In the case of black holes, lights are pulled into by great gravitational force exerted by the dying stars (which forms into a black hole). If light has no mass, how is light affected by gravity?
In: 784
Fun titbit: we actually don’t know for sure that photons have no mass! I don’t think they have mass, and hear me out before you down-vote this post for being “clearly wrong”:
First off, we indeed have no good reason to expect photons to have any rest mass and Occam’s razor thus suggests to believe they really do not have any. Massless means always moving at the speed of “light”, but in the Theory of Relativity c is actually the speed of causality (and gravity, but that is a similar can of worms). Conversely, having mass means to never be able to move at full c.
Actual real light _could_ always be slower and it is in many cases such as in a medium, for example air or glass; that’s why c is _at best_ the speed of light _in vacuum_.
But what if photons have an extremely tiny rest mass after all? Would physics break? Actually not, things just become much more ugly and complicated (see below). But reality ultimately doesn’t care about our beauty standards.
We actually other particles that by our best guesses _should_ have a rest mass, yet so far we never saw any, nor were we able to measure any difference between c and the speed they move at: neutrinos. Or rather, at least two of the three neutrino types. We know this, assuming our physical understanding, because they can turn into each other over time (called “neutrino oscillation”), and this implies that the three types must have different rest masses. Hence at most one of them can have none, the others must have some!
As said above, this is the only proper evidence for neutrinos having any mass at all, other attempts to find a difference have failed. So could the same be true with photons after all? Might we just not have found the proper experiment that shows they also must have mass? There are even other examples of numbers in physics that _seem_ to be exactly 0, but beyond it being really, really neat, we have no confirmation.
What our experiments show is that the mass of photons, if any, is absurdly small. We can find bounds on it in many ways, from direct measurements to theoretical predictions such as the maximal range of electromagnetism: the range of a force is limited by the particles it acts with, _exchange particles_, and only massless ones allow infinite range; that’s why the nuclear forces only act in small distances, the respective particles have mass, while photons are the exchange particle of electromagnetism. Yet we can only measure speed to some precision and electromagnetism only is confirmed to act within millions of light years. Even if it acts over many billions of light years that is still not enough to confirm true infinities.
People also often say that this or that implies that light _must_ move at c in vacuum. But those statements are always based on theories that have this as a basic assumption, an axiom, to begin with! (Interestingly, the theory of relativity does not truly need this assumption.)
When googling for such arguments (try it!) one finds fancy-sounding argument involving words such as “gauge invariance” (that is a hard one to explain properly, so I will just blackbox it), and indeed the latter would ultimately only work properly if photons are massless. But maybe… we are wrong and there is no “gauge invariance”?
Physics can be adapted to work with a mass-y photon. As said, it would just be much more ugly by our standards. Most things should remain unchanged, at least within any measurable approximations.
So, should we really think light moves slower than c, and photons have rest mass? No, most likely not. Multiple meta-physical reasons such as Occam’s razor tell us to assume the simpler theory, which clearly is massless photons. But it also tells us to always keep an eye out for the small chance that this assumption(!) is wrong.
It’s not bending the light. It is bending the structure of the surrounding spacetime the light is moving through. The universe the light is propagating through is what’s being “bent.”
The fact that something is there, that affects the space-time, is enough. Light does not need any mass to bend around a gravity field, because space itself does it, the light is still happily travelling “straight” forward.
Gravity isn’t a force. It’s a manifestation of curved or perhaps flowing spacetime.
Everything that exists within spacetime follows the curve/flow that is represented by gravity.
Draw a line on a piece of paper, all the way across
Then fold the paper where the line is folded too
Black holes work in a similar way to that, where light still travels in a straight line but the space around it doesn’t really want it to
Gravity doesn’t bend light because light doesn’t weigh anything; gravity bends the road light travels on. You specifically asked about black holes. Anything heavy sitting on a bed makes a dent in the bed, which bends the stripes on the bedspread around it. Anything heavy in space makes a dent in space, which bends the rays of light around it. A lightweight dent is shaped like a bump. A really heavyweight dent is shaped like a funnel. A really, really, really heavyweight dent is shaped like a hole. The bedspread stripes on the sides of a hole run in circles around the hole and never lead back out. That’s a Black Hole: where space is bent so much that all “straight paths” inside it are bent into circles that never point away from the center. It’s like laying a balloon flat, crawling inside it, drawing straight lines on it, and having someone else inflate it and tie it shut with you in there. All the lines that were straight when you drew them now just lead around the inside of the balloon. That’s “bent space”, a black hole.
Imagine you have a big bed sheet, and 4 people are holding it tight at each corner. This creates a large, flat, plane — imagine this is the “fabric” (no pun intended) of space.
Now, cut a piece of yarn long enough to stretch straight across the sheet in any particular direction you like – this represents a particle of light traveling through space.
Now, take a softball or some other heavy ball and place it on the sheet. It will pull the entire sheet down, and the straight piece of yarn will now appear not straight. It’s still straight – you placed it straight, but the fabric underneath the yarn is now warped by the mass of the ball weighing on it.
This is how space works. Massive objects dont “pull” on the light with their gravity, they change the underlying fabric that the light is moving through. They pull the fabric of space “down” so that light moving in a straight line is now “bent” around the massive object as it follows the curve the heavy object has created. We call that a “gravity well” but its really caused by the mass of the object.
ELI5 best effort….
Draw a straight line on a piece of paper, that’s your light.
Now bend the paper, that’s space time bending due to being in the proximity of a mass.
Your line is still straight.
It doesn’t bend light itself, it bends the space-time continuum, like when a stretched out fabric gets stretched and compressed by items you put on it.
Gravity is not a force; mass just bends space-time. It seems a minor difference, and given how long we’ve worked successfully with the force assumption, in some sense it is. But that assumption doesn’t explain why light is clearly observed to bend around black holes — which is why we needed a new one that does.
Latest Answers