Though special relativity forbids any signal moving faster than the speed of light, general relativity allows the expanding universe to carry galaxies away from us so fast that light from those galaxies can never reach us.

Suppose, for the sake of a thought experiment, we have very powerful telescopes and very long lives. We watch the light from a distant galaxy red shift until...nothing.

Does that galaxy remain "a scientific object?" Can we ask "will a star in that galaxy supernova during the next N days?" or must we reject this as a scientific question because the light speed barrier renders either answer non-falsifiable?

Can someone point me to literature in the philosophy of science at least close to this question?

Also, a less philosophical version: What do conversation laws say, from an operational standpoint, if whole galaxies can effectively disappear for a given observer?

  • $\begingroup$ How does light no longer being able to reach us relate to any conservation laws? $\endgroup$ Commented Apr 15, 2019 at 20:12
  • $\begingroup$ physics.stackexchange.com/q/100911/25301, physics.stackexchange.com/q/107748/25301, etc. $\endgroup$
    – Kyle Kanos
    Commented Apr 15, 2019 at 20:26
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    $\begingroup$ This seems like a philosophy question followed by a physics question. The philosophy question is off topic. The physics question has been asked and answered previously: physics.stackexchange.com/questions/2838/… $\endgroup$
    – user4552
    Commented Apr 15, 2019 at 20:53
  • $\begingroup$ The red shift is itself a philosophical question since the red shift is calculated by comparing the chemical spectra of the galaxies to the chemical spectra in the lab and then inferring a velocity red shift of the galaxy. But the red shifts are quantized and the high red shifts are as sharp as low red shifts. Matter is quantized but the velocity of large objects doesn't appear to be quantized. In short, try posting on scif.stackexchange.com. $\endgroup$ Commented Apr 15, 2019 at 21:47
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    $\begingroup$ You will no doubt find more users familiar with the philosophical literature at the philosophy SE site. philosophy.stackexchange.com $\endgroup$
    – D. Halsey
    Commented Apr 16, 2019 at 20:16

1 Answer 1


Sure, no event from the objects past the Hubble radius will be able to affect you if the universe does not change drastically. If it was to start shrinking in the future, then what happened outside our horizon today could end up influencing our physics tomorrow.

It is correct to say that what happens outside the Hubble horizon is non falsifiable. Therefore it matters cum grano salis: you may use our physics laws to get an idea of what is happening beyond the Hubble radius (like "yeah, I am very positive saying that the star will go supernova N days after we see it crossing the horizon"), but you can never disprove anything, which makes those predictions completely useless for scientific purposes. That does not mean that it makes zero sense to say that the star will go supernova after N days, it means that you cannot use this statement to make any rational leverage on any scientific argument.

Conservation laws are a little advantaged in this matter: you can define a conservation law on a ball of a certain radius (like: the momentum change inside this ball is equal to the incoming forces through its boundary). By chaining a lot of balls (smaller than Hubble radius) together you can patch regions which are bigger than the Hubble radius and therefore formulating the initial conservation law on scales bigger than Hubble radius.

Even though the mathematics seems solid, this conservation law reasoning is non disprovable too: you see that a lot of matter is vanishing through the Hubble horizon; how can you be sure there is the same amount of matter on the other side after it went through? If one replies: "I suppose the conservation of matter works on smaller scales than Hubble radius and I patch a lot of conservation balls together", how can you go the ends of the observable universe and prove if he is wrong?

Even so, these conclusions are the ones we tend to take for granted because of the so called Hoccam Razor: instead of finding a new explanation as for why the conservation laws cease to be valid we just assume they are, since we have no reason to believe we are special observers. But could it be possible to falsify the statement that we are not special?

I am no expert in philosophy so correct me if I was mistaken, I just felt like giving it a shot. Unfortunately I am not aware of any literature besides obviously Popper's classic "Conjectures and Refutations", which you seem to be familiar with.


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