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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54 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.
    Joe

  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 !

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