# Really exceeding the speed of light?

1. Oct 31, 2004

### Darwin

The following comes from Earth, a Smithsonian publication, © 2003, p. 24.
The early universe was small, hot, and dense. Ever since, it has been expanding and cooling. Within a nanosecond (a billionth of a second), it was hundreds of millions of miles in diameter. . . .​

Considering a speed limit of 186, 283 mps, how would such an expansion be possible?

2. Oct 31, 2004

### mathman

Space itself was expanding. The concept is called inflation. Nothing was by itself travelling faster than the speed of light, but things were being separated faster by being carried along by space.

3. Oct 31, 2004

### marcus

I agree with mathman and like the explanation!
Basically, to repeat what he said:
the distance separating two things can increase
even tho neither is moving in the space around it
and there is no known limit on the rate at which such a distance can be increasing

it can be increasing at twice or three times the speed of light, no problem

many of the distant galaxies that have been observed and have had their redshifts measured are receding at speeds faster than light. Astronomers typically just tell you the redshift---and sometimes the distance---and let you figure that out for yourself, rather than making it explicit.

Last edited: Oct 31, 2004
4. Nov 1, 2004

### RingoKid

I really do have a hard time wrapping my head around this one and as yet no one has been able to answer some simple questions. I hope it's different here...

things moving apart at faster than the speed of light but it's only the space between increasing and not the things actually moving

do you see my problem ???

You say the galaxies can be receding at faster than c, so what of the matter making up those galaxies ???

What then of someone on a galaxy shining a flashlight back at us while moving away faster than light ??? Would we only see it when our galaxy catches up to the previous position of said galaxy and pick up the photon trail ??? What then if we are moving away from the galaxy at faster than c also ??? Would we ever see the flashlight

5. Nov 1, 2004

### marcus

Ringo,
You can imagine speaking another language besides English, right?
Let's imagine we both speak a version of English where "recede" does not mean "move". Two things, neither of which is moving, each of which is sitting still, can recede from each other if the distance between them keeps on increasing.

The RATE that the distance is increasing is called a "recession speed" and that is a different type of speed from a "motion speed".

Motion is only measured locally, by comparing nearby things. And it only involves comparatively small speeds----nothing ever catches up with and passes by a photon---everything is limited to move at photon speed or less.

This local motion is all comparatively slow. The rates of recession can be much greater----but that is not local motion, that is increase in distances that are already very large, nothing is catching up to and passing something else, just the whole business expanding.
We dont say that things are moving apart faster than light. Imagine that they are all sitting still. they are merely receding from each other at rates proportional to the distance between them..so very large distances increase at very high rates.

the first part of what you said (things moving apart at faster than the speed of light ) is wrong.
the second part of what you said (but it's only the space between increasing and not the things actually moving) is right

well, of course if a galaxy is receding from us FTL then the matter making up the galaxy must be receding from us FTL. no big deal. nothing says that it is illegal for it to do that! It is not like it is MOVING in some way that would register in a local reference frame.

You ask "What then of someone on a galaxy shining a flashlight back at us while moving away faster than light ???"

But that couldnt happen. A galaxy could not be moving FTL because that would violate 1905 special relativity, which is about local frames and motion in local coordinate systems.

You should rephrase the question to be about recession speed.

What then of someone on a galaxy shining a flashlight back at us while the galaxy is receding faster than light ???

In that case you have to learn how to use Morgan's calculator to find out what happens, because it depends on when (in the history of expansion) it happened, and how far away it was at the time of the flash.
There is no single answer, about whether the flash could reach us or not.
the ratio of expansion rate to distance has not been constant in the
history of the universe. Maybe in the case of that flash the expansion ratio decreased some, and allowed the flash to get here! Maybe it did slow down some, but not enough, and that particular flash will never get here. Or maybe it eventually will but it is still on its way. You are in 1915 General Relativity territory now----it is not a straightforward flat-out linear graphpaper scene. So you should learn to use Siobhan Morgan's calculator to answer the question about the flashlight.

Fortunately Siobhan made the calculator easy to use, for her beginning Astronomy students at the univ. of Iowa. How hard can it be?
The link is in the Astronomy and Cosmology reference shelf----that A&C stickythread in this forum.

Last edited: Nov 1, 2004
6. Nov 1, 2004

### DaveC426913

"...things moving apart at faster than the speed of light but it's only the space between increasing and not the things actually moving

do you see my problem ???..."

Using the venerable balloon analogy:

Imagine reducing spherical space to a 2D plane. It's shaped like the surface of a balloon. Every bit of matter in it is an object on the *surface* of the balloon. No object can move across the surface of the balloon faster than the speed of light. Light itself travels across the balloon's surface at c.

But - the balloon is inflating. There is no law that puts a limit on how fast the balloon can inflate. Objects on the balloon will get farther apart as the balloon expands - without violating c.

(BTW, the balloon analogy also shows nicely how it is possible for space to expand without there being a definable "centre" of expansion. Every point on the balloon sees all other objects recede from it as if it were at the centre of the expansion - i.e. it's all relative!)

Dave

7. Nov 1, 2004

### marcus

a great explanation! shows the difference between recession speed (rate at which distances increase, between points on the balloon surface)
and motion speed (a bug creeping across surface of balloon while the balloon is expanding)

your discussion of this (using balloon analogy) could be extended to
cover how the cosm'l redshift occurs by the stretching out of lightwaves (and not by the doppler effect)

there is a Ned Wright animation of the balloon picture with dots and squiggles on the balloon. dots are galaxies, squiggles are photons, the photons actually move----the dots merely get farther apart.

Dave, do you hppen to have the URL for the ned wright balloon animation?