Cradle to Grave

In Physics by Brian Koberlein55 Comments

Gravity is perhaps the best known of the four fundamental forces. It’s also the one that’s easiest to understand. At a basic level, gravity is simply the mutual attraction between any two masses. It’s the force that lets the Sun hold the planets in their orbits, and the force that holds the Earth to you. The force is always attractive, and the strength of the force between two masses depends inversely on the square of their distances, making it an inverse square force. But gravity’s simplicity is just a veneer that hides a deeply subtle and complex phenomenon.

When Newton proposed his model of universal gravity, one criticism of the model was how gravity could act at a distance. How does the Moon “detect” the presence of Earth and “know” to be pulled in Earth’s direction? A few ideas were proposed, but never really panned out. Since Newton’s model was so incredibly accurate, the action-at-a-distance problem was largely swept under the rug. Regardless of how masses detected each other, Newton’s model let us calculate their motion. Another difficulty came to be known as the 3-body problem. Calculating the gravitational motion of any two masses was straight forward, but the motion of three or more masses was impossible to calculate exactly. The motion could be approximated to great precision, and was even used to discover Neptune, but an exact, general solution for three masses would never be found. Newton’s idea was simple, but it’s application was complex.

In the early 1900s, we found that gravity wasn’t a force at all. In Einstein’s model, gravity isn’t a force, but rather a warping of spacetime. Basically, mass tells space how to bend, and space tells mass how to move. General relativity isn’t just a mathematical trick to calculate the correct forces between objects, it makes unique predictions about the behavior of light and matter, which are different from the predictions of gravity as a force. Space really is curved, and as a result objects are deflected from a straight path in a way that looks like a force.

An approximate potential for quantum gravity. Credit: John Baez

An approximate potential for quantum gravity.
Credit: John Baez

But despite its simple approximation as a force, and its beautifully subtle description as a property of spacetime, we’ve come to realize over the past century that we still don’t know what gravity actually is. That’s because both Newton’s and Einstein’s models of gravity are classical in nature. We now know that objects have quantum properties, with particle-like and wave-like behaviors.  When we try to apply quantum theory to gravity, things become complicated and confusing. In most quantum theory, quantum objects exist within a background framework of space and time. Since gravity is a property of spacetime itself, fully quantizing gravity would require a quantization of space and time. There are several models that attempt this, but none of them have yet achieved a fully quantum model.

Usually our current understanding of gravity is just fine. We can accurately describe the motions of stars and planets. Seemingly odd predictions such as black holes and the big bang have been confirmed by observation. Every experimental and observational test of general relativity has validated its accuracy. Large objects with strong gravity can be described just fine by classical gravity. For small objects with weak gravity our approximate quantum gravity is good enough. The problem comes when we want to describe small objects with strong gravity, such as the earliest moments of the big bang.

Without a complete theory of quantum gravity, we won’t fully understand the earliest moment of the universe. We know from observation that the early observable universe was both very small and very dense. From general relativity this would imply that the universe began as a singularity. Most cosmologists don’t think the universe actually began as a singularity, but without quantum gravity we aren’t exactly sure. Even if we put the quantum aspects of gravity aside, there is still a part of gravity we don’t understand. Within general relativity it is possible to have a cosmological constant. Adding this constant to Einstein’s equations causes the universe to expand through dark energy, just as we observe. While general relativity allows for a cosmological constant, it doesn’t require one. The cosmological constant agrees with what we observe, but there are other proposed models for dark energy that agree as well (at least for now). If dark energy is really due to the cosmological constant, then the constant must be very close to zero, at about 10-122. Why would a constant be so incredibly close to zero? Why does it even exist when general relativity doesn’t require it?

We don’t know, and without that understanding, both the origin and fate of the universe remain mysteries.

Tomorrow: Electromagnetism was the first unified theory, combining the forces of magnets and charges. The result gave us a new understanding of light, and led us down a path toward a theory of everything.



  1. If Einstein showed that gravity is not a force, why is it said to be a fundamental force?

      1. Like everything else ,The Universe , outside the Universe , Our existence , Gravity is something that Humans don’t understand yet.
        I believe that we will always have ever changing theory’s but will never know for SURE what all of it is.

        1. Author

          That’s nonsense. We understand gravity quite well. Even if a better theory comes along, Newton’s model will work as well as ever, and so will Einstein’s. Your brand of ignorance peddling needs to stop.

          1. That’s just what I mean, you talk about a better theory, my point is there will always be a better theory that works at the time. All I am saying we “probably” (there’s that quantum term again) will never know the complete truth.
            The results for infinity >0< = prove it. LOL
            Best Regards

          2. then duplicate gravity in your lab and tell me how it comes out.

          3. Author

            Good grief. We’ve done countless gravity experiments in the lab. Arguing that we must somehow “recreate” gravity to understand it is like arguing that biology can be understood simply by producing a human in the lab. That’s easy, and only takes two willing test subjects and nine months.

          4. if that s nonsense just explain us WHAT GRAVITY IS… REALLY… NOT JUST HOW IT BEHAVES… know some formulas DO NOT MEAN WE KNOW HOW IT IS generated or REALLY WORKS… ITS LIKE QUANTUM MECHANICS… god…

          5. We understand and can describe how gravity effects the natural world. We still have NO idea HOW it does what it does. Warping the fabric of space is hyperbole, and doesn’t really tell us what it’s doing. What is the fabric of space made of? Is it the aether concept that was employed in the past.

            If you know what gravity is, then please do tell. I for one, would love to hear your explanation.

          6. I think we understand WELL what gravity DOES – just as we understand well what MAGNETISM DOES. If we understood it well enough – we’d be alble to manufacture it – and anti-gravity to boot – to my knowledge we can’t do either.

      2. Maybe light can only travel through the medium of what we currently call “spacetime”?? Since spacetime seems to be warpable, maybe we should give spacetime a name less confusing? I suggest “glorp”. Glorp is warpable whereas spacetime is …. huh!?

    1. All of the four fundamental “forces” are aspects of, and arose from, the original unified field. This field differentiated into all known forces when the universe expanded during the “Big Bang”, some of which in turn condensed into matter. As a result, the known universe is essentially chaotic, expanding in the macrocosm and contracting in localized regions called “galaxies”. This is why every galaxy has a “black hole” at its’ core. Thus, the universe is unstable, and has an inherent tendency to revert to the stable and coherent original unified field. In effect, the entire universe is reverting, although not in a smooth and coordinated way. Like a wild river, this reverting field gives rise to eddies and vortices, which may be expressed as violent forces or emerging matter. But ultimately, all matter and forces will revert to a coherent field.

    2. The Bib Bang is not observable as they state in the article. First of all no one was there to see it. Second of all it’s a scientific impossibility. How can nothing exoplode and create everything?

  2. I don’t think that I will agree that gravity is easy to understand. I think rather it is easy to experience and to measure. Saying we understand gravity is like saying we understand spacetime. As good as we are doing, we’re not quite there yet.

  3. Gravity is easy to understand but Einstein is partially wrong. Gravity is not an effect of spacetime warping, but spacetime warping is an effect of gravity. Also gravity is a force but one that has not as of yet been defined.

  4. we as humans are trapped in 3 dimensions of space and time, so our observations are skewed ,in fact it took 1 man Einstein and only 1 , to think out of the box when it came to understanding relativity an all its seemingly impossible outcomes ,so it is with all new avenues. God said I stretch out the universe and I bring into existentence things that were not before!

    1. Bob free..if you’re going to include space and time, “we as humans are trapped in 4 dimensions” (at least), as time itself is considered a dimension

  5. What if the presence of gravity is evidence of the existence of closed spacetime curves? Wouldn’t that fulfill gravity as not a force, yet still fundamental as a force due to the constant looping that closed spacetime curves cause? I may need help working this logic out, it’s at the tip of my mind but not fully out there yet..

  6. Para 5 “For small objects with weak gravity we our approximate quantum gravity is good enough”

    Delete “we”?

  7. Gravity is what keeps everything on planet earth from spinning off into space? Really?
    Spin any round object (big or small) and pour water on it and see what happens… it will fly off, and the faster it spins the faster it will fly off. I think that is called Centrifugal force. And the more mass, the greater the centrifugal force.

    Gravity holds the oceans on the spinning, at over a thousand miles per hour at the equator, globe? What holds the gases, our atmosphere on this rapidly spinning ball? What keeps our atmosphere from being sucked into the vacuum of space, gravity?

    Wouldn’t the centrifugal force (from the spinning globe) be much greater at the equator on any object residing there, since its spinning at over 1000 mile per hour, than at the poles, where the rate of spin would negligible? So gravity compensates for mass and centrifugal force? For it is strong enough to hold unfathomable amounts and weights of objects and mass, such as water, rocks and everything not nailed down to the earth at the equator, but is gentle enough not exert enough force to crush those same objects near the north or south poles? Please explain this to me.

    1. Author

      Yes, gravity keeps everything from spinning off into space, even the oceans and the atmosphere. If you say “the equator is spinning at 1,000 mph!” that sounds huge, but the Earth itself is huge, so the centripetal force is actually quite small. What matters is the total force, and even at the equator gravity is much stronger than any centrifugal force. A simple calculation finds that your weight at the equator is only about half a percent less than it is at the poles. If you weighed 200 pounds at the poles, you would still weigh 199 pounds at the equator. That’s still plenty of weight to keep you firmly planted on the ground.

  8. I believe photons have causality with relation to gravity. The relationship between matter and photons. Photons emitted by matter vs matter cancel each other out, creating a negative pressure between two objects, against the mass of photons matter is immersed in! positive pressure!! Gravity.

    1. Water is heavier than air, and boats hold the air within a cavity which allows it to float. (generally)
      Once a boat has water enter that cavity, it displaces the air, enveloping the cavity and filling it. Another way is to contain enough weight to overcome the buoyancy of the air contained within the cavity, pushing the boat under water to the point that it would enter the boat. That is how a boat sinks.

  9. Could gravity be a pressure on the surface area of a molecules hence the denser mass the greater pressure in human terms ( heavier) ?

  10. I’m pretty sure the confusion with ‘gravity’ is something we can blame on the great Albert Einstein. He was so far ahead of his time, in catching up, we have been working backwards. It’s been 100 years since his greatest work. If he was still alive, he would have figured out the details by now. Our science community tries to emulate his genius but do we know how smart he really was? He predicted black holes which we discovered and observed as cosmic vacuums, but they don’t actually “swallow” much at all. His construct of space-time has done more to restrict out observations than broaden them and this whole notion that ‘gravity is not a force’ hasn’t sunk in. This word “Gravity”, in cosmology, is a term created in ignorance and the use of the word adds to our vacuity. It is time scientists truly think outside the box as he did.

  11. Brian.. pleazzzzee

    Big Bang is still nothing but a theory, Black Holes are nothing but theory… these are [b]still[/b] the realms of theoretical physics.
    Your brazing of TomR shows your ignorance of science in general, the politics of research and popularist science.

  12. Gravity is causes by the bending of space-time (warping/time dialation) by a massive or energy concentrated object. When this happens it causes outter -space to accelerate to that center of mass and that is why we are pushed towards earth and why gravity is less on Everest than at sea level. Gravity is not attraction by objects. Gravity is actually push from outerspace. Measurements at different heights will confirm this. Of course, objects like the Sun and moon have a play in the gravitational push on earth.

    1. The push theory makes more sense to me – so how can we make localised warping of this incoming force ie bend the incoming force around us, make anti gravity boots – besides rockets etc do we know of any examples of anything escaping from this pushing force? even light is bent or overpowered by gravity

      And what about rockets? – in the atmosphere the expulsion of rocket fuel pushes against the atmosphere – why does a rocket motor make forward thrust in a vacuum?

  13. Thanks for a succinct and thoughtful write up. And Kudos for putting up with some of these fatuous comments.

    Maybe next we can tackle the curvature of the Internet Article and its tendency to capture the orbits of comments, of apparently any context density.

  14. Black holes are often thought of in terms of a whirlpool image, but no; that is too limiting! Think of them as points in space with omni-directional capacity, collecting space debris over time

  15. Might super massive black holes be “connected” by spooky action at a “distance”?

    Could gravity be a constantly produced phenomenon?

  16. What if gravity was a force similar to a magnetic , but instead of metal with rather water

  17. What if – all particles making up atoms are linked (strings) to all other particles in the Universe so that each particles can sense all other particles even over vast distances. This link (or string) has a pull on all other particles (gravity) with closer particles and larger numbers of particles having a stronger pull (i.e. Earth/Moon) or Earth/Sun or the weaker link between the Milky Way and Andromeda Galaxies, for example. The string would be an unknown force – yet to be discovered and waiting I guess – for a new “Einstein”. This would explain why the Moon knows it is in close proximity to the Earth and the appropriate pull (Gravity) would be directly proportional to the number of particles (atoms) making up the Earth and on the Moon………Just a thought. Robin Faulkner-Smith.

    1. As per gravity theory , Earth get attracted to wards sun and revolves around sun. The Moon is also nearly at same distance from sun. Then why does MOON does not revolve round sun as earth does but revolves round earth!. As per gravity therory . It only says it is force of attraction proportionate to mass of object and inversely proportionte to distance betweeen obsject, WHY THE MOON ATTRACTS TOWARDS EARTH AND NOT TOWARDS SUN ? WHEREAS EARTH ATTRACT TOWARDS SUN both being nearly at same distance from sun?

      1. Author

        The moon actually does orbit the Sun. It also orbits the Earth. If you look at the path of the Moon relative to the Sun, it is a slightly wobbly orbital path around the Sun. The wobble is due to Earth’s pull. If you look at the motion of the Moon relative to Earth, it appears to move around the Earth. There’s a good animation of these effects here.

  18. I think dark energy is billions of black holes created from the oldest galaxies in the Big Bang and on the fringe of the universe. These collapsed galaxies are all around us and the source of the gravity pulling younger galaxies toward them at an accelerating rate. That’s why the universe appears to be expanding at an accelerating rate in all directions. The fringe of the universe is mass heavy because more than half of all the galaxies ever created have already collapsed into black holes that are pulling everything younger toward them. They are also eating each other and that will cause the universe to warp in the direction of greatest mass, eventually consuming itself, or maybe not. Who knows?

  19. How does gravity pull or push a feather or a ton of steel at the same speed?

    1. Author

      In Newtonian gravity, the amount of gravitational pull is proportional to an object’s mass, so a ton of steel is pulled harder than a feather, making them fall at the same rate. This is known as the equivalence principle. In general relativity, the curvature of spacetime makes all objects fall at the same rate regardless of their mass.

  20. I find the study of gravity interesting but like many things in life I’ll never have the full understanding of it, so pardon the questions. I’m trying to learn, not criticize.
    If the condition of gravity is part of space/time curvature caused by mass warping, then what is it about mass that can warp space/time in the first place? Isn’t the flow of time the same on each pole as the equator, or does it slow down as it permeates the matter of the earth?
    If space/time permeates all things, then does it also permeate all dimensions? If so, then are some of these matter warpings that we assume are dark matter gravity actually originating in other dimensions? Again, if so, then are we chasing proof of dark matter when it’s really proof of mass in other dimensions?
    Thank you for your insight.

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