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Comment by throwaway290 | Hacker News Reader

> The vast majority of matter in the universe is dark—it is entirely invisible and detected only through its gravitational effects

They state like dark matter is a fact. Isn't it a hypothesis?

> The FRBs shine through the fog of the intergalactic medium, and by precisely measuring how the light slows down, we can weigh that fog, even when it's too faint to see

Light slows down??

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Light is always light and has always the same speed but its path in a gas is less straightforward than in a vacuum because of the interactions with atoms. It takes longer to get through. Its speed as we can measure it is c divided by the refractive index of the gas, if I'm not wrong.

Same thing for light in a liquid or in a solid. Example: speeds of radio waves in networking cables https://en.wikipedia.org/wiki/Velocity_factor#Typical_veloci...

Yep. And then journalists wonder why people don't like them. How much harder would it be to write "measuring how much longer would it take for the light to get to us" without making people feel gaslit next time they are told speed of light is obviously always the same

Is assuming the audience has knowledge of fundamentals or the intellectual curiosity of their own to pursue it wrong? If every article had to describe the most basic aspects of physics, they would be books and significantly harder to understand due to breathiness.

If it was written for scientific audiences it would use proper terms instead of factually wrong "light slows down". It's not "easier to understand" when it is wrong. Speed of light does not change.

The speed of light in a vacuum does not change. The speed of light in a non-vacuum medium can be different than the speed of light in a vacuum, however. And light passing from one medium to another changes speed (and is refracted). See https://en.wikipedia.org/wiki/Refractive_index

Weird, those of us who knew what refraction was knew what they meant just fine. Saying light "slows down" in alternative mediums is the normal way to communicate it.

This was written on a college website and probably assumes an audience that is familiar with basic physics, since refraction is something we learn in grade school.

Also, who doesn't like journalists? I appreciate journalists a lot, even when they make mistakes. They provide a valuable service.

1. It is a phenomenon not a hypothesis. Dark matter is a collection of observational facts that indicate an unknown source of gravity. 2. Yes, in any medium lights slows down. This is what refractive index measures.

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