# Why isn't gravity faster than light?

## Curved light and stretched time

However, Einstein's general theory of relativity becomes really radical where it goes beyond the interaction of mass and gravity. Because in his universe of curved space-time even light and time are subject to the influences of gravity.

Atomic clocks - here an output only the size of a chip - are now accurate enough to measure the gravitational stretching of time. © NIST

### Great gravity - stretched time

As early as 1905, Einstein proved in his special theory of relativity that time cannot be an absolute quantity. The time expansion is a direct consequence of the speed of light as a fixed natural constant. So that it can always remain the same, regardless of whether a viewer races towards this light beam or stands still, the time must pass at different speeds for the respective viewer. The stronger his acceleration, the more pronounced the time expansion is for him.

And this is where gravity comes into play: With his equivalence principle, Einstein established that acceleration and gravity have the same effect. So if acceleration stretches time, then gravity must do it too. Consequently, time passes more slowly in the vicinity of large masses. And indeed: It is precisely this effect of gravitational time expansion that can be measured today with atomic clocks - even on Earth. For example, on a mountain peak around 3,000 meters above sea level, time passes around 30 nanoseconds faster per day than at sea level.

The curvature of space-time deflects the light - large masses bend the light towards them. © scinexx

### Diverted light

Einstein anticipated the effect of gravity on light in his thought experiment with the elevator. The invention of space-time curved by masses also explains why this happens. If gravity bends space-time, then a ray of light must also follow this bend, the physicist argues. Something like that was completely unimaginable in Newton's universe. Because light has no mass, it could not be influenced by gravity according to its law of gravitation - at least that was the unanimous opinion at the time.

Einstein's curved spacetime puts an end to this. According to her, gravity acts just as attractive on light as it does on masses. If a ray of light races past a heavy body like the sun, it is therefore deflected towards its gravitational “hollow”. Initially, however, Einstein's contemporaries did not really want to believe that this is actually the case in the cosmos. Only an astronomical experiment confirmed this effect and thus proved that large masses can even deflect light.