The universe as we know it is made up of three spatial dimensions, right? The size and location of everything can be plotted out on a graph of X, Y and Z axes – at least, that's what our everyday experience tells us. But some physics models suggest the existence of extra dimensions that we can't perceive. In an effort to find evidence of these dimensions, researchers have studied gravitational waves and come up empty.
Although it's probably the one we're most familiar with, gravity is the weakest of the four fundamental forces that govern the universe (the others being electromagnetism and the strong and weak nuclear forces). One proposed explanation is that some gravitons – hypothetical particles that would carry the gravitational force – "leak" into other dimensions over long distances, weakening gravity's influence here where we can observe it.
It's an intriguing idea, but how do you test it? According to a team of astrophysicists from the University of Chicago, you measure gravitational waves – ripples in the very fabric of spacetime – as they propagate across the universe, and see if the signals are weaker than they should be.
Gravitational waves were first predicted by none other than Einstein more than a century ago, but they weren't detected until 2015. That first event, as well as a few others in the years since, were caused by pairs of black holes merging, which are all but invisible to most other instruments.
But that changed last year, when astronomers were treated to a spectacular stellar show as two neutron stars collided. Along with gravitational waves, the explosion created clear electromagnetic waves, in visible and infrared light, gamma rays, X-rays and radio waves. This event turned out to be a far more useful tool for astronomy, since those different signals can all be compared to tell us far more about the event – and the universe – than gravitational waves alone could.
And so, the new study has used this data to look for evidence of extra dimensions. The reasoning goes that if they do exist, gravitational waves would leak into these dimensions, while the electromagnetic waves would be unaffected. That should create a discrepancy between the gravitational waves and the other signals, as the former would appear to be much weaker than the others.
But as it turns out, all of the signals, gravitational waves and all, were in agreement about the distance to the neutron star smashup. According to the team, that means that gravitational waves do propagate in three spatial dimensions, as general relativity predicts.
That doesn't necessarily rule out the existence of these other dimensions, the team says. They could still make their presence known by influencing gravitational waves that travel longer distances. What we do know is that 3D space still holds up on a scale of well over 100 million light-years.
As an interesting side note, earlier this year Japanese researchers searched for extra dimensions by testing gravity way down the other end of the scale – at the subnanometer level. They also found no evidence of hidden dimensions down to 0.1 nanometers.
The new research was published in the Journal of Cosmology and Astroparticle Physics.
Source: University of Chicago
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