Be Excellent To Each Other

So... They sped up light?

</futurama>

I posed this to Lave the other day. The speed of light was always thought to be constant, but it's now thought in the early universe it was different. So probably faster, like.

Einstein didn't actually say that things couldn't travel faster than light, as far as I know. He simply said that things that started moving at below the speed of light couldn't move faster than light. So perhaps the first bits of the universe were already moving at superluminal speeds

Maybe, but there is no physical evidence for that. It's just a theory.

Grim..., your book is talking about the Planck epoch.

DON'T MAKE ME EXPLAIN SPECIAL AND GENERAL RELATIVITY.

I think the difference is, light moves through spacetime, and the inflation is actual spacetime getting bigger. Any photons or other particles will still be limited by c

Grim..., your book is talking about the Planck epoch. Googling around suggests the universe did not expand much in this time, indeed, it was still not much bigger than a single atom is now. Can you doublecheck what you book is saying?

Grim..., your book is talking about the Planck epoch. Googling around suggests the universe did not expand much in this time, indeed, it was still not much bigger than a single atom is now. Can you doublecheck what you book is saying?

I think his book is talking about the time straight after the planck epoch, as per my wiki link.

I know nothing about this, other than that it is possible for things to move faster than the speed of light (but, apparently, only if they've always travelled faster-than-light). Is that not true?

So how would the police catch you on this road?

A bad analogy I've just come up with that I doubt will hold if you think about it too hard is a motorway (made of rubber). The speed limit might be 50 mph, but that doesn't stop the road being pulled like a rubber band and stretching faster than fifty miles an hour. And it wouldn't make the car suddenly break the speed limit either.

So how would the police catch you on this road?

Vote to transfer the Doctor title from Gaywood to Malc, please.

But they could only do that if they were originally travelling much faster than 51mph. Plus the wear on the road would make it impossible to repair.

So it's relative? (Perhaps a better analogy would have been that the Earth is moving?)

But would the plane still take off?

I fear for the future of this thread. Also, Lave, why haven't you brought up the expanding balloon metaphor yet?

Lave, I'd be aware of the bogs due to the pressure in my bladder, the roadsigns pointing out the services, and the fact I'm on the road and roads have services. What about my piss stream?

According to the big bang theory all stars move outwards on the perimeter of a sphere from the center. The moving speed of this matter is much less than the speed of light. If that is correct how can telescopes pointed to the center today pick up any light emitted from stars that were formed billions of years ago when this light should have passed us long time ago? How does the red shift in the light frequency determine a distance?

Your questions stems from a slight misunderstanding about how we picture the expansion of the universe. As you write, one way to think about this, is to picture stars (actually galaxies, which are just bound conglomerations of stars) attached to the perimeter of a sphere, which itself is expanding, carrying the galaxies with it. Personally I like the balloon analogy, where one pictures an inflating balloon with little ants crawling around on the surface, representing the galaxies.

The key point in either analogy is that our three-dimensional space is represented by the (2D) SURFACE of the sphere/balloon. In this analogy it doesn't make sense to look at the center, since the center of the balloon isn't part of space. It lies somewhere in hyperspace, but our observations have nothing to say about this point. As the universe expands, the fabric of space itself is actually growing, the universe is getting larger, just like the surface area of the balloon. On average every ant on the surface is moving away from every other ant. I say "on average", because one must allow for the possibility of an ant (or a galaxy) moving relative to the underlying space, at a rate greater than the expansion itself. A real life example is M31 (Andromeda galaxy). It happens to have a large peculiar velocity, in a direction towards us, and is actually blueshifted. On average, however, all galaxies are moving away from all others.

It's also not true that this recession velocity must be less than the speed of light. Einstein's special relativity does state that nothing may travel faster than the speed of light, but this holds for objects moving with respect to an underlying reference frame. Einstein's theory says nothing about how fast space itself can expand. Two galaxies that are receding from each other at twice the speed of light due to the expansion of the underlying space, are not able to exchange any kind of information, since this information is confined to travel through the expanding space itself, at a speed no greater than the speed of light. Thus Einstein's theory is not violated in any way.

If you think through this expanding balloon analogy in more depth, you will discover that the rate of recession between two ants must be proportional to the distance between the two. Again, this distance would not be calculated by drawing a straight line from one ant to the other, piercing the surface of the balloon, but instead by measuring the distance from one to the other along the surface of the balloon. Think the distance between Sydney and London (10562 miles / 16997 km) - what's meant is the path along the surface of the earth, not the length of a hypothetical tunnel through the center of the earth.

So, the fact that we can pick up light emitted by galaxies billions of years ago is explained by the fact that the universe (the surface of the balloon) has expanded to an incredible size, and the photons we receive from these galaxies cannot travel outside of our three-dimensional world.

I think the above should have also clarified why redshift is proportional to distance. As you correctly state, the redshift depends on the relative speed between the objects. By the sphere/balloon analogy you now understand that in our expanding universe the average recession rate is proportional to distance and hence the direct relation between distance and redshift - Hubble's Law.

I feel sorry for the poor buggers driving the other way along the road. They'd never get home.

#expansion" class="postlink">Here's a reasonable writeup:


Edit -- applause to Lave for surely being the first person in the history of physics to describe a streak of piss as blueshifted.

What about the guys in the glittery Skyline going the other way? How will they ever get home?

Whats a glittery Skyline?