Excitement Over Gravity Waves Comes Crashing Down | Hacker News Reader

Excitement Over Gravity Waves Comes Crashing Down

nautil.us

27 pointsdnetesn9 comments

>These swirls would bolster the idea that the infant cosmos expanded faster than the speed of light during a brief growth spurt called inflation.

what speed of light they are talking about - today's one or the one that was ruling Universe back then? Physical constants do change with changes in the Universe (otherwise the visible Universe which was long time ago inside its current Schwarzschild radius - 10B in today's dollars ... err ... light years - would have supposedly crossed it to get to grow to the 46B light years the Universe is today. Either it crossed the Schwarzschild radius or the Schwarzschild radius of the same visible Universe was "deflated" back then and got inflated together with the Universe inflation - that means physical constants change. Actually the constants may be the same - for the speed it is just length of the different, "deflated", space divided by the different, "deflated", time)

The speed of light is a dimensionful quantity -- it has dimensions of length over time. Dimensionful quantities cannot be measured, only dimensionless ones can be measured: we can only measure pure numbers.

When we say we measure the speed of light, we are really measuring the ratio between that and some other arbitrary speed defined by our measurement device. Therefore, it makes little sense to ask how the speed of light changes over time; we can only ask this question after we specify how we measure, and then it's no longer a fundamental question.

It may be a bit surprising that the speed of light is actually a consequence of universal constants interacting, and not merely measured via reference benchmarks [0]. At least for me, finding that Maxwell's equations distill an equation that happens to be a velocity in a vacuum was extremely gratifying. There's a notion that perhaps if the constants were different, then c would be different as well - both in scale and even dimensionally; this is, I think, what the parent is referencing.

EDIT: I'd like to restate why c is neat. There's a lot of history to it, but it's sort of a form of a number ending up so important that it overtook the archetypical "and a constant c". I highly recommend the article from Phillip Gibbs that goes into this in depth [1]. Your comment actually shows up near the beginning of the long answer in the form of paragraph 5 of The Long Answer, so you'll likely enjoy the additional detail.

[0] http://en.m.wikipedia.org/wiki/Maxwell's_equations#Vacuum_eq...

[1] http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLigh...

These universal constants (vacuum permittivity and permeability) are themselves dimensionful quantities, so the same comment applies to them. They can only be measured by comparing to a reference, and there is no natural sense in which they can be said to change over time.

By the way, these constants are no more 'universal' than the speed of light itself.

The fundamental constant you're looking for is the fine-structure constant (often referred to as alpha). It's a dimensionless ratio of physical constants, and if its value changes, physics is different.

I mean, sure, it's going to be turtles the whole way down if you're looking to define the universe without resorting to physical quantities (that is, dimensioned units). We don't have anything like a Grand Unified Theory yet, wherein we define Physics by means of some elemental mathematics (in the form that we can bootstrap computation from lambda calculus via things like the Y Combinator [0]). Besides, we occasionally give dimensional names to dimensionless quantities (my favorite being the engineering unit the inverse radian) [1]. It's a useful abstraction, and it seems confusing to complain about it.

So we resort to what we know, which is what we see. We can't see more than the surrounding universe, but we can try extrapolating from what seems to work. Attempting to stifle that inquiry seems a bit... I dunno, pointlessly antagonistic.

This article shows science in action, doing what it does best: taking observations and reconciling them with other observations via theory, and then fixing theory to match. It seems our universe follows certain equations which are themselves dimensionful, so I'm not sure why we wouldn't attempt to benchmark the universe against itself - we don't have much else to use. Even dimensionless numbers are only marginally useful, like alpha; it may "exist" and manifest as our universe, but it's not especially predictive.

[0] https://www.youtube.com/watch?v=FITJMJjASUs (one of my favorite videos - I still don't grok it, but I'm getting closer!)

[1] http://arxiv.org/pdf/1409.2794.pdf

Maybe the speed of light has changed, but your reasoning is completely wrong.

The big bang theory is based on solutions to the equations of General Relativity, the same equations that predict black holes. Since these equations do allow for the big bang theory (without any changes to the speed of light), your verbal argument must not properly capture the theory.

More info is here http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/... or just google "big bang black hole"

I agree with the last quote in that list. This is a really wonderful example of science working the way it's supposed to.

Sometimes I wonder if ppl actually know there's more stuff in the world then what the eye can see.

Like that there's more stuff in the universe, then just the glowing stars.

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