<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>sclarisse</author><text>As others have noted, the speed of light (in a vacuum) will always constant. To be nice and pedantic, you can always slow it down by making that light go through a medium, where it will go slower (e.g. any prism that makes a pretty rainbow). It’s a cheat of sorts, but the speed of light isn’t some special property of light: it’s a property of spacetime. There is a maximum speed of anything at all, and unimpeded light goes this speed. For most astrophysics purposes this doesn’t worry us much, as space is essentially empty.&lt;p&gt;You can also “slow light down” by just making it go further: a few clever mirrors will do this easily. This is even more of a cheat, as the light itself isn’t any slower, it just gets where it was going a little later because it went further.&lt;p&gt;In a sense that’s what’s happening here: the spacetime is being stretched on one axis and squeezed on another, as gravitational waves pass through it. It isn’t by much, which is why we need a whole galaxy to measure it.</text><parent_chain><item><author>c7DJTLrn</author><text>So when we say that a star is N light years away and therefore it would take N years to reach at the speed of light, is that not true? If the speed of light is changed by gravitational waves.&lt;p&gt;How does this play into our understanding of the size and expansion of the universe?</text></item></parent_chain></comment><story><title>Evidence for a gravitational wave background from a galaxy-sized detector</title><url>https://astrobites.org/2023/06/28/drop-the-bass-evidence-for-a-gravitational-wave-background-from-a-galaxy-sized-detector/</url></story>
<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>nathan_compton</author><text>The way to really think about this is that there really isn&amp;#x27;t any objective thing except proper time and the manifold of the spacetime you&amp;#x27;re interested in. The idea of things taking an objective amount of time as seen by all observers is only an approximation valid at low speeds and low curvature in a small area. The only objective thing is the causal structure.</text><parent_chain><item><author>c7DJTLrn</author><text>So when we say that a star is N light years away and therefore it would take N years to reach at the speed of light, is that not true? If the speed of light is changed by gravitational waves.&lt;p&gt;How does this play into our understanding of the size and expansion of the universe?</text></item></parent_chain></comment><story><title>Evidence for a gravitational wave background from a galaxy-sized detector</title><url>https://astrobites.org/2023/06/28/drop-the-bass-evidence-for-a-gravitational-wave-background-from-a-galaxy-sized-detector/</url></story>