Even Incredibly Dense Neutron Star Falls Like A Feather

  • Researchers test Einstein’s general relativity theory on a triple star system that is 4,200 light years away. 
  • They found that both neutron star (heavy and dense) and white dwarf star (less massive) fall towards an outer dwarf at the same rate. 
  • Any acceleration difference between inner white dwarf and neutron star is too small to be detected.

Albert Einstein’s general theory of relativity suggests that all bodies fall at the same rate, regardless of their composition or mass. This universality of free fall tells that all objects accelerate identically in an external field of gravity.

This was first stated by Galileo in 1589: regardless how heavy objects are, they all fall at the same rate in the absence of atmosphere. The same concept was demonstrated with a hammer and a feather on the Moon in 1971.

Einstein’s understanding of gravity, i.e. relativity theory works fine on Earth. It has passed all experiments — from careful lab researchers to observation of bodies in our solar system — but does it work on far distant celestial bodies? What about the dense and massive objects in the universe: are their effects of uniform gravitational field and constant acceleration distinguishable?

Many alternatives to general relativity theory suggest that extremely dense objects, like neutron stars, have slightly different falling rate than lesser mass objects. This difference arises due to objects’s what they call gravitational binding energy. It’s the gravitational energy that holds the object together.

Recently, an international team of researchers tried to solve this mystery by analyzing one of the most extreme scenarios in the universe. This is the most precise and strict test ever conducted.

A Stellar Triple System

Direct experiments of this theory using Solar System planets are limited by the weak self-gravity of the bodies, and experiments using pulsar-white-dwarf binaries have been limited by the Milky Way’s weak gravitational pull.

Therefore, scientists used a triple star system, named PSR J0337+1715, to test this theory. The system is nearly 4,200 light years away from Earth, and it consists of a neutron star orbiting a white dwarf star (with an orbital period of 1.6 days), and both of them are orbiting an another distant white dwarf star (with an orbital period of 327 days).

Dense Neutron Star PSR J0337+1715 | Credit: YouTube

It was discovered by the National Science Foundation’s Green Bank Telescope in 2011. Since then, the stellar triple system has been observed regularly by the Arecibo Observatory (Puerto Rico), Westerbork Synthesis Radio Telescope (Netherlands) and Green Bank Telescope (USA).

The Results

All these telescopes helped measure how each object in this system moves with respect to the others. The neutron star is actually a pulsar that rotates 366 times in one second, sending faint radio wave pulses, which are detected by the Green Bank Telescope.

These pulses give exact location — within several hundred meters — of the neutron star, and some crucial information like where the neutron star is heading and where it has been so far.

Reference: Nature | doi:10.1038/s41586-018-0265-1 | NRAO

If alternatives to general relativity theory were correct, then the inner white dwarf star and neutron star would each fall towards the outer white dwarf star at different rates. This would happen because neutron star is more massive and dense than inner white dwarf, and thus has higher gravitational binding energy.

After numerous careful observation and examination, the researchers discovered that any difference in acceleration between inner white dwarf and neutron star is too small to be detected.

However, if there is any difference, it would not be more than 3 parts per million (or 2.6 × 10−6 to be exact). Once again, this proves Einstein right, and puts serious constraints on alternatives to general relativity theory.

Read: How Big Are Neutron Stars?

The researchers claim that their result is ten times more accurate than the previous best gravity test. Moreover, future observations and results from the Gaia mission will significantly improve the light-bending limit obtained via Cassini, providing better indirect weak-field constraints.

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