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Comment by benj111 | Hacker News Reader

So its kind of touched on in the lensing part, but gravity acts on light.

So the black hole is slowing down the light travelling towards us.... But the speed of light is a constant so... 'time' is slowed down?

So does the mean that the light is 'older' than the 54 million years it took to reach us?

I'm over 50% sure there's a flaw in my reasoning somewhere here...

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The light is redshifted when it "climbs out" of a strong gravity field but still moves through space at a constant speed. https://en.m.wikipedia.org/wiki/Gravitational_redshift

Ok that makes sense.

2nd stupid question: I though gravity acting on light was a 'light as particle' thing, rather than 'light as wave' thing? If that were the case gravity acting on light as particle manifesting in light as wave doesn't seem consistent.

I think most of the time it's best to think in terms of waves, but every now and then they exhibit particle-like behaviour. Any such attempt at simplification hits problems though.

I think the best approach is to be pragmatic and simply follow the observations and what the experiments tell us rather then force it into one paradigm or the other and complain when it doesn't make sense. This is sometimes referred to as the "Shut up and do the math" approach.

A photon does not switch from particle behavior to wave behavior. Physicists do. These are two different models for the same thing.

In some ballistics problems, you'll consider a flat earth (baseball problems) in some others a spherical one (spaceflight). It is about which model makes the math simpler.

I'd say physicists are pretty constant in their coffee seeking behaviour (the dominant operation mode).

A photon being a particle or wave is mainly about pedagogical convenience, it is neither but has properties of both.

Depending on your metaphilosophical models, you might also say that both waves and particles have light-like properties.

The equivalent of 'slowing down' for light is stretching out (increasing) the wavelength. It reduces it's energy in the same way that gravitic deceleration reduces an object's kinetic energy, but without affecting it's 'speed'.

The model of gravity as distorting 4D spacetime helps this make sense; just as atomic clocks run slower in Earth's gravity well, an atomic clock placed near a black hole would also run much slower. Similarly light leaving the black hole has to move through curved spacetime to get out of the gravity well.

This also circumvents all the wave/particle questions; it doesn't matter which conceptualisation you choose for the light, it's the space the light is moving through that is distorted.

http://blogs.discovermagazine.com/d-brief/2018/11/28/atomic-...

I think the answer is yes, and it’s called the Shapiro time delay effect, but I don’t trust my comprehension of anything about GR: https://en.m.wikipedia.org/wiki/Shapiro_time_delay

Also, not to confuse you more, but for light, time actually stands still since it is moving at the speed of light.

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