On April 8, of this year, a total solar eclipse occurred across the United States that caused much public awe and excitement. The path of totality ran from Texas on a northeasterly trajectory which took it through upper New England – a rare opportunity for millions of Americans to bask in the awe and wonder of temporary night occurring in mid-day.
Many Americans booked airline tickets to be in the path of totality where our close friend, the moon, comes precisely between our earth and the solar furnace which provides us daylight hours and sustains all life on this planet. Aside from weather problems in some locations, few who made plans to “be there” were disappointed as daylight slowly dissolved into total night producing skies in which even faint stars were visible. Viewers were in awe. Birds and other creatures became confused and excited and then quiet over a nightfall which came on relatively suddenly and completely out of sync with their biological clocks. Even local temperatures dropped several degrees during the eclipse.
In 1919, a very famous total eclipse occurred on the Island of Principe, off the coast of west Africa. A small band of scientists and technicians set sail for that very remote destination with a mission of overriding importance. Their purpose? To test the validity of Albert Einstein’s 1915/16 theory of general relativity.
The 1919 Principe Solar Eclipse and Albert Einstein: Time Magazine’s Person of the Century
A revolutionary central tenant of Einstein’s earlier theory of special relativity, dating from 1905-1907, declares that mass and energy are equivalent entities in the world of physics. It seemed plausible to Einstein, at that time, that they might even be inter-changeable, in some unknown way, in accordance with Einstein’s derivation that e=mc2 where e=energy, m=mass, and c=the constant speed of light (300 million meters per second – a very large number). Accordingly, a very small quantity of mass is theoretically equivalent to a very large energy! The two atomic bombs which ended Japan’s role in WWII proved that the exchange can indeed take place whereby tremendous energy is liberated through nuclear reactions which only slightly reduce the mass of fissionable material in the bomb’s core.
From 1909 to 1916, Einstein was working feverishly to extend special relativity to a new, advanced theory of general relativity which, unlike special relativity, applies to the physics of systems under gravitational influences – like our solar system…and the universe at large. The nature of this endeavor was mathematically far more intense than any physics Einstein had tackled before. By the time he finally published his perfected conclusions in 1915/16, his mind and body were ravaged by the effort, leaving him near physical and mental collapse.
The four-dimensional space/time framework of special relativity was now revealed as subject to a “curvature” caused by bodies of mass in the system. Essentially, mass curves (or warps) four-dimensional space/time in its region and it is this curvature which determines the (natural) motion of the planets around the sun and not some force of attraction between bodies of mass as Isaac Newton had declared in his Mathematical Principles of Natural Philosophy (aka the Principia). Newton’s equations for his force-based “universal gravitation” worked perfectly for most practical purposes…and still do. Today, NASA routinely computes spacecraft orbits using Newton’s three laws of motion and his law of universal gravitation. But Einstein, by 1911, was convinced that space/time curvature due to bodies of mass in the region is the real root-explanation of gravity, not Newton’s long-revered theory of an attractive force between bodies of mass.
By 1916, the mathematics and physics all came into harmony for Einstein, and he was absolutely convinced that general relativity would overturn one of physics’ most sacrosanct theories – Newton’s theory of universal gravitation. There were, however, many sceptics in scientific circles – in no small part due to the difficult mathematics required to even comprehend general relativity.
A Brief Review of Space, Time, and Gravity – from Newton to Einstein
Newton’s concise view of the universe (from 1687):
-Conventional three-dimensional absolute space.
-An independent “arrow” of absolute time whose master clock applies simultaneously everywhere throughout space: all clocks run at the same rate.
-Gravity: a force of attraction between any two bodies of mass in the universe whose magnitude is directly proportional to the product of the two masses and inversely proportional to the square of the separation distance.
-The gravitational force of attraction between the sun and planets and Newton’s famous (and still valid) three laws of motion are what determine the planets’ elliptical paths around the sun.
Einstein’s modified view of the universe (after 1916):
-We reside in four-dimensional space/time where time is a fourth dimension integral with three-dimensional space.
-Space is not absolute. There is no definable reference-point (think coordinate origin) in space.
-Time is not absolute (no master clock). There is no such thing as “simultaneous events” occurring across space.
-Distance and time measurements in one system are relative only to those in another system. To repeat: there is no standard ruler or clock in the universe.
-There is no force of gravitational attraction between two bodies of mass as postulated in Newton’s theory of universal gravitation. A curvature in four-dimensional space/time occurs in regions near a body of mass (like the sun). A body passing by the sun (like a planet) is not subject to an attractive force keeping it in orbit; instead, a planet naturally follows the curved space/time around the sun’s mass, thus defining its orbit.
Comment on Einstein’s contributions: The concept of four-dimensional space/time along with the relative natures of space and time emanated from the special theory of relativity ca. 1905-1907. The redefinition of Newton’s force of gravitational attraction came from Einstein’s 1915/16 general theory of relativity in the form of a curved four-dimensional space/time around regions of mass.
How to Prove That Einstein Was Correct? Enter the 1919 Principe Total Eclipse!
On October 14, 1913, Albert Einstein wrote a famous letter to astronomer George Ellery Hale of the Mount Wilson observatory, near Los Angeles. I have seen the original letter in the Huntington Library Collection – a memorable experience. Einstein asks Hale if it would be possible to measure the deviation from a straight-line path which might result when a ray of starlight passes close by a sizeable mass like the sun. Einstein reasoned that any such detected deviation could prove his theory of curved four-dimensional space/time in the regions surrounding such a mass. A ray, or beam, of starlight is essentially electromagnetic energy which, historically, was assumed to travel in a straight line through Newtonian three-dimensional space. Any deflection of the light path would indicate the presence of curved four-dimensional space/time in the region near the sun’s mass – per Einstein’s proposed theory of general relativity. Newton’s universe, as revealed in his masterwork book, the Principia, would not produce such a deflection.
However, since Einstein’s revelation in 1907 that e=mc2, the energy/mass equivalence inherent in that most famous equation in physics suggests that the mass aspect of a light beam’s energy would, in fact, be susceptible to an attraction to the sun’s mass in accordance with Newton’s interpretation that gravity represents an attractive force between any two bodies of mass – the sun and the mass aspect of the light beam, in this case!
Given the above statement, one might ask how a measured deflection in the starlight’s path would possibly prove Einstein’s contention that curved space/time is the cause of the deflection, and not Newton’s force between masses?
The answer: the calculated deflection for Einstein’s curved space/time is twice the value which would be result from the gravitational attractive force of Newton! Clearly, any deflection measurements made in the field would require extreme care and accuracy in order to conclusively prove Einstein’s general theory.
A Total Eclipse of the Sun Is Required
Recall that on October 14, 1913, Einstein had written a letter to the famous astronomer, George Ellery Hale, of the Mount Wilson Observatory in Los Angeles, California. Einstein asked if it might be possible to detect and measure any deflection in the normally straight-line path of starlight as it passes close-by the sun on its way to earth.
From the above version of the sketch Einstein made in his letter to George Hale, one can see how any deflection in the beam of starlight passing close to the sun could be determined by comparing the calculated actual position of the star in question with the observed apparent position of the star as seen through the astronomer’s optical equipment. A total eclipse of the sun would be required to have any hope of seeing the star in question next to a normally very intense solar brightness. The greater the deflection of the light beam, the greater the difference between the calculated actual vs. observed apparent positions of the star in the heavens.
In Einstein’s 1913 letter to Hale, he had assigned a 0.84 arc-second deflection as the expected value. He later, in 1915, refined his estimate to be roughly 1.75 arc-seconds – close to the measured value recorded at Principe in 1919. Note: one arc-second is 1/3600 of a degree – very small.
The 1919 Total Eclipse Verified Einstein’s Claim for Curved Space/Time!
The observed angular deflection at Principe of approximately 1.75 arc-seconds of angle was close enough to that predicted by Einstein in 1915 as his general theory of relativity was being published. To repeat, the calculated angular deflection to be expected using Newton’s theory of gravitation based on an attractive force between any two bodies of mass yields an angular deflection of approximately half the 1.75 arc-seconds observed and predicted by the curved space/time hypothesis of general relativity!
Once Arthur Eddington and colleagues thoroughly combed through the myriad photographic plates and data logs involved in team’s efforts at Principe, the announcement of confirmation was made and carried world-wide.
The interest and adulation of the global audience that quickly followed changed Albert Einstein’s life forever. He was no longer regarded as an ivory-tower scientist who made former, radical claims about light, space, and time (some of which were, as yet, not convincing even to scientists of stature). With general relativity, he had successfully embellished even his previous, startling claims for special relativity. The total eclipse at Principe had revealed the genius and the validity of his foundational efforts over the previous decade. Now, he had, literally overnight, achieved a world-wide pop-icon status that could hardly have been imagined possible. Einstein’s catapult to fame and public recognition will, likely, never be equaled again – certainly not by mere entertainers. The global public could not get enough of the doe-eyed professor/philosopher with the curly, unruly hair that we saw so often in later life. Ordinary peoples of the world, 98 per-cent of whom had no real sense of Einstein’s accomplishments were, nevertheless, suddenly in awe of this other-worldly figure because his claims that seemed so contrary to common-sense, were being now accepted by the best and brightest minds in the world.
His subsequent elevation in the scientific world was akin to riding a high-speed elevator directly to the pinnacle of scientific fame. For most of us who know something of science and its historical development, Einstein is the only serious challenger to the genius of Isaac Newton who has long occupied the lonely top of the intellectual/scientific pyramid.
Unlike Newton, Einstein was, inherently, a very sagacious philosopher in addition to his scientific status. His numerous and oft-published quotations reveal both a philosopher’s acuity and a rascally sense of humor peppered with self-deprecating commentary. My yardstick for scientific greatness asks the questions, “Who has most changed our concept of the world around us and the way we live our lives today, and who else, at the time, could have possibly advanced science in the same way to the same degree?” In my opinion, Newton and Einstein are in a class of their own.
My Motivation for This Blog Post
The public enthusiasm for the recent eclipse of April 8 made me think of Einstein and the famous 1919 Principe total eclipse which proved Einstein correct in his assertions for general relativity. Einstein’s 1915/16 publication of general relativity has been referred to as one of the most remarkable and sublime creations ever to emerge from the human mind: this from people who really understand the content and magnitude of the effort.
I love that the public became so enthralled over the recent eclipse, but I am also saddened over the lack of awe and wonder for the many scientific miracles inherent in nature that we so often take for granted while barely noticing. The story of Principe is the story of just another total solar eclipse – a relatively mundane fact of nature compared to the incredible scientific content of the accompanying story – the confirmation of general relativity. It saddens me that, today, there is not more public curiosity – not to understand, but to merely know and wonder about such things. It has been my goal to help the cause by sharing some information!
First Edition Offprint of Einstein’s Theory of General Relativity – 1916
Reason and Reflection 
Wow! Way above my level of comprehension! However, acknowledge that he’s a rock star and beyond brilliant!
thanks!
Einstein’s relativity theories and energy/mass equivalence are not easy concepts for anyone to grasp, Linda – not to feel discouraged! You got the central theme of the post, though – the massive effect of the 1919 eclipse on Einstein’s image and reputation, world-wide! Hi to Gil!!