An international team of researchers believe they've figured out why strange quantum behaviours occur at the level of individual particles, but aren't in the classical, everyday world. And it all comes down to Einstein's general theory of relativity and the effect of gravity on time.

The research also explains why the cat in Erwin Schrödinger's famous thought-experiment is alive the whole time it's in the box.

The idea behind Schrödinger's cat, which was thought up by the Austrian physicist in 1935, was to demonstrate something known as quantum superposition - this is where an object is simultaneously in more than one state until the moment it's measured. For example, a cat inside a box would be both dead and alive, and would only truly be one or the other once someone opened the box to look at it.

But while quantum superposition has since been demonstrated using atoms and molecules, we've never seen the weird quantum behaviour occur in anything bigger. As a result, researchers believe that something - most likely environment interference - must be suppressing these quantum behaviours in objects involving more than one particle. But now a team of researchers has shown that a phenomenon known as 'gravitational time dilation' is also involved.

Gravitational time dilation is the phenomenon that makes time slow down near a massive object, such as the Earth. This was predicted by Albert Einstein 100 years ago, and it's since been confirmed to exist using super-precise clocks. In fact, someone who works on the ground floor of a building will age slower than their co-workers who sit on the 10th floor - but only by around 10 nanoseconds a year.

But now researchers from Austria, the US and Australia have found that this same effect is stamping out quantum behaviour in composite objects made up of more than one particle.

All particles vibrate ever so slightly, but this new research shows that this vibration is affected by time dilation, so it slows down on the ground and speeds up at higher altitudes. At the individual scale this isn't a huge deal, but when many particles form an object, the effect suppresses quantum behaviour and forces objects to behave the way we expect in everyday life.

"We've shown how changes in time caused by gravity affect the ability of an object to be in different quantum states simultaneously," Magdalena Zych, a physicist from the University of Queensland in Australia who was involved in the project, told Stuart Gary from ABC Science. "Even the small effect of the Earth's mass will still impact the quantum state of an object, and the bigger the object the greater the effect."

The results have been published in Nature Physics and could shed some light on the interplay between quantum theory and general relativity, the two great theories of the 20th century, and may also explain some of the things we see happening in the rest of the Universe.

"It remains to be seen what the results imply on cosmological scales, where gravity can be much stronger," said co-researcher Časlav Brukner from the University of Vienna in Austria in a press release.

So far this work is all theoretical, and hasn't been observed in experiments, but the team proposes that wave-matter experiments in the near future could confirm their results.