So I saw a short on YouTube talking about how they proved a prediction Einstein made about double pulsars falling towards each other at the rate of 7 mm a day. My question; how the hell can they measure two stars that are roughly the size of LA, but the mass of our sun, moving 7 mm closer each day while in orbit of each other lightyears away? At that distance it’s gotta be a couple pixels on whatever they’re using to view them right?
In: Planetary Science
“A couple of pixels” is a reasonably accurate estimation. We cannot measure the distance between them this way. We can detect things like alignment of the two, or when a pulsar’s pole is pointing our way, however.
There is a workaround. Orbits follow a set of rules. And if the masses of the two don’t change significantly but the orbit becomes smaller, then it also becomes faster. And these events would become more frequent at a very specific and very predictable rate.
We might not have telescopes that can measure the distances between two stars to within a centimeter. We do, however, have very very accurate clocks.
Nobody can actually *see* the pulsars, it’s all about timing. The pulsars each produce a sweeping “lighthouse beam” of radio waves that sweeps past us, creating a regular sequence of pulses (hence the name). When a pulsar is closer to us in its orbit the pulses arrive a bit early, when it’s far away the pulses arrive a bit late, so by measuring that we can precisely calculate how long it takes to complete an orbit.
By observing for many years using clocks with nanosecond accuracy, we can see the orbital period changing slightly, and from this (and knowing how gravity and orbits work) we can calculate the change in the radius of the orbit.
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