Q: Why does “curved space-time” cause gravity?: A better answer.

Physicist: The original post is here.

The curvature of space alone has almost no effect on the movement of objects until they are moving really fast.  With the exception of only the most extreme cases (black holes), space is very, very close to flat.  For example, the total stretching of space due to the Earth amounts to less than 1cm.  The precession of Mercury’s orbit is another example of the tiny effect of the curvature of space (and it is tiny).  Literally, there’s a little more space near the Sun than there “should” be, and as a result the direction in which Mercury’s orbit is elliptical moves.  It takes a little over 3 million years for it to go full circle.

In almost all cases the vast majority of an object’s movement is tied up in its forward movement through time. The curvature of spacetime (not just space) is responsible for gravity. Literally, near heavy objects, the “future direction” points slightly down. So anything that moves forward in time will find its trajectory pointing down slightly.  This takes the form of downward acceleration. This acceleration (time pointing slightly down) is entirely responsible for the motion of the planets, and every other everyday experience of gravity.

In flat space traveling forward in time has no effect on your movement through space. In curved space (e.g., near a large blue mass) parallel lines can come together, and moving through time leads to movement downward.

It may seem a little confusing that, once you’re moving, the explanation doesn’t change and falling is still caused by movement through time.  Well, there is some effect caused by spacial movement and the spacial part of the curvature, but these effects are almost completely overwhelmed by the effects from the time component of the velocity (much, much bigger).

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70 Responses to Q: Why does “curved space-time” cause gravity?: A better answer.

  1. G Crawford says:

    Assuming that a free falling object on earth is not pulled by gravity but is seeking a future time direction how does that object know that is the future direction?

  2. Gerard. Scally says:

    This is for joe Tomas:
    Joe did you get my response about Meryl: this is not for R. Crawford

  3. Joe Thomas says:

    Hi Gerard,
    My apologies for delayed reply. I was not able to study Meyl after that !!
    But question remains why pharma cos dont want to pursue things that may work.
    Am on a tour so couldnt catch up. Shall email you as soon as im back to base.

  4. Cisla says:

    Apologies but if curved spacetime is The new space plus absolute time – and geodesic trajectories are the new straight line, this “tiny” curvature as you put it fails to explain macroscopic effects like why a ball thrown in the air comes down in as little a space as my bedroom… Space must be very very curved down here and time moving very slow for such a macro phoenomenon !

  5. G says:

    About this concept of less space around massive objects, it has been linked to π. As you would probably know, the circumference is just slightly more than 2 times longer when compared to the length of its diameter. For simplicity, let’s say, exactly 2 times longer than its diameter. If the diameter is 1 unit, the circumference would be 3 units.

    The question is: does this relation hold true for a perfect circle around the sun ?

    Please, provide a complete analysis mathematical or descriptive.

  6. Barry says:

    Wouldn’t it matter how far away from the sun as compared to a circle in flat (special relativit) space?

  7. larry leonardi says:

    all that exsists is a singularity

  8. Mantra says:

    Most here forget Einsteins Equivalence Principle etc …
    The Earth is accelerating at near or close to 68,000 m.p.h equivalent to 30 km /s or 18.5 miles per second ! around the Sun ….

    The Earth is spinning at 1,600 kmh or 1,000 mph .. on its axis … 465 meters per second or 1,525 feet per second ….

    When you toss an object up into the air its already traveling at 68,500 mph …. it has the kinetic energy of the earths acceleration eventually the Earth catches up to the object … The Earth and The Moon is in Free Fall around the Sun ” The whole Solar system is in free fall around the Galaxy … ! at 514,000,000 Miles per hour So you see Gravity equals acceleration + Curved space time ….

  9. G Crawford says:

    Don’t quite think that’s right as I understand it curved SPACETIME exerts a pressure on free falling object I.e. an apple and that kind of pushes it to earth while you’re on the earth stationary you are accelerating though time and you feel a push from the Earth on your feet or backside that you interpret as gravity. The General theory of relativity is highly mathematical I do not fully understand this took Einstein 10 years to figure it out the main maths is differential geometry and tensor calculus this he took 1 year to learn from his old classmate Marcel Grossman circa 1911-2 now a professor of descriptive geometry at his old uni.

  10. Gerard. Scally says:

    I am trying to write a reply to G Crawford . I am not a Physics scientists and I am angry that. “ASK A MATHEMATICIANS” who keep sending me e-mail and NO BODY ANSWERS QUESTIONS FROM REPLYS. I’ll try one more time.

    From what I can gather G Crawford thinks that Gravity is a force placed on falling objects but claims not to understand Einstiens General theory because it took him 10 years to develop. The fact that newtons F=MA. and GMM/R2 can predict weight and gravity well enough to land on the moon makes me wonder about the LINEAR ALGEBRA or whatever Einstein was using to predict why things fall. Are you falling when you get on a weight scale. NOW THAT II EXPRESSED MY CONSERNS I WOULD LIKE TO OFFER A NEW WAY TO LOOK AT THE DOUBLE HOLE EXPERIMENT AND THE SPEED OF LIGHT LIMITATION.

    If you consider that electron wave interference only occurs when you do not use light to look at the double hole experiment.then I say that adding the light energy changes THE SPACE TEXTURE Close to the electron beam causing the beam to stop bring a wave and become a particle beam. The electron are always there but in some space texture they are waves and in another they are particles. Which brings me the final suggestion tha speed of light is space texture specific so that instantaneous(faster tha night) changes of quantum entanglement are representation of information transfer using another space texture,

    Don’t worry Mr Crawford you don’t have to respond
    Gerry Scally. [email protected]

  11. Mantra says:

    There are two competeing Theories* 1 Newtonian Gravity 2 General Relativity ..

    Newtonian Gravity although very precise boils down to an approximation …

    General Relativity is more accurate for the past 100 yrs and not so long ago gravitational waves for the first time has been detected by LIGO ….

  12. Gerard. Scally says:

    I’m a Ockum’S razor king of guy. Relative speed although I clearly exit has no impact on local gravity—- Newton is good enough

  13. special relativity is calling that E= m c^2 e=energy , c=speed of light to follow this equation we can conclude that the force = momentum which diverts the meaning of both , another thing is that suppose we project two balls with the same velocity and have equal masses, at the same time, from the same point in the same frame of reference then the two balls should follow the same space -time curvature in their front movement and will come back following the same geodesic lines on their free fall then they should hit the ground at the same point and at the same time as if they are one mass , now let us consider one mass representing the addition of the two masses and do the same experiment by projecting the two original masses and project the some of the two masses then definitely they will not hit the ground at the same time and at the same point because they will not follow the same space time curvature and the same geodesic lines , i am afraid to say that the explanation of relativity theory to free fall may not seem to be as solid as in newton explanation that the force of gravity attraction is the reason behind free fall , and more over what happened to the acceleration of gravity which is 9.81 m/second square ,where is this constant in relativity ?, also time and space do not have the same units so to identify one point it has to be the point at the intersection of projections of all dimensions (x,y,z) all of the same units but how we can we project time to identify that very point if we are using space time system , please if you have clear understanding of the above mentioned points kindly advise – thanks for your anticipated cooperation

  14. Ryann says:

    Can the above be used as a justification to why time slows down the deeper we go in a gravitational field?

  15. mamoun mohamed mukhtar says:

    thanks for your message , if i get it correct space curvature has no effect on object movement unless it is moving very fast , suppose we have object on free fall with normal speed then if it is not the gravity and we already excluded space time curvature as above then what is it that makes that object to fall ? either we should admit that it is solely gravity or it is the object is following geodesic lines attributed to space time curvature , please kindly give us a specific verification assuming that objects are moving with only normal speeds

  16. Bernie Goetz says:

    Curved space seems like baloney. Does curved space effect a magnetic field?

  17. John Nolan says:

    I have never seen this explanation of gravity before, that it is almost entirely due to the curvature of time, as opposed to space. Could the author, or any kind commenter, direct me to other references which discuss this at length? ( Thanks)

  18. mamoun mohamed mukhtar says:

    then what causes free fall if not the gravity and not the time space curvature?

  19. Little curious says:

    Wait, Why is time causing gravity? To my understanding i thought gravity affected time. Could you please explain.

  20. BobJ says:

    Usually when someone can’t explain a complex topic in simple terms is because they don’t really understand it.

    I know that they try to say gravity is just a curvature of space/time but it also may be due to gravitrons. We have not found them but that does not mean they don’t exist.

    I prefer to think of gravity as a attractive force which pulls us down to the earth and it also can pull tug on the fabric of space/time. I don’t think of space time creating gravity its the other way round. More massive objects have more Gravitrons which attract things like a magnet even spacetime. Its weak so it takes something the size of the sun to bend it enough to see its effects. ie. Seeing a star that is behind the sun because the gravity of the sun curves the space time line.

    Much like a bowling ball on a sheet of rubber bends the rubber so you could roll a small marble at the bowling ball on the sheet and it might curve right around the edge of the ball.

    Just seems easier to think of gravity this way until someone figures it out.

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