Gravity Waves

In Relativity by Brian Koberlein6 Comments

When most people think of waves they typically think of water waves. Drop a pebble in a calm pond and you can watch the waves spread out over the surface of the water. Waves occur in water because it takes time for the water displaced by the pebble to push against the surrounding water. The displaced water moves outward, which pushes water further out, which pushes water even further out, and so on. The energy of the displacement moves through the water at a particular speed. That “speed of water” varies depending on the depth of the water and other factors, but it is never instantaneous.

Light waves (electromagnetic waves for those who like precision) are a bit different. Light doesn’t need a material to travel through. You could say light is its own carrier. Still, if you make a flash of light, the waves spread out in all directions just like our pebble in a pond. And light makes waves for the same reason. Disturb an electromagnetic field, and it takes time for that disturbance to spread. The speed of that disturbance is what we call the speed of light.

In Newton’s universe, there’s no such thing as a gravity wave. Newton held that gravity acted instantly. Since Newton’s gravity acts at a distance, it has to act instantly. If it took time for the gravity of the sun to travel to earth, our planet would be attracted to where the sun was, not where it is, and our planet would go flying off into an unstable orbit.

But this leaves us with a bit of a problem. Special relativity says that nothing should travel faster than light, so how could gravity act instantly over large distances. The solution to this problem is of course Einstein’s theory of general relativity. In general relativity gravity is not an action over distance, but a bending of space and time. Our little planet doesn’t orbit the sun because its getting intant messages, it orbits the sun because the space it travels through is curved. That space is curved by the mass of the sun. In general relativity matter tells spacetime how to bend, and spacetime tells matter how to move.

But if you disturb spacetime by moving masses around, that disturbance takes time to propagate. There’s a speed to that spactime propagation, and it happens to be the speed of light. Since gravitational disturbances propagate at a finite speed there must be gravity waves.

gravitywavesIf we imagine a little water bug on our pond, we know it would bob up and down as the wave flows past it. Even if we couldn’t see the water, the bobbing of the water bug would tell us the wave was there. The same is true with gravity waves. We can’t see gravity waves, but we should be able to see their effect. In the picture above I’ve drawn what would happen to a ring of small masses as a simple gravity wave passes by (although I’ve made the effect much larger than it actually is).

There’s been a great deal of effort to detect gravitational waves this way, but so far we haven’t succeeded. But even though we haven’t detected them directly, we know they exist because of the way binary pulsars behave. But I’ll leave that for another time.

Comments

  1. 4th paragraph “intant messages” should be “instant.” We all like typing precision.
    Almost everyone gives a fluid dynamics analogy for gravitational waves. Most drop a stone in a smooth pond, you have a volunteer water bug. What if, a big huge giant if, the attractive force of gravity which we perceive as a stationary monopole is complemented by the repulsive force which does indeed travel the length of the universe instantaneously. We would need to amend relativity, “nothing (in the electromagnetic spectrum) can travel faster than light speed. All attractive force waves propagate inwardly making them really hard to see, repulsive force waves move so fast, making them nearly impossible to detect. Yep, it’s one of those big old giant “ifs,” that comes with inflation.

  2. What is the difference between space being curved or a gravity field of varying potential being present? This seems like linguistic difference for the same.

    The question of instantaneous becomes important because any change of a distance between gravitating body is immediately reflected by changing gravity. If an object moves radially from the Sun by 7 .8 kilometers in a second and the Sun is is ~8 light minutes away, does the distance change by 7 .8 km occurs within 1 second or within ~8 minutes and 1 second?

    How can you explain Tom Van Flanders claim that the Earth accelerates toward a point 20 arc seconds in front of the visible Sun, where the Sun will appear to be in 8.3 minutes indicating instantaneous propagation of gravity.?
    If that is true then no chances for gravitational waves which is so far consistent with accumulating failed attempts to detect them despite reasonable expectations.

  3. If the universe has to be stable, with objects spread out at Millions of light years apart, something must be responsible it. We know that from Earth frame, it takes 2.54 Million light year for the photons from Andromeda galaxy to reach us. Similarly, any gravity wave would also require 2.54 Million light year to reach us, if they travel at speed of light. However, for the photons and gravitons, due to length contraction, the time required to travel the 2.54 Million light year distance is “ZERO” in the reference frame of the photon or graviton. So, they”instantly” reach every where, Millions and Billions of light years away. According to GR, the distance reduces to ZERO for massless particles, when they travel at speed of light. So, can this explain the stable universe that is spread Billions of light year apart?

  4. Great stuff. I really enjoy reading your posts. Question: In the 2nd paragraph of ‘gravity waves’ you write ‘just like our pebble in a pond’ should that not say ‘just like waves in a pond’

    Thanks, Gor

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