If there are other universes, would our universe feel their gravity?
Mike Follows Sutton Coldfield, West Midlands, UK
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We can鈥檛 be certain. Recent astronomical evidence supports the view that the universe is infinite in extent.
At the same time, Einstein鈥檚 general theory of relativity suggests that our universe is finite but lacking a boundary or outer edge. As an analogy, imagine that our universe is the surface of a sphere; this would give it a limited size without a boundary.
This ushers in the possibility that parallel universes exist 鈥渂eyond鈥� our own. If this were the case, we might expect to see peculiar motion of galaxies in our universe as they are pulled towards a massive neighbouring universe.
Using the European Space Agency鈥檚 Planck spacecraft, scientists have looked for evidence of this in the cosmic microwave background radiation, but have found no sign of it.
Quantum mechanics, however, hints at a multiverse. The famous double-slit experiment shows that an electon behaves both as a wave and a particle, and can pass through a pair of adjacent slits at the same time.
In 1925, Erwin Schr枚dinger came up with his eponymous equation to describe this wave-like behaviour. The equation implied that many different states are possible, even if only one is seen. This was 鈥渟olved鈥� by the Copenhagen interpretation, which holds that the act of observing the particle banishes all the other states.
Later, Hugh Everett III devised his many-worlds interpretation of quantum mechanics, suggesting that all the states not seen in our universe are seen in parallel universes. A phenomenon called quantum entanglement would allow all the parallel universes to act in unison, each exhibiting a different state.
But the jury is still out on whether we live in a multiverse.
Roger Savidge Shoreham-by Sea, West Sussex, UK
The term 鈥渦niverse鈥� is widely accepted to mean everything that exists, which would clearly exclude the possibility of other universes existing, except in our imagination. Therefore, any meaningful discussion of 鈥渙ther universes鈥� requires a more restricted definition.
One such definition is that 鈥渙ur universe鈥� includes everything whose existence we can verify 鈥� in other words, everything that is capable of interacting with us, even if, like dark matter, it only does so gravitationally. Anything that affected us gravitationally would therefore be a part of our universe.
It is possible to conceive of the existence of a form of dark matter that doesn鈥檛 even interact gravitationally. Such matter and anything composed of it would truly occupy another universe, even if contiguous with our own in space and time.
鈥淥ur universe could be an insignificant dot in a multiverse of billions of galaxies of universes, and could feel their gravity鈥�
However, the existence of such other universes could never form the basis of a scientific theory since it is unfalsifiable. It can never be more than speculation.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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