Talk About Old News:
From the Associated Press:
This story is sooooo 7.5 billion years ago.STAR EXPLODES HALFWAY ACROSS UNIVERSE The explosion of a star halfway across the universe was so huge it set a record for the most distant object that could be seen on Earth by the naked eye.
A star 7.5 billion light years away exploded, giving off the brightest gamma-ray burst afterglow ever seen.
The aging star, in a previously unknown galaxy, exploded in a gamma ray burst 7.5 billion light years away, its light finally reaching Earth early Wednesday.
The gamma rays were detected by NASA's Swift satellite at 2:12 a.m. "We'd never seen one before so bright and at such a distance," NASA's Neil Gehrels said.
If anyone is in the mood for a discussion of the metaphysics of this, here goes: We tend to think of time as having a past, present, and future. The past is determined (cannot be altered or undone), the future is undetermined (different possible futures may occur based on our choices, random events, etc.), and the present is where the two meet. The problem comes with trying to figure out when "now" is. I snap my fingers and say "now." A person across the room perceives that a few milliseconds or after me. So "now" is different for me and the observer. If there can be no real border between the past and the future, either the past is still indetermined or the future is already determined.
I don't see the metaphysical difficulty here. If I send a postal letter to you on Tuesday and it arrives on Wednesday, the "moment of the letter" is Tuesday for me and Wednesday for you. It doesn't mean there is no border between the past and the future, or that the past is indetermined or the future determined. It just means that it took a day for the thing I experienced to reach you and become the thing you experienced. Or so it seems to me; maybe I'm missing something.
Nothing new here (except relative to that 7.5 billion-year-old starburst).
Thanks for the response. Can you clue me in as to what I am missing? I enjoy learning, and if you can teach me, I would appreciate it.
One of the passages in Greene's interesting book explains the concept of inflation [the expansion of the universe following the Big Bang, not monetary inflation].
The observable universe is ~13.8 billion years old, which only means that our "event horizon" has that limit; the actual universe is bigger. Much bigger.
Prof. Greene explains physicists' [very conservative] calculation of the universe's expansion during inflation this way: if our observable universe were the size of a grain of sand, the actual size of the universe is roughly comparable to the size of the earth.
The reason we can't see it all is because of Hubble's red shift, where the more distant an object is, the more that light is shifted toward a lower frequency. At the limit of the observable universe, the frequency of light becomes essentially zero. So we're in the middle of a bubble, the size of which is set by the red shift.
Interestingly, an observer 7 1/2 billion light years away is also in the center of their own bubble - shared with us - just as we are in the center of our bubble. The frame of reference is always the observer.
Anyway, that's Greene's [almost math-free] explanation of reality. For his reasoning of how the past isn't set in stone any more than the future is, check out the book. It's too much heavy slogging to go into here.
Can you recommend a good treatise on strict liability, space-time hyperbolic orthogonality and the mailbox rule?
A topic such as this cries out for economic analysis. Surely Judge Posner has written on it. :)
Basically, our intuition says there's a three-dimensional "space" that things move around in as a one-dimensional "time" passes. Together, they make up a four-dimensional "spacetime". We call the points of this combination, "events".
Think of it like a flipbook, where a bunch of two-dimensional pictures are stacked up into a three-dimensional stack, and then viewed in sequence. We think of each page in the book as a moment -- a "now". An object sitting still traces a vertical line through the book (since it stays in the same place), while a moving one traces a diagonal line.
So here's your letter example in this picture: I send you a letter at event A in spacetime, and you receive it at event B. The path the letter takes in space gets pulled up into a path in spacetime, which keeps track of when the letter was where. So your interaction with the letter is in one page of the flipbook (one "now") while mine was in an earlier page (a different "now"). All events -- all physics (except some things about quantum mechanics, which I'm ignoring) -- is purely local in spacetime, dealing only with what's happening at this point in spacetime, and not caring what's happening elsewhere.
The problem is this: we're thinking of a 3-D slab sliced up into 2-D pages (or in real life, a 4-D spacetime sliced into 3-D "nows"). But why did we have to choose that way of slicing up space into pages?
When you slice up spacetime, you say events are "simultaneous" if they lie on the same slice. But then I come along and slice it up in a different way; maybe at an angle to your slices. Then I have my own notion of "simultaneous", and it's not going to agree with yours. We can find events you say happen at the same time, but I say happen at different times. This throws off the whole notion of what "now" means. And, amazingly, it turns out that this actually happens.
When I'm staying still, I'm slicing up my flipbook into horizontal pages, which are perpendicular to my vertical "world-line". I see you moving along, tracing out your diagonal world-line. But as far as you're concerned, you're the one standing still. So you make your "now" pages perpendicular to your own line. Your pages don't agree with mine!
The thing is, we don't notice this in practice because the speeds we're used to seeing in the real world are so insanely small that the little difference between your notion of simultaneous and mine never really get that far off. It only makes a difference when you're going some significant fraction of the speed of light.
For instance, there are particles in cosmic rays that we know only last a fraction of a second in our laboratories, but we see them making the journey from the edge of our atmosphere to the surface of the Earth in a few seconds. How can it survive that long? Because to the particle the trip only lasts a fraction of a second. The clock it carries disagrees with ours because it's moving so fast with respect to ours.
So, what's going on out in space? We've got beams of light travelling from the event "star goes boom" to the event "detector in the telescope triggers". We know exactly how fast light moves, and we can figure out how far away the star was. We define the distance in time (how long ago) to be whatever it would take for light to travel that distance, since if we use our method of slicing up spacetime into "nows", that's roughly the page the star went boom on. Now we're back to a situation roughly analogous to your letter, and the same analysis above applies.
Oh, I agree that shared concepts of time become complicated when v/c is no longer extremely close to zero. I didn't think CEB's question raised that scenario, but was rather dealing with more run of the mill experiences where we can treat v/c as essentially zero.
Smokey, how about:
At the limit of the observable universe, the frequency of light becomes essentially zero
or Like the future, the past can change, too.
or The observable universe is ~13.8 billion years old, which only means that our "event horizon" has that limit.
Now then, watching the History channel, I understand if this happens a bit closer to Earth, we are all toast, literally. I think even the roaches.
Ya think?! Are they sure it just wasn’t the Vogons making way for a hyperspatial express route?
Actually, speaking of the space time continuum thingy, we could already be toast and just not know it.
Every concept you referred to was taken directly from Prof. Greene's book. Feel free to argue with him if you disagree [but keep in mind that he's got a physics doctorate and was a Rhodes Scholar].
Thinking about these very strange concepts, which are extremely counter-intuitive, caused the same type of consternation for Newtonian classicists a century ago when special and general relativity came on the scene.
The concepts mentioned do not disagree with other posters here. Maybe I wasn't as good at explaining them as others. But that does not make anything I quoted from Prof. Greene "pure nonsense."
Rather than simply dismissing these concepts as nonsense, perhaps you can refute them with your own hypothesis. If it's better than Greene's, there's a Nobel waiting for you. But it's playing a weak hand to simply say, in effect, "Why, that's crazy!"
I suggest reading Greene's books for a better understanding than I can give here. For under ten bucks you can get a lot of knowledge.
That said, when Greene builds a TARDIS I hope he invites me along.
The good news is that CERN comes online this year or next, and it may be able to help verify some of the predictions of string theory,
You obviously have never met a politician.
Your explanation was the only one that made sense to me. If there's any solace in that, then take it, because I assume it indicates nothing good about the chances of what you said being accurate.
The problem is not you, but Greene. What he is writing of is nearly pure speculation, not scientific fact. This is, sadly, the state of modern theoretical physics. (I think string theorists have been spending too much time in the humanities department.)
I'm not trying to be snarky but that is a nonsensical question. I take it that you do not have a degree in physics. Unfortunately you are trying to get a simple answer without having an understanding of the physics. That is not easy to do. That is also why reading Greene's book can be confusing.
I enjoyed Greene's book because it is speculation. But what else do we have when we're discussing the future of physics? It's all speculation; nothing's been proven. That doesn't make it less interesting. And I would rather get my physics speculation from a physics prof than from L. Ron Hubbard or Al Gore.
Thanks to all for the comments. For those still questioning time travel, maybe this guy has the answers.
Where are the fundamentalists?
In particular:
Assuming special relativity applies, what is the observed wavelength in our reference frame, of a light wave of wavelength L (in the source reference frame) emitted by a source moving directly away from us, in the limit as the source's speed relative to us approaches the speed of light?
Simpleminded hacking through the special relativistic doppler equation sure looked to me like the observed frequency would approach zero (or the observed wavelength approach infinity).
That's another way of saying you can't get here from there in the limit, which is what I thought Smokey referred to as an "event horizon".
But I haven't thought carefully about anything remotely like this stuff for over forty years, so I was hoping you or someone else who knows the subject well would clarify those issues Smokey raised.
Bottom line is that the Universe consists of "events", happenings at a certain location at a certain time. Each event has a pair of "light cones" associated with it, one backwards in time, and one forwards in time.
The backwards light cone contains the set of other events that may have influenced the event we're examining. The forward light cone contains the set of events that may be influenced by the event we're examining.
Between the two light cones is the set of events that can have no causal connection with the event we're examining. A supernova seven billion light years away, going off "now", whatever that means, is an event that is causally disconnected from the event of me posting this message.
Turns out the light cones are relativistically invariant. (In fact, they can be used to construct a definition of relativistic invariance.) Causality looks the same to every observer. It's the one really fixed aspect of special relativity.
Light travels precisely on the light cones -- that's why they're called light cones. Nothing travels faster than light, which is why the light cones mark the boundaries of events that might possibly be causally connected.
Don't know if this advances the discussion at all, but who of us can resist showing off his special expertise?
In the old series, the Doctor stood perfectly still and explained thoroughly the science fiction theory behind the current incident.
Now, I prefer the explanation of the past as found in The Lion King. Specifically when the wise, historian monkey wacks Simba in the head with a stick and then in response to Simba's complaint admonishes him with his own logic "what does it matter? It's in the past."
Most stars, like our sun, burn for about 10 billion years and in that time convert five percent of their original mass into energy. A collapsing star that produces a gamma ray burst, of which this particular event is just an extreme example, converts forty percent of its mass into energy in TEN MINUTES!
Damn. When's the next one?
According to the article, astronomers at an observatory detected what appeared to be a supernova. Later they realized that the arrival of the light from the supposed supernova had coincided with someone striking an old-fashioned wooden match on the casing of the telescope.
One reader rejected the obvious explanation, however. Instead, he suggested that the chemicals in the head of the match had unusual endochronic properties, and that the ignition of the match had caused the supernova to have occurred millions of years in the past.