## Penrose: seeking the limits of quantum

World-famous mathematician and physicist Sir Roger Penrose is Lorentz Professor 2011 at Leiden University. But his links with Leiden go back much further than this. Together with Leiden experimental physicists Dirk Bouwmeester and Tjerk Oosterkamp he is seeking the limits of quantum mechanics.

### Idea

It started with a visit by Roger Penrose to Dirk Bouwmeester's study in Oxford, some ten years ago. Penrose had an idea for an experiment. It embraced a complex panoply of satellites and photons. Dirk Bouwmeester's response was: 'Can't we think of something that will fit on an optical bench?' As a result Bouwmeester has spent the past seven years carrying out the experiment that he and Penrose dreamed up that day.

### Superposition

Penrose wanted to look for the limits of quantum mechanics. It is something he has been interested in for many years, he explains. In theory, quantum mechanics should always apply, but the fact is that this is not what we perceive. Take one of the phenomena in the quantum world, known as superposition, where particles can be found in several places at the same time. Penrose: 'That sounds strange and it is something we don't see in everyday life. Apparently there is a limit to this superposition.' In his view, the cause for this can be found in the theory of gravity.

Penrose is also well known for the Penrose tiles, the breakthrough he made together with physicist Stephen Hawking in the theory of gravity and his ideas about awareness as a quantum mechanical process.

### Curvature

Particles have a mass and they therefore cause a curvature in space time, much as a heavy ball makes in a sheet that has been pulled taut. This is the theory of gravity. If a particle were to be in two places at the same time, it would therefore cause two curvatures in space time. Penrose argues that this costs extra energy, which means that superposition would cancel itself out in time. And the theory goes that the heavier the object, the faster this will happen. Penrose's argument is not a complete theory, and operates outside the standard laws of physics.

### Mirror

The experiment that Bouwmeester started seven years ago should prove whether Penrose is right. 'In our experiment we want to try to make a miniature mirror - the diameter of a human hair - oscillate and not oscillate both at the same time,' Bouwmeester explains. 'The mirror is in superposition, then. If we see that the mirror loses this superposition as a consequence of its mass, this will confirm Penrose's assumptions. 'This is by no means easy to do. It's an enormous technical challenge, because it has to be carried out in a vacuum, at ultra-low temperatre and without any disturbance.'

### Bizarre

If Penrose is proved right, this will shed new light on the relation between gravity theory and quantum mechanics, the two theories that scientists have been trying to combine for a long time. We need a theory that can bridge the gap between these two,' according to Penrose. Even if he is wrong, the outcome will still be remarkable. 'It would mean that there are no limits to quantum mechanics, and that we humans are also in different states at the same time,' says Bouwmeester. 'That's extremely bizarre, but by no means impossible.'

### Co-operation

The work being done by Penrose and Bouwmeester also inspired Tjerk Oosterkamp to seek the limits of quantum mechanics. His approach is different: he brings the stylus of a scanning electron microscope into superposition. 'It's an interesting idea, that needs to be developed further,' comments Penrose. 'My input may well have some significance.' Penrose is positive about the collaboration with the Leiden physicists. 'We have already had many fruitful discussions,' he comments. Now it just remains to see whether Penrose's assumptions really can be confirmed.

Lecture by Professor Roger Penrose:

*Are we Able to See Through the Big Bang, into Another World?*
*10 June 2011
*
More information

*(10 June 2011)*

### Links

- Roger Penrose Lorentz Professor 2011
- Vici subsidy for Dirk Bouwmeester (in Dutch)
- Bizarre double life in quantum mechanics (in Dutch)
- Staff page of Dirk Bouwmeester
- Tjerk Oosterkamp appointed Professor of Experimental Physics (in Dutch)
- Fundamentals of Science Research profile area
- Studying Physics in Leiden, bachelor's and master's