• Question: Is there any recorded evidence of the existence of white holes? And if so, would you think that they would be easier to observe and study than black holes?

    Asked by anon-255268 on 27 May 2020.
    • Photo: Rosie Hayward

      Rosie Hayward answered on 27 May 2020: last edited 27 May 2020 4:16 pm

      White holes would be easier to observe than black holes I assume, as matter can escape them. There are white hole solutions to Einstein’s field equations, but there is little evidence of their existence as far as I’m aware. The possibility of observing white holes was first pointed out in the 70s, but since then I believe there has been a single gamma ray burst, in 2006, which could be attributed to a white hole. More evidence is needed to confirm their existence at this point, but that’s all the more reason to stay interested in them!

    • Photo: Susan Cartwright

      Susan Cartwright answered on 27 May 2020:

      Currently, there is no evidence for white holes. They should be reasonably easy to see, since unlike black holes they should actually emit radiation. However, we’ve never seen anything that was only interpretable as a white hole (it’s been suggested as a possible explanation for one particular gamma-ray burst, but there are more conventional expanations for that (admittedly unusual) burst as well).

      There are solutions to Einstein’s field equations (the equationns of general relativity) that describe white holes, and physicists usually expect that anything that is allowed by theory will occur, if perhaps very rarely (this is often expressed as “anything not forbidden is compulsory”). But, of course, general relativity probably isn’t the complete and final theory of gravity (since it does not combine well with quantum mechanics), so perhaps white holes aren’t allowed after all.

      To “discover” a white hole, you would have to find something that cannot be explained by any known mechanism other than a white hole. We don’t have anything yet that satisfies that criterion: things that have been proposed to be white holes have all had plausible alternative explanations. That doesn’t mean that we won’t find such evidence in the future – that’s why research is interesting!

    • Photo: James M Monk

      James M Monk answered on 28 May 2020:

      It’s been a while since I studied any general relativity, but as I remember it a white hole is an artifact implied by a simplified mathematical description of a black hole. Relativity causes time dilation – a clock in a strong gravitational field runs slow relative to an observer outside that field – so from the point of view of an observer well away from a black hole it takes a literally infinite amount of time for an object to cross the event horizon and fall inside the hole. Of course, from the point of view of something – or someone – falling into the hole, it happens rather quickly, and they see the entire future history of the universe unfold outside the hole.

      In this simple picture, since the black hole already exists in the outside observers time, and has material inside it, that material would have needed to have fallen into the hole an infinite amount of time ago. This is where the white hole comes in – it is an inaccessible region of the simplified picture of a black hole that lies infinitely far in the past.

      In reality the black hole formation must be more complicated and does not need an infinite amount of time. The reason for that is when an object falls into a black hole, the event horizon of the combined black hole + falling object actually expands a little bit to meet the falling object. I imagine this requires a much more complex mathematical description, which removes the need for the white hole to actually exist. In the simple picture the white hole exists as an imagined infinite past to allow the black hole to be described by “simplified” mathematics (where simplified is still quite complex!).

      I remember that there is some interesting conjecture that the big bang, and hence maybe the universe itself, is a kind of white hole. This is in the sense that a black hole singularity – the very centre of the hole – always lies in the infinite future from the point of view of an outside observer. The big bang on the other hand is equally inaccessible as a singularity in the past, and might therefore be regarded as a white hole.

    • Photo: Nabil Iqbal

      Nabil Iqbal answered on 8 Jun 2020:

      There is no recorded evidence for a “white hole”; they’re actually quite different from black holes. While a black hole is a real thing that exists in the universe, most people believe a “white hole” is actually just a mathematical curiosity — it is a solution to the equations of Einstein’s general relativity, but nobody really believes that they should exist in real life.

      To understand why: it turns out that if you take a star, and wait till it runs out of fuel, it will sometimes eventually collapse into a black hole, under very general conditions. Two black holes colliding make a bigger black hole, and so on and so forth. But the same isn’t true for a “white hole”; there is no reasonable physical process that results in a while hole forming! So most physicists believe there’s no great reason to believe that white holes actually exist in nature, and any evidence for one would have to be very strong indeed!

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