Seems like the answer to "does light have mass" is a huge resounding "maybe."
this post was submitted on 26 Jun 2026
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It's "yes, but not rest mass." - light has energy and energy is mass. (Specifically m=E/c²)
In theory a sensitive enough scale could measure the difference between a charged battery and a drained one* without the battery gaining or losing any matter. Not even electrons, since each one leaving the negative end is replaced by one going in the positive end, and vice versa when charging. This is the type of mass light has, just pure energy.
*I did the math a while back and if I remember right for a typical AA size Ni-MH battery the difference would be about the weight of a bacterium.
So I'm just an idiot when I guess if a battery is good by weight vs other batteries I have. Two heavy ones must be good!
Maybe I should actually get a battery tester......
Yeah, just get a tester. In reality the weight difference is immeasurably small (like completely swamped by dust particles)
Just lick the batteries! No need for fancy tech!
So the answer is "kind of"?
Black holes warp the fabric of space so much that not even light can escape once it passes the event horizon.
It has nothing to do with speed, it's simply that all possible paths lead into the black hole. Photons travel through the fabric of space like a car on a road. The car can't go off the road, so if the road curves, the car will have to curve with the road.
There's a lot more to it, but the most simple way to think about it is that a black hole's gravity is so strong, it warps space itself. And this warping of space basically acts like a funnel that funnels everything to the center of the black hole. Once an object or particle or whatever passes the event horizon (the point of no return) it is basically being funneled into the black hole by the fabric of space being twisted into the black hole.
Forgive me still have a knowledge boner...no sarcasm. But in your comparison then if a black hole is like a funnel then at the edge of it has a certain speed and speeds up when you keep going deeper? So in someway (hate to bring up fiction) Interstellar was correct or semi correct in it's portrayal of a black hole going into it? (not the timey wimey stuff). But with gravity being so strong if it warps space then that mean's just looking at one should give a viewpoint of something that is distorted to all around it? But the question I ask is the event horizon of a black hole...is that the way we can view speed through that lense, but with that said any event horizon at least should have an escape velocity like we have on earth? ......on a side not thank you for having a civilized discourse with me. It's hard to find. ...much love and if I don't reply have a great weekend.
Interstellar was somewhat correct about the event horizon. In theory there also could be an escape velocity but since nothing moves faster than light nothing can escape it. Since light (speed). is the ultimate barrier for velocity.
And every mass bends the space, the small the mass the small the dent.
In science fiction and with a real theorem people "move" faster than light. But using the Alcubierre drive would require so much energy that it is (for now(?)) impossible to achieve. The drive also doesn't propel the spacecraft faster than light but bends the space around it, so technically you would more so move the space around you, instead of traveling though space.
In regard to speed, the speed of light is actually the speed of causality, which is the absolute limit at which one event can influence another. It is an absolute speed limit. Light can travel at this speed limit because it is a massless particle/wave. Anything with mass would need to have infinite energy to travel at the speed of light because the faster something is traveling, the more energy it takes to accelerate it further.
Inside the event horizon, these same rules apply because it is still space and time inside the event horizon. Even light that is traveling at the speed limit cannot escape. You just can't go fast enough to escape once you are inside the event horizon, because the speed limit of the universe prevents you from going fast enough to escape. Basically you would be on your way to being spaghetti-fied as you approach the black hole, or maybe not. There is a whole lot we don't know about the inside of black holes, so who knows what could happen in there. There are theories that say that super massive black holes could have different effects on things, but I only know of them and not how to explain them.
Outside of the event horizon, you can technically escape by accelerating away. But it would require so much energy that you wouldn't want to cut it close, I would think.
Gravity does bend space and that does cause light to curve along with space. If there's enough mass, it can bend space enough to essentially make a lens that can magnify light coming from behind. A black hole can bend space so much you can see the opposite side of its accretion disk above and below the event horizon.
If you're interested, I watch a lot of Antov Petrov, Cool Worlds, Dr. Becky, and PBS Space Time on YouTube. That's where I have learned most of this stuff. PBS Space Time can be a bit hardcore sometimes, but they have tons of information.
Since light cannot pass through a black hole does that mean light has mass?
No, it doesn't. Light is a wave. What is the weight of a musical note?
Also why does light form a singularity in a black hole?
It doesn't, the singularity is the name by which the actual mass of the black hole is known by. In short singularity is the mass in the middle, black hole is the phenomenon caused by that mass, but they're mostly the same thing.
Is that like a fixed point on a map or something?
Sorta, think of a black hole like a drain emptying a huge pool, you can feel it sucking the water the closer you are to it. The singularity is the drain, but from the outside you can feel the water being pulled from much farther away, and that's the black hole.
And can you travel to that fixed point after the black hole has its way with it?
What point? The singularity? No. That is the black hole, it's like asking whether you can travel to the sun after the star had its way with it, you're using two words that mean the same as if they were different things.
And if the velocity of a black hole is so intense that it exceeds the speed of light, then would that mean we have a new speed to consider?
Black holes don't have any velocity, they just are. Think on the drain example I gave, the drain is not moving, but the water around it is.
If so can you explain what speed is that is faster than light?
No speed is faster than light. Again, light is a wave, not a physical object, imagine the drain again, you're making ripples in the water, and you see that those ripples get near the drain and are "pulled down", you might conclude that the ripples are attracted to the hole, but in reality it's just that the medium they're moving on (water) is being pulled into it and so ripples on that medium get dragged along.
Light is a wave until it starts acting as a particle.....
That is an excellent breakdown!
- Light in a vacuum does not have rest mass, because there is no frame of reference where you can measure light at rest, because light famously always moves at the speed of light in any reference frame. You could define mass as including the kinetic energy, and physicists did at one point define mass that way, but it led to less intuition and more complicated math and definitions, so they dropped it to only really consider rest mass. (Good video on it here: https://www.youtube.com/watch?v=wiBsfvW5AWY)
- What exactly do you mean by the velocity of a black hole? Do you mean the velocity it's moving through space with respect to an observer? Is this about how the universe can expand at a speed that's calculated to be above the speed of light?
- Once it's in a black hole, it's not coming out.
This is a long playlist but I recommend watching the videos that jump out at you: FloatHeadPhysics is really good at helping explain relativity and quantum mechanics without complicated math and building intuition. https://www.youtube.com/playlist?list=PLawLaqps30oAcpVd4r-wj8hGodzpPRYTT
Look man Ive spent many years tryna understand this shit and in fact nobody does. Event horizons aren't really even a fixed place, and you could pass one without realizing if the BH is big enough. Then there's time dialation. Speed of light is relative to the observer
My rec is to expect no answers but if you find it fascinating, then spend the next year watching PBS Spacetime on Youtube and stay rock hard, nerd metaphorically speaking
Of course you need to read Einstein first. It all starts there.
For example one of his first articles https://dn760106.eu.archive.org/0/items/EinsteinsOriginaltextZurRelativittstheorie/Physik_Einstein_text.pdf
I think photon's don't have mass but they do have energy and that can mean they act in the same way? In that they are subject to gravity. That could be very wrong though, I'm going off my own non-scientist memory of a popular science book.
Light does have mass, it is just immeasurably small. That's how a solar sail would work: the light would be physically pushing the sail. It's not a lot, individually, but there's a lot of light hitting it, so it pushes it forward.