Einstein’s theory of General Relativity completely changed the way we think about
space, time, and gravity when it dropped, pun intended, over one hundred years ago.
General relativity says that objects warp space and time, and this warping is experienced
as gravity.
This idea has some pretty mind-bending implications that ripple down from it, and now scientists
have strong evidence that spinning objects put space-time in a twist.
The analogy most of you have probably seen is that space-time is like a sheet, or a trampoline.
Put a bowling ball on the trampoline and it warps the taut surface.
While that’s a helpful image to have in your head in most cases, it doesn’t cover
all the effects a massive object can have on spacetime, like when it’s rotating.
Three years after Einstein published his theory, he and two Austrian mathematicians, Josef
Lense and Hans Thirring, realized that a spinning object should also drag spacetime around with it.
So if I may, I’d like to substitute the trampoline analogy with something I’m always
thinking about: noodles.
Picture spacetime as a plate of pasta, and a spinning object as a twisting fork stuck
into it.
Dragging the noodles of spacetime around is known as the Lense-Thirring effect, or frame
dragging, since it’s literally dragging the local frame of reference around with it.
Of course, like most things with general relativity, you don’t notice them on a small scale.
When I spin around as fast as I can nothing swirls around me in a vortex, even though
it kinda feels like that when I stop.
Even an object as large as the earth produces miniscule frame dragging, though we have observed
it in the past.
NASA’s Gravity Probe B was launched in 2004 and orbited in a polar orbit, so, perpendicular
to the rotation of the Earth.
The satellite was equipped with a set of gyroscopes that measured precession, or wobble, due to
frame dragging.
It isolated the effects of frame dragging by compensating for other sources of drag
like Earth’s gravity, solar radiation, and the atmosphere.
Gravity Probe B then observed a star for a year and measured how much the churning spacetime
threw off its gaze.
In the end, the change was tiny: just 39 milliarcseconds, or 0.00001 degrees.
That’s next to nothing, but in extreme and rare cases, frame dragging can be hugely noticeable.
In January of 2020, scientists published their observations of just such a special case.
The scientists watched a binary star system with some unusual qualities.
One star is a white dwarf about the size of the Earth but much chunkier, about 300,000
times the density.
This white dwarf is spinning so fast it completes a rotation in a matter of minutes.
That mass and speed means it drags space-time around it much more strongly than Earth does.
Of course the question is, how do you observe the space time around it actually twisting?
And that’s where the white dwarf’s companion comes in.
Whipping around it like an excited chihuahua is a neutron star just 20 kilometers across,
but 100 billion times the density of Earth.
This neutron star is also spinning rapidly, causing it to shoot off powerful beams of
electromagnetic radiation.
These types of stars are known as pulsars, because their spinning beams look like regular
pulses to distant observers, kind of like a cosmic lighthouse.
The pulsar in this system is orbiting the white dwarf in a different plane than the
white dwarf’s rotation, so the entire orbit tumbles around.
By measuring how the timing of the neutron star’s pulses varied, they could figure
out how much frame dragging was caused by the white dwarf.
Simple, right?
Well, it took them almost 20 years to untangle exactly what was going on, but in the end
they concluded the white dwarf exhibited frame dragging 100 million times stronger than Gravity
Probe B observed around Earth.
The scientists are extremely happy with the results of their research, which confirms,
once again, Einstein’s notion of how the universe works...
and makes me hungry for spaghetti.
Frame dragging can also account for the precession in the jets of matter shot out by black holes.
Oh, you didn’t know black holes shot out jets of matter?
Well, it just so happens that I made a video about that, too— and you can watch it here!
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