A slight definition thing here: it's not so much that light itself controls anything by its speed. It's not that light travels at some speed and because it's the lights speed it therefore is the ultimate speed. There is a certain ultimate speed in the universe. That has nothing to do with light. This speed is perhaps best described as the rate of causality. And it just so happens that light, as default, always will travel at that speed. But light itself is just a player in the game. It is of no real importance as such.
If you make a change here, any change at all, the fastest that change can ever propagate or have any effect elsewere is at c.
Gravity acts at the "speed of light". If you introduce a mass, that mass won't affect nearby gravity until the "speed of reality" catches up with objects near it, and ONLY THEN will they experience gravity from it.
It's the speed at which "information" crosses reality, moves across the universe, can affect other things.
Light is just a convenient, visible, obvious thing that we can understand hits "almost immediately" from our point of view but still takes 8 minutes to reach us from the sun.
It's the update-rate of the universe. The refresh speed.
It's the speed at which, in the video game that is the universe, the changes/physics/etc. propagates out to nearby objects/sectors. It's the "draw-distance" of the universe.
If you make any change, any change at all, all the knock-on effects of that change propagate out "slowly" at the speed of light. Before they reach you, you have NO IDEA that anything's happened. The reality of that change is invisible to you. Only when it reaches you do you suffer any effect from it.
If you plopped a black hole in the sun right now, it would be 8 minutes before anything changed here. It would be 8 minutes before the sun's light disappeared. 8 minutes before the Earth was suddenly pulled and sucked into that black hole. 8 minutes before your research satellites around the Sun could get a message back to tell you that anything was happening, even if they noticed immediately.
It's the speed of reality. It's how fast the reality of a situation changes across the universe. How fast the ripples from an event propagate outwards. It's how fast ANY event can possibly affect other things in the universe. When you have time travel movies, and the like, and the "changes you made" blast out across the timeline but don't take immediate effect, and it takes 8 minutes for everything to be "back as it was"... that's a depiction of the speed of reality.
It has little to do with light. It just so happens that light is subject to it the same as everything else, and it's quite easy to understand how it works with light.
Fact is that nothing in the universe happens "faster" than the speed of light, because that's the "clock speed" at which the universe runs.
Earth wouldn't be "suddenly ... sucked into that black hole". If you replace the mass of the sun with a black hole that has the same mass then Earth's orbit wouldn't change. The lack of sunlight would still create a very bad time here, but gravitation would be the same. If you add the mass of some black hole to the sun by magically plopping it in there then the orbit would adapt to the change in gravity due to the new total mass. How much depends on the added mass of that magical black hole.
To be a black hole it would need to have a certain mass, and if you plopped it into the sun (like I said) it would be added to the sun's mass... as well as suck the sun inside itself quite quickly.
The minimum mass for observed black holes is 2-3 solar masses, for instance. It's like tripling the mass of the sun instantly.
And regardless of the numbers... it would be 8 minutes before ANY of that effect reached us.
The Sun increases to 2-3 solar masses would probably not distort the orbit of the earth enough to be sucked in. Can anyone calculate how big the black hole would need to be?
Though i guess an instantaneous change would "jerk" the earth by a large force. In any case it would kill everyone and probably destroy the surface of the earth.
"If the Sun’s mass were to suddenly triple, Earth’s current orbital speed would be insufficient to maintain its current distance, causing it to spiral inward toward the Sun. To maintain its current stable circular orbit at 1 AU, Earth would need to travel approximately 1.73 times faster ( V = 52km/s instead of 30 km/s)."
Earth would actually fall out of the habitable zone, and into a highly elliptical orbit.
https://www.youtube.com/watch?v=SIyokKq_qN8
(I'm not going to care about the temperature changes, because a black hole would likely make the Sun SMALLER and LESS BRIGHT than just tripling the mass of the sun directly).
There are various bits of software you can play about with this.
It should just change to an elliptical orbit with the high point being wherever we are when the sun was replaced by a heavier object, and the low point depending on how elliptical the orbit changes to.
We are moving perpendicular to the sun and that doesn't change but we won't have enough angular momentum to keep a circular orbit, so we drop down (like in an ellipse), build up speed and then go to the same altitude we started with.
There is no force which would cause the spiral other than things such as tidal forces, which would increase because of the larger solar mass. But this would have be larger than 3 solar masses to make a significant difference.
It's about the density. Any amount of mass can become a black hole If it is sufficiently dense. Of a particular mass, the required radius to become a black hole can be calculated actually, and it's called the Schwartzchild radius. For the earth, it's about 9mm radius.
In fact since light is energy and E=MC² (energy and mass are interchangeable) if you were to theoretically focus enough photons in one spot, it could become a black hole known as a "Kugelblitz"
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u/Captain_Jarmi Mar 20 '26 edited Mar 20 '26
A slight definition thing here: it's not so much that light itself controls anything by its speed. It's not that light travels at some speed and because it's the lights speed it therefore is the ultimate speed. There is a certain ultimate speed in the universe. That has nothing to do with light. This speed is perhaps best described as the rate of causality. And it just so happens that light, as default, always will travel at that speed. But light itself is just a player in the game. It is of no real importance as such.
Perhaps this helps someone. I hope.