stupid physics questions

[pogo]

FTL communication or travel is impossible - just as impossible as time travel. With that in mind although our ability to see the stars and beyond them has improved colossally we are still no closer to them than our ancient ancestors. In that sense our universe is no bigger.

You could even say it is smaller. While the ancients looked at the stars and saw mythical beasts, gods and heroes that were as real to them as many historic figures, we know the stars are incredibly remote and unreachable, trillions of miles away. The universe we can reach is far smaller and emptier than that of our ancestors, consisting only of a few planets, their moons, a few dwarf planets, and a lot of large rocks.

[gumboot]

FTL communication or travel is impossible - just as impossible as time travel.

Yes, but if it was suddenly realised, one day, that that was incorrect because somebody forgot to carry the 1, then the immediate consequence would be that things beyond the observable universe would become observable.

And I was going to put figures to that, but immediately I fail to understand them.

If the observable universe is 93 billion light years across (47 billion light years in radius) but only 14 billion years old, then how is it not the case that an observation made of an event occurring at the edge of the observable universe does not actually pre-date the observable universe itself?

[edit] nevermind, it's right there on wikipedia. That event actually happened 14 billion light years away but has moved an additional 33 billion light years further away over the subsequent 14 billion years.

Same as how time works in the bible, I think.

[zip accident]

[edit] nevermind, it's right there on wikipedia. That event actually happened 14 billion light years away but has moved an additional 33 billion light years further away over the subsequent 14 billion years.

Same as how time works in the bible, I think.

Yeah, that is a bit strange, but it seems to be what they say.

Is it an implication of that, that galaxies should appear to be closer together the further away they are?

[gumboot]

I couldn't begin to try to speculate on that one until I could reconcile the implications of "moving" 33 billion light years in only 14 billion years.

[Chontler]

The universe expanding is not the same thing as galaxies moving within it it or further away. We're like ants running on a balloon that God is blowing up. Er, I mean some mysterious force. Anyhow, it may also be that the balloon can be inflated faster than light.

[gumboot]

It sounds so much like when people talk about how in the bible people used to live for hundreds of years and it all made perfect sense because time was different back then.

[pogo]

The universe expanding is not the same thing as galaxies moving within it it or further away. We're like ants running on a balloon that God is blowing up. Er, I mean some mysterious force. Anyhow, it may also be that the balloon can be inflated faster than light.

Not that mysterious. It all comes from the Big Bang, the event that started the universe. We don’t know what caused the Big Bang. What was there before it, even if it makes sense to talk about before the Big Bang as everything including time starts with it. But the universe since is relatively well understood.

There are still a few unknowns. For a period we think the universe expanded much faster than it is expanding now – cosmic inflation – and we don’t know why. There’s also dark matter and dark energy, which we think makes up most of the matter and energy of the universe but is so far undetectable. But the broad outline of how the universe was created and is expanding is known.

[gumboot]

I may have done my maths wrong, or used an inappropriate definition of light year. If an event was 13 billion "light years" away when it occurred then 6 billion years ago the waterfront would still be more than 6 billion "light years" away because that gap has expanded, so it would not be arriving any time soon.

Right?


So at the birth of the universe, how far away was the most remote observable event?

[pogo]

At the birth of the universe everything was in the same place, at the Big Bang.

The physical universe is thought to be bigger than the visible universe, but there is no contradiction in that. The best model is the one of a balloon being inflated, with the universe, e.g. us, on the surface. The laws of physics as we know them are those of that surface, so we can travel no faster than light across it, and can see no more than 14m light years or so. But the balloon inflation is not governed by this, so can be, and is thought to be, larger than the visible universe (though we cannot see it directly to confirm this).

[gumboot]

Doesn't help me with the maths. That only says that there was _nothing_ 13 billion "light years" away at the birth of the observable universe.

So let me ask a simpler question.

6 billion years ago, an event 6 billion light years away occurs. When will the wavefront from that event arrive at Earth?

[pogo]

Now, almost by definition. If something is six billion light years away it means its light takes six billion years to reach us, so we are seeing it as it was six billion years ago. For the large scale universe we don’t have any other way of measuring things. We can’t get out there with a measuring stick and check the distances are the same. Measuring distance using time may seem odd but it makes sense as according to relativity they are one and the same thing.

[zip accident]

6 billion years ago, an event 6 billion light years away occurs. When will the wavefront from that event arrive at Earth?

I see your question, the expanding universe means there's more space. I think the answer you get is going to be "Now, there is no ether, there is no medium the light travels through, it leaves, it arrives, there is nothing in between."

[gumboot]

Now, almost by definition. If something is six billion light years away it means its light takes six billion years to reach us, so we are seeing it as it was six billion years ago. For the large scale universe we don’t have any other way of measuring things. We can’t get out there with a measuring stick and check the distances are the same. Measuring distance using time may seem odd but it makes sense as according to relativity they are one and the same thing.

But since the wavefront left the event, six billion years ago, space has expanded. By the time it reaches us, the distance will be maybe twice as large as it was when the light started its journey.

If the wavefront took two discreet steps to reach us, then three billion years ago we might say that it it had covered half the distance, but then we consider that space has expanded and so it has more than another three billion light years to travel. So now it must take more than three billion years to cover the remainder of the distance.


Conversely, if you say that a light year is always a relative measure, then the logic that declares that the observable universe extends further than the age of the universe doesn't make any sense, and the radius of the observable universe must then match the age of the observable universe.

[gumboot]

6 billion years ago, an event 6 billion light years away occurs. When will the wavefront from that event arrive at Earth?

I see your question, the expanding universe means there's more space. I think the answer you get is going to be "Now, there is no ether, there is no medium the light travels through, it leaves, it arrives, there is nothing in between."

By "no ether" you mean no halfway point? So what if you put a relay station at three light years and a transmitter at a further three light years and then repeat the experiment? Say, have the station transmit its own ping when it observes the event, and then compare the arrival times of the two events at earth.

Does the definition of get distorted by the expansion of space? Would I ned to transmit another event in the reverse direction and have the relay station compare arrival times only to discover that they do arrive at the same time and yet ...umm... mumblemumble somethingorother...

[zip accident]

6 billion years ago, an event 6 billion light years away occurs. When will the wavefront from that event arrive at Earth?

I see your question, the expanding universe means there's more space. I think the answer you get is going to be "Now, there is no ether, there is no medium the light travels through, it leaves, it arrives, there is nothing in between."

By "no ether" you mean no halfway point? So what if you put a relay station at three light years and a transmitter at a further three light years and then repeat the experiment? Say, have the station transmit its own ping when it observes the event, and then compare the arrival times of the two events at earth.

Your experiment doesn't work, because the rate of expansion of the universe is ... complicated to think about. It is (presumably) constant per unit distance, but the distance matters because at several billions of lightyears the expansion is faster than light, whereas between lightyears it definitely isn't anything like that fast, maybe as slow as the speed of sound or less.