Particle Horizon Question

[brucewarren]

I'm going to have a stab at this. Feel free to laugh when I get it all wrong.

Since the speed of a light emitting object has no effect on speed of light, the light from the remotest object must get here eventually, but of course by then the image will be out of date. What we see in the telescope is not what the universe is but what the universe was. There will have been a time when the universe was still small enough for the light from the remotest bit, as it was then, to have got here by now.

So we can see the whole universe as it once was. We cannot see it as it now is.

Of course now we run into the philosophy of the tree falling in the woods. If the bear doesn't see or hear the tree as it falls on his head, does the tree exist?

To some folk if a thing cannot be measured it doesn't exist. To others it does.

So does the up to date universe we can't see, as opposed to out of date version we can, exist? I think it does but your mileage may vary.

[RegisterMe]

the light from the remotest object must get here eventually

I think that's incorrect because the universe is expanding. Imagine two light bulbs plugged through a rubber sheet. They start ten feet away. Now stretch the sheet so that they are twenty feet away. The speed of light is the same but the distance it has to cover to be seen by the "other light bulb" has doubled.

This is why though the universe is "only" ~14 billion years old the radius of the observable universe is ~46.5 billion light years.

[RegisterMe]

Actually I think what I wrote above is wrong, and that Bruce is right. I'm now thinking that the "edge" of the observable universe is a time based horizon, and with the passage of time it gets further away from us.

[Observe]

I realize we are talking about 'space' expanding the distance between galaxies, but half in jest, I wonder if the universe is expanding, then the 'space' between the atoms in our body is expanding too? At some time in the future, will I be twice my size, but since the measuring rod will also be twice size, no evidence of expansion will be seen on this small scale?

[EDIT] If 'space' is expanding, isn't time also expanding. Otherwise, the speed of light would not remain constant?

[RegisterMe]

[EDIT] If 'space' is expanding, isn't time also expanding. Otherwise, the speed of light would not remain constant?

Nope. Space is expanding, time remains constant, the speed of light remains constant, hence redshift.

[pjknibbs]

Actually I think what I wrote above is wrong, and that Bruce is right. I'm now thinking that the "edge" of the observable universe is a time based horizon, and with the passage of time it gets further away from us.

No, I think you had it right the first time. The furthest thing we can see is the cosmic microwave background radiation, which you've presumably heard of? That's the echo from the time when the universe had cooled to the point where it was no longer opaque and photons could start moving freely. It's not in the visible spectrum because it's so massively red-shifted by distance.

The interesting thing about that radiation is that it's everywhere--no matter what direction you look in, you'll see it. I always find that fascinating, myself--theoretically, the universe should have been a much, much smaller place when that radiation started its journey around 300,000 years after the Big Bang, but we nonetheless see it in every direction we look.

[brucewarren]

@RegisterMe I also thought you made a good point but my conclusion was five feet not ten.

If light travels from Lamp A to Lamp B it occurred to me that once it has set off a photon would no longer care about A. A could be going at heck of a pace and it wouldn't matter because it's in rear..

B though could be a different story. If B is getting further away then the poor photon has got a bit further to go to catch up.

That said it comes down to whether the lamp B, or us lot with the big telescopes, are legging it away at more than the speed of light. I assumed we weren't because it's not supposed to be possible for anything with mass to go that fast.

However if we're not moving all that fast but space is expanding rapidly then I'm out of my depth. Can space expand at greater rate than a photon can catch up?

[euclid]

.... Can space expand at greater rate than a photon can catch up?

Yes, much faster, but this does not contradict the maximum speed of information within the space.

Cheers Euclid

[Morkonan]

My understanding, is that while we measure photons traveling at the speed of light, for the photon, the travel time is instantaneous. From our point of view, photons emitted from the first stars, have taken 14 billion years to get to our eyes. However, from the photon point of view, it took no time and the distance was zero. Or, do I have things all wrong?

Well, if the photon could be conscious, then the answer to your question would be "Yes, more or less."

The photon doesn't locally experience "time." And... then, there's a big debate... and some really cool experiments.

So at the moment at least some people think we can in fact see the entire universe?

No, but what we can see is, mostly, the only "Universe" that matters to us. It's possible that some of what can see, at the most extreme, has already passed beyond the horizon long ago and we're just seeing the photons that have reached us.

I thought there were bits we couldn't see because the light from those places couldn't reach us due to expansion, but we knew they must be there. (sorry if I'm sounding dumb now, I'm a bit confused)

No, you are correct. (Well, that's what cosmology says for now, at least.)

A couple of notes:

IIRC, there have been some cosmological musings on whether or not the speed of light has always remained a constant. It may be that, in the early universe, it was different.

According to Inflation, still somewhat debated, but fairly firm, expansion did occur at faster than the speed of light for a brief moment in the early universe. However, at the present, expansion itself can not progress "faster" than the speed of light. BUT, the relative rate of expansion between any two points in the Universe can be greater than the speed of light. So, the speed of expansion could be, for instance, .6 c. Between any two points, however, that relative rate of expansion would be 1.2 c. That's why, again "relatively speaking" light from one object could, at some point, never reach another, not because of the actual velocity of the rate of expansion, but the relative rate of expansion between the two points. If you were able to travel to a different observation point where that relative rate of expansion between you and that object was less than c, you could then see the light from that object that has reached that point.

Another note: Gravity is stronger than the force of the expansion. It is propagated at the speed of light.

And, a link that explains why a distant point appears to recede from us at faster than the speed of light: https://www.space.com/33306-how-does-th ... light.html (Special vs General Relativity and the importance of local frames)

[RegisterMe]

Another note: Gravity is stronger than the force of the expansion. It is propagated at the speed of light.

A note - gravity doesn't kill you when you fall out of a tree. The ground does.

[mrbadger]

Another note: Gravity is stronger than the force of the expansion. It is propagated at the speed of light.

A note - gravity doesn't kill you when you fall out of a tree. The ground does.

Technically that's still gravity killing you, because the ground is preventing you from moving in the direction that gravity demands that you should.

But then what were you doing in the tree to start with? Silly person.

That reminds me of a time when I was a kid and this boy was going to fall out of a tree and asked me to catch him. I was about ten.

I figured if I tried I'd get hurt, and I had told him not to climb out on that branch that was now breaking, so I said no.

He fell and broke his arm. I didn't feel guilty. Kids are real sociopaths, aren't they

[Tamina]

Following this discussion since the last couple of days with great interest, it reminds me of a discussion I had once with the professor of astronomy of my university, at launch.
After a while of counter questioning and discussing* every answer he provided (initial topic was 'Why is the universe flat?') he eventually ended the discussion along the lines with: We don't know anything and every conclusion we make is based on assumptions that have proven to be wrong, many times. Yet, they are the closest we can get to it.
^-^


*My subjects included classical and quantum physics.

[pjknibbs]

We don't know anything and every conclusion we make is based on assumptions that have proven to be wrong, many times. Yet, they are the closest we can get to it.

One of the most important assumptions we have to make is that the laws of physics are the same everywhere in the universe, but of course we can't actually prove that because we'd have to travel to everywhere in the universe to do it.

[mrbadger]

One of the most important assumptions we have to make is that the laws of physics are the same everywhere in the universe, but of course we can't actually prove that because we'd have to travel to everywhere in the universe to do it.

We can see and measure a fair bit of it. Not to well right now, but that will improve.

[Morkonan]

We don't know anything and every conclusion we make is based on assumptions that have proven to be wrong, many times. Yet, they are the closest we can get to it.

One of the most important assumptions we have to make is that the laws of physics are the same everywhere in the universe, but of course we can't actually prove that because we'd have to travel to everywhere in the universe to do it.

There have been several recent observations that "could" point to that being "not true." Some of these are less credible than others, but the important thing, somewhat philosophically speaking, is that we have to consider the possibility that they are not immutable, steady, permanent constants. That is scientific blasphemy, of course, and only mentioned in hushed tones and in darkened rooms. Though, the idea that these laws may have been slightly different in the early universe is a bit more accepted, these days. It has to be noted that if these laws were ever different or could ever change, then the baby has truly been thrown out with the bathwater and great foundations of Science might tremble. Not fall, mind you, just tremble a bit, as Samson rampages around the temple of Science. I'm not saying that there's any solid observations for this occurring after the early stages of the Universe, by the way. Though, some bits and pieces incorporate this idea more than others.

Just a personal note:

I wonder how many people actually try to examine their surroundings. Even a basic smattering of human knowledge, tainted with a few curses by Science or not, would point out that we're living in a sea of air. Plants communicate to each other. Mold irritates us because it's designed to kill its competition. Aside from some really quantum weirdness, the only thing keeping sand on the ground is gravity. Every photon that impacts one's eyes not only elicits a reaction from cells that, by all rights, shouldn't notice such quantum silliness, but those photons have taken every possible path on the way to that eyeball from wherever they came from. Does coffee taste the same to two different people? Is a ball rolling across the floor actually chasing itself? Is decoherence just an excuse to normalize reality?

(reality)(-1)=Experience is now more comfortable for those who try to think about it

I am very thankful I don't indulge in narcotic substances... I don't need them.

[Hank001]

After reading the posts here and seeing all the research at local U trying to find where extra photons appearing through the slits are coming from, and my own experience in astormony I have came to the concusion that at some point these questions pass from physics and into the realm of philosophy and perhaps theology. Especially since a few are giving up as well saying it's showing our human limitations in how we perceive reality that are skewing are data. In otherwords we must ask the question, can we even form the correct questions? (In my words they are "Copping Out") This happens most when the obseved data fails to prove their thoery. I personally think this is a Darwinistic way to cull the least out of the debate. Obseve how the universe really works and if it doesn't match what you expect it to, it's not the universes fault. The universe goes on quite well and could care less what you think. Of course it takes some experts decades to come to this conclusion, and when they do some simply dismiss it. I have found few physical principles that exceede the pure motive force of achademic hubris.

[Bishop149]

Yes, the particle horizon would move if you did what you suggested. The reason this isn't actually a problem is because what you suggested is impossible--you can't travel faster than light, after all.

Ok, how exactly do wormholes factor into all of this.
In my very limited understanding of such things things moving faster than light is not necessarily impossible, there are a number of theoretical means by which it might happen, a wormhole being one.
When physics does tend to break however is when "information" can travel faster than light, and hence most of those theoretical things tend to also have something implicit in the mechanism that scrambles any kind of coherent info that might be transmitted.

So what about a wormhole, suppose one existed and you flew your spaceship formed of nicely and distinctly organised matter into it. . . . . would you emerge a random collection of matter and energy?

[Hank001]

Does light travel faster than light?
Some theory is in abeyance until a platform moving a X velocity shoots a beam of light towards a like beam shot from another platform traveling towards the first at the same velocity. Most say relativity says the added velocity won't matter. The closing velocties will still be at the speed of light. The ones crying "That's impossible" can set out there at the covergence point and measure it themselves. Then tell me what the outcome was. As for black holes, worm holes and their relation to the topic? Put them in the same category friend. "Things you can speculate about, but shouldn't be crazy enough to want to go out and gather data on." Yet until someone does, and survives to report the results, I'll stay on the fence.

[brucewarren]

Not a wormhole fan.

Assuming you managed to find one bigger than an atom across I think the loss of information would be the least of your worries. You'd be torn apart by the tidal forces before you even got close. Whether or not your lifeless corpse managed to make it through at that point would be rather moot.

[Observe]

So what about a wormhole, suppose one existed and you flew your spaceship formed of nicely and distinctly organised matter into it. . . . . would you emerge a random collection of matter and energy?

It's an interesting question that speaks to whether any proof may exist of previous universe incarnations. If the figurative spaceship emerges (new universe analogy), will it have some evidence of its previous structure?