# Gravity Doesn't Work How You Think It Does

Most of us grew up believing that gravity is just the mass of two objects attracting each other. We probably still think that, really, since who spends their time reading up on gravity? Well, that's WRONG.

Our old friend MinutePhysics explains that gravity is actually based on energy and momentum, which means even light has gravity (yeah yeah, extra credit if you already knew that). So basically: Suck it, Newton.

And also? All of your sixth-grade word problems in maths were wrong, because they didn't account for the curvature of the Earth. [MinutePhysics]

Of course, gravity doesn't work the way he thinks it does either. Special relativity was superseded by general relativity, which included a redefinition of Newtonian inertia (which is the actual factor of gravity, not momentum) to work on geodesics that account for space-time curvature. The theories have been constantly refined since.

Or in short, the reality of gravity is way more complicated than most people are able to understand, including the video author. So instead, we use approximations that are less precise but equally as accurate in 99.9% of cases.

No, Special Relativity doesn't account for gravity at all. General Relativity is a generalization of Special Relativity that *does* take into account gravitation. General Relativity is the current theory used to describe gravity in modern physics. It's also pretty likely incomplete, but no one has devised anything better in all cases yet.

Your first two sentences are partly correct. Special relativity doesn't 'account' for gravity, but it does deal with gravity in the form of gravitational potential at average gravitational strengths. General relativity isn't 'special relativity + gravitation' though. The only thing 'special' refers to is that special relativity only applies in the special case of an inertial frame of reference. In fact, at non-relativistic strengths and distances special relativity is accepted for gravitational calculations by the scientific community in much the same way Newtonian physics is for medium scale calculations. General relativity applies to all frames of reference and does so in part by factoring curvature of space-time, and replaces gravitational potential with the metric tensor which allows it to work with calculations involving stronger gravitational effects, among other things.

As I said, special relativity was superseded by general relativity, which included a redefinition of Newtonian inertia to work on geodesics that account for space-time curvature. For the most part your response seems to be agreeing with me, so it seems a bit of a non-sequitur that you said 'no' at the beginning of it.

Last edited 08/12/12 8:22 am

this coming from zombie jesus.

Of course photons have gravity. If they didn't there wouldn't be black holes. They would just be called holes.

Sorry bud, but that is so far wide of the mark, I'm hoping you're just being funny.
I do so love reading armchair physicists who all pop up saying "I knew all about that" just after someone else says something.
Even if that something is also wide of the mark.

Black holes are called black holes because photons are unable to escape the gravity well they cause. If photons did not have a gravitational field (small though it is) they would not be effected by the gravitational field of black holes.

If you're so up with physics, why don't you explain why photons aren't affected by the gravity of black holes? Oh wait... they are.

Ok, time to get my arm chair physics on: Time slows down as gravity increases in a region of space. Is it incorrect to assume this plays a part in why light cannot escape from beyond the event horizon of a black hole?

Only the relative observation of time slows, and it's due to the acceleration caused by gravity rather than gravity itself. As far as the photon is concerned, its own perception of time hasn't changed at all, and it falls into the black hole normally and is annihilated. What Tony was referring to is the bending of light around the proximity of the event horizon, which causes gravitational lensing, rather than the light that actually passes through the event horizon and can't escape.

And yet, still way wide of the mark.
Photons = no mass
Keep guessing armchair boy.

What does mass have to do with anything? we are talking about gravity. things that have mass generaly have more gravity, but that is not always true.

Whatever education system the author went through is sorely lacking if these concepts weren't imparted by the end of it. Pratchett had it right with "lies-to-children". "A lie-to-children is a statement that is false, but which nevertheless leads the child's mind towards a more accurate explanation, one that the child will only be able to appreciate if it has been primed with the lie".

http://en.wikipedia.org/wiki/Lie-to-children

Indeed, a lot of the science that is introduced is simplified to make it easier to get the major underlying elements ingrained.

take chemistry for example.

First, its electrons in nice orbits, but its far closer to being an electron cloud.

The noble gases called such because they don't react with anything, except that every noble gas below neon on the periodic table can react with fluorine at the very least, even if it has to be in specific conditions.

So yeah, I do agree that while it may be lying, it is mostly about the complexities where unless you plan to work in the field, you don't really need to know about them.

I'm sorry but he didn't actually explain gravity to me in that little flic, and as far as I can tell, mass for the uneducated is easier to understand than relativity. So until someone explains it in terms my feeble mind can understand, big shit still pulls small shit towards itself and small shit is not as gravitationally massive as big shit.. mmmkaay..

In fact, we know very little about mass generation. Only theories. All very complex.
Funny to see, like a smartass trying to explain that Newtonian gravity is wrong. In fact it is correct to a scale. Same goes to general relativity, and quantum mechanics.
Until mass generation is a proven theory, this is all just a nice talk.

It bothers me a bit that it's a revelation that light is affected by gravity. Because black holes are described everywhere as having a gravitational pull that not even light can escape (and this is seen in that the black hole is completely devoid of light), it seems logical that it'd be fairly well known that light is affected by gravity.

On a slightly related note, I'm reading a great book at the moment about anti-gravity. I can't put it down.