The Shapiro effect is a gravitational time delay; light is slowed down when it travels through a gravitational field:


Is this the same thing as gravitational acceleration or deceleration of light as per the effect directly measured by Pound-Rebka? or is it a different effect that can only ever result in a time delay of light, and never a time reduction over its path?

Given the symmetry of the Shapiro radar experiment, where radar was bounced between Venus or Mercury and Earth, one might expect an equal amount of gravitational acceleration and deceleration along the path of the light - resulting in no delay. But Shapiro measured a delay?

An analogy: If I cycle my bike along a straight road with the wind blowing at 90 degrees to my direction of travel, I still experience greater drag than if there was no wind, even though none of the winds velocity vector is in the same direction as my direction of travel. Is the Shapiro effect somewhat similar, in that light will be delayed when travelling through a gravity field, no matter the direction of the light or orientation of the field?

Sorry if my analogy is a bad one, but hopefully someone gets what I am on about.


1 Answer 1


The speed of light is reduced near to a massive object.

If the object weren't present the light would cover the distance $d$ between its source and us in some time $t$ given by the usual expression:

$$ t = \frac{d}{c} $$

When a large object is in the way this changes the geometry of space time in two ways. Firstly the distance $d$ gets slightly longer, and secondly time is dilated near the object. These two effects work together to reduce the speed that light travels, so the average velocity of the light as it covers our distance to us is less than $c$.

The end result is that the time the travel takes is greater than $d/c$, and this is the Shapiro delay.

  • $\begingroup$ The question is "Is the Shapiro effect the same thing as gravitational deceleration of light?". I see what you are saying, do I infer that it is not the same thing? $\endgroup$ Commented Mar 24, 2017 at 22:05
  • $\begingroup$ @user2800708: the point is that light doesn't accelerate towards the massive object then decelerate away from it. The speed of light depends only on the distance from the object so so it decreases on both the approach and departure parts of the trip. The light actually slows down as it approaches the mass. $\endgroup$ Commented Mar 25, 2017 at 6:02
  • $\begingroup$ Is the light red-shifted by this time dilation? Or it keeps the same frequency just takes longer to get to its destination? $\endgroup$ Commented Mar 25, 2017 at 19:04

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