How Einstein Abolished the Aether - with John Spence

The fact that light travels at the same speed in all frames of reference is one of the greatest revelations of modern science.

How have physicists from the ancient Greeks to Einstein tackled the problem of how light travels and what discoveries did it lead them to along the way? John Spence tells the stories of some of the greatest experimental scientists in history as they searched in vain for the mysterious 'aether' and a frame of absolute rest in the universe.

Prof John C.H. Spence is a Fellow of the Royal Society and the Richard Snell Professor of Physics and a Regents Professor at Arizona State University. He is also the director of science for the NSF BioXFEL Science and Technology Center on the application of X-Ray Free-electron lasers to structural biology.

This talk was filmed in the Ri on 11 March 2020.

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[physicshacks]

" The space is endowed with physical properties, in this sense therefore there exists an ether( aether) . The space without ether is unthinkable " .
- source : Lecture " Ether and theory of relativity" by Albert Einstein at leiden University (1920, publushed in 1922)

[hvanmegen]

I can watch these videos for hours on end.. they never cease to fascinate.. thank you for putting these online!

[box-botkids]

Did he really abolish the aether? I think he merely re-labled it 'space-time".

[nazgullinux]

After watching, and listening, to this talk more than 4 times back to back, he never explains "how" einstein abolished aether.

[likesrush]

Just yesterday I learned that Faraday made a prophetic remark about the nature of vibrating electricity being accountable for all we see (light). And today I get to see it in text. I'm so happy to see these lectures. I love the history because I remember distinctly that Maxwell was the first the measure the speed of light. I remember he arranged spinning disks with mirrors around them. These were spaced by 1.5 km or just a crazy distance to aim mirrors so accuratly, by yourself, without a motorbike. He spun the one at home base, if you will, and the pulsing light would locknphase when it completed this circuit and then he did the math and came really close. Today, I see that so many people measured it long before. I'm astounded. I had assumed that the speed of light was suspected to be a constant for a long time but then with Maxwell doing all the math for everyone, there isn't a lot left for Einstein to do, right? e= mc^2. It's like I=mv^2. Einstein got way too much fame for so little. Oh well, he didn't stop there. Great lectures !!! I wish I had paid more attention to my high school calculus book. It had a biography of the mathematicians in the margin. Like Laplace and etc. I'd be so fascinated now but in high school, I just had too much to do as it was. I wish there was a thorough history of mathematics from geometry through calculus and beyond. I mentioned Laplace because Newton gets the credit for calculus but that's not true. Laplace came up with a solution to the very same problems independently and simultaneously. I used Laplace transforms and more in calculus. Laplace's method is very popular for many types of problems because it's easier. Newton's is just different. I think that alone is extraordinary. There should be a show on just that neck and neck between Newton and Laplace. I can't remember much because I wasn't interested in high school. When you get old, you really appreciate how much work these men did to advance something they really didn't know would become so useful in the distant future. There is a LOT of math that had no use at the time, but later became fundamental in describing electricity in capacitors and filter circuits and so many things the mathematicians couldn't dream of. That's fascinating to me. Why'd they work so hard? Like imaginary numbers. The sq rt of -1. It can't exist and yet it's used in electrical engineering constantly. Makes you wonder if ordinary electronics does employ a 5th dimension where the (sq rt -1) is an ordinary thing in that universe. You go there and someone picks up one and says "of course there's a sq rt of -1, here, catch." Anyway it's amazing that match and the external objective reality match up at all. Is there any reason why they have to? No. But we've come a long way with that assumption. That's the history I'd like to see. Plus Laplace getting the proper respect as one of 2 people who invented calculus. In the case of calculus, by the way, there was simply not a way to express something that describes the conditions at an instant of time, if it weren't infinitesimally small in duration. It was math to fix math. That's not so much a math with unknown uses in the future. The uses were what we needed to express. We knew the need for it before we had it. That's different than those that worked on math that had no practical application and wouldn't for centuries, which is mind blowing, in my opinion. These things seem, like Faradays' intuition, to have a prophetic note to it. "I know it's weird but I got to solve it, useful or not. I can't explain it."
I can't find the drive to clear the floor I walk on. So... I'm in awe of their dedication to math, back then. And it'd make another interesting show to see the types of "useless" math people are working on today that may drive how we understand something we may not discover for yet another 200 years or more. right?

[Zikar]

"Hippolyte Fizeau used a rotating comb and a mirror and measured the speed of light, only being off by 5%."
That's seriously impressive.

[RockBrentwood]

7:30 "Maxwell got the *correct* equations..." no he didn't; that's a folklore myth that's bandied about in the Physics community, but it is totally wrong. His equations differed *substantially* from what we now call Maxwell's equations -- and one of the biggest differences is that they had a fixed frame for light propagation (called the "stationary frame"); and that *only* in this frame would the constitutive relations 𝗗 = κ 𝗘, 𝗕 = μ 𝗛 be isotropic. (So "stationary frame" is more properly denoted "frame of isotropy"). In addition, he also stated incorrect relations (and transformation laws) for 𝗕 because he failed to distinguish it from 𝗛, always writing it as μ𝗛 ... until he started calling it its own name 𝗕 by the time he wrote the Treatise. But even then he *still* got the transformation properties of 𝗕 wrong (it's a pseudo-vector & 2-form, while 𝗛 is a vector & 1-form) and consequently wrote down the wrong constitutive law for 𝗕, which Thomson had to correct.

The equations, when made consistent with Relativity, are the Maxwell-Minkowski equations, which could be written as
(1) {𝗕 = ∇×𝗔, 𝗘 = -∇φ - ∂𝗔/∂t}, {∇·𝗕 = 0, ∇×𝗘 + ∂𝗕/∂t = 0} for the magnetic potential 𝗔, electric potential φ, electric force 𝗘 and magnetic induction 𝗕,
(2) {∇·𝗗 = ρ, ∇×𝗛 + ∂𝗗/∂t = 𝗝}, {∂ρ/∂t + ∇·𝗝 = 0} for the electric induction 𝗗, magnetic force 𝗛, current density 𝗝 and charge density ρ,
(3) The constitutive relations {𝗗 + α 𝗚×𝗛 = κ (𝗘 + 𝗚×𝗕), 𝗕 - α 𝗚×𝗘 = μ (𝗛 - 𝗚×𝗗)}, with permeability μ, dielectric coefficient κ and a velocity 𝗚 that references the frame of isotropy.

The equations Maxwell wrote correspond to the case α = 0, while for Relativity, one needs α > 0. In addition, he failed to include the - 𝗚×𝗗 term because he was still confusing 𝗕 and 𝗛 -- its inclusion was a correction made later by Thomson (and verified experimentally c. 1900 by a husband and wife team). The "stationary frame" referred to in late 1800's papers and in the opening part of Einstein's paper is 𝗚 = 0. Einstein's objection (stated therein) is that there would continue to be a 𝗚-dependence for (3) even in a vacuum, when there ought not to be; and that there should be nothing to single out any specific speed 𝗚 in a vacuum, so that the stationary case 𝗚 = 0 should hold for the vacuum in all inertial frames of reference.

In contrast, the Maxwell-Minkowski equations (which are the ones required by Relativity) - have α > 0 and single out a unique speed c ≡ √(1/α) - which is the invariant speed postulated by Relativity. And it just so happens that in the case where κμ = α (i.e. the vacuum), equations (3) for *all* cases of 𝗚 become *almost* *equivalent* to the equations for the "stationary case"; i.e. the isotropic relations 𝗗 = κ 𝗘, 𝗕 = μ 𝗛. And that's where the comment he made in his 1905 paper that 𝗚 becomes "superfluous" comes from.

Note the " *almost* " by the way. The equations are *not* equivalent to the isotropic relations if the medium is rarefied to a vacuum κμ > α → κμ = α and κμG² → 1 in such a way that (κμ-α)/(1-κμG²) approaches a finite non-zero limit; i.e. if the frame of isotropy is at light speed. The irony of this, of course, is that it corresponds to the very case alluded to in the very question (and the answer to it) that sparked Einstein's foray into relativity "what it is like to travel alongside a light beam?" A residual dependence on G² remains in the limit in that case. As a result, there continues to be a lingering vestige of an "aether frame" *even* in Relativity. An experiment to verify this, given the high speed of the medium required, would probably be something involving plasma physics.

[nathanlansford]

Actually, Einstein did NOT abolish the aether--he himself returned to it after eleven years of rejecting it.

[kevinlung]

If nothing is nothing, then nothing does not exist. Therefore there is always something in empty space, but it has no name yet if it is not called aether .

[Zamicol]

I'm listening to this again a year later.

Whoa! This talk is fantastic. Thank you!

[xDRTeK]

Yesterday, I was taking a stroll in a local library. Found a fascinating book The Odyssey by Homer in modern English. I previously had the Illiad, but in the old English, the king's english as one would speak. However, now as I watch this presentation, I am reminded that knowledge is massive and we cannot preserve it all if knowledge is stored in diverse forms. Surely, if put on paper and on some digital storage it must be preserved, but that is not what I mean. It is only truly preserved if commanded to memory. If only to keep the human element of experimentation for permanent recollection, that would give us the future generation a sense of continuity. The world we live in is a form of causality. Knowledge is a propagation of thought. The idea that we relinquish some of our previous knowledge from earlier innocent existence is foreboding. I will bookmark this video. It holds a sentimental value.

[dimension]

His book Light Speed is super easy to understand. I bought the hard cover. I never buy books except for Feynman.
Such a confusing puzzle this defunct business of the aether.
A great story about Bradley and
David Hughes. Fascinating read and what could be better than to see the author at RI.
Mahalo again RI.

[ZeedijkMike]

RI just keeps publishing great lectures. Yet another hour wel spend.

[adamh]

Awesome lecture. I have a pretty solid understanding of the discussed math and theory, but also know how important it is to make this stuff approachable. This lecture has a huge amount of information, while never requiring an advanced understanding of the principles. The stories and history of the experimental procedures that went into providing evidence or proving these theories was fantastic, most of which I had never heard before. Math and physics stories were always my favorite part of school, when I had talented instructors.

[Markoul]

All what Einstein did is to replace the term "Aether" with the term "fabric of spacetime" both meaning "not nothing" and can be treated as a medium. The only difference is that although Aether can be bend and stretched also with Electromagnetism besides gravity, the Einstein's fabric of spacetime can be bend and stretched only by large gravitational masses. So no, he did not abolished the Aether but just renamed it to vacuum space and changed its properties.

[Chris.Davies]

6:18 - Yep, this is what Quantized space looks like. With it being created by quanta at the sub-Planck scale, which are denser where mass is higher, and which behave like a superfluid, offering practically no resistance at solar system distances, but they have an effect on EM radiation as it travels between stars and galaxies. It also easily explains orbital mechanics without trying to bend space, and why there's no such thing as a graviton.

[PhysicsHigh]

Good historical overview about the nature of light, it’s speed determination and independence of frames of reference. Though I do think the Einstein portion was rushed. The title in the thumbnail isn’t quite correct as he did not really explore the quantum nature of light

[bokchoiman]

I love hearing about theories being confronted and re-adjusted. A perfect representation of the tenacity of the human experience.

[utcsjakie]

Title is a bit misleading watched whole video and havent seen 1 believable fact of why Einstein ''abolished'' the Aether. Why speak so much about Light. Gravity, magnetism and energy space and time are also affected by concept the Aether.

[seanmccann]

In 1849 the hz value for the note "A" was not equal to 440hz. It was actually A= 432hz. If you tune your instruments to A=432 you tend to get better harmonics and depth because the instruments were designed with that frequency in mind. A=440 was not made the standard until sometime around WWII. Can not remember the exact date.