Understanding the Speed of Gravity: The Role of Massive Objects in the Cosmos

Imagine a scenario where two massive objects suddenly appear on a distant point in the universe, say, 10 light years apart from each other. A compelling question arises: how long does it take for the gravity of these objects to affect each other?

Challenges of Instantaneous Appearance

The fundamental law of physics, general relativity, does not allow for the instantaneous appearance of massive objects. According to Einstein's theory, matter cannot travel faster than the speed of light. Thus, objects cannot assemble or explode at speeds greater than light.

However, to better understand the concept, we can consider a more thought-provoking scenario. Imagine two massive objects initially separated by a significant distance. These objects are in an unstable situation where a slight nudge could cause them to collide within 10 light years. This raises the question: how long will it take for the gravity of one object to affect the other?

Observational Insights

The recent advancements in gravitational wave detection have provided us with valuable insights into this phenomenon. Gravitational waves are ripples in the fabric of spacetime caused by the acceleration of masses, such as merging neutron stars. The detection of events like these has given us a concrete understanding of how gravity propagates through the universe.

In a notable event recorded as GW170817, the merger of two neutron stars each with a mass around 40 solar masses was observed. The gravitational waves produced by this event offer a precise constraint on the speed of gravity, revealing that it is indeed the same as the speed of light to within an astonishing accuracy of 3 parts in 100 trillion.

Based on this, the direct answer to our initial question is that if a gravitational event occurs in a given location, it would take approximately 10 years for the gravity of that change to travel 10 light years away. This result aligns with the predictions of general relativity, which stipulates an exact travel time of 10 years.

Contradictory Perspectives

Other perspectives on the question highlight the importance of precise definitions. For instance:

Travis Coleman's Perspective: If the massive objects are suddenly assembled from pre-existing matter in the shortest possible time, then the effect of gravity would theoretically take no time at all to propagate. Andrew Jonkers' Perspective: If matter is generated ex-nihilo (i.e., from nothing) and with no inherent gravitational field, then the effect of gravity would take 10 years to spread out 10 light years.

Both perspectives are correct, but they arise from different definitions of "suddenly appear." By carefully defining the scenario, one can derive the appropriate answer.

Conclusion

The speed of gravity remains a fascinating area of study within the framework of general relativity. While it aligns with our current understanding, the nuances of how gravity propagates in different scenarios provide deeper insights into the nature of space, time, and mass in our universe.