Can you bend a laser around the Moon?

Thanks to Professor Michalis Zervas and Professor Simon Hooker for the answer!

In the far flung future, when we need to send messages out into the cosmos, we could do worse than using lasers. According to NASA, future Laser communications will enable 10 to 100 times more data transmitted back to Earth than current radio frequency systems. But what if there’s something in the way? Well perhaps gravity can help us out. Here to explain is University of Southampton’s Professor Michalis Zervas…

On a cosmic scale, it is known that light and, therefore, a laser beam can be deflected or bent around a celestial body, like the moon, due to a fascinating effect called gravitational lensing.  Massive objects, such as a stars or galaxies, warp the spacetime around them, and this warping causes the path of light to curve as it passes near the celestial object.

However, compared to stars and galaxies, the moon has much, much smaller mass and, as a consequence, creates a very slight gravitational lensing effect.  Therefore, its impact on laser beam bending, although theoretically present, it will be extremely subtle and typically not observable with current technologies.

So, although it will be possible to deflect a high-power laser beam around and beyond a big star or a galaxy, it cannot be done around the Moon.

Gravity is a no go for the Moon then, but what about man made stuff like mirrors and special beams? To talk us through that, the University of Oxfords Professor Simon Hooker...

Setting aside the problem of getting the mirrors in the right place, and keeping them there, you’d find that they’d need to be very large. This is because the laser beam will naturally expand on its way to the moon due to a process known as diffraction. Perhaps paradoxically, first expanding the laser on Earth helps, but even if you launched a beam with a diameter of 1 m, it would expand to nearly 200 m on arriving at the moon. So you’d need to put some pretty big mirrors in space!

Something else you might try is a special kind of optical beam known as a “Bessel” beam. This can be made by passing a laser beam through a glass cone, which converts the beam to a conical beam. We can think of a conical beam as being made up of lots of beams all heading from points around the rim of a circle to the axis of the glass cone. These beams all overlap at points along the axis to give a very bright central spot, that looks just like a focused laser beam. Bessel beams have “self-healing” properties. This means that if an obstacle is placed in its way, the central bright spot will be blocked. But, all the laser light away from the axis will go round the object, and this can “re-build” a bright central spot at points downstream. So, it looks just as if the beam has re-built itself —or self-healed!

Could we use this self-healing property to project a beam “through” the moon? In principle yes, but at the moon the diameter of the conical beam would need to be many times that of the moon (so that the light going round the moon can re-build the Bessel focus beyond the moon). On Earth, the beam would need to be bigger still … which would be expensive and unwieldy. So, on balance, I think that it would be easier to wait a few days for the moon to move out of the way!