# What is gravity?

From an early age I thought that scientists knew what gravity is, the actual thing and not just the effect it has on us. One time, to my disbelief, I read somewhere that we do not know what it is. I actually have no memory of where I heard that we don't know what it is. Anyway, for the rest of my life from when I discovered that we don't know what gravity is till now, I have just assumed that that was true. We don't know. However, I recently had a teacher who said we do know what gravity is, I forget his explanation though.

So my question is: Do we know what gravity is? If so, what is it? If not, why don't we know, and what do we need to find out?

jtbell
Mentor
How would we know whether it's the "actual thing"?

Chronos
Gold Member
Gravity is the negative reaction of space to the positive energy of matter.

Fernando Freire
Unfortunately, we don't actually know for sure what causes gravity....or even that there is such a thing as gravity . There are theories that involve particles called gravitons but we haven't ever actually observed them.

The truth is that gravity is just a math model that that helps us predict stuff pretty well. Keep a close eye on cern...maybe we'll see some gravitons in our lifetime!

Chronos
Gold Member
Gravitons are virtual particles. We can only infer their existence by their effective action on mass possesing particles. That evidence is pretty compelling right now.

Gravity is the negative reaction of space to the positive energy of matter.

Unfortunately, we don't actually know for sure what causes gravity....

This is the real issue. We can describe what gravity is and what it does.

The reason it is there is the real mind blower.

Drakkith
Staff Emeritus
This is the real issue. We can describe what gravity is and what it does.

The reason it is there is the real mind blower.

I don't see this as an issue at all. I don't even think it's possible to know whether or not we know what something "really is". If we can accurately predict what will happen using a model, does it matter if the model and underlying theory is "really true" or not? Even if we were able to predict things with perfect precision and accuracy, how could anyone know if we "really" knew what something is?

I don't see this as an issue at all. I don't even think it's possible to know whether or not we know what something "really is". If we can accurately predict what will happen using a model, does it matter if the model and underlying theory is "really true" or not? Even if we were able to predict things with perfect precision and accuracy, how could anyone know if we "really" knew what something is?

I think perhaps the problem is that we don't fully understand how a force can act at a distance. I think that understanding how gravity acts at a distance is something that will help further our understanding and help build better models.

Drakkith
Staff Emeritus
I think perhaps the problem is that we don't fully understand how a force can act at a distance. I think that understanding how gravity acts at a distance is something that will help further our understanding and help build better models.

Care to elaborate on how we don't fully understand how a force acts at a distance?

Care to elaborate on how we don't fully understand how a force acts at a distance?

It was my understanding that the graviton is a theoretical particle?

Drakkith
Staff Emeritus
It was my understanding that the graviton is a theoretical particle?

Yes, it has not been observed. I'm not sure if the standard model of particle physics even predicts its existence or not.

Yes, it has not been observed. I'm not sure if the standard model of particle physics even predicts its existence or not.

And so virtual particles are what mediate the fundamental forces, yes?

So it's my understanding that a lack of understanding and a lack of knowledge of the existence of the graviton means that the understanding of how the fundamental force of gravity acts at a distance is not complete.

We have models like Newtonian gravity and General Relativity which describe how it works on macroscopic things, but on a quantum level, we don't have a complete understanding. Or at least, that is *my* understanding. I've only got a rudimentary education of basic quantum mechanics principles.

Drakkith
Staff Emeritus
And so virtual particles are what mediate the fundamental forces, yes?

Only in Quantum theories, not in GR.

So it's my understanding that a lack of understanding and a lack of knowledge of the existence of the graviton means that the understanding of how the fundamental force of gravity acts at a distance is not complete.

I think GR explains it pretty well.

We have models like Newtonian gravity and General Relativity which describe how it works on macroscopic things, but on a quantum level, we don't have a complete understanding. Or at least, that is *my* understanding. I've only got a rudimentary education of basic quantum mechanics principles.

Sure. But we don't know which one is correct for gravity, or rather which one is more correct and which way we should go to develop the next theory or resolve the discrepancies. Keep in mind that GR is the most accurate theory of gravity we have currently.

Chronos
Gold Member
Drakkith said:
I don't see this as an issue at all. I don't even think it's possible to know whether or not we know what something "really is". If we can accurately predict what will happen using a model, does it matter if the model and underlying theory is "really true" or not? Even if we were able to predict things with perfect precision and accuracy, how could anyone know if we "really" knew what something is?

I agree strongly. Physicists, when asked to explain what something IS, can only describe as best they can: what it does, how it works in relation to other stuff etc..... till the cows come home, if there is a strong need for clarity. Try describing what a bicycle IS, for practice!

For gravity, physicists have available for description the language of mathematics, including an esoteric kind of geometry-cum-tensor yak which so far provides the best predictive and quantitative description of this particular mystery. Backus should realise what she/he IS: an evolution-conditioned, hard-wired, all -talking, -writing and -describing species of African ape (great to BE one such).!

From an early age I thought that scientists knew what gravity is, the actual thing and not just the effect it has on us. One time, to my disbelief, I read somewhere that we do not know what it is. I actually have no memory of where I heard that we don't know what it is. Anyway, for the rest of my life from when I discovered that we don't know what gravity is till now, I have just assumed that that was true. We don't know. However, I recently had a teacher who said we do know what gravity is, I forget his explanation though.

So my question is: Do we know what gravity is? If so, what is it? If not, why don't we know, and what do we need to find out?

The "why" game, known to every four year old.

As a question. Whatever the answer, ask "why?" Repeat until the adult says "Because I said so."

The why game seldom gets beyond five answers. Two is fairly typical.

look think of a blast. what happens when a blast takes place? what happens to the things near the blast took place! every thing is pushed away! or we can say the blast was a distribution of high pressure into low pressure zones! Now think of its reverse. think of a negative blast! the blast will pull things! or low pressure zone will pull high pressure into it to balance pressure. Now if we see a negative blast has the same property of gravity! so maybe gravity is negative explosion that pulls mass!

Drakkith
Staff Emeritus
look think of a blast. what happens when a blast takes place? what happens to the things near the blast took place! every thing is pushed away! or we can say the blast was a distribution of high pressure into low pressure zones! Now think of its reverse. think of a negative blast! the blast will pull things! or low pressure zone will pull high pressure into it to balance pressure. Now if we see a negative blast has the same property of gravity! so maybe gravity is negative explosion that pulls mass!

Absolutely not. Please refer to the rules about personal theories and speculation.

You should have posted this one in the SR/GR sub-forum...

To me, general relativity implies that gravity is the fictitious force caused by the fact that we live in universe which is curved by the presence of energy.

As an example of this understanding, one should not say that light is bent by the sun, but rather light travels on a straight line (geodesic) which appears curved in the coordinate system we use to talk about events in the universe. Analogously, flights generally travel along mostly straight lines between cities but these appear curved on the maps we use.

The Earth is not pulling you towards it. As you hurdle forward in time along your timelike geodesic, the distance between your geodesic and the Earth's geodesic is shrinking because of the spacetime curvature. You feel a force because you push against the Earth so that you maintain a constant distance from its center.

If you go with the assumption that gravity (and dark energy) live in its own dimension and space/time is essentially the surface area on this extra dimensional bubble (gravity / dark energy) then:

When this extra dimension shrinks, you get the gravity effect on the space time (i.e. space time is pulled together). When the extra dimension expands then space/time stretches. The effect on space and time by gravity and anti-gravity (dark matter) is indirect. In the example of a bolloon (the surface is space / time, the inside is gravity's dimension) if the air is added inside the balloon, the volume (gravity dimension) expands and space time stretches (dark energy), when air is removed the gravity dimension contracts and the surface of the bolloon tightens (gravity).

Just my thought is that gravity is the dimension that comes after the 3rd and that time lives in either the 1st dimension or a dimension that comes before the 1st dimension.

This is the real issue. We can describe what gravity is and what it does.

The reason it is there is the real mind blower.

Drakkith
Staff Emeritus