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--------------------------------- 2609 - LIGHT - how to measure its speed?
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- If the Sun were to suddenly wink out of existence, we’d still receive light from it for just over 8 minutes after it disappeared! But what about gravity, and the Earth’s orbit? Would the Earth simply fly off in a straight line?
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- Or, would the Earth continue to move in its planetary orbit for some time, and perhaps suffer some more interesting effects? Believe it or not, this is one of the most severe differences between Newton’s old school theory of gravity and Einstein’s General Relativity. Newton says if you have two masses separated by a distance, and that determines the force. You take one of those masses away, and the force goes away. Instantly.
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- According to “general relativity“, things are much more intricate, and very different from the simple Newtonian picture. It isn’t mass that causes gravity. Rather, all forms of energy, including mass, affect the “curvature of space“. The incredibly large mass of the Sun dominates the curvature of space, and the Earth travels in an orbit along this curved space, just like all the other bodies in the Solar System.
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- If you simply removed the Sun, causing it to wink out of existence, what would happen? In general relativity, it’s true that space would go back to being flat, but it wouldn’t do so right away at every point. In fact, just like the surface of a pond when you drop something into it, it snaps back to being flat, and these disturbances send ripples outward!
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- In Einstein’s theory of gravity, these ripples move at the speed of light, not instantaneously. This tells us that the distortion of spacetime due to matter and energy ought to propagate at “c“, and that the speed of gravity ought to be equal to the speed of light in a vacuum.
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- It turns out, that while Newton doesn’t care what your velocity is, Einstein does. The Sun, as it is right now, won’t have its gravity affect Earth for another 8 minutes, and the gravity that the Earth feels right now pulling it towards the Sun is actually pulling it towards where the Sun was 8 minutes ago!
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- The changes in the gravitation that Earth experiences is due to the fact that the positions and momentum of all the objects in the Universe, including the Sun, are changing over time, changing the curvature of space in our vicinity.
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- If that was the only thing that was different from Newtonian gravity, Einstein’s theory would be wrong. The predictions that we’d get for planetary orbits, based on where objects like the Sun and the other planets were 8+ minutes ago (or whatever the light-travel time for the planet in question was) are different enough from even observations a century ago that General Relativity would have been determined to be false right away.
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- This effect on its own demanded that, if Newton’s theory was right, the speed of gravity be at least 20 billion times faster than the speed of light!
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- There is another piece to the puzzle. The Earth, since it’s also moving, kind of “rides” over the ripples traveling through space, so that it comes down in a different spot from where it was lifted up. It looks like we have two effects going on: each object’s velocity affects how it experiences gravity, and so do the “changes” that occur in gravitational fields.
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- What’s amazing is that the changes in the gravitational field felt by a finite speed of gravity and the effects of velocity-dependent interactions cancel almost exactly! The inexactness of the cancellation is what allows us to determine, observationally, if Newton’s “infinite speed of gravity” model or Einstein’s “speed of gravity = speed of light” model matches with our Universe.
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- We know that the speed of gravity should be the same as the speed of light. The Sun’s force of gravity out here, by us, is far too weak to measure this effect. In fact, it gets really hard to measure, because if something moves at a constant velocity in a constant gravitational field, there’s no observable affect at all. What we would want is a system that has an object moving with a changing velocity through a changing gravitational field.
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- That would take something with intense gravity, like a neutron star orbiting another stellar-mass object extremely close together! Occasionally, we get very lucky, and a neutron star emits very regular blips of light, pulsing with incredible precision: this makes it a pulsar!
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- In very rare cases, we even have two neutron stars orbiting one another! If one of these neutron stars is a pulsar aimed at us, we can test whether gravity moves at the speed of light or not! We have discovered multiple independent binary pulsars with this exact configuration!
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- Not only is the gravitational source ,star #1, moving, but the other object ,star #2, is changing its velocity, as it changes its direction in orbit around the gravitational source! Remarkably, this effect causes the orbit to ever-so-slowly decay, which leads to time changes in the pulses!
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- The predictions from Einstein’s theory of gravity are incredibly sensitive to the speed of light, so much so that even from the very first binary pulsar system, we have constrained the speed of gravity to be equal to the speed of light with a measurement error of only 0.2%!
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- We were able to do a more direct measurement when a chance coincidence lined up the Earth, Jupiter, and a very strong radio quasar all along the same line-of-sight! As Jupiter moved between Earth and the quasar, the gravitational bending of Jupiter allowed us to measure the speed of gravity, ruling out an infinite speed and determining that the speed of gravity was between 2.55*10^8 and 3.81*10^8 meters-per-second, completely consistent with Einstein’s predictions.
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- ( 3*10^8 meters/second = 186,300 miles / second)
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- The proposed Laser Interferometer Space Antenna (LISA) would have been sensitive to exactly these types of gravitational waves, and could have measured the speed of gravity directly. However, it is the indirect measurements from very rare pulsar systems that give us the tightest constraints, and tell us that the speed of gravity is between 2.993 × 10^8 and 3.003 × 10^8 meters per second.
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- This is an amazing confirmation of General Relativity and a terrible difficulty for alternative theories of gravity that don’t reduce to General Relativity!
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- And now you know not only what the speed of gravity is, but where to look to figure it out! Now if I just could figure out how fast time traveled?
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------------------------ Other Reviews to learn about light.
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- 2537 - LIGHT - stranger than you can imagine? - Is “light” energy what is warping space and time? What about Dark Energy that is the repulsive force expanding space in the Universe? Is “light” responsible for this as well? “Light” has a much bigger role to play in the Universe than what we can “see”. All that we do not see is stranger still.
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- 2402 - LIGHT - There is not enough light. We can never understand the Universe by studying starlight. There is just not enough of it. Starlight comes from the energy, or radiation, from ordinary matter and ordinary matter comprises less than 5% of the Universe. 30% more is Dark Matter and 65% more is Dark Energy. We need a telescope that can see in the ”Dark”.
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- 2387 - LIGHT - how light moves particles. Einstein had developed the theory of special relativity that revolutionized the way we understand light. To this day, it provides guidance on understanding how particles move through space.
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- 2310 - Getting Light from Electricity and Magnetism. Light, electricity, and magnetism seem to me to be three separate things. How can they be connected into electromagnetic waves? We had to discover each one at a time before Einstein even connected them with mass and space.
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- 2306 - How far away is the Sun? When at sea how can we use the Sun to determine our position, longitude and latitude? How can the moons of Jupiter be used as a clock? How can that clock be used to determine the speed of light? Learn from Ole the Danish astronomer:
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- 2210 - There is a star that is looping around the blackhole at the center of our galaxy. Theory of Relativity tells us that light would loose energy leaving a gravity well and it would gain energy entering the gravity well. This energy difference shows up in the change in wavelength of the light. Einstein’s math is tested again.
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- 2260 - Getting light from electricity and magnetism? How do the electric and the magnetic forces, or, waves create light. Or, is it the opposite? Is light that creates the electric and magnetic forces? See here, to learn the answer.
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- 2182 - Light is traveling energy. In order to make the speed of light constant space had to get bigger and time had to get slower. A photon traveling at the speed of light experiences all of space with no time at all. The speed of light times itself equals energy. E = c^2 . Speed of light times itself * ( the magnetic constant + the electric charge constant) = 1
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- 2181 - The photon is the force carrier for all electric repulsive and attractive forces and all magnetic attractive and repulsive forces.
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- 2179 - How did we earn the speed of light?
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- 2178 - Light’s mysteries?
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- 2005 - Half the human brain is used in visualization. Here is how it works.
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- 1845 - Why is the sky blue?
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- 1389 - The power of light?
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- 768 - The wave - particle duality of light?
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- February 7, 2020 2609
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--------------------- Friday, February 7, 2020 --------------------
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