# How does gravity slow light without an equal and opposite action? [closed]

This is regarding the gravity well sort of explanation. People claim that gravity doesn't slow light by saying it curves the road by which it travels but if space is indeed without properties and empty, it can exert no force upon light therefore the light would travel unless interacted upon by a force, there exists pushes and pulls ( though the "pull" of gravity is more an influx therefore a push as well) regarding types of forces so it would seem reasonable to assume that gravity interacts with light(push or pull), though perhaps in a manner of conflicting forces resulting in the observed "curvature". Similar to throwing a ball at a clockwise tornado, it might enter straight from one side but the left spin coupled with the straight momentum from the throw would guide it around the center and as it traveled through. (Referring to light passing though the field) I would assume perhaps this might provide similar data as the other theory due to the path of light still being curved but I was curious as to 1. what problems could be found in this theory 2. Regarding a black hole, could a possible explanation be that the spin/influx of the field at event horizon becomes so great that it travels faster than light thereby inflicting a force upon light that is greater than the force responsible for facilitating the existence of light. Like swimming through a stream that gets faster as you go until it's force is greater than yours. (Stream:gravity as swimmer:light)

## closed as unclear what you're asking by user10851, Brandon Enright, Colin McFaul, Kyle Oman, Kyle KanosJun 27 '14 at 15:13

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• Hi sir simplicity, and welcome to Physics Stack Exchange! It's not really clear why you would ask this question. Where did you hear that gravity slows light? Could you edit the question to expand on this? – David Z Jun 27 '14 at 0:43
• Gravity does not slow light. Newton's laws are not valid in general relativity/on a cosmological scale. What is it you are asking? Could you rephrase your question? – ACuriousMind Jun 27 '14 at 0:43
• Gravity does slow light as measured by a distant observer (local observers measure no change). If the question is whether the distant observer measures an equal and opposite reaction on the gravitating body then it's an excellent question. – John Rennie Jun 27 '14 at 7:00

If you measure the speed of light using the Schwarzschild coordinates then you will calculate a speed that is less than $c$ when the light passes near a massive body. In this sense gravity does slow light. There are already several questions hereabouts that discuss this.

The issue tends to be a bit controversial because there are differing interpretations of what happens. An observer making a local measurement, that is measuring the speed of light at their location, will always measure the speed to be $c$. The distant observer gets a different value because the coordinate systems of the distant and local observer do not match.

Anyhow, the light does exert a force on the massive body. If you shine a laser close to the Sun, so it is deflected by gravitational lensing, then the momentum of the Sun changes by an equal and opposite amount to the momentum of the light beam. The change is far to small to be measurable, but it is there.

However things are a little more complicated than this. The laser beam does not exert a force on the Sun directly. The light is responding to the local curvature of spacetime. In doing so it changes that curvature very slightly, and that change propagates through spacetime to the Sun where it causes the Sun to accelerate. The mechanism is the same as a massive body being deflected by the Sun's gravitational field.

• Fascinating, I didn't know that you can actually meaningfully (if controversially) talk about slowed light in empty space. Could you expand on how a distant observer can actually measure the speed of light far from him? I would naively think that this requires detecting the photon by some means, thus being not distant. – ACuriousMind Jun 27 '14 at 17:19

Gravity in General Relativity does not slow light down. What happens is, in the presence of a celestial body, spacetime curves around it. Light coming from behind the body will follow the curved path of spacetime as described here.

The speed of light in a vacuum never changes. However, there are some effects of gravity that could make it seem like it is slowing down to an outside observer.

Gravity can bend light by essentially changing the shape of a straight line in space-time. This could result in a slightly longer path than the light would have taken anyways, thereby "slowing" the light by increasing the total distance travelled.

Black holes seem like they slow light in that they have a point at which no light can escape their gravitational pull. However this isn't done by slowing down the light, rather it's a side-effect of the fact that as you get closer to the black hole the escape velocity (the speed at which you need to go in order to escape the gravitational pull of the black hole) increases until at some point (called the event horizon) the escape velocity exceeds the speed of light, and even it can't escape. But again, this doesn't slow down the light - all the light - even the light that can't escape from within the event horizon - will still be travelling at the speed of light (just think of it going around in circles if it's within the event horizon).

And that doesn't even take into account the time dilation at (and beyond) the event horizon. But even then, no matter how the fabric of space-time gets manipulated, the light going through it (in a vacuum) never changes.

The only thing that does (kind-of) slow down the speed of light is when it goes through a medium such as water or glass. The light still goes the speed of light, but it gets absorbed and then re-emitted a ton and (again, to an outside observer) looks like it slows down.

• So... are you saying that light travels in a vacuum (emptiness) or that it travels in the medium you called spacetime. Because gravity as a force cannot exert a force upon nothing... Also, it is my understanding that a force must be applied for an effect to occur, so how can a black hole pull faster than light while not requiring light to slow. I could only fathom light being able to be trapped if it were traveling at the same speed and thereby negated. – sir simplicity Jun 27 '14 at 3:38
• Space-time isn't a medium, like glass or air. It's not something that can be picked up, stored, or moved elsewhere. What you and I call 1 meter, 1 second, or a straight line (our concept of space and time) aren't absolute but can be different depending on where you are and what you are doing. Things with mass "bend" both time and space towards themselves, which adds a very small amount of acceleration towards other masses. This acceleration is gravity. The black hole isn't pulling on the light so much as it's twisting up the road. That's why so many call it "the fabric" of space-time. – Freudian Slip Jun 27 '14 at 7:07
• I hold that space cannot be curved, for the simple reason that it can have no properties. It might as well be said that God has properties. He has not, but only attributes and these are of our own making. Of properties we can only speak when dealing with matter filling the space. To say that in the presence of large bodies space becomes curved, is equivalent to stating that something can act upon nothing. I for one, refuse to subscribe to such a view. Nikola Tesla – sir simplicity Jun 27 '14 at 9:22
• Long ago [I] recognised that all perceptible matter comes from a primary substance, of a tenuity beyond conception and filling all space – the Akasa or luminiferous ether – which is acted upon by the life-giving Prana or creative force, calling into existence, in never ending cycles, all things and phenomena. The primary substance, thrown into infinitesimal whirls of prodigious velocity, becomes gross matter; the force subsiding, the motion ceases and matter disappears, reverting to the primary substance.12 – sir simplicity Jun 27 '14 at 9:27
• newdawnmagazine.com/articles/… – sir simplicity Jun 27 '14 at 9:28

It doesn't slow light but it does bend it and I have a hypothesis that it does so because it's "faster" than light----sort of. Think of a black hole where light emitted from its center would b pulled back down into it.

If u tie two toy jets with the same mass together in zero gravity n they pull apart at the same speed they'd move nowhere. Now if jet A travels just a meter/s faster jet b will begin to change its trajectory as it's now being pulled in the opposite direction.

Of course I could b wrong seeing as I know absolutely d*ck about physics but thinking about the laws of force here it seems gravity can reach speeds higher than c relative to its quantity.

This form can only be applied if u think of gravity as a wave like light. Now if u think of it as a force (which seems a lil more common sense[ish]) then gravity isn't faster than light but a force strong enough to disrupt an object with such a speed. But again, as force is directly relatable to speed one can argue the force behind gravity can pull faster than the speed of light given a big enough quantity of gravity (of course in this case mass and other variables must be factored in).

Again I kno d!ck about physics so this is purely abstract thought but u gotta admit it makes (common) sense as an abstract thought, no?

-my edit- Lol I said it was jus an abstract thgt I'm just getting to learn physics ease up on me fellas u all had to learn at the beginning too. Yes @Kyle thanks for the correction "directly related to acceleration" and also as u said it moves at the speed of light and I read this a few days back

http://www.csa.com/discoveryguides/gravity/overview.php

BUT! I'm still curious if the quantity (by quantity I mean the strength of gravity, I just like quantity better it starts w/ a Q) is relevant because velocity required to escape gets lower the farther u get from the center of anything because gravity's pull is weaker right? Well could it be the event horizon is where there's just enough gravity to reach just over the speed of light I mean c<√(2GM/R) can be true so y couldn't gravity be traveling faster than the light? Just what I c in the equations if I'm wrong it's cool I'm open to learn.

And yes light travels through the vacuum of space which is the space time medium from my understanding. Also I'm there w/ u I can't fathom light being trapped unless it's speed was matched or beaten as in the example of jets A and B. If at the same speed they're "trapped" and won't move and if A speeds exceeded B's then B gets pulled. Seems like some kind of BASIC physics but honestly idk any equations that will prove so for Anna's sake I'll stop it there lol.

• Gravity also moves at light speed. And force isn't (generally) directly relatable to speed (only in particular cases like drag), it is directly related to acceleration. – Kyle Kanos Jun 27 '14 at 2:45
• After the time of Newton abstract thought has small probability to describe the physical world. The language of physics is mathematics. If an abstract thought is not in the language of mathematics it cannot be discussed as a physics hypothesis. – anna v Jun 27 '14 at 3:09
• Today's scientists have substituted mathematics for experiments, and they wander off through equation after equation, and eventually build a structure which has no relation to reality. Nikola Tesla – sir simplicity Jun 27 '14 at 9:11