Have you ever wondered why the trajectory of a plane rarely follows a straight line on a flat map, despite the plane actually flying on a straight path to its destination? That’s because the surface of the Earth itself is not flat but curved, and a straight path does not always match what our intuition suggests when working with this flat representation of reality.
Physicists encounter something similar when dealing with what they call space-time. In essence, ‘space-time’ is a fancy concept for describing where and when everything is located, and consists of the three-dimensional space we are used to fused with time as a fourth dimension (which makes us permanent travellers even if resting motionless in space, since we move along the direction of time). In space-time, curvature exists as well, but remains as elusive to us as Earth’s curvature does on a map.
Space-time tells matter how to move ...
So how does space-time curvature affect us? Nature is intrinsically ‘stubborn’: without an external force, a moving object would not change their direction of motion. If, like in the case of the aeroplane, an object follows a straight path in a curved geometry, an observer blind to this curvature (like us) would misinterpret the apparently curved path as a result of an external force acting on the object, seemingly redirecting it from going straight.
… Matter tells space-time how to curve
How does space-time get warped in the first place? It is matter itself that causes this effect. The more massive an object is, the more it distorts the ‘fabric’ of space-time.
The combination of this cause and the effect it has on the movement of other matter is exactly what Isaac Newton described in his theory of gravity as a ‘pull’ that objects with mass exert on each other. In the framework of curved space-time, this force is however not real, but like in the example of the plane and world map, just perceived by us. In fact, the Earth on its elliptical orbit is not attracted by the Sun, but rather follows a straight path through space-time from its position ‘here’ and ‘now’ to the same spatial location one year later. Similarly, the force Newton would have had to use to keep his famous apple from falling to the ground is not a counter-force opposing gravity, but rather the force needed to actively pull the apple onto a curved path in space-time that keeps it resting in its place through time.
These realisations, as part of his famous theory of General Relativity, are probably one of Einstein’s greatest achievements.
Did you know?
• Technology built into every modern smartphone can be used to measure space-time curvature
Satellite navigation relies on highly-precise timing signals from satellites high above the Earth. But as space-time is warped by our planet, time itself progresses more slowly the closer you get to Earth. This effect is usually negligible for the trilateration used to calculate our position, as the satellites are on circular orbits with fixed altitude and thus fixed clock speeds. However, a mishap during a launch for Europe’s satnav system Galileo in 2014 left two of its satellites on elongated orbits instead, climbing and falling about 8,500km twice per day. Researchers used this unique opportunity to track the changes in the satellite’s clock speeds and measure the space-time curvature, confirming the predictions of Einstein’s General Relativity to the highest accuracy yet.
For more trivia, see: www.um.edu.mt/think
Sound bites
• In 2017, the Nobel Prize went to researchers at LIGO for detecting gravitational waves. These two huge laser interferometers in the US look for distortions in space-time by measuring changes in the length of long tunnels with a sensitivity of the size of an electron. Besides warping space-time around them, Einstein further predicted that massive objects, when accelerated, also create distortions that travel freely through space-time – the larger the mass and acceleration, the bigger such ripples. In fact, LIGO’s first observed waves in 2015 originated from two distant, massive black holes closely orbiting each other before their final merger. This detection opened the door to a completely new field of astronomy, with a European site (Virgo) and a future ESA space mission (LISA) also competing in it now.
https://www.nobelprize.org/prizes/physics/2017/press-release
• As in the case of matter, the path of light is affected by the curvature of space-time, bending around massive objects. This lensing effect allows scientists to search for the elusive dark matter by looking for its footprint in space-time itself. For instance, it was successfully exploited to perform a cosmic ‘CT scan’ to reveal the otherwise invisible underlying structures of the cosmic web.
https://www.mpia.de/en/news/science/2014/09
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