If the aether is at rest and the Earth orbits the Sun at speed v≈30 km/s, then on Earth we should feel an "aether wind" blowing past us at speed v. Light moving along the wind vs. across it would then be measurably affected — like a swimmer in a river.
What medium was light thought to need before 1905?
The luminiferous aether.
What did the Michelson–Morley experiment try to detect?
Earth's motion (velocity v) relative to the aether, via a light-speed difference between arms.
What was the experimental result?
A null result — essentially no fringe shift was observed.
Round-trip time for the parallel arm?
t∥=c2L1−v2/c21
Round-trip time for the perpendicular arm?
t⊥=c2L1−v2/c21
Why is the cross-arm light speed c2−v2?
The beam aims upstream; total speed c with current v gives cross-component c2−v2 (Pythagoras).
Leading-order expected time difference?
Δt≈c3Lv2 (proportional to β2).
Why doesn't the parallel arm's fast/slow legs cancel?
You spend more time in the slow (c−v) leg, so the round trip is longer than no-wind.
What postulate of SR did the null result support?
The speed of light c is the same for all observers (constancy of c).
What classical patch tried to save the aether?
FitzGerald–Lorentz length contraction.
Order of magnitude of β=v/c for Earth's orbit?
∼10−4, so β2∼10−8 — tiny but detectable by the interferometer.
Recall Feynman: explain to a 12-year-old
Imagine two identical swimmers in a flowing river, both swimming the same speed in still water. One swims down the river and back; the other swims straight across and back. Even though they're equally fast, the river current makes their times slightly different. Scientists thought space was filled with an invisible "river" called the aether that light flowed through, and the Earth was zooming through it. They built a machine that raced two light beams — one along the "river," one across it — to catch the time difference. But the beams always tied. No matter how they turned the machine or what time of year, perfect tie. That meant there was no invisible river at all, and light always travels at exactly the same speed for everyone — the idea that launched Einstein's relativity.
Bhai socho — 1800s ke scientists maante the ki light ko travel karne ke liye ek invisible medium chahiye, jaise sound ko air chahiye. Us imaginary medium ka naam tha aether. Logic simple tha: Earth sun ke around 30 km/s se ghoom rahi hai, to is aether ke andar Earth move kar rahi hogi, matlab humein ek "aether wind" feel hona chahiye — bilkul jaise chalti car ki khidki se haath bahar nikaalo to hawa lagti hai.
Ab Michelson aur Morley ne ek interferometer banaya. Ek light beam ko beam splitter se do parts mein toda — ek beam wind ke parallel jaata hai aur wapas aata hai, doosra wind ke perpendicular. River-swimmer analogy yaad rakho: current ke saath-against jaane wala swimmer aur across jaane wala swimmer, dono ki speed same hone par bhi time alag lagega. Isi time difference se interference fringes shift honi chahiye thi. Maths se predicted shift tha around 0.4 fringes, aur unka machine 0.01 fringe tak detect kar sakta tha — to shift saaf dikhna chahiye tha.
Par twist ye hai: kuch bhi shift nahi mila! Chahe machine ko ghumao, chahe saal ke kisi bhi time karo — light dono directions mein exactly same speed se chal rahi thi. Iska matlab: ya to aether hai hi nahi, ya phir light hamesha har observer ke liye same speed c par chalti hai. Yahi se Einstein ka famous second postulate aaya — speed of light is constant for everyone. Isliye ye "failed" experiment actually physics ka sabse important experiment ban gaya, jisne special relativity ka raasta khola.
Key formula yaad rakho: parallel arm ko 1−β2 se divide karo, perpendicular ko 1−β2 se (Pythagoras se c2−v2 aata hai), aur difference ≈Lv2/c3 nikalta hai. Bas isi tiny difference ki talaash thi — jo kabhi mili hi nahi.
