Light/Dark energy/Dark matter

Question

So I have been working on my comic book project that takes place on another planet. But in my story, there will be warp gates and there will be other civs that can travel between stars. I am determined to make my story as scientifically accurate as possible. I watched a number of videos to try to understand how time, space and light works but I surely don't have a grasp of probably... anything.

To my question: So nothing can exceed light speed. Light can be imprisoned by black holes' gravity. That means light is affected by gravity. So can light create a similar effect like gravity or anti-gravity itself? When it shoots its beam, it also backfires and whatever is behind it gets pushed back. Then this force looks so insignificant in small quantities that we cannot make any tests or get healthy results on Earth, because of Earth's gravity and lacking of necessary amount of light to test it. But universe-wide, considering universe is like a Christmas tree, all objects somehow emit light and get pushed back, like a helium balloon with a small hole. If an astronaut farts on Earth nothing happens, if he does it in space, he should start going(right? -.-')

OR, maybe light doesn't have a backfire effect yet it can push stuff. It can also clash with incoming light and create something like magnetic repulsion effect and that pushes anything behind them(it also has to require a huge amount to be able to test it).

Or maybe both? Any of these would explain how gravitational force is bigger than light's backfire effect when objects are close. But then after gravitational forces are escaped, light becomes the king and starts showing its secondary effects, pushing everything from each other. This would also explain dark matter and dark energy, why everything in universe getting apart, why scientists cannot solve light's secondary effect aka dark matter/dark energy because it requires zero-g and at least galaxy-wide lab to test.

Sitting here writing about it got me thinking, maybe light normally doesn't have a speed limit when there is no other light to clash. Now there is light everywhere and incoming light says "No buddy, you cannot go faster than me because I am incoming". Maybe 300.000km/s is the first gear for light and it wants to accelerate but incoming light stops its speed. Maybe 300k is the minimum speed it can go? That would kind of explain how universe got bigger, faster than speed of light in early stages, when there was no other light. This would also be impossible to test since it would require a planet big space in utter darkness I guess

Anxiously waiting for corrections. I plan to put this to my story if at least current scientific knowledge doesn't say a hard "no"

Ps: I am surely humbling myself but if anybody gets a reward out of this theory, I want the money you hear?!

Answer 1

Light is a form of energy, and like other forms of energy (such as "mass") it has a gravitational effect. Two photons travelling in space will be attracted to each other by gravity. Of course, the attraction is very weak, since the amount of energy in a photon is very small. In principle, you could make a black hole just by focussing enough light into a small enough region of space (This is called a Kugelblitz, and is strictly theoretical).

Light also carries momentum. So when something is hit by a beam of light, or when something emits a beam of light, it is pushed back. This is how a light-sail works. You can use sunlight to push a spacecraft, but you need a very big and very lightweight sail.

But photons have no charge and don't interact by the nuclear forces either, So photons can't "clash". And it is a hard "No" on "This would explain dark matter/dark energy". While we don't know what these are, we know they are not an effect caused by photons.

It is also a hard "No" on the speed of light being caused by interactions between photons. Although it is called the "speed of light" it really has nothing to do with "light". It is the speed at which information can be transferred, by any particle, and a fundamental constant. The interesting fact is that light always travels at this speed relative to any observer, no matter how fast the observer is moving relative to the source of the light.

Finally, the early universe had a lot more light than the current universe. The early universe was hot and dense. It glowed! As pela notes, until 50000 years the gravitational effect of light was greater than the gravitational effect of matter. The universe now is very dark, and light continues to travel at exactly the same speed.

Of course, it is your story. And if you have warp gates, then you aren't really doing "hard" science fiction. So if it suits your story you can bend the science as much as you want.

Answer 2

So can light create a similar effect like gravity or anti-gravity itself?

Yes, despite having no rest mass, light has relativistic mass, and therefore causes a gravitational pull. (edited)

I watched a lecture by Alex Flournoy on YouTube who pointed out nicely that it is not light that puts the limit on how fast things can go. Light is just an example of something without mass, and things without mass travel at the speed of light (roughly $3\times10^8 m/s$).

Maybe 300.000km/s is the first gear for light and it wants to accelerate but incoming light stops its speed.

An experiment that would test this idea would be to create an environment without light and then measure the speed that light travels through it, and compare this to the speed of light as it travels through a room with a lot of light. I think this experiment has been done many times, whether it was directly or indirectly.