# Same as it ever was?March 14, 2012 8:17 AM   Subscribe

Is the speed of light constant on the timescale of the universe?

From my understanding the speed of light in a vacuum is constant in all reference frames, at 299,792,458m/s. The only way for something to move faster than this is for the space between two reference frames to expand, increasing the amount of distance between them that light has to travel (the two objects on a partially inflated balloon analogy). Such an expansion happened immediately after the big bang, and the universe has been expanding (albeit slower) ever since.

What I don't really understand is why the vacuum of today should be the same as the vacuum of 5 billion years ago. Sticking with (and probably breaking) the balloon analogy, as the balloon is inflated the properties of the rubber change. A wave moving from one point on the inflated balloon to another should move faster than before, due to the tension. For light through a vacuum, I guess this would mean the vacuum is 'more empty'? It has less vacuum energy?

Either way, it seems like light should be moving faster as the universe expands. The change in speed would be incredibly minor on any timescale we can measure, but over billions of years significant. Since I haven't seen this theory anywhere outside of my head, I assume I'm missing something.

What am I missing? Is there some compensatory change in the flow of time, or am I misunderstanding relativity?
posted by Orange Pamplemousse to Science & Nature (12 answers total) 2 users marked this as a favorite

Since I haven't seen this theory anywhere outside of my head, I assume I'm missing something.

It has been brought up before.
posted by vacapinta at 8:25 AM on March 14, 2012 [1 favorite]

increasing the amount of distance between them that light has to travel (the two objects on a partially inflated balloon analogy). Such an expansion happened immediately after the big bang, and the universe has been expanding (albeit slower) ever since.

But if you measured that distance between the two points before and after the expansion, wouldn't you get the same measurement? Things aren't moving farther apart in a fixed "space" dimension; space itself is expanding. That's why light is redshifted fro distant objects - the frequency changes (because the waves are more spread out), but it's still moving at the same speed. This would be related to the whole dimensional contraction/time dilation thing that you'd see at relativistic speeds.

I'm just a layman, so there's probably something I'm missing or have horribly mangled too, but that's how I understand the thinking on it.
posted by LionIndex at 8:29 AM on March 14, 2012

And isn't the expansion of the universe accelerating?
posted by LionIndex at 9:22 AM on March 14, 2012

What I don't really understand is why the vacuum of today should be the same as the vacuum of 5 billion years ago. Sticking with (and probably breaking) the balloon analogy, as the balloon is inflated the properties of the rubber change. A wave moving from one point on the inflated balloon to another should move faster than before, due to the tension.

Yup, you broke the analogy. The "tension" in the balloon doesn't have an analogue in the actual mathematical models we use, any more than there needs to be someone inflating the Universe with his or her cosmic lungs.
posted by Johnny Assay at 9:27 AM on March 14, 2012 [1 favorite]

I think Johnny Assay has it.
posted by xbonesgt at 9:32 AM on March 14, 2012

Then am I misunderstanding the idea of vacuum energy? If "empty" space has a constant amount of energy (ie. the same now as in the distant past), doesn't the universe's expansion mean that the amount of total (vacuum) energy is increasing?
posted by Orange Pamplemousse at 9:48 AM on March 14, 2012 [1 favorite]

Why do you suppose that the volumetric vacuum energy of space will necessarily impact the value of c?
posted by IAmBroom at 9:51 AM on March 14, 2012

Because light passes through the vacuum, I suppose. If the nature of the medium changes, shouldn't transit through it?

Looking over the wiki article, I might be wondering more about the permittivity of the vacuum, not so much light's speed through it.
posted by Orange Pamplemousse at 9:58 AM on March 14, 2012

Remember that C isn't just the speed of light. It's also the conversion constant for matter and energy (or C² is). If C were changing over the life of the universe, then distant galaxies wouldn't look like they do.
posted by Chocolate Pickle at 12:25 PM on March 14, 2012

I have a similar theory that time is speeding up as the universe expands, but we don't notice (at least not in a quantifiable way) because we are "stuck" in the time stream. Perhaps the change in time 'keeps up' with the expansion of space.

Or I'm talking nonsense.
posted by tacodave at 2:58 PM on March 14, 2012

Is there some compensatory change in the flow of time

time is speeding up

Normally when we talk about speeds it's change per unit time. What would it mean for time to speed up?
posted by aubilenon at 3:41 PM on March 14, 2012

Is The Speed of Light Constant?

Observational evidence against a time variation in Planck's constant (Planck's constant h relates the energy of an electron to its frequency, but frequency and wavelength are related via the speed of light, so if c varies then either the planck constant varies inversely or electron energy varies directly); the paper also cites some other papers that address experimental evidence of the actual time-constantness of so-called physical constants.

Fine-structure constant > Is the fine structure constant actually constant? The speed of light, the electron's energy, the permittivity of free space, and Planck's (reduced) constant can be placed in an equation whose value is a unitless constant (not in meters or seconds or any other SI units, just a number). This is called the fine structure constant. If the speed of light varied, but the other involved constants did not, then the Fine Strucure Constant would vary. However, there are severe limits placed on the possible variation of the Fine Structure value on a cosmological time-scale.

Finally, in honor of π day, you may be interested in: Time variation of a fundamental dimensionless constant, a paper from April 1 2009 marshalling the evidence for, well, time variation of a fundamental dimensionless constant.
posted by jepler at 5:15 PM on March 14, 2012

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