If scientists show us unintuitive, weird stuff that breaks every notion of common sense but is happening every moment in the Universe, and new age mysticism presents us with similar incredible claims; how do we tell them apart?

What gives theorists in science the right to far-fetched and perhaps improvable flights of imagination which, for us to do the same, would be categorised as un-scientific nonsense?

At the head of Speculation Patio, this article looks at a single experiment which strikes at any common sense view of how the Universe works, yet we can see with our own eyes that it's true.  And then comes the question of what gives scientists legitimacy to speculate about subjects like the Universe next door, if may never be able to test it empirically.

Challenge: Do you think speculation undermines science?  Do you think that scientific speculation justifies everything mysticism and religion have asked us to believe on faith?  Write your views in the Comments section at the bottom of this article.

The Double-Slit Experiment

There is nothing speculative about the experiment described below.  It is true because observation has shown it to be true.

Take a bowl with two slits cut into the bottom and fill it with sand, and place it on a platform of two bricks.  The particles of sand will form two piles on the ground.  This is how we'd expect particles to behave.

Now take two pebbles, and throw them into a bath of water.  Circles of ripples will form where the pebbles hit the water.  Where the ripples of the pebbles collide, we see in interference pattern.  Where the head of two ripples collide we see a bigger ripple; where the head of one ripple meets the trough of another, the two cancel and we see still water.  On the bath tub we see areas of columns of high water and columns of still water.  This is how we'd expect waves to behave.

The double-slit experiment made the same test with light, to decide whether light was a particle or a wave.  And the first result was interesting, but not mind-blowing: although photons of light are indeed particles, in the experiment light behaved as a wave. Light coming through two slits interfered and columns of light could be seen on a screen behind the double-slit.

Scientists wanted to know more, so they set up an experiment which would fire photons one-at-a-time through the slits.  Like throwing one pebble into the bath, there should now be no interference pattern.  But that's not what happened: as photons were fired individually through the slits, an interference pattern formed on the back plate.  It was as if each photon travelled through both slits at the same time and interfered with itself!

To get to the bottom of this mystery, a detector was set-up by the slits to see which slit (or both (?!#!)) the photo would travel through.  What they found was startling: with the detector switched on, the behaviour of the photons changed: instead of making a wave interference pattern on the back plate, there were just two columns: the photons were now behaving as particles.

Photons had laid down the gauntlet for the experimenters, who tried everything they could imagine to out-wit the sneaky photons:

• They turned-off the detector, and the wave interference pattern returned.  They turned it back on, and the photons behaved as particles.
• They left the detector switched-on but stopped recording the result, and the wave interference pattern returned.  They re-started recording, and the photons behaved as particles.

The double-slit experiment has been repeated many thousands of times by scientists who would achieve fame and fortune by disproving its implications.  Physicist Richard Feynman often said that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment.

What it shows us is that something as counter-intuitive, as objectionable to common sense as a particle being in two places at the same time is not a theory, but an inevitable conclusion drawn from evidence.

Theories Without Experimental Evidence

The Scientific Method is to let reality speak for itself.  From observation create a theory; create a falsifiable hypothesis and test it; analyse and draw conclusions.  Secure against mistakes and bias with repeatability by independent experimenters.  The Scientific Method has shown us the beginnings of the Universe, split apart the components of atoms and shown us what's inside.  It has weighed galaxies and read the geometry of space itself from the "last scattering surface", 300,000 years after the Big Bang.

And the tool which has given us the predictive power to make all those hypotheses which have been proved by experiment is maths. The mathematical model of Einstein's relativity has been experimentally validated infallibly, time-upon-time, even decades after his death.  Peter Higgs and his colleagues predicted a new particle 30 years ago, for which the experimental discovery has just now won him the Nobel Prize.

Credit is due to the theorists, but also to the experimentalists who have crafted exquisitely accurate devices to measure the distance from London to New York within a small fraction of a human hair's width.

But the age of experiment may be coming to an end.  There are experiments we can make in principle, even if they would be impracticably expensive.  But there are also experiments we have no apparent hope of running.  For example, how would we measure the physics of another Universe which is forever beyond our reach?  (Is it, in fact, beyond our reach?)

For the moment, we have mathematical models.  Theories made by you or I or the New Age Mystic down the street is that scientific theories - no matter how bizarre they sound when described in popular form, have mathematical consistency with the Universe we know.  If you have a pet theory, then do the maths and get the peer reviews, and we'll feature it here in Speculation Patio, until such time - if there comes such a time - when it can be validated by experiment.

Further Reading

These videos explain and demonstrate the Double-Slit Experiment:

• Derek from Veritasium re-runs the original Double-Slit Experiment...
  
  
   
• ...and then with single photons.
  
  
   
• A Royal Institution Video by Jim Al-khalili, where he tells the full spooky story of trying to measure the photon travelling through both slits at the same time.
  
  
   
• The technical stuff: Double-Slit Experiment on Wikipedia