The Alcubierre drive is one of the better known warp drive on paper models – where a possible method of warp drive seems to work mathematically as long as you don’t get too hung up on real world physics and some pesky boundary issues.
Recently the Alcubierre drive concept has been tested within mathematically modeled metamaterial – which can provide a rough analogy of space-time. Interestingly, in turns out that under these conditions the Alcubierre drive is unable to break the light barrier – but quite capable of doing 25% of light speed, which is not what you would call slow.
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OK, so two conceptual issues to grapple with here. What the heck is an Alcubierre drive – and what the heck is metamaterial?
The Alcubierre drive is a kind of mathematical thought experiment where you imagine your spacecraft has a drive mechanism capable of warping a bubble of space-time such that the component of bubble in front of you contracts bringing points ahead of you closer – while the bubble behind you expands, moving what’s behind you further away.
This warped geometry moves the spacecraft forward, like a surfer on a wave of space-time. Maintaining this warp dynamically and continuously as the ship moves forward could result in faster-than-light velocities from the point of view of an observer outside the bubble – while the ship hardly moves at all relative to the local space-time within the bubble. Indeed throughout the journey the crew experience free fall conditions and are not troubled by G forces.
Some limitations of the Alcubierre drive model are that although the mathematics can suggest that forward movement of the ship is theoretically possible, how it might start and then later stop at its destination are not clear. The mechanism underlying generation of the bubble also remains to be explained. To warp space-time, you must redistribute mass or energy density in some way. If this involves pushing particles out to the edges of the bubble this risks a situation where particles at the boundary of the bubble would be moving faster than light within the frame of reference of space-time external to the bubble – which would violate a fundamental principle of general relativity.
There are various work-around solutions proposed, involving negative energy, exotic matter and tachyons – although you are well down the rabbit-hole by this stage. Nonetheless, if you can believe six impossible things before breakfast, then why not an Alcubierre drive too.
Now, metamaterials are matrix-like structures with geometric properties that can control and shape electromagnetic waves (as well as acoustic or seismic waves). To date, such materials have not only been theorized, but built – at least with the capacity to manipulate long wavelength radiation. But theoretically, very finely precisioned metamaterials might be able to manipulate optical and shorter wavelengths – creating the potential for invisibility cloaks and spacecraft cloaking devices… at least, theoretically.
Anyhow, metamaterials capable of manipulating most of the electromagnetic spectrum can be mathematically modeled – even if they can’t be built with current technologies. This modeling has been used to create virtual black holes and investigate the likelihood of Hawking radiation – so why not use the same approach to test an Alcubierre warp drive?
It turns out that the material parameters of even so-called ‘perfect’ metamaterial will not allow the Alcubierre drive to break light speed, but will allow it to achieve 25% light speed – being around 75,000 kilometres a second. This gets you to the Alpha Centauri system in about seventeen years, assuming acceleration and deceleration are only small components of the journey.
Whether the limitations imposed by metamaterial in this test are an indication that it cannot adequately emulate the warping of space-time – which the Alcubierre drive needs to break light speed – or whether the Alcubierre drive just can’t do it, remains an open question. What’s surprising and encouraging is that the drive could actually work… a bit.
Further reading: Smolyaninov, I. Metamaterial-based model of the Alcubierre warp drive.