Follow Slashdot blog updates by subscribing to our blog RSS feed


Forgot your password?

Comment: Re:TL;DR (Score 1) 100

by Rei (#49815749) Attached to: Does a Black Hole Have a Shape?

Have you never seen anything about the twin paradox? Even at the most superficial level introduction of relativity, you get that time slows down when something is moving relative to the observer.

This is ridiculous. Even "at the most superficial level introduction of relativity "you should know that if a person departs earth moving at "nearly C" and comes back, far less time will have past for them than someone who stayed on Earth the whole time.

From the perspective of the people on the ship, the journey is no longer a distance of 4.3 light years. If the spaceship is going 90% of c relative to Earth, then in the spaceship's frame it will take them 2.08 years to make that trip, and also in their frame they will observe it takes light 1.87 years to go from Earth to Alpha-Centari.

First off, apart from trying to add confusion, why did you change the velocity from the one I gave? Secondly, from a trip travel time perspective, it doesn't matter whether you view it as time dilation or length contraction. The trip at 0.999c takes 70 days from the perspective of the crew. That's the beginning and end of it right there. From their perspective, it's as if they got there moving far faster than the speed of light, as if there were no limits on how fast they could keep accelerating. With an infinite supply of energy, they could travel the 4,3 light years in what they perceive to be 7 days, 7 hours, 7 minutes, or 7 seconds (let's ignore G-forces here, or how to have such vast quantities of energy at their disposal). The crew of a spacecraft experiences no "upper limit" to how fast the universe will allow them to traverse a distance.

Comment: Re:Compare editing a CSV with a spreadsheet (Score 2) 343

by Rei (#49815649) Attached to: SourceForge and GIMP [Updated]

What are you talking about? I just did "echo 1,2 > test.csv" then opened test.csv in OpenOffice Calc, then saved it as test2.csv from the save dialog. No complaints. Then I clicked to close it. No complaints about unsaved changes. Did you actually try that out before you commented? I don't have any of the other programs you mention on this computer, so I'll pick another - let's try OpenOffice Writer. Made a text file, opened it, saved it as a .txt file, it asked me for the encoding, I confirmed it, I clicked closed, and it closed without trying to force me to save as an .odt.

I'm sorry, but GIMP's change is totally broken behavior. The most common workflow for GIMP (as you can see from all of the rage on the forums when these changes occurred) is not long complex workflows, but simple changes to jpegs or pngs. Open, change it, save it, close it. What sort of moron do you take people for to think that you have to "protect" them from choosing a format of file that doesn't save layers, and instead try to make them always save whatever they do in a format that no other programs support? As if a dialog warning them that it doesn't save layers and asking them if they want to flatten it, like Gimp used to do, isn't enough? What on earth is the point of *banning* people from typing in a file with the suffix that they want to use in the save menu, and instead making them choose an entirely different menu? Actually two different menus, depending on context, only one of which has a keyboard shortcut. It's just ridiculous. We're not preschoolers, we don't need the hand-holding.

Comment: Re: TL;DR (Score 1) 100

by Rei (#49815373) Attached to: Does a Black Hole Have a Shape?

An interesting side effect of this would be that it would actually be theoretically possible to send a probe into a black hole and get a signal back from it. If you're REALLY, REALLY, REALLY patient, that is ;)

(more realistically, one would likely try to probe the insides by making mciro black holes inside colliders and trying to get them to consume particles before they collapse, then looking for traces of information in the aftermath of the collapse)

Comment: Re: TL;DR (Score 1) 100

by Rei (#49815345) Attached to: Does a Black Hole Have a Shape?

And from the traveler's perspective the universe is consistent and there's no information loss either. They still see an apparent horizon, a place where time appears to stop, but they never reach it, it always recedes ahead of them. To them, the area beyond that apparent horizon is also not part of spacetime, but nothing ever manages to enter it so no information appears to be lost.

They of course eventually get ripped apart by tidal forces, but their information doesn't disappear into a "no-hair" singularity, it remains to be released when the black hole evaporates. As a black hole evaporates, time showing the particles falling deeper and deeper into it becomes observable to the outside world (albeit incredibly distorted and with the matter ripped to bits).

Again, that's at least my understanding of Hawking's "black holes don't actually exist" concept, and it makes logical sense to me. From the perspective of a traveler, they're just falling to their deaths in an extreme sort of collapsed star. From the perspective of an outside observer, they've fallen into a spot where a the collapsed star has ripped a hole in spacetime that won't start back up (from our perspective) until the "hole" boils off. Nothing ever lost, nothing ever undefined, always part of our universe, just effectively frozen temporarily in time. From our perspective.

Comment: Re:stopped using sourfeforge after filezilla (Score 4, Informative) 343

by Rei (#49814167) Attached to: SourceForge and GIMP [Updated]

Not only do they bundle it with adware, but they've apparently sabotaged GIMP too - for example, they apparently changed the save dialog so that you can only save XCF files and have to click through a "you have unsaved changes" warning when you export to a different format. They added an very difficult to precisely adjust sliders to things like brush size. They took out 16 bit color support. Basically, sourceforge has really totalled GIMP. ;)

Comment: Re:For future reference (Score 2) 343

by Rei (#49814117) Attached to: SourceForge and GIMP [Updated]

Yeah, because this is untimely, facturally inaccurate, five years old, and equivalent to a story about the sun being made of freshly chopped artichoke hearts? Meanwhile things like this and this are timely wellsprings of useful information?

And care to respond to the people mocking your "busy weekend" excuse, given that your weekend appears to be five days long and your "the main reason it's late" post which gives an entirely different reason for the delay?

Comment: Re:TL;DR (Score 1) 100

by Rei (#49814025) Attached to: Does a Black Hole Have a Shape?

No, from both perspectives it seems like the time of the other has slowed down, including Earth from the perspective of the ship.

This is not only incorrect, but impossible. If my time is speeding up and I'm looking at your watch, it's going to appear to be slowing down, and vice versa. Two parties both slowing down will appear indistinguishable from either party, rendering the concept of time dilation pointless.

The rest of what you wrote is what I wrote, just phrased differently: that from the perspective of the people on the ship, the 4,3 light year journey only takes 70 days. They are not capped in their rate of travel by some cosmic absolute "speed limit". Only to a third party to which they are moving relative will their velocity appear to be unable to reach or exceed c.

Comment: Re: TL;DR (Score 1) 100

by Rei (#49813973) Attached to: Does a Black Hole Have a Shape?

Numerous pages, including this one from NASA, say that from an outside perspective it takes an infinite length of time for an object to cross the event horizon. Here's an "Ask a Physicist" column about black holes that says that time distortion reaches infinity at the event horizon. The Wikipedia article on event horizons says the same thing.

From our perspective, nothing ever passes the event horizon and thus the information is never lost.

Comment: Re: TL;DR (Score 2) 100

by Rei (#49813639) Attached to: Does a Black Hole Have a Shape?

No, from an outside perspective it's never inside the event horizon. Only from the perspective of the matter entering the black hole does it cross. Saying "by then it's well within the event horizon" is simply not accurate from an outside perspective. No data collected from Earth will ever correspond to a reality in which the object has passed the event horizon.

Which is why there's no information paradox: the information is never in an unreachable state from any perspective.

Comment: Re:TL;DR (Score 2) 100

by Rei (#49813447) Attached to: Does a Black Hole Have a Shape?

Another one that I see a lot of people having trouble with: that of there being a universal speed limit. I'm surprised at how many people think this means there's a speed limit from all perspectives.

If we launch some incredible 100fold-staged antimatter spacecraft capable of reaching 0.999c toward Alpha Centauri 4,3 light years away, from the perspective of people on Earth, it'll never reach or exceed c and will take a touch over 4,3 years to get there. But from the perspective of people onboard the spacecraft, they're reaching their destination in only 70 days. From our perspective, their time slows down 22,4-fold; from theirs, Earth time has speed up. We see their velocity as capped off at c; to them, it's as if they can just keep accelerating without limit.

Now, it's not exactly like "going really fast"; everything around them seems pinched toward the forward direction and shifted to blue, like this - the same situation as where we see light emitted from particles moving at relativistic speeds relative to us (such as a black hole's event horizon) doppler shifted and distorted. If the occupants of our spacecraft go fast enough, even the cosmic microwave background will be shifted into the visible spectrum. ;)

Comment: Re:TL;DR (Score 5, Insightful) 100

by Rei (#49813401) Attached to: Does a Black Hole Have a Shape?

AFAIK: black holes are not sphere shaped, from our perspective - they're shell shaped. From our perspective as an outside observer, the singularity does not exist. From our perspective, time has slowed down on each particle moving into it from a near stop; they never actually pass the event horizon. Even the mass of the parent star that formed the black hole never reaches the event horizon as it is defined at the point in time that the star is collapsing, even though the event horizon may in time swell to a size that extends beyond where a collapsing particle was. Any light emitted from a doomed collapsing particle which manages ultimately escapes will do so on an escape trajectory that will always appear to come from outside the event horizon, no matter how much the black hole grew while it was in transit. From our external perspective, the particle never entered; the area beyond the event horizon is not part of spacetime to us. Now, as for an entity moving into the black hole, the perspective is different - the "hole" is quite well defined spacetime and they can enter just fine. But from our perspective, that entity never entered - it just slowed down to a virtual stop, stretched out across the event horizon.

Again, AFAIK, from my reading of the answer to the Hawking information paradox.

We love metrics that are continuous. We perceive the world with a Euclidean metric. And we generally don't have trouble understanding metrics distorted from the euclidean, such as the taxicab metric. Even the concept of a metric with points that bend space, simple gravitational distortion, is something we can usually grasp well after we get used to the concept. But people have trouble picturing a metric where space is warped by gravity so much that there exist regions where our euclidean mind insists must be there but actually aren't.

Comment: Re: Design flaw? (Score 1) 71

by Rei (#49807453) Attached to: Third Stage Design Problem Cause of Most Recent Proton Failure

Unfortunately, when it comes to rockets, a couple percent difference in performance means a huge difference in the first stage.

The Russians make quite good early-stages for their rockets - but they've long had trouble with the upper stages. The N1 being the glaring exception, even the first stage was big trouble... they just couldn't handle the necessary level of QA for such a complex design to work, at least not on the budget they were given.

Comment: Re: Design flaw? (Score 1) 71

by Rei (#49805171) Attached to: Third Stage Design Problem Cause of Most Recent Proton Failure

It also made them a lot more sensitive to the manufacturer, however. Underfunding a project almost certainly led it to being a disaster (the N1 rocket being a classic example). They generally were willing to sacrifice performance for ease of production and quantity - it was very much a widespread phenomenon.

A friend of mine once served as a translator for the military during one of the late mutual nuclear disarmament treaties (don't remember which one). She described them as pretty much a scam, in that both sides wanted to get rid of their old weapons anyway and it gave them an excuse to put funds toward development of new, treaty-compliant weapons. But anyway, they were allowed to inspect any area small enough to contain a "treaty limited item". To figure out whether they could inspect it, the teams were equipped with sophisticated laser measuring devices - if it determined that the space was large enough, they could inspect it. The Russians were really impressed with it. They sent their teams over with... a stick. If the stick fit, they could inspect it. ;)

Another example she mentioned was driving licenses. You know, if you get stopped in the US, they take your license, run your number through their computer, it connects to remote databases, they look up past offenses, they register a new one, etc, and give you your license back. In the USSR it was much simpler: the officer took your license and punched a hole in it. If you had too many holes, they kept it. ;)

I know it's such a stereotype that the Russians preferred low tech solutions vs. the US, but she said that the stereotype was totally well deserved. ;) She also found that they weren't as inclined to get a joke. She and some the guys on her team found it rather sad that these incredible weapons delivery systems were just being destroyed, literally crushed - devices that could deliver a payload anywhere on Earth with precision. So for fun they did some calculations for what they would have to do to reengineer one such that you could load up frozen pizzas onto racks and have them cook on reentry then parachute to the surface - they figured that given the reengineering and operations costs, a person ordering a very large order of pizza could get them for about $20 per pizza, delivered by decommissioned ICBM. They wrote up a formal "proposal" with all the calculations and budgetting and had her talk with what she described as the Soviet equivalent of a colonel about their "Intercontinental Pizza Delivery System". She said he stared at her like she was mad. ;) Totally didn't get the concept that it was a joke and thought that the US team was honestly proposing a private pizza-delivery-by-ICBM venture.

Don't panic.