Question: Would it be possible to neutralise gravity with a light optical tweaser above and gravity below?

Susan Skelton answered on 21 Nov 2013:

Hi again, u12laversb!
This is a fantastic question! The answer is very much yes, but only for small objects – around the size of a cell.

I’ll tell you a bit about my optical tweezers system I built in the lab…
In my optical tweezers system I have one light beam which I can use to hold particles (say a cell, for example) and move them wherever I want in space. My light beam travel upwards, from bottom to top: this means that as well as holding the cell, it pushes it upwards.

But the cell also experiences a force caused by gravity, which pulls it downwards towards the floor. To be able to hold the cell and prevent it being pushed upwards by the laser beam, or fall downwards due to gravity, I have to make sure the upwards (light) force and downwards (gravity) forces are the same strength.

So in this case I DO use the optical tweezer to neutralise the gravity force on the cell: I stop the cell from falling down by pushing it upwards with the light beam.

It would be really nice if we could do this for bigger things like people: to switch off gravity so we could leap 10 feet into the air. 🙂

Unfortunately this won’t work for bigger things because they are just too heavy! The force on an object due to gravity is:
F = m x g,
that means the gravity force is the mass of the object multiplied by the gravitational acceleration of the Earth (about 10). Big things like people have a big mass so they also have a big gravity force!

In fact, a person is about 400000000000000 times heavier than a cell! That means we would need to use a laser beam that is 400000000000000 stronger than the one we used on the cell to neutralise gravity on a person!!

Such a powerful laser beam would be so dangerous it would burn a hole right through you instantly!

So unfortunately I don’t think we could use an optical tweezers to neutralise gravity on people, but it’s very useful for studying much smaller things to learn about the nanoworld. Things on this scale are far smaller than our fingers (and even our tools!) so they are really difficult to pick up and study. Optical tweezers lets us do that, which allows us to manipulate atoms, molecules and cells, and even build microscopic nano-machines!