particular way we can make things apparently not exist.
Then things get weird, because space and time are linked in a very particular way (time is a lot like space – in fact it is identical from a Physics point of view to a Cartesian axis of imaginary numbers… but you don’t care about that so I’ll get back to the other stuff…).
So if you can hide something in space, why can’t you hide it in time? Short answer- you can.
This means that we could make an event (which is the space-time equivalent of a point in space) appear not to happen. Said event would, from a certain perspective, appear not to have happened.
For example the cracking of a safe or a journey in some mimicry of teleportation.
So how does this work? The way it was explained by researchers at Imperial College London was like a major road. If you are down the road from me and I want to cross, then you might expect to see a break in the traffic where I walked out in the road, causing alarmed motorists to slow down.
The invisibility works a lot like this, only we artificially slow all the motorists down (say with a temporary speed limit); we then allow the motorists to return to the normal speed, as if they were only delayed by the pedestrian crossing. In this way, delayed motorists are able to catch up with the other, non-delayed, cars.
That is to say, you interrupt the stream of information (photons in the real world, cars in our analogy) and then speed up the information which has been delayed to patch the signal back together.
You, an observer a mile down the road, might never realise that something had occurred because from a sufficiently great distance, the difference between motorists who had slowed and those who had not, would no longer be seen.
Naturally trying to do this for a bank robber remains impossible due to the amount you’d need to speed up light (in fact we need to find a medium to slow light down rather than speed it up, the latter being impossible, but this is a technicality). The larger the event (in space-time) to be hidden and the closer the observer of the event is, the harder our disappearing act will be. Crossing a school-load of children is very disruptive to traffic compared to a pedestrian; equally for the delayed traffic to merge with the other traffic, a larger distance is advantageous.
And so, sadly, the funding for theoretical physics must still come from government grants rather than bank theft… for now at least…
Image sourced under the Creative Commons license.