What is the speed of Gravity?

Speed of Gravity? Surely that seems an absurd idea, but I wouldn’t discuss if there wasn’t a catch in it.

According to Newton, Gravity was absolute, it is transmitted instantaneously between objects. ‘It doesn’t travel, it simply exists’. Einstein’s Special Theory of Relativity concluded that the Speed of Light is the ultimate speed, and nothing can travel faster though space, than the speed of light. Here, he was contradicting his hero, Isaac Newton, but Einstein’s theory was correct; even Gravity cannot travel faster than the speed of light!

EVEN GRAVITY HAS A SPEED?!

Yes, and in a more technical form, speed of Gravity means the speed at which gravitational field propagates or transmits. General theory of relativity concluded that Gravity, travels at exactly the speed of light in vacuum! Experimental measurements also hint at the same finding. This can be a difficult concept to grasp, as I have just ruined much of your understanding of Gravity, but please read on.

If you remember the model of Gravity (general theory of relativity) that I had discussed in ‘What’s wrong with Gravity?‘, it reveals that Gravitational pull is created because matter bends space-time.

Consider a spandex/ rubber sheet stretched to form a diaphragm. If an object is placed in the center, the sheet will bend, but not instantaneously. It will take some time for that bend to be transmitted to the edges as you can easily see in the below image! Similarly, the unbending also lags behind the performer having already left the trampoline. Just like ripples in water take some time to travel outwards and dampen.

Trampoline bending

In this analogy, the bending of trampoline fabric was travelling from center to edges at the speed of sound, as no information travels faster through matter faster than the speed of sound, but in case of space-time, it is the speed of light!

Gravitational waves propagate at the speed of light, and hence some fundamental particles might be thought of as mediating this force between objects. This particle is known as Graviton, which is a massless hypothetical particle since it is impossible to detect with our current instruments as it is weakly interacting with other particles. I had briefly discussed about Graviton’s in the ‘Standard Model of Physics and Theory of Everything’.

Few consequences of the speed of Gravity being limited:

Hypothetically speaking, assume that the Sun vanishes, instantaneously. Since light takes about 8:20 minutes to reach the Earth from the Sun, we will not know for about 8:20 minutes that the Sun is missing by either its light or gravitation. Our daily lives would continue as such until we receive the darkness. At the same moment, the gravitational field of the Sun will vanish for us, so the Earth will not revolve around the Sun any more and move in a tangent to its orbit in a straight line, but only after 8:20 minutes. Until then, the Earth will revolve as if the Sun still exists.

But if Gravity was instantaneous, then as soon as the sun vanishes, we would be expelled out of our orbit, but still see the light coming from the Sun for another 8:20 minutes or so. We wouldn’t know that the Sun has disappeared, and would be confused on why we are not orbiting the Sun any more, while light is reaching us. So, in a way it is good that the gravity takes the same time as light for it to reach from the Sun to us. This keeps a check on the Sun-Earth interaction I suppose.

If the Moon disappeared, tidal waves will still form for 1.62 seconds later. That’s the time light takes from Moon to reach the Earth.

Light from the Sun reaches Neptune, the furtherest planet from the Sun approximately 4 hours to reach, so at the instant when the Sun vanishes, Neptune will keep on orbiting the vanished Sun for another 4 hours as if nothing has happened!

The Solar System, is about 27000 light years from the center of the Milky Way Galaxy, orbiting the Super Massive Black Hole (Sagittarius A*). If Sagittarius A* suddenly vanishes, the Solar System will keep orbiting the center for another 27000 light years as if nothing had happened! Of course you wouldn’t be alive to observe it!

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