I like to read up on science reporting when I can, and that’s what made an article about more proof for general relativity catch my eye. You see, I’m a longtime Einstein skeptic. Bear with me, because I know iconoclasm always has a tendency to come off a little like the ravings of a crackpot.
At a pretty fundamental level, the idea that photons always travel at exactly the speed of light is, well, stupid. That’s not to say it couldn’t be true, but it’s outlandish and I think there are perfectly sensible explanations that work better. I understand the math behind the concepts of time dilation, length dilation, etc., but there’s really nothing about those equations that’s incompatible with merely describing what people would observe from another vantage point. Furthermore, the clock paradox makes no kind of sense whatsoever. The idea of photons being emitted at the speed of light is a lot more sensible.
Still, there are a lot of supposed proofs of Einstein’s theory, or at least experiments and data supporting it. Atomic clocks run slower in space, which our GPS system actually depends on—but I think this is of a piece with all other proofs, in that when you get down to cases, every single system of measurement we have is based on forces that propagate at the speed of light. The theory predicted the correct orbit of Mercury, but the mere fact of gravity propagating at light speed (rather than instantaneously) does the same. The theory predicted light is bent by gravity, which is the subject of the article, and in fact it is; but there are other perfectly sound explanations for such a phenomenon, as I will explain.
There’s one other wrinkle to consider, which the article in question even mentions:
Scientists expect that Einstein’s theory of general relativity, which is already incompatible with quantum physics, would at some point no longer hold true in extreme conditions.
Here’s the problem: Every time quantum physics goes up against general relativity head to head, quantum physics wins. I’ve never heard of an exception. Physicists know that at some point, one or the other, or both, has to break. When we know which one will break and how, we’ll be closer to a unified theory. In the meantime, though, general relativity is accepted as darn near gospel.
The article basically says that gravitational lensing has held up under scrutiny under extreme conditions around a pulsar. Light is bent by gravity, which makes perfect sense if gravity is merely a function of space being warped by mass. But light being bent by gravity is not incompatible with Newtonian physics, depending on how you view the math.
Gravity is merely a force between two objects, and according to current theory—which I frankly suspect is incomplete—it’s proportional to the mass of both objects and decreases with distance. I think the law of gravity is incomplete based on the same data that suggests dark matter and dark energy are real things. The fact that we haven’t really found dark matter, let alone that we can’t pin down the gravitational constant to more than a few decimal places, suggests maybe there’s a better explanation and the formula we have needs modification. But that’s a subject for another discussion.
Assume the law of gravity as we know it is perfectly fine, and there is dark matter and all that to shore up the holes. It’s not an unreasonable assumption, barring better information. So with a photon having no mass, there’s no force, hence no gravity; hence light should travel in a straight line, right? Nope.
The equation for gravitational force is F = Gm1m2/d2. The formula for how that relates to acceleration is F = ma. But if the photon’s mass is zero, that’s 0 = 0 and it’s a pretty meaningless equation. It makes more sense, therefore, to look at acceleration caused by gravity, rather than gravitational force, which would be a = Gm/d2, m being the mass of the other object. The zeroes effectively can cancel out, and the photon is moved by gravitational attraction from another object but not vice-versa. Conservation of momentum isn’t broken because the photon has no mass.
Alternately, it’s possible that the photon does have mass, but it’s so infinitesimal that even our most sensitive measurements can’t read it. Nonzero mass of any size would mean the force of gravity was nonzero, and then the math works just fine. Given that photons carry energy, and E=mc2 is still sound as far as I know, it does seem like a photon should have mass. Apparently the idea that the mass of a photon is zero isn’t really all that set in stone.
Either way, you don’t need Einstein. So seeing predictions of gravitational lensing hold up proves nothing. All it really proves is that photons are bent by gravity—it tells us nothing of why. Theorists were trying to see whether gravitational lensing works the same in extreme conditions or not. It’s still valid research because if it didn’t hold up it would tell us something interesting, but the fact that it works just as predicted doesn’t mean Einstein’s answer is the only one.
Update: A different article referring to the same study suggests this is less about lensing than about gravity waves. Noted, though I don’t think it steps on the point: Many things predicted by general relativity are explainable by other means.