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.

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A prototype of the 'impossible' fuel-free engine that some say power a spacecraft to Mars in just 10 weeks. The design is now set to undergo peer-review. Many maintain the system goes against the laws of physics

WHAT IS AN EM DRIVE? The concept of an EM Drive engine is relatively simple. It provides thrust to a spacecraft by bouncing microwaves around in a closed container. Solar energy provides the electricity to power the microwaves, which means that no propellant is needed. The implications for this could be huge. For instance, current satellites could be half the size they are today without the need to carry fuel. Humans could also travel further into space, generating their own propulsion on the way. But when the concept was first proposed it was considered implausible because it went against the laws of physics. Its allegedly fuel-free nature also means that the drive may directly contradict the law of conservation of momentum. It suggests it would produce a forward-facing force without an equal and opposite force acting in the other direction. Advertisement

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.

The concept of an EM Drive engine is relatively simple. It provides thrust to a spacecraft by bouncing microwaves around in a closed container. Dr Mike McCulloch, a scientist at Plymouth University, says something known as 'Unruh radiation' may be behind the bizarre performance of drive

One of the images from an as-yet unpublished paper on the EMDrive, revealing the magnetic fields of the device. The concept has generated widespread excitement about its potential to be used in deep space travel. But many argue this is simply hype. Now a new theory aims to show exactly how it could work

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.'

HOW THE EMDRIVE PRODUCE THRUST WITHOUT AN EXTERNAL FORCE Dr Mike McCulloch of Plymouth University has a new explanation about how the EmDrive works The EmDrive creates thrust by bouncing microwaves around in an enclosed chamber, and uses only solar power. 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 Mike McCulloch of Plymouth University has a possible explanation based on a new theory of inertia. 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. 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, causing sudden jumps in acceleration. 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. Advertisement

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 concept of an EmDrive engine is relatively simple. It provides thrust to a spacecraft by bouncing microwaves around in a closed container. Solar energy provides the electricity to power the microwaves, which means that no propellant is needed

The propulsion system may make deep space travel as in Star Trek a realistic possibility. 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'

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.

WHAT IS THE LAW OF CONSERVATION OF MOMENTUM? The basic laws that are applied in the theory of the EmDrive include Newton's laws including the law of conservation of momentum and the law of conservation of energy. At its most simple, the law of conservation of momentum can be explained by studying collisions. During a collision between two objects - object 1 and object 2 - the forces acting on and between the two objects are equal, and opposite in direction. These forces act for a set amount of time depending on the strength of the force and the shape and size of the objects, but regardless of how long this lasts, the time on each object is the same. The EmDrive appears to violate this law because it seemingly produces a forward-facing force that powers it through space without an equal and opposite force acting in the other direction. However, the electromagnetic wave momentum that is created in the resonating cavity travels to the end walls. At this point, the momentum gained and the momentum lost by the electromagnetic wave is equal, which suggests it complies with the law. Advertisement

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's test set-up for the futuristic EM Drive in its 'Eagleworks' lab. The system is based on electromagnetic drive, or EM Drive, which converts electrical energy into thrust without the need for rocket fuel

'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,'

While there has been some scepticism around the EMDrive, in April Nasa released results of its own test which showed that the EMDrive did in fact create thrust. Pictured is an experimental set-up to test the system

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.