Happy 95th Anniversary, Relativity

StartsWithABang (3485481) writes "It's hard to believe, but there are people alive today who remember a world where Newtonian gravity was the accepted theory of gravitation governing our Universe. 95 years ago today, the 1919 solar eclipse provided the data that would provide the test of the three key options for how light would respond to the presence of a gravitational field: would it not bend at all? Would it bend according to Newton's predictions if you took the "mass" of a photon to be E/c^2? Or would it bend according to the predictions of Einstein's wacky new idea? Celebrate the 95th anniversary of relativity's confirmation by reliving the story."

Comments

95 years but

[rossdee]

its less than that time if it was travelling at significant speed

Re:95 years but

[barlevg]

I realize you were making a joke based on a perception common in popular culture, but the truth is that the Schrodinger's Cat paradox has a simple resolution: the cat *cannot* be both alive and dead because the detector (which detects whether the decay has occurred and which triggers the release of the poison if the decay occurred) collapses the wave function of the particle. There's no such thing as a passive detector. So while a subatomic particle could indeed exist in a superposition of "decayed" vs. "not decayed," the second you go about asking the particle whether it's decayed (that is, when you set up the detector), the wave function collapses, and no superposition is possible.

Re:95 years but

[barlevg]

The photons themselves are still traveling at c. What's "slowing them down" is that they're being absorbed and re-emitted by the atoms in the medium. The speed of light is absolute.

Simultaneity is in the eye of the beholder.

[mpoulton]

95 years of confusing the heck out of second-semester physics students! You didn't think you signed up for a calculus-based philosophy class with numerical answers to epistemological questions...

Re:Simultaneity is in the eye of the beholder.

[barlevg]

The truth is, relativity doesn't have to be as confusing as it's usually made out to be. The most accessible explanation I've found for time dilation came from Brian Greene's The Elegant Universe :

Suppose you have a race car that can only go 100 m/s, no faster, no slower. Suppose it's racing down a very wide track that's 1km in length . Depending on the angle at which the car travels, it may cross the finish line in 10 seconds, 20, 50 or however long, just no less than 10 seconds. So similarly, we can think of our journey through the universe as happening along a "time" direction as well as three "space" directions: the faster we travel through space, necessarily the slower we travel through time, but no matter what, we're travelling at c.

The math even works out, in terms of c=sqrt(v_x^2+v_t^2) where "v_t" (your velocity through time) is c*dtau/dt.

This analogy obviously only gets you so far, and the real "wow" of relativity comes from the concepts of simultaneity (I wish more SF authors realized that FTL and time travel are the same friggin' thing), but especially for non-majors this is a great way to get one's foot in the door and begin to understand what is a pretty alien concept.

Re:Simultaneity is in the eye of the beholder.

[Jason Levine]

I never found relativity to be that hard or confusing. Now quantum mechanics on the other hand.... *shudders*

Re:Simultaneity is in the eye of the beholder.

[TopherC]

Funny, I read that book (which is excellent) but don't remember that analogy. But I think you're talking about special relativity, not general relativity. The best GR explanation I've seen is an article Lost in Hyperbolia. For me that explanation worked perfectly.

Now I remember reading in various places that the solar eclipse data on GR was not actually conclusive. Bad science. The earlier work Einstein did that explained the precession of mercury's orbit was actually the first confirmation of GR. Also, of course, confirmation is not a word that is ever used correctly in science. The precession of mercury's orbit disproved Newtonian gravity but failed to disprove GR. The bending of starlight by the Sun would have been an even more impressive failure-to-disprove GR if the data were actually conclusive.

Re:I don't believe in relativity

[Anonymous Coward]

It's nowhere to be found in Genesis.

So now that we have had the official American view on the matter, any other nationalities care to chime in?

Re:I don't believe in relativity

[The Rizz]

Not sure what a British rock group has to do with a German/Swiss/American physicist's work, but whatever...

Re:I don't believe in relativity

[Anonymous Coward]

It's nowhere to be found in Genesis.

Sure it is! How do you think Methuselah lived for 969 years? Time dilation, dude.

Re:I don't believe in relativity

[TapeCutter]

Einstein was a (mono-cranium) Tasmanian, his theory of roll and rock has been well documented. The drummer in Genesis had been accused of robbing a train and fled the UK, he went to the most remote place on the planet he could think of (Oz) and witnessed Einstein giving a lecture about the theory of "roll and rock. You will notice 'Bert had, by this time, renamed his theory to the more familiar "rock and roll" which is what Ronald Biggs heard and took with him to Genesis - (BTW, that black haired beauty with big brown eyes in the lecture video is a young Marie Curie).

Re:Does mass matter?

[TapeCutter]

Newton's primary insight is the gravitational field, ie: two bodies attract each other with a force proportional to the combined masses and the distance between them. That he invented calculus to prove it and wrote it all down in his "Principia" is why he is remembered. A photon is neither a hammer , nor an atom. Photons did not have mass so they were believed to be unaffected by gravity. Einstein came along and said mass and energy are two forms of the same thing and a photon would be affected by gravity. The experiment in TFA allowed the universe to make the final call.

Trivia: Newton's Principa contains only two explicit assumptions, one of them was the assumption that "time is constant".

Re:tachyon

[itsdapead]

Lorenz contraction, says the tachyon.
Why the long face? asks the barman.
A tachyon walks into a bar.

The poetry of the Universe

[wjcofkc]

When contemplating phenomena in this universe, I find that in a small number of situations, a rudimentary understanding can be more readily had by a humble and feeble intellect such as mine if I simply drop the speed of light squared from the equation. C squared is where things get strange. Consider the following: A star 100 million light years away ignites. From out relative position and motion, we measure the light as traveling at ~186,300 miles per second over a distance of 100 million light years. As far as we are concerned, it took a long time to get here. Now for the tricky part. As everyone here I am sure knows, time slows down the closer you get to the speed of light, coming to a standstill once the cosmic speed limit is reached. As a consequence, as soon as the light from that star was generated, it was instantly already here. From our perspective, it took 100 million light years to get here. For the perspective of the light itself (so to speak), the transit time was 0. Apply that to light that is older than the Earth and it becomes a real mind-fuck. In fact, kick back and expand on that concept in many different ways. At least this is according to Dr. Tyson. Despite the complexities, E = MC squared is elegant mathematical poetry.

Re:Does mass matter?

[AthanasiusKircher]

Newton's primary insight is the gravitational field, ie: two bodies attract each other with a force proportional to the combined masses and the distance between them.

It's worth noting that this insight was not at all unique to Newton. There was, in fact, a major dispute in the scientific community about who came up with this idea at the time, since Robert Hooke had already published on this notion. Other scientists had basically also postulated similar ideas in the decades before the Principia.

That he invented calculus to prove it and wrote it all down in his "Principia" is why he is remembered.

Yes -- Newton may have been the first to explicitly identify the specific inverse square relationship (rather than a general form relationship mentioned in the first quotation above), and he had the mathematical apparatus to prove how it all worked.

But it's also important to be clear that the idea of a "gravitational field" or an "unseen force acting at a distance" was a very spooky and strange notion to contemporary scientists in Newton's era. In fact, such ideas were commonly associated with occult ideas; they didn't fit in with the conception of a simple mechanistic universe. Thus, Newton's idea of some strange unseen "force" acting across vast distances would seem like invoking the power of God or angels or some mystical astrological "force" today.

Because of that, many scientists were initially very suspicious of Newton's methodology. Newton therefore wrote a clarification as an appendix to the second edition of the Principia explicitly saying he was NOT assuming the existence of unseen forces and fields. Instead, he claimed his model was valuable simply because the mathematics were an accurate model. (Some historians have argued that this was in fact the most important element of Newton's revolution in thought: he argued for the acceptance of a mathematical model as a scientific explanation, even if we can't explain the underlying causes of that model.) Of course, Newton was a pretty weird guy and believed in all sorts of things that modern science would think weird, so obviously he thought the unseen forces were real. But it's interesting that he worked so hard to distance himself from such ideas at the time -- to be in accord with science of the time, the "force" in his model was thus to be considered a mere mathematical contrivance, rather than how the universe actually worked.