In the 1800s, when pneumatic tubes shot telegrams and small items all around buildings and sometimes small cities, the future of mass transit seemed clear: we'd be firing people around through these sealed tubes at high speeds. And it turns out we've got the technology to do that today – mag-lev rail lines remove all rolling friction from the energy equation for a train, and accelerating them through a vacuum tunnel can eliminate wind resistance to the point where it's theoretically possible to reach blistering speeds over 4,000 mph (6,437 km/h) using a fraction of the energy an airliner uses – and recapturing a lot of that energy upon deceleration. Ultra-fast, high efficiency ground transport is technologically within reach – so why isn't anybody building it?
I know light's speed in vacuum is constant
Now it seems that physicists have come up with a new way of changing the speed of light in a vacuum. Over two years, and colleagues at the University of Glasgow, together with of Heriot-Watt University in Edinburgh, designed an experiment that can determine whether light with a certain "spatial structure" travels substantially slower than regular light in a vacuum. The researchers created a source that emitted pairs of photons simultaneously. One of the photons went straight to a highly precise photon counter, while the other went via two liquid-crystal masks, which imparted their profile onto the passing particle of light.
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Of course, light can appear to slow down if it travels through a dense medium – a result of the photons having to interact with the medium and take an indirect route through it. In water, the speed of light is roughly 225,000,000 m s–1, while in glass it is roughly 200,000,000 m s–1. The change can be even more drastic – particularly in highly "nonlinear" materials, in which light's speed can be reduced to just a few metres per second. Strange effects can also occur in a vacuum, including the Gouy phase shift, which happens when a beam of light is focused to a point and results in a tiny increase in its "phase velocity".
The speed-vac is used to concentrate small-volume samples