The Astrophysical Hotseat

[extro...]

Ok, was reading a discussion on slashdot about a recent news item about some recently observed event that happened "near the edge of the observeable universe". Details not important - the question being, what is "the observeable universe"? The proposed answer was that it is a sphere with the earth at the center, with radius equal to the speed of light times the age of the universe (anything beyond that, the light from it couldn't reach us given the time that has passed thus far). Does this make sense? If so, could anything be beyond "the observeable universe"? If everything was in one place before the Big Bang, wouldn't anything being beyond the observeable universe imply that it moved, relative to our galaxy/solar system/planet, faster than the speed of light? I'm geussing there's a very fundamental misunderstanding here (possibly regarding implicit assumptions about the Big Bang).

[Jack_Ian]

Look for information on the Hubble Distance to answer this question. The truth is that we can see things that are further away than that because the light started moving towards us when we were closer together.
Imagine something a light year away from us now. Light from this object starts moving towards us. However, the universe is expanding, so by the time it gets to us (after one year), it seems to have come from an object that is further away than 1 light year but the light is a little "stretched" due to the expanding universe.

Do things exist beyond the observable universe? YES
In fact there are things which are currently observable that are receding away from us at faster than the speed of light so that in the future they will no longer be in our observable universe.

FASTER THAN THE SPEED OF LIGHT? Are you mad?

They are not moving at faster than the speed of light. They are receding away from us at faster than the speed of light. There's a difference.

This is brought about by the expansion of the universe. It's kind of like an ant walking away from a point on an expanding balloon (which can expand forever without bursting). If he walks at 1 mph, then after one hour he will be more than 1 mile away from his original starting position. The difference depends on how fast the balloon is expanding. Even items that are not moving will appear to be moving away from each other and the further they are away from each other, the faster they seem to move.

I wrote this a little quickly, since I have to leave now.
I hope this makes some sense. It will probably read like garbage when i read it myself again tomorrow.

[Jack_Ian]

[Chuck]

[austinap]

What are the current theories on the constancy of the speed of light? Ive glanced over a couple articles recently about 'recent findings... not so constant' but didn't have time to get very far into them (the professor actually came back for her office hours). She's since put up new articles and I can't find the one she had.

[Samadhi]

The speed of light depends on the medium, but it's upper limit is vacuum and that's constant at ~3*10^8m/s
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[austinap]

I understand that portion of it, but my question is whether or not its upper limit (in a vacuum) is really constant, and if it is, then why? Ive heard this challenged recently. At least a fair number of people seem to think that the speed of light is gradually slowing down.

[alphatango]

The theory's been around for a while, although the consensus of mainstream science seems to be that it lacks sufficient evidence and still contains some fairly fundamental flaws. Unfortunately, some overzealous pseudo-scientific creationist groups have seized on the theory and continue to make rather ridiculous claims (many, for example, use it to support a very literal interpretation of Genesis and place the age of the earth at around 6000 years).

Rather than reproduce all the arguments here, it's probably best to point you first to the Wikipedia link on the subject. The creationist/evolutionist versions of the story are more or less comprehensively given here and here, respectively, although I wouldn't regard either as coming from a rock-solid scientific viewpoint.

Some recent experimental research from 2003 on two galaxies containing supermassive black holes seems to put some fairly tight constraints on the theory -- be warned, you might have to work through a fair bit of the paper to get the result you're after.
_________________
"Hanging is too good for a man who makes puns; he should be drawn and quoted." -- Fred Allen

Keeper of the Eternal Flame of the Inner Geek.

[austinap]

Yeah, well creationists will jump on anything they see as a scientific 'flaw' to push their agenda. Actually it gets pretty funny after you learn not to give them any credibility whatsoever. I'll check out those links a little later though and stop back with any questions, now its off to learn about the regulation of the complement cascade and what can go wrong with it (immunology) Interesting stuff, but I'm tired of learning pathways and acronyms.

[Phloxy*]

I'd like to start by thanking you for this amazing thread. It's been one hell of a read.

I have some questions:
1 . Is the rate at which every (well, almost every) galaxy moves away from other galaxies increasing? Is there a cap on the speed they can reach (other than the speed of light)?
2. Do you believe in quark stars?
3. What holds neutron stars together? Is it possible for neutrons to stick together like this on our planet?
4. Relativity predicts that blackholes are a singularity and have no "memory". Doesn't this conflict with quantum theory? Is one of them incorrect or is the idea of black holes incorrect? (I've heard other theories like "dark energy stars" that I like ----> http://www.newscientist.com/article/mg18925423.600.html)
5. Do you believe we'll ever have a unified theory? If so, when?
6. Why didn't the very early universe turn into a big black hole? There was just so much matter squeezed into one place.
7. When you you think the mysteries of Dark Matter and Dark Energy will be solved?

[Samadhi]

The dog doesn't post much anymore so I'll take a stab at what I know. I can think of several other GL members who could probably provide better answers than me.

1. Your qualifier is appropriate, clearly there are galaxies that are moving towards each other (our impending collision with the Andromeda galaxy is a good example). But yes. Apparently from blue shift measurements from supernovae (I forget which specific type) the expansion of the universe is increasing so, on average, the speed at which galaxies are moving away from each other is increasing. As for a cap....I don't know. Observation will definitely be a problem as the speed approaches C.

2. I'll pass on this. IMO, a solid opinion requires a PHD (or equivalent knowledge) and I'm far from that.

3. Gravity holds neutron stars together. As for whether neutrons could stay together like that here on earth. Sure, but there's a critical amount. I mean two neutrons won't stick together for long. Their gravitational attraction is so much weaker than any of the other forces that they will feel from other particles that they'll eventually split up. It takes a certain critical amount of neutrons crammed so close together that their gravitational attraction overcomes any of the other fundamental forces that would pull them apart.

4. Stephen Hawking recently said that they do have memory, as it were. Humble man that I am, I will cede to his opinion.

5. Probably better answered by a phd but I think membrane theory is unification. I'm nowhere near 100% on this.

6. I don't have the answer to this. I know a few facts about the early universe but not enough to claim.

7. It seems that dark matter is not the reason for various observed anomalies. From what I read, the explanation seems to be "vacuum energy" (or whatever it's called). This also is the preferred explanation for the acceleration of the expansion of the universe.

[Lepton]

Don't mind if I take a stab at some of these...

1. Incidentally, the recessional velocity of distant objects are the things that cosmologists work with. The so-called "redshift" (denoted everywhere as z) is the ratio of the recessional velocity to the speed of light. The furthest visible objects (generally quasars) have z ~ 6. They are, indeed, receeding at something like 6 times the speed of light. This doesn't violate Einstein's relativity, since they are not actually moving that fast; rather, it is spacetime that is growing and pushing us apart.

2. Quark stars are a bit of a long shot, in the opinions of most astrophysicists, as far as I can tell. The so-called "MIT Handbag Model" is the working model, but it's little more than speculation. The problem is that quark stars would be governed by physics that we don't understand. In addition, the equations that govern black holes seem to be doing fairly well in experimental tests, so far, so there is little or no observational impetus for quark stars. Nonetheless, an interesting idea.

3. Samadhi has again done a good job. I will add that the strong nuclear force seems to keep the nuclei of atoms pretty close together, too, so there are some similarities between the cores of heavy nuclei and neutron stars.

4. Hawking's calculation on this issue is quite impressive. As last I heard, none were willing to either stand for its accuracy or point out a flaw. It will be a while before we understand the quantum information properties of black holes... if it is even possible to solve this mystery without a more fundamental theory. I think that the field is being driven by speculation right now. And that's a good thing.

5. Samadhi alludes to the generalization of string theory that is also known as "M-Theory". At the moment, people working on this theory seem to be in a bit of a rut, but the theory still holds a lot of promise to contain everything that a unified theory of physics should: the known laws, the known particles and forces, an explanation of the early cosmology, and possibly even a motivation for the physical constants (pi, speed of light, etc). That's a bit ambitious, though.

The question, though, is whether we will have such a theory. My opinion is unimportant, because we should search for it whether it exists or not. In any case, it seems unlikely that we will have a useful, fundamentally deeper understanding of the workings of the universe in the near future, although hopefully one will come along eventually.

6. Yep, lots of matter, but spacetime itself was expanding. As I mentionned earlier, the fabric of spacetime isn't constrained to travel at less than the speed of light. In fact, the expansion velocity was probably infinite at the moment of the big bang. Think of a plot of y = sqrt(x) if you like, where y is the size of the universe, and x is time. The velocity is the slope of this line, and the slope is infinite at x = 0.

7. Hey, slow down about dark energy, eh? =] It was first postulated in the 90's, and not taken seriously until the '98 observations (from Type IA Supernovae) that the universe's expansion was accelerating. The best guess at the moment seems to be that dark matter is WIMPS (weakly-interacting massive particles, such as are predicted by some theories of particle physics that are the precursors to string theory), and that dark energy is caused by a scalar field (of the sort which might arise in certain brands of string theory and some other similar theories).

Many are pinning their hopes on the LHC (Large Hadron Collider) at the Centre European pour Researche Nucleaire. This particle accelerator should be able to product the lowest-energy WIMPS, if they exist, within a year or so of being turned on in 2007. Exciting times.

[Phloxy*]

Great answers. Thanks, Samadhi and Lepton. That really did clear up a few things

[Chuck]

[raekuul]

Okay... here's one for you all...

As you all know, I personally think that the speed limit of the known universe, e.g. the speed of light, is obeyed by all things that can detect it. Subluminals travel and observe subluminally, superluminals travel and observe superlunimally, and luminals just occupy their own little limbo.

What are the implications of the statement "Light is the fastest travelling matter that has been detected by modern scientists"?

[Samadhi]

You're misconstruing what's going on. Can something be colder than absolute zero? Not in this universe.
Can something travel faster than light? Not in this universe.
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[raekuul]

I might not be the expert here, but my personal perception of this is:

Absolute Zero is the lowest observabe temperature- anything colder is always percieved as at by us.

Luminal velocities are the fastest observable velocities- anything faster would have to be undetectable by us or any subluminal species. Same goes for superluminals- the boundary is lightspeed.

[groza528]

[Samadhi]

Exactly. Just as nothing can travel faster than light because it's not possible.
_________________
And he lived happily ever after. Except for the dieing at the end and the heartbreak in between.

[raekuul]

[Coyote]

I'll admit I don't know a lot about this subject, but isn't the light-speed limit supported by a bunch of scary-looking mathematics, rather than just observation of how fast things move?

I mean, it's not as if Einstein was smoking some really good weed with a bunch of his scientist buddies and said "Woooah...dudes, what if everything we see is all relative, but no matter how fast we go light always looks like it's going the same speed?" and one of the other scientist was like "Yeah man, but what if you're going faster than light?" and Einstein is like "STFU, n00b--you can't go faster than light--nothing can go faster than light!" and all the other scientists are like "Lol, awesome, lets make it a theory!"
_________________
Hard work may pay off in the long run, but laziness pays off right now.

[Bicho the Inhaler]

Coyote's right. Historically, Einstein posited his theory before there was observation to support it. It's not an assumption based on observation, but a consequence of more fundamental assumptions.

If you don't know the theory behind the speed-of-light limit, then how can you argue seriously about it?

[extro...*]

I think it's substantially different than "absolute zero is lowest possible temperature" though, which seems trivial to me: you can't have less heat than no heat - there is no "negative heat".

Speed of light as a fundamental speed limit follows from theory, as opposed to basic definitions.

[Chuck]

You could say something similar for the speed of light case. The length of an object along its line of motion approaches zero as the object's speed approaches of the speed of light. There's no such thing as negative length so you can't go faster than light.

[raekuul]

[Jack_Ian]

I'd answer that question, but I know I'm actually just dreaming all of this and I expect to wake up soon.

Seriously though. It has been mathematically proven that it is impossible to prove everything that is actually true. Even if we know it to be true.

What's important is that we have a useful model. The truth or otherwise of that model is irrelevant.

If something cannot be observed, directly or indirectly, then it is irrelevant. It can serve no function to add such complication to our model.

[groza528]

I agree with extro... here. I do not dispute that nothing is faster than light, but I do think that the concept of a minimum (i.e. absolute zero) is a lot easier to understand than a maximum (speed of light.) That being said,
"Assumptions" are based on observations. When Bicho said 'fundamental assumptions' I believe he meant that relativity was not one large assumption, but the result of inferences that come from observed phenomena. We don't *know* that the sun will rise tomorrow, but we can assume it will based on the clearly established pattern and what we know about the earth's rotation. You might say "But I observe the sun rising every day," and that may be true, but you certainly have not observed it rising *tomorrow* so how can you be sure it will?
I do not know enough about the subject to know by what observations and inferences it was decided that nothing could travel faster than light, but I assume that physicists would not let a clearly unsupported theory see the light of day (no pun intended). That's not to say it's be-all end-all; science has been disproven repeatedly. Especially when it's based entirely upon observation. I'm talking to you, Ptolemy.

[extro...*]

[Jack_Ian]

[extro...*]

I'm familar with the incompleteness theorems, but I think they're often abused in being used to suggest implications far beyond their scope, which is limited to what can be proved about integer arithmetic with any given formal system ... not what can be proved about integer arithmetic in general (a typical proof of Godel's theorem proves that their is a true unproveable (in a given system) statement by demonstrating how to construct it, and then proving it is true), and certainly not what can be proved about other things in general.


BTW, regarding "Given a computably generated set of axioms ...". Here's an exercise: Show that for any recursively enumerable (also called partially computable) set of axioms, there is a logically equivalent computable (also called recursive) set of axioms.

[Lepton*]

An appropriately useful and rigorous definition of temperature doesn't lend itself so easily to the intuitive definition of absolute zero that has been described in this thread. The idea that temperature is the same as moving particles is an analogy and doesn't reflect physical reality. Like a good analogy, it is close, though.

The speed of light being an absolute limit comes from a combination of Newtonian mechanics with electrodynamics. It is, nonetheless, more true than the absolute zero hypothesis. Why?

Testability. The absolute zero hypothesis predicts that a number of different quantities will asymptote or diverge as the temperature goes to zero. Experimentally, these are confirmed. The speed of light hypothesis predicts that particles get more massive and stretched out as their speed goes to c. Ditto, confirmed. However, we can also view particles that travel exactly at the speed of light and have no mass. We cannot view systems that (for example) have a temperature of absolute zero and emit no blackbody radiation.

Thus, while it has been contended above that the absolute zero hypothesis is in a sense more true, I contend that the speed of light hypothesis is better confirmed by experiment - the only metric by which a scientist might compare theories.

[Jack_Ian]

[extro...*]

[Jack_Ian]

[raekuul]

[Jack_Ian]

[Lepton*]

[extro...*]

[Jack_Ian]

[Lepton]

I was thinking specifically of some work that is similar to the following:
Think of waves at a shore. The waves themselves travel at a given velocity, but if you watch where the waves are breaking you might notice that when the crests of the waves are not parallel to the shore the wave appears to travel the length of the shore at a much higher velocity. So no particle is moving faster than c, although the effect is exceeding that limit. Information cannot be carried faster than c in this manner.