If light has no mass why does it get sucked into black holes?
In: Technology
One way to look at it is that gravity warps the “fabric” of space. A massive object in empty space makes “straight” lines bend toward it. And the more massive the object, the greater the warping and the more those lines bend.
And just like a ball rolled or kicked along an uneven surface, the faster you go, the less this warping affects you.
Light moves really fast, so the effect is relatively small. But black holes’ mass is enormous, so it bends those straight lines enough to capture light that comes too close.
Gravity doesn’t pull on light the same way that it pulls on masses. But gravity isn’t just an attraction between masses, it is more properly understood as a warping of spacetime.
What this means is that while an outside observer might see light as being bent around a black hole, from the point of view of the light it is always traveling in a straight line as it feels no attractive force from gravity (not having mass). Instead space itself is bent, such that a straight line near the black hole isn’t parallel to a straight line a greater distance away.
Light is pulled in and trapped by the black hole because space is warped in such a way that there are directions that lead into the black hole but beyond a certain point there are no directions that lead out. This point of no return is called the “event horizon”.
Another way to look at is that light follows the plane of space (this is really simplifiex) and black holes warp that plane.
Think of a bed sheet pulled tight across a bed. You roll a golf ball across the middle it it will just go in a straight line. Now, if you place a bowling ball on the sheet, when you roll the golf ball, it will roll closer to the bowling ball becaue of the depression it makes.
The “depression” in space that black holes make are infinitely deeper than the bowling ball on the bed.
Mass creates gravity. Gravity pulls on all objects equally, regardless of mass. A feather and a brick will fall side by side if dropped in an area without air. Light falls the same way as well.
Things only get weird when you realize that light’s speed cannot change, so it must somehow fall without slowing down.
There are many theories.
It’s possible light has a very low mass that we can’t measure.
It’s also possible gravity affects not only mass, but also energy and momentum (thus light)
While light does not have a mass, it is still a particle of matter so it can still be pulled into the black hole. The point of no return is what is known as the event horizon. Once something passes the event horizon, it can no longer escape. The size of the event horizon is determined by what is know as the Schwarzchild radius of the black hole. This is why you could survive crossing the event horizon of a supermassive blackhole (millions of solar masses) However you could not survive crossing the event horizon of a stellar mass black hole. The tidal forces at the event horizon of a stellar mass black hole are much stronger due to the event horizon being closer to the singularity, or the “center point”, of the black hole. Also, black holes (or any other massive object) bends space-time around it. This effect is the primary mechanism of Gravitational Lensing, which allows you to see what is on the other side of the black hole, albeit an incredibly warped picture of it
Lights traveling in a straight line. But that straight line is bent towards black holes because mass warps spacetime. And black holes have a lot of mass, so it bends spacetime a lot
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