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Einstein's core idea about gravity just passed an extreme, whirling test in deep space

In this illustration, a pulsar (PSR J0337-1715) is shown with two white dwarf companions. The green mesh illustrates the curvature of space-time caused by the different masses. (Size and distances of the three components are not to scale.)
In this illustration, a pulsar (PSR J0337-1715) is shown with two white dwarf companions. The green mesh illustrates the curvature of space-time caused by the different masses. (Size and distances of the three components are not to scale.)
(Image: © Michael Kramer/MPIfR)

Once again, physicists have confirmed one of Albert Einstein's core ideas about gravity — this time with the help of a neutron star flashing across space.

The new work makes an old idea even more certain: that heavy and light objects fall at the same rate. Einstein wasn't the first person to realize this; there are contested accounts of Galileo Galilei demonstrating the principle by dropping weights off the Tower of Pisa in the 16th century. And suggestions of the idea appear in the work of the 12th-century philosopher Abu'l-Barakāt al-Baghdādī. This concept eventually made its way into Isaac Newton's model of physics, and then Einstein's theory of general relativity as the gravitational "strong equivalence principle" (SEP). This new experiment demonstrates the truth of the SEP, using a falling neutron star, with more precision than ever.

The SEP has appeared to be true for a long time. You might have seen this video of Apollo astronauts dropping a feather and a hammer in the vacuum of the moon, showing that they fall at the same rate in lunar gravity.

But small tests in the relatively weak gravitational fields of Earth, the moon or the sun don't really put the SEP through its paces, according to Sharon Morsink, an astrophysicist at the University of Alberta in Canada, who wasn't involved in the new study.

"At some level, the majority of physicists believe that Einstein's theory of gravity, called general relativity, is correct. However, that belief is mainly based on observations of phenomena taking place in regions of space with weak gravity, while Einstein's theory of gravity is meant to explain phenomena taking place near really strong gravitational fields," Morsink told Live Science. "Neutron stars and black holes are the objects that have the strongest known gravitational fields, so any test of gravity that involves these objects really test the heart of Einstein's gravity theory."

Neutron stars are the collapsed cores of dead stars. Super dense, but not dense enough to form black holes, they can pack masses greater than that of our sun into whirling spheres just a few miles wide.

The researchers focused on a type of neutron star called a pulsar, which from Earth's perspective seems to flash as it spins. That flashing is a result of a bright spot on the star's surface whirling in and out of view, 366 times per second. This spinning is regular enough to keep time by.

Related: 8 ways you can see Einstein's theory of relativity in real life

This pulsar, known as J0337+1715, is special even among pulsars: It's locked in a tight binary orbit with a white dwarf star. The two stars orbit each other as they circle a third star, also a white dwarf, just like Earth and the moon do as they circle the sun.

(Researchers have already shown that the SEP is true for orbits like this in our solar system: Earth and the moon are affected to exactly the same degree by the sun's gravity, measurements suggest.)

The precise timekeeping of J0337+1715, combined with its relationship to those two gravity fields created by the two white dwarf stars, offers astronomers a unique opportunity to test the principle.

The pulsar is much heavier than the other two stars in the system. But the pulsar still falls toward each of them a little bit as they fall toward the pulsar's larger mass. (The same thing happens with you and Earth. When you jump, you fall back toward the planet very quickly. But the planet falls toward you as well — very slowly, due to your own low gravity, but at the exact same rate as a feather or a hammer would if you ignore air resistance.) And because J0337+1715 is such a precise timekeeper, astronomers on Earth can track how the gravitational fields of the two stars affect the pulsar's period.

To do so, the astronomers carefully timed the arrival of light from J0337+1715 using large radio telescopes, in particular the Nançay Radio Observatory in France. As the star moved around each of its neighbors — one in a quick little orbit and one in a longer, slower orbit — the pulsar got closer and farther from Earth. As the neutron star moved farther away from Earth, the light from its pulses had to travel longer distances to reach the telescope. So, to a tiny degree, the gaps between the pulses seemed to get longer.

As the pulsar swung back toward Earth, the gaps between the pulses got shorter. That allowed physicists to build a robust model of the neutron star's movement through space, explaining precisely how it interacted with the gravity fields of its neighbors. Their work built on a technique used in an earlier paper, published in the journal Nature in 2018, to study the same system.

The new paper, published online June 10 in the journal Astronomy and Astrophysics, showed that the objects in this system behaved as Einstein's theory predicts — or at least didn't differ from Einstein's predictions by more than 1.8 parts per million. That's the absolute limit of the precision of their telescope data analysis. They reported 95% confidence in their findings.

Morsink, who uses X-ray data to study the mass, widths, and surface patterns of neutron stars, said that this confirmation isn't surprising, but it is important for her research.

"In that work, we have to assume that Einstein's theory of gravity is correct, since the data analysis is already very complex," Morsink told Live Science in an in an email. "So tests of Einstein's gravity using neutron stars really make me feel better about our assumption that Einstein's theory describes the gravity of a neutron star correctly!"

Without understanding the SEP, Einstein would never have been able to develop his ideas of relativity. In an insight he described as "the most fortunate thought in my life," he recognized that objects in free fall don't feel the gravitational fields tugging on them.

(This is why astronauts in orbit around the Earth float. In constant free fall, they don't experience the gravitational field that holds them in orbit. Without windows, they wouldn't know Earth was there at all.)

Most of Einstein's key insights about the universe begin with the universality of free fall. So, in this way, the cornerstone of general relativity has been made that much stronger.

Originally published on Live Science.

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  • Xinhang Shen
    It seems that many physicists still don't know that Einstein's relativity is logically wrong and can be easily disproved through Lorentz Transformation of clock time i.e. our physical time between two inertial reference frames:

    Let's look at the twin paradox which is designed to demonstrate that relative speed would generate time dilation as predicted by special relativity which claims that when the speed of a clock relative to an observer was close to the speed of light, the observer would see the clock slow down close to stop. But, it is pretty ironic as shown on Wikipedia, the final conclusion of the twin paradox becomes that, after a high speed space travel, it is the acceleration of the traveling twin (not his speed relative to his twin brother) that made him younger than his twin brother staying on the earth because both twins had experienced exactly the same speed relative to each other during the entire trip. Is it funny that the original argument that relative speed generates time dilation is completely lost, although relativists still think that the paradox has been solved? In fact, this paradox has simply confirmed that relative speed can never generate time dilation and special relativity is wrong.

    Actually Einstein's relativity has already been disproved both theoretically and experimentally for more than four years. The fatal mistake of Einstein's relativity is that it uses Lorentz Transformation to redefine time and space, and the newly defined time is no longer the physical time we measure with physical clocks. The claim of the constant speed of light is very similar to the claim that everybody had the same height if the height is measured with a new ruler - an elastic band a ruler. Obviously, such claims do not make any sense.

    In a physics theory, the physical time shown on a physical clock is T = tf/k where t is the theoretical time, f is the frequency of the clock and k is a reference frame independent calibration constant.

    In Newton's mechanics, the absolute Galilean time makes frequency f a reference frame independent constant. Therefore, we can set k = f to make the clock show the theoretical time i.e. the absolute Galilean time t: T = tf/k = tf/f = t.

    But in special relativity, the relative relativistic time makes frequency f a reference frame dependent variable and can't be eliminated by setting k = f in the clock formula. Thus, clock time can't be simply calculated by the formula: T = tf/k != t in special relativity. Therefore, we need to verify whether clock time T and relativistic time t have the same property in Lorentz Transformation.

    When a clock is observed in another inertial reference frame, we have t' = rt and f' = f/r and T' = t'f'/k = rt(f/r)/k = tf/k = T, where r = 1/sqrt(1 - v^2/c^2), which means that the physical time T won't change with the change of the inertial reference frame, and is Lorentz invariant and absolute, completely different from relativistic time. That is, a clock still measures the absolute time in special relativity.

    Some people may argue that relativistic time has to be shown on two clocks. OK, here is it.
    If you have a clock (clock 1) with you and watch my clock (clock 2) in motion and both clocks are set to be synchronized to show the same physical time T relative to your inertial reference frame, you will see your clock time: T1 = tf1/k1 = T and my clock time: T2 = tf2/k2 = T, where t is relativistic time of your frame, f1 and f2 are the frequencies of clock 1 and clock 2 respectively observed in your inertial reference frame, k1 and k2 are calibration constants of the clocks. The two events:

    (Clock1, T1=T, x1=0, y1=0, z1=0, t1=t)
    and
    (Clock2, T2=T, x2=vt, y2=0, z2=0, t2=t)

    are simultaneous measured with both relativistic time t and clock time T in your reference frame. When these two clocks are observed by me in the moving inertial reference frame, according to special relativity, we can use Lorentz Transformation to get the events in my frame (x', y', z', t'):

    (clock1, T1', x1'=-vt1', y1'=0, z1'=0, t1')
    and
    (clock2, T2', x2'=0, y2'=0, z2'=0, t2')

    where

    t1' = r(t1-vx1/c^2) = r(t-0) = rt
    t2' = r(t2-vx2/c^2) = r(t-tv^2/c^2) = rt/r^2 = t/r
    T1' = t1'f1'/k1 = (rt)(f1/r)/k1 = tf1/k1 = T1 = T
    T2' = t2'f2'/k2 = (t/r)(rf2)/k2 = tf2/k2 = T2 = T

    in which r = 1/sqrt(1-v^2/c^2).

    That is, no matter observed from which inertial reference frame, the two events are still simultaneous measured with physical time T i.e. the two clocks are always synchronized measured with clock time T i.e. clock time T is absolute, but not synchronized measured with relativistic time t'. In real observations, we can only see clock time T but not relativistic time. Therefore, clock time is our physical time and absolute, totally different from relativistic time in Lorentz Transformation and thus relativistic time is a fake time without physical meaning. The change of the reference frame only makes changes of the relativistic time from t to t' and the relativistic frequency from f to f', which cancel each other in the formula: T= tf/k to make the physical time T unchanged. This proves that even in special relativity our physical time is still absolute. Therefore, special relativity based on the fake relativistic time is wrong.

    That the physical time (i.e. clock time) is absolute has been clearly confirmed by the physical fact that all the atomic clocks on the GPS satellites are synchronized not only relative to the ground clocks but also relative to each other to show the same absolute physical time, which directly denies the claim of special relativity that clocks can never be synchronized relative to more than one inertial reference frame no matter how you correct them because "time is relative".

    You will find the mathematical proofs that in special relativity, the real speed of light still follows Newton's velocity addition law, and both time dilation and length contraction are simply illusions in my peer-reviewed journal paper and conference paper which are available free of charge at: https://www.researchgate.net/publication/297527784_Challenge_to_the_Special_Theory_of_Relativity and https://www.researchgate.net/publication/297528348_Clock_Time_Is_Absolute_and_Universal
    Reply
  • Hayseed
    The puzzle of a constant velocity has always been a mystery. This is because everyone has been taught that EM waves are alternating and continuous, like media waves are. This is probably because of what we measure when we detect them.

    But media waves have different and relative velocities. Why not light? How does one solve this mystery with an omnipresent time.

    EM radiation is the instant emission, of discreet volumes(or lengths or durations) with a 50% duty cycle, is does not have frequency until detected. It has duty cycle. Alternation is not required. Single pole particles emit.

    If you understand RF, one precision rectified half-wave sine signal, fed into a dipole will show you this at the absorber/receiver.
    Reply
  • jeffrey
    Xinhang Shen said:
    It seems that many physicists still don't know that Einstein's relativity is logically wrong and can be easily disproved through Lorentz Transformation of clock time i.e. our physical time between two inertial reference frames:

    Let's look at the twin paradox which is designed to demonstrate that relative speed would generate time dilation as predicted by special relativity which claims that when the speed of a clock relative to an observer was close to the speed of light, the observer would see the clock slow down close to stop. But, it is pretty ironic as shown on Wikipedia, the final conclusion of the twin paradox becomes that, after a high speed space travel, it is the acceleration of the traveling twin (not his speed relative to his twin brother) that made him younger than his twin brother staying on the earth because both twins had experienced exactly the same speed relative to each other during the entire trip. Is it funny that the original argument that relative speed generates time dilation is completely lost, although relativists still think that the paradox has been solved? In fact, this paradox has simply confirmed that relative speed can never generate time dilation and special relativity is wrong.

    Actually Einstein's relativity has already been disproved both theoretically and experimentally for more than four years. The fatal mistake of Einstein's relativity is that it uses Lorentz Transformation to redefine time and space, and the newly defined time is no longer the physical time we measure with physical clocks. The claim of the constant speed of light is very similar to the claim that everybody had the same height if the height is measured with a new ruler - an elastic band a ruler. Obviously, such claims do not make any sense.

    In a physics theory, the physical time shown on a physical clock is T = tf/k where t is the theoretical time, f is the frequency of the clock and k is a reference frame independent calibration constant.

    In Newton's mechanics, the absolute Galilean time makes frequency f a reference frame independent constant. Therefore, we can set k = f to make the clock show the theoretical time i.e. the absolute Galilean time t: T = tf/k = tf/f = t.

    But in special relativity, the relative relativistic time makes frequency f a reference frame dependent variable and can't be eliminated by setting k = f in the clock formula. Thus, clock time can't be simply calculated by the formula: T = tf/k != t in special relativity. Therefore, we need to verify whether clock time T and relativistic time t have the same property in Lorentz Transformation.

    When a clock is observed in another inertial reference frame, we have t' = rt and f' = f/r and T' = t'f'/k = rt(f/r)/k = tf/k = T, where r = 1/sqrt(1 - v^2/c^2), which means that the physical time T won't change with the change of the inertial reference frame, and is Lorentz invariant and absolute, completely different from relativistic time. That is, a clock still measures the absolute time in special relativity.

    Some people may argue that relativistic time has to be shown on two clocks. OK, here is it.
    If you have a clock (clock 1) with you and watch my clock (clock 2) in motion and both clocks are set to be synchronized to show the same physical time T relative to your inertial reference frame, you will see your clock time: T1 = tf1/k1 = T and my clock time: T2 = tf2/k2 = T, where t is relativistic time of your frame, f1 and f2 are the frequencies of clock 1 and clock 2 respectively observed in your inertial reference frame, k1 and k2 are calibration constants of the clocks. The two events:

    (Clock1, T1=T, x1=0, y1=0, z1=0, t1=t)
    and
    (Clock2, T2=T, x2=vt, y2=0, z2=0, t2=t)

    are simultaneous measured with both relativistic time t and clock time T in your reference frame. When these two clocks are observed by me in the moving inertial reference frame, according to special relativity, we can use Lorentz Transformation to get the events in my frame (x', y', z', t'):

    (clock1, T1', x1'=-vt1', y1'=0, z1'=0, t1')
    and
    (clock2, T2', x2'=0, y2'=0, z2'=0, t2')

    where

    t1' = r(t1-vx1/c^2) = r(t-0) = rt
    t2' = r(t2-vx2/c^2) = r(t-tv^2/c^2) = rt/r^2 = t/r
    T1' = t1'f1'/k1 = (rt)(f1/r)/k1 = tf1/k1 = T1 = T
    T2' = t2'f2'/k2 = (t/r)(rf2)/k2 = tf2/k2 = T2 = T

    in which r = 1/sqrt(1-v^2/c^2).

    That is, no matter observed from which inertial reference frame, the two events are still simultaneous measured with physical time T i.e. the two clocks are always synchronized measured with clock time T i.e. clock time T is absolute, but not synchronized measured with relativistic time t'. In real observations, we can only see clock time T but not relativistic time. Therefore, clock time is our physical time and absolute, totally different from relativistic time in Lorentz Transformation and thus relativistic time is a fake time without physical meaning. The change of the reference frame only makes changes of the relativistic time from t to t' and the relativistic frequency from f to f', which cancel each other in the formula: T= tf/k to make the physical time T unchanged. This proves that even in special relativity our physical time is still absolute. Therefore, special relativity based on the fake relativistic time is wrong.

    That the physical time (i.e. clock time) is absolute has been clearly confirmed by the physical fact that all the atomic clocks on the GPS satellites are synchronized not only relative to the ground clocks but also relative to each other to show the same absolute physical time, which directly denies the claim of special relativity that clocks can never be synchronized relative to more than one inertial reference frame no matter how you correct them because "time is relative".

    You will find the mathematical proofs that in special relativity, the real speed of light still follows Newton's velocity addition law, and both time dilation and length contraction are simply illusions in my peer-reviewed journal paper and conference paper which are available free of charge at: https://www.researchgate.net/publication/297527784_Challenge_to_the_Special_Theory_of_Relativity and https://www.researchgate.net/publication/297528348_Clock_Time_Is_Absolute_and_Universal
    Reply
  • jeffrey
    One of the guide lines is to avoid excessively long discussions. It seems you have presented a entire paper. Perhaps you could present it to a journal for publication. Then it could be better reviewed by your peers.
    Another guide line recommends to limit your conservations to subjects that are debatable and do not have a over whelming amount of evidence.
    I am sure you have done a great job on your paper and it really needs a proper venue as this is more casual setting.
    Reply
  • jeffrey
    Hayseed said:
    The puzzle of a constant velocity has always been a mystery. This is because everyone has been taught that EM waves are alternating and continuous, like media waves are. This is probably because of what we measure when we detect them.

    But media waves have different and relative velocities. Why not light? How does one solve this mystery with an omnipresent time.

    EM radiation is the instant emission, of discreet volumes(or lengths or durations) with a 50% duty cycle, is does not have frequency until detected. It has duty cycle. Alternation is not required. Single pole particles emit.

    If you understand RF, one precision rectified half-wave sine signal, fed into a dipole will show you this at the absorber/receiver.
    Reply
  • jeffrey
    What are media waves?
    Reply
  • Hayseed
    A media wave is a wave, that requires a media to propagate.
    Reply
  • TorbjornLarsson
    Nice test, and another test passed!

    But, nitpicking the article, the the equivalence principle is the equivalence of gravitational and inertial mass, where the weak principle is the Galilean principle and the strong principle holds for bodies that are so large that they contribute a gravitational field.
    Reply
  • TorbjornLarsson
    Xinhang Shen said:

    It seems that many physicists still don't know that Einstein's relativity is logically wrong and can be easily disproved through Lorentz Transformation of clock time <snip> .

    Self promotion.

    How can scientists know anything but the empirical facts at hand - which well tested fact and theory the article points out has passed another test - when "logical proof" has nothing to do with it?

    Any counter evidence can be published in peer review - your ideas has not been.
    Reply
  • TorbjornLarsson
    Hayseed said:
    The puzzle of a constant velocity has always been a mystery. This is because everyone has been taught that EM waves are alternating and continuous, like media waves are. ... Why not light?

    I think you have confused, perhaps in writing, two different concepts.

    Constant velocity is not a mystery.

    My science hero, which work underlies Einstein's equations describing general relativity by the way, Emmy Noether showed that basic nature laws derives from symmetries and imply "charge" conservation https://en.wikipedia.org/wiki/Noether's_theorem ]. Intrinsic symmetries of quantum fields hence imply conservation of various field charges such as electromagnetic charge, and extrinsic symmetries of space hence imply conservation of charges such as momentum and angular momentum . Why quantum charges are discrete, say 1 or 1/3 units of electron charge, while space charges are continuous is more complicated - still not a mystery though. (The naive idea is that quantum physics often express discreteness such as discrete energy levels in an atom. Such discreteness comes out of putting constraints such as potential barriers on the atom fields. Essentially you get intrinsic, constrained, physics of discrete charges and extrinsic, default not constrained. physics of continuous charges. )

    So, good, constant velocity is a given default state, if space exist and nature has laws.

    The universal speed limit of massless field particles such as photons is due to precisely relativity. It comes out of preserving laws among different observers. In the same vein the equivalence principle comes out of preserving global laws.

    So, good, now we know how to have laws in nature.
    Reply