- EmDrive creates thrust by bouncing microwaves in chamber
- Scientists aren't sure how this happens as it defies the laws of physics
- Dr Mike McCulloch suggests it is caused by effect of 'Unruh radiation'
- At low accelerations, the radiation causes inertia to increase unexpectedly
- Research on the engine is due to undergo peer review in coming months
An 'impossible' fuel-free engine, that could take humans to Mars in just 10 weeks, has been shown to work - but no one knows why.
The so-called EmDrive creates thrust by bouncing microwaves around in an enclosed chamber, and uses only solar power.
Many argue the concept is simply hype, pointing out that the design goes against the known laws of physics.
Now, one scientists claims he has a new theory that could explain exactly how the EmDrive would work.
The idea for an EmDrive was proposed in 2000 by a researcher named Roger Shawyer.
Since then four independent labs, including one at Nasa, have recreated the drive.
They still, however, have no idea how it creates thrust.
The system is based on electromagnetic drive, or EM Drive, which converts electrical energy into thrust without the need for rocket fuel.
According to classical physics, the EM Drive should be impossible because it seems to violate the law of conservation of momentum.
The law states that the momentum of a system is constant if there are no external forces acting on the system – which is why propellant is required in traditional rockets.
But Dr Mike McCulloch of Plymouth University believes he has possible explanation for its behaviour.
His hypothesis is based on a new theory of inertia.
This describes the resistance of all massive objects to changes in motion or accelerations.
Why inertia exists at all has puzzled scientists for centuries.
McCulloch's suggests inertia arises from an effect predicted by Einstein's theory of general relativity called 'Unruh radiation'.
The Unruh radiation effect states that if you're accelerating in a vacuum, empty space will contain a gas of particles at a temperature proportional to the acceleration.
In other words, the universe heats up as objects accelerate.
According to McCulloch, inertia is the pressure that the Unruh radiation exerts on an accelerating body.
When the accelerations involved are smaller, such as is the case with the EmDrive, the wavelength of Unruh radiation gets larger.
At extremely small accelerations, the wavelengths become too large to fit in the observable universe.
As a result, inertia may only take on whole-wavelength units over time, causing it to become 'quantized.'
This means it can only in some multiple of a unit of measure.
At very low accelerations, inertia jumps unexpectedly in value – an effect already seen when spacecraft perform a flyby of Earth, causing them to move faster than scientists calculate they should.
The same effect could be happening in the EmDrive by reducing the size of allowed wavelengths of Unruh radiation.
McCulloch says because photons have an inertial mass, they must experience inertia when they reflect.
But because of the EmDrive's truncated cone, the Unruh radiation in tiny.
The cone allows Unruh radiation of a certain size at the large end but only a smaller wavelength at the other end, according to an in-depth report by MIT.
This means the inertia of photons inside the cavity change as they bounce back and forth. To conserve momentum, they are forced to generate thrust.
McCulloch says there is some evidence that exactly this happens. 'This thrust reversal may have been seen in recent Nasa experiments,' he says.
If the EmDrive works on a large-scale, researchers say the new drive could carry passengers and their equipment to the moon in as little as four hours, or to Mars in 10 weeks.
A trip to Alpha Centauri, which would take tens of thousands of years to reach under current methods, could be reached in just 100 years. And as the thrusters are solar powered, propulsion would be generated along the way.
When London-based Sawyer came up with concept in, the only team that took him seriously was a group of Chinese scientists.
In 2009, the team allegedly produced 720 millinewton (or 72g) of thrust, enough to build a satellite thruster. But still, nobody believed they had achieved this.
Last year, Pennsylvania-based scientist Guido Fetta and his team at Nasa Eagleworks published a paper that demonstrates that a similar engine works on the same principles.
Their model, dubbed Cannae Drive, produces much less thrust at 30 to 50 micronewtons - less than a thousandth of the output of some relatively low-powered ion thrusters used today.
On the NasaSpaceFlight.com, those allegedly involved in the project claim that the reason previous EM Drive models were criticised were that none of the tests had been carried out in a vacuum.
Physics says particles in the quantum vacuum cannot be ionised, so therefore you cannot push against it. But Nasa's latest test claims to have shown otherwise.
'Nasa has successfully tested their EmDrive in a hard vacuum – the first time any organisation has reported such a successful test,' the researchers wrote.
'To this end, Nasa Eagleworks has now nullified the prevailing hypothesis that thrust measurements were due to thermal convection.'
However, Nasa's official site said earlier this year: 'There are many 'absurd' theories that have become reality over the years of scientific research.
'But for the near future, warp drive remains a dream.'
But not everyone agrees. The engine is due to undergo peer review soon.
The International Business Times last month spotted a comment on the that Nasa Spaceflight forum which suggests more testing is set to be done on the engine.
One member of the EmDrive team, Paul March, wrote: 'The Eagleworks Lab is NOT dead and we continue down the path set by our Nasa management.
'Past that I can't say more other than to listen to Dr Rodal on this topic, and please have patience about when our next EW paper is going to be published. Peer reviews are glacially slow,'
Eagleworks is an experimental lab at Johnson Space Center that was created to explore alternative propulsion technologies.
In November, the EMDrive team posted their first update on the engine in months, and it seems to suggest that the futuristic engine does, in fact, work.
At the time, March said Nasa has managed to remove some of the errors from earlier tests - but still found signals of unexplained thrust.
Among the criticisms of previous experiments were that the tests were not carried out in a vacuum.
March posted on the Nasa Spaceflight forum in reply to an unpublished paper that claims the unaccounted thrust is generated by something known as the Lorentz force.
The Lorentz force is the force that is exerted by a magnetic field on a moving electric charge.
But March says his tests prove this is not true.
'I will tell you that we first built and installed a second generation, closed face magnetic damper that reduced the stray magnetic fields in the vacuum chamber by at least an order of magnitude and any Lorentz force interactions it could produce,' he said
'And yet the anomalous thrust signals remain.'
March also says that in the latest developments, thermal expansion of the thruster is taken into account to reduce all possible sources of error.
But the thrust can still be seen, and engineers still can't explain what's causing it.
Current plans are to try to recreate the Eagleworks results under higher power.