Could you be levitated with a tractor beam?

Is there any scientific backing at all behind being levitated up through air so aliens can abduct you? Well incredibly, there just might be. The Public University of Navarra’s Asier Marzo...

Asier - A tractor beam, it's a source of energy. And this energy could be, lasers, could be sound, could be electrons, some kind of radiation. But the important thing is that it can attract objects towards the source.

Will - How might we be able to do that then? It's very easy to push stuff away, but pulling stuff towards you is much more difficult.

Asier - With optics, it's a bit simpler: you focus a laser and at the focal point of that laser, a very small particle, and I mean like one micrometre, less than one micrometre - so cells or tiny pieces of ash - it'll be trapped in the focal point. So if you move the focal point of that laser, if you make it focus closer and closer, that particle will move with it. When we use sound, it's a bit more tricky because you need to create something that is called a hollow trap, some kind of sound that has a silent core and the object gets trapped in that silent core. And again, you do something with the wave so that that silent core moves towards the source.

Will - So what's going on in that focal point or that silent core that is essentially attracting what you want to levitate to move towards you.

Asier - If you think of a sound field that has high amplitude - it's quite loud everywhere, but there is a point that is silent and you drop an object there, it will remain there because it will be trapped there. There is vibration all around the object and it'll be pushed towards that silent core. If it moves a bit to the left, the vibration will push it back. So in theory, it gets trapped in that silent core. And if you can modify the position of that silent core, the object will move with it.

Will - It's almost as if the object you want to move is stuck in the middle of a huge crowd of people and you can't move in any direction because it'll always push you back into the centre.

Asier - Exactly like that. Maybe it's not a solid crowd that will not allow you to pass, but these people are moving and pushing in random directions, but not in that silent core.

Will - What sort of sound or what sort of noise do you use to try and levitate things?

Asier - In general we try to use ultrasound for two reasons. First of all, it carries more power. The higher the frequency, the more energy these waves are going to have so the harder they will push the objects. And the other reason that it's more comfortable, when we work at 40 kilohertz we cannot hear it and it's easier to focus. So it's not such an annoying experiment as when we use audible sound.

Will - So what are the current limits of this then? How large an object can you move with this sort of acoustic levitation?

Asier - There are two limits. One is how dense the object can be and that depends on the power that you put on the sound. Maybe you can levitate something that is as dense as water - one gram per centimetre cubed - and then with the same device, you may not be able to levitate iron, which is around seven grams per centimetre cubed. But if you put more power into the levitator or tractor beam, you will be able to levitate those objects. But there is a more fundamental limit and that limit depends on the wavelength of the sound that you use. And basically, objects cannot be larger than half of the wavelength. To give an example, when we do levitation in air, we like to use 40 kilohertz, 40,000 hertz. So the sound vibrates 40,000 times per second, and that gives a wavelength of 8.6 millimetres. Therefore, objects that are larger than four millimetres cannot be trapped because they don't fit in the wave.

Will - Purely hypothetically, if we wanted to levitate a human, what conditions would have to change? Presumably they'd have to be a massive shift in what we can currently do to get to that point.

Asier - Indeed, we should first lower the frequency so that we can trap larger objects. If we use 40 kilohertz, you may be able to trap a small bee, so we will need to reduce the frequency probably down to, depends on how big the person is, and maybe if the human is rolled in a ball, but for sure you will need to go down below 400 hertz. And the problem with that is that it becomes audible, it becomes very noisy and also you lose a lot of power because less frequency, less power in the waves. So you will need to increase the volume a lot. You will need a lot of decibels. In fact, you will need so many decibels that it's maybe not physically possible to create that amount of decibels in the air.

Will - Presumably you'd get to a point where you'd create so many decibels that the person you're trying to lift would just explode?

Asier - Indeed. First of all, you will of course go deaf. And then you will heat up and I think at that level you may have difficulties breathing, your eyes may increase the pressure. So for sure it will not be a pleasant experience in the case that it was possible.

Will - So this is not a very practical means of abducting humans then?

Asier  Not at all, at least not with sound. And I think people usually get disappointed because they say, can you levitate a car? Can you levitate a human? And you cannot even levitate an apple. Acoustic levitation using sound, it is much, much more useful for small things.