By the time it reaches the top, the top wall of the spaceship is already moving away. Suppose the spaceship is not moving initially, when the light is emitted upwards from the bottom. Instead of thinking about light moving upwards against gravity, we can think of it as moving upwards in a giant spaceship that accelerates upwards at the same rate as the gravitational pull at that point. European Southern Observatory How starlight responds to a black hole The motion of the S2 star passing through the extreme gravitational field near the supermassive black hole in the centre of the Milky Way. Intuitively, we expect the light to lose some energy climbing away from the black hole. Consider light moving away from the black hole, say from a distance of 100m to 101m miles. In this case, we can use the principle to calculate gravitational redshift. Remarkably, the equivalence principle allows you to convert gravitational problems (like how does light behave near a massive object) to a non-gravitational problem (how does light behave in an accelerating spaceship without gravity). In both cases, if you drop a ball in the spaceship, it will accelerate downwards at 9.8 metres per second over each second that you watch it. This says that if you’re inside a closed spaceship without windows, you can’t tell whether you’re just sitting on the ground and feeling the Earth’s gravity or if you’re accelerating upwards in deep space, with no gravity. To understand it properly, one first has to understand the equivalence principle. Gravitational redshift forms the very heart of how general relativity works. The star with the fastest orbital speed has shown the gravitational redshift effect predicted by Albert Einstein. With new infrared technologies, scientists have managed to obtain sharp images of a dozen stars at the very centre, after corrections for blurring caused by the atmosphere. Such observations are difficult for many reasons, including the thick layer of dust between us and the galaxy’s centre. The second cause of redshift is gravity – and this gravitational redshift is the effect detected in the latest results from the international team led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany. However, the siren always sounds the same to someone in the ambulance. This explains, for example, why an ambulance siren sounds a higher pitch (frequency) to someone when the ambulance is approaching them and a lower pitch when moving away. In general relativity, redshift arises for two main reasons. Redshift is a term that describes how light appears redder (that is, at a lower frequency) to an observer, compared to the point it was emitted. This is the first time that astronomers have detected the effect in light from stars near the central black hole in the Milky Way. In July 2018, they revealed that they have directly observed the subtle “ gravitational redshift” effect predicted by general relativity, the leading theory of gravity developed by Einstein. For the past 30 years, scientists at the European Southern Observatory have been investigating the motion of stars near the massive black hole at the centre of our galaxy.
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