The trouble with the speed of light

It’s hard to observe the universe when it’s constantly moving away from you
Two of the biggest misconceptions about cosmology are that the universe must have a limit, and that our ability to learn about it doesn’t.

Although we know it had a beginning in time, nothing in our observations suggests that the universe has an edge. The universe seems pretty much the same in all directions. It’s a basic tenet of cosmology that there’s no “special place” in the Universe – it’s called the cosmological principle.

It’s true that things are very different if you’re situated on a planet inside a galaxy rather than floating in empty space. But when you zoom out for a large‑scale view, one region of space looks more or less like another. The universe could just go on forever.

However our knowledge of the universe does have an unsurpassable limit. There are parts of the universe we will never be able to see at all, and for the most distant things we can see, our knowledge about them is stuck in the past.

The culprit here is the speed of light. Light travels at about 300,000 kilometres per second. The distance light travels in a year, about 9.5 trillion kilometres, is called a light-year.

The fact that light has a speed introduces a delay between when light leaves something (say, the Andromeda galaxy) and when it gets to us (2.5 million years later).

The farther away something is, the longer its light has taken to reach us, meaning the snapshot we get is more and more out of date. The most distant galaxy we can see is so far away that its light took 13.4 billion years to reach us.

Because of the light speed delay, it still looks to us like a baby galaxy that existed in a baby universe, only 400 million years after the Big Bang.

Looking even further out, we can see the cosmic microwave background, the afterglow of the Big Bang itself. That light has travelled 13.8 billion years to reach us – almost the entire age of the universe.

This is the most distant light we can see, but since the universe has been expanding since the beginning, the part of the universe it comes from is not, as you’d expect, 13.8 billion light years away, but more like 46 billion light years away.

This distance of about 46 billion light years is called the particle horizon, and it defines the edge of the “observable universe”.

It’s a basic tenet of cosmology that there’s no 'special place' in the universe – it’s called the cosmological principle

Energy bursts
Energy bursts spotted possibly traveling faster than the
speed of light
It’s not a real edge in space – we have reason to believe that the universe must be exponentially larger than the region we can observe directly – but it is a real edge in our knowledge.

If there is, right now, a galaxy 50 billion light years from Earth, we can’t possibly see it: even starlight that left that galaxy as soon as it came into existence couldn’t possibly have crossed 50 billion light years in the time between the beginning of the universe and today.

What’s worse, we will never see it, or anything else beyond the horizon, no matter how long we wait for the light to arrive.

Nothing can travel faster than light through space, but space itself doesn’t have that constraint, and if it’s expanding in all directions, there will be points far enough away from each other that they’re getting further apart faster than light can travel between them.

The light leaving that galaxy now not only has to travel through the space between us, it has to contend with the fact that the space between us is getting larger all the time. And the expansion is speeding up. If you’ve ever accidentally set a treadmill to a speed that’s faster than you can run, you know how that light beam feels.

As time goes on, we’ll be able to see less and less of the universe around us. Eventually, everything outside our Local Group of galaxies will slowly fade as the conveyer belt of spacetime pulls it away from us faster and faster.

Astrophysicists of the distant future won’t be able to see the rich and beautiful universe we have the privilege to see now.

So we better make the most of it while we still can.

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July 10, 2016 at 11:29 PM delete

Fascinating and depressing. Probably why Earth has never had alien visitors and why space travel as seen in science fiction will never happen. There is probably lots of life out there but this law of physics means we will never meet up with it.

All the life out there, thinking about the other life out there (as we are), but all equally condemned to a life of lonely isolation.

July 10, 2016 at 11:30 PM delete

So even if we can figure out how to be the equivalent of being 'higgless'( no discernible mass) we are still trapped by the universal constant? So we may get the invite for tea but find out we turned up to late. Just Fermi sat their saying

July 10, 2016 at 11:32 PM delete

I saw the atomic clock on the plane experiment to prove there is a difference in time  a few years ago. As a Sifi fan but clueless about real physics and maths. If I travel at say 10% of the speed of light for 100 years say, what would be the difference between me and an earth based clone working in a cafe in birmingham in time dialation? Apart from the ability to make bacon sandwiches or number of books read? 8^)

July 10, 2016 at 11:36 PM delete

Photons can't travel faster than c and they also cannot travel slower than c because they are massless. The rest mass is defined as
and if this is zero, like for photons, you may easily show that pc2/E=c

and this is how the speed is defined for an energy-momentum vector.

More conceptually, nothing can move faster than light in special relativity because according to special relativity, it would be equivalent to traveling backwards in time which would violate causality. A trajectory
where V>c may be Lorentz-transformed to another trajectory where
where a

is negative, so the trajectory goes backwards in time. Because the laws of physics must have the same form in this frame, it means that the identity of the cause and the effect may be switched. Because there's no sharp ordering between the cause and the effect, you may kill your grandfather before he met your grandmother, making your own existence inconsistent.

Such things can only be avoided if the cause preceded its effect from the viewpoint of all inertial systems which is equivalent to the effect's belonging to the future light cone of the cause, i.e. to the speed limit c
on any velocity that any signal or material object may have.

July 10, 2016 at 11:39 PM delete

Just to be a wiseass, I'll point out it's certainly possible to get light propagating with a phase velocity faster than c (where c=speed of light in vacuum). It's also possible to get light propagating with a group velocity faster than c. Folks do it all the time. Of course, this doesn't answer your question, but I'm not sure anything will. Does Jerry Schirmer's formula help you understand why light travels at the speed of light any more than pointing out "that's the way our universe is"? It's an experimental "fact" that light in vacuum always travels at the same speed.