“It isn’t the mountains ahead to climb that wear you out; it’s the pebble in your shoe.” -Muhammad Ali
It’s been a very busy week here at Starts With A Bang — as most full weeks after vacation often are — but there’s a whole lot to look forward to and look back on this past week. Not only did we have a new podcast come out for The Mac Observer, but we also had an overflowing week of stories to cover, including:
- Is the Universe expanding faster than expected? (for Ask Ethan),
- No one, not even Newton or Einstein, was the Muhammad Ali of physics,
- Fly over NASA’s greatest-ever view of Pluto (for Mostly Mute Monday),
- Escape from a black hole? Not in this Universe,
- Why dark matter?,
- How do black holes eat? Like Cookie Monster!, and
- Can the Universe expand faster than the speed of light?
Not only do people keep making my book out-of-stock at Amazon (I love it!), but there’s a potentially new one in the works; stay tuned! In the meantime, it’s onto your slightly belated version of our Comments of the week!
A diagram of the cosmic distance ladder. Credits: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU).
From Denier on the Scienceblogs synopses of a couple of our articles: “It doesn’t seem to be linked correctly.”
This is my bad; Scienceblogs has recently “upgraded” their WordPress to a far worse user interface experience for linking. Because I like to have links open in a new tab, I have a series of extra clicks to make, and if you miss, it undoes the entire link. I missed a couple of times last week, and I’m trying to be more careful now. Tiny things that you’ll never know behind-the-scenes, but I’m committed to keeping a good experience for you!
Ali taunting a fallen Sonny Liston in the first round of their second fight. Image credit: AP, via http://ift.tt/1XwKwDG.
From Ragtag Media on Muhammad Ali and science: “Leonardo da Vinci is about as close as you could come to an “Ali of Science”.
And to do this you have to appreciate his work as a whole.”
There are a whole slew of differing opinions out there, and I’ve heard the following:
- Newton
- Einstein
- Tesla
- da Vinci
- Galileo
- Maxwell
- Alfvén
- Zwicky
- Boltzmann
and that was just after one article. There’s no universally good way to measure greatness, but I’d say rising up to conquer the greatest “problems” facing your field at a time when the field and the “problem-challengers” are especially rich is the closest. After all that, I’d probably go with Einstein, for the fact that coming up with General Relativity was such a herculean leap that novel, new challenges are still being devised, and are still falling at its feet 101 years later. But all of them had demons they couldn’t conquer. Honestly, I feel Ali did, too; after the final Frazier fight, perhaps both of their finest hours, neither man was never the same.
The dark (night) side of Pluto, showcasing layers of atmospheric haze and possible low-lying clouds (foreground) nearer to the surface. Image credit: NASA/JHUAPL/SwRI.
From Denier on NASA mission classes: “This is a spectacular display of the old “Faster-Better-Cheaper” mission philosophy. New Horizons came in under $800 million. Kepler, even with its ongoing study, is still under a total of $600 million. The Opportunity rover is at roughly $400 million.”
And there’s a lot to be said about that. You see, there’s always a trade-off when you do anything between those three goals: fast, good and cheap. “Pick two,” is the resolution I’ve heard. In the late 1990s, NASA committed to “faster, better, cheaper” as the strategy, and what they found is that they could do things faster and cheaper, but that it wasn’t better. The flagship class of missions — the Apollo-class missions, the Hubble-class missions, the James Webb Space Telescope, WFIRST, future 10-meter-class space telescopes or LISA — are truly incredible science factories that no fast, cheap mission (or series of missions) can add up to.
Image credit: NASA, James Webb Space Telescope science team.
Hubble has given us our best optical views on the Universe for more than 25 years, 24 hours a day. Yes, fast and cheap missions can do amazing things, but they’re not as good as the ones that are more expensive. Like Louis CK said,
“If you want to get a big thing in life you gotta make a big effort, you gotta try hard, you gotta do things you’re not used to doing.”
And part of that means investing in the biggest class missions there are. Sure, we can do some amazing things on a (relatively) shoestring budget, but they’re not going to be as good as what we could do with a greater investment. We could’ve done WFIRST for way cheaper, too, but it wouldn’t have found as many objects, been able to do spectroscopy, or been able to do all it’s planning on doing for planetary science. Tentative launch date: 2024.
A selection of some of the most distant galaxies in the observable Universe, from the Hubble Ultra Deep Field. Image credit: NASA, ESA, and N. Pirzkal (European Space Agency/STScI).
From See Noevo on dark energy: “If Dark Energy is changing (e.g. growing “denser”, or growing stronger), what if anything would this mean for the law of the conservation of energy?”
So you are a little late to the party, because you apparently missed the Ask Ethan from a week and half ago where I talked about this exact finding. Now, that find does not indicate that dark energy is increasing in strength, but it could be doing so if it were a scalar field. In particular, if dark energy either had an equation of state where w < -1 (or where it’s pressure was greater in magnitude than the energy density), or if dark energy had a w that was changing over time (i.e., a w’ that was ≠ 0), we could get a stronger dark energy in the future. It would mean nothing for the global law of conservation of energy, because there is none; energy is not defined in General Relativity. For individual particles on a quantum level (where there is a defined conservation law), it would mean nothing until the final instants before the Big Rip, as energy would still be conserved in all interactions until that moment.
Opportunity beginning its mission at Eagle Crater. Image credit: NASA / JPL-Caltech / MSL team.
From Wow on the cost/effectiveness ratio of missions: “Maybe the difference isn’t the cost, but the effort spent in making the cost effective and ignoring being “cost effective”, which would tend to lead to cutting corners under the *expectation* that these edge cases aren’t important.”
I think it’s also important to keep a little perspective in mind. When we prepare for missions, we try and maximize the chance of success of the main mission, and to make contingency plans for achieving as much of it as possible if there are some malfunctions. Philae was a disaster, but Rosetta was a tremendous success. Opportunity is still a hero; Spirit was less so; the Mars Phoenix lander was a disappointment (failing after only a fraction of its mission was performed); and Mars Climate Orbiter and Mars Polar Lander — both part of “faster, better, cheaper” — both failed spectacularly. It’s easy to point to the biggest successes of that style of missions and go, “SEE!” But the reality is that we’re not as successful as we’d like to be in any of our missions; doing something new always comes with challenges we never could have anticipated.
Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://ift.tt/10eTU31).
From MP on black holes and entanglement: “Is it possible to shoot an entangled particle into a black hole and observe the local particle?”
Yes. And then you’ll learn a tiny bit of information: what one of the spins of one of the particles that went into the black hole was. That, oddly enough, doesn’t teach you anything; angular momentum is one of the types of “hair” that we already know black holes have!
Image credit: Tommaso Dorigo of http://ift.tt/1PoTMB7.
From Denier on infinities and theories: “There are those that say physical infinities are just an artifact of the unfinished character of a theory.”
What most laypeople don’t realize is that pretty much all quantum field theories have infinities in them, and aren’t “complete” theories in any way. You like to think that if you can write down all the possible interactions at all loop-orders and sum them up, you’ll get the full probability amplitudes (and scattering cross sections) of all the things a particle can do. But you won’t; all field theories, if you approach them from this perturbative method, will eventually give you nonsense (infinite) answers. That’s because they give you not a converging series as you go to higher and higher loop orders, but an asymptotic series, which diverges past a certain point.
One of the big, big problems for String Theory, by the way, that people really don’t like to talk about is this: this “feature” of quantum field theories (i.e., this problem of calculation) gets worse with every extra dimension you add. There is still plenty of room for physics to grow.
Image credit: Wikimedia Commons user Stefania.deluca.
From Michael Kelsey on how you measure a galaxy’s rotation curve, and its relative redshift or blueshift: “It only works for relatively nearby galaxies, where the angular size of the galaxy is large compared to the telescope resolution (the “point spread function” or PSF, typically a few arcseconds for ground-based telescopes).
In such a situation, you can use a “long slit” spectrograph, which has an aperture limiter to pick out a narrow strip of the object of interest, and get a spectrum along that whole strip (see the Wikipedia article for a cool picture).
To measure rotation, you don’t actually care about the _absolute_ red or blue shift, but rather the _difference_ between the shift at different points across a galaxy image. If the galaxy is rotating, then the left side will have a shift slightly different, and opposite in sign, compared to the right side, with respect to the average or middle value.”
I can’t believe I haven’t talked about this in this level-of-detail before! So to add onto this, here’s the picture (for nebulae, not galaxies) that Michael refers to:
Long-slit spectroscopy for nebula. Image credit: Wikimedia Commons user Larryisgood, under c.c.a.-by-s.a.-3.0.
For galaxies, we can do this for starlight via standard spectroscopy, we can do it farther out using radio (21-cm) spectroscopy, and we can factor in the tilt of the galaxy as well (because we only measure the edge-on component). We can’t measure the rotational speed directly, or for face-on galaxies, because the timescales are too great. But from a snapshot of a rotating galaxy, this rotation curve is a piece of cake. It’s no wonder, though, that we didn’t really begin to do this in earnest until the 1970s with the work of Vera Rubin: the technology really wasn’t there yet!
This artist’s rendering shows a galaxy being cleared of interstellar gas, the building blocks of new stars. Image credit: ESA/ATG Medialab.
From Omega Centauri on how black holes eat: “[I]f you consider an isolated particle interacting with a BH, either it goes into the event horizon, and vanishes without emitting radiation, or it swings around it, and leaves with the same kinetic energy (relative to the BH) that it arrived with. But matter in bulk, is going to suffer from extreme tidal distortion, which allows it to gain energy. But we still have to conserve overall energy (mass-energy), so some matter must be swallowed or at least left in a more tightly bound orbit, in order for other matter to radiate and/or be thrown out at high velocity.”
Think about this: a black hole is extremely massive but extremely compact. Its event horizon is tiny compared to masses you’re used to. The Earth, for example, is 6371 km in radius and has a mass of 6 x 10^24 kg. If the Earth were a black hole, it would have the same mass but be 0.44 cm in radius, or more than 100000 times smaller in all three dimensions. Most of the space dust passing by our vicinity don’t even collide with our atmosphere; they get gravitationally “slingshotted” out elsewhere. Now scale that up: every factor of 10 in mass gives you another factor of 10 in radius for a black hole.
Image credit: Wikimedia Commons user Lunch, of a 2-D projection of a Calabi-Yau manifold, one popular method of compactifying the extra, unwanted dimensions of String Theory.
From Richard Fraser on growing dimensions: “Suppose for a moment that the 1st dimension segways, a line that on 1 end splits into the second, which explodes into a 3rd dimension, the creation of the 3rd creates motion and distance. If the Big Bang was the birth of the 3rd dimension would universal expansion be the 3rd dimension pursuing time at its maximum dimensional expansion rate? It’s clear working backwards that higher dimensions must be built off lower dimensions, so it indicates that lower dimensional movement gives rise to the dimension above it by reaching its maximum dimensional expansion limit.”
It actually works the other way, in most physical Universes we can construct: it’s very difficult to make a new dimension “spring” out of a lower-dimensional space. How would you turn a piece of paper into a three-dimensional solid, for example? You can crumple it up into a ball, but all you’d have, to an observer on that two-dimensional paper, was a topologically defective sheet of paper with kinks and discontinuities. Instead, it’s easier to have a dimension disappear as it evolves over time, while others grow. A fun example — first worked out by Steve Detweiler (R.I.P.) and Alan Chodos — was to take a five-dimensional Kasner metric and watch it evolve into a four-dimensional expanding Universe.
Image credit: Chodos and Detweiler (1980), retrieved from http://ift.tt/1VV4aaL.
Steve was the guy who taught me GR back in grad school, and I had the opportunity to meet Alan Chodos — an excellent writer and communicator — back in 2004 or so. This is a result that’s periodically forgotten and periodically re-found again, but always interesting. Extra dimensions are messy but fun, and a useful but problematic tool for theorists. Going down is easier than going up!
Public domain timelapse photo by flickr user comedynose (Pete), illustrating fast, relativistic motion. Image retrieved via http://ift.tt/1UkPNLJ.
And finally, from G in an effort to put me out of a job: “The answer to this should be obvious to regular readers.
Spacetime itself is what expands, and at the difference between two sufficiently distant points, the rate of expansion is faster than c. This we observe in the red-shift and the progressive loss of distant galaxies from view.”
“Can the Universe expand faster than the speed of light?” That was the question. And the answer is exactly as G claims. The tough part for most people (non-regular-readers of this blog) to wrap their heads around is that the expansion of the Universe is a rate, but it’s an unusual rate: a speed-per-unit distance. In our Universe’s case, it’s around 70 km/s/Mpc. But the speed of light is 300,000 km/s, so if you’re more than 300,000/70 = 4300 Mpc away (or about 14 billion light years), you’ll appear to recede faster than c. The technical details of the expansion are less than 5% different from this, so this easy rule-of-thumb gets you 95%+ of the way there!
Thanks for a great week, and let’s start the next one imminently!
from ScienceBlogs http://ift.tt/1PoUfn7
“It isn’t the mountains ahead to climb that wear you out; it’s the pebble in your shoe.” -Muhammad Ali
It’s been a very busy week here at Starts With A Bang — as most full weeks after vacation often are — but there’s a whole lot to look forward to and look back on this past week. Not only did we have a new podcast come out for The Mac Observer, but we also had an overflowing week of stories to cover, including:
- Is the Universe expanding faster than expected? (for Ask Ethan),
- No one, not even Newton or Einstein, was the Muhammad Ali of physics,
- Fly over NASA’s greatest-ever view of Pluto (for Mostly Mute Monday),
- Escape from a black hole? Not in this Universe,
- Why dark matter?,
- How do black holes eat? Like Cookie Monster!, and
- Can the Universe expand faster than the speed of light?
Not only do people keep making my book out-of-stock at Amazon (I love it!), but there’s a potentially new one in the works; stay tuned! In the meantime, it’s onto your slightly belated version of our Comments of the week!
A diagram of the cosmic distance ladder. Credits: NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU).
From Denier on the Scienceblogs synopses of a couple of our articles: “It doesn’t seem to be linked correctly.”
This is my bad; Scienceblogs has recently “upgraded” their WordPress to a far worse user interface experience for linking. Because I like to have links open in a new tab, I have a series of extra clicks to make, and if you miss, it undoes the entire link. I missed a couple of times last week, and I’m trying to be more careful now. Tiny things that you’ll never know behind-the-scenes, but I’m committed to keeping a good experience for you!
Ali taunting a fallen Sonny Liston in the first round of their second fight. Image credit: AP, via http://ift.tt/1XwKwDG.
From Ragtag Media on Muhammad Ali and science: “Leonardo da Vinci is about as close as you could come to an “Ali of Science”.
And to do this you have to appreciate his work as a whole.”
There are a whole slew of differing opinions out there, and I’ve heard the following:
- Newton
- Einstein
- Tesla
- da Vinci
- Galileo
- Maxwell
- Alfvén
- Zwicky
- Boltzmann
and that was just after one article. There’s no universally good way to measure greatness, but I’d say rising up to conquer the greatest “problems” facing your field at a time when the field and the “problem-challengers” are especially rich is the closest. After all that, I’d probably go with Einstein, for the fact that coming up with General Relativity was such a herculean leap that novel, new challenges are still being devised, and are still falling at its feet 101 years later. But all of them had demons they couldn’t conquer. Honestly, I feel Ali did, too; after the final Frazier fight, perhaps both of their finest hours, neither man was never the same.
The dark (night) side of Pluto, showcasing layers of atmospheric haze and possible low-lying clouds (foreground) nearer to the surface. Image credit: NASA/JHUAPL/SwRI.
From Denier on NASA mission classes: “This is a spectacular display of the old “Faster-Better-Cheaper” mission philosophy. New Horizons came in under $800 million. Kepler, even with its ongoing study, is still under a total of $600 million. The Opportunity rover is at roughly $400 million.”
And there’s a lot to be said about that. You see, there’s always a trade-off when you do anything between those three goals: fast, good and cheap. “Pick two,” is the resolution I’ve heard. In the late 1990s, NASA committed to “faster, better, cheaper” as the strategy, and what they found is that they could do things faster and cheaper, but that it wasn’t better. The flagship class of missions — the Apollo-class missions, the Hubble-class missions, the James Webb Space Telescope, WFIRST, future 10-meter-class space telescopes or LISA — are truly incredible science factories that no fast, cheap mission (or series of missions) can add up to.
Image credit: NASA, James Webb Space Telescope science team.
Hubble has given us our best optical views on the Universe for more than 25 years, 24 hours a day. Yes, fast and cheap missions can do amazing things, but they’re not as good as the ones that are more expensive. Like Louis CK said,
“If you want to get a big thing in life you gotta make a big effort, you gotta try hard, you gotta do things you’re not used to doing.”
And part of that means investing in the biggest class missions there are. Sure, we can do some amazing things on a (relatively) shoestring budget, but they’re not going to be as good as what we could do with a greater investment. We could’ve done WFIRST for way cheaper, too, but it wouldn’t have found as many objects, been able to do spectroscopy, or been able to do all it’s planning on doing for planetary science. Tentative launch date: 2024.
A selection of some of the most distant galaxies in the observable Universe, from the Hubble Ultra Deep Field. Image credit: NASA, ESA, and N. Pirzkal (European Space Agency/STScI).
From See Noevo on dark energy: “If Dark Energy is changing (e.g. growing “denser”, or growing stronger), what if anything would this mean for the law of the conservation of energy?”
So you are a little late to the party, because you apparently missed the Ask Ethan from a week and half ago where I talked about this exact finding. Now, that find does not indicate that dark energy is increasing in strength, but it could be doing so if it were a scalar field. In particular, if dark energy either had an equation of state where w < -1 (or where it’s pressure was greater in magnitude than the energy density), or if dark energy had a w that was changing over time (i.e., a w’ that was ≠ 0), we could get a stronger dark energy in the future. It would mean nothing for the global law of conservation of energy, because there is none; energy is not defined in General Relativity. For individual particles on a quantum level (where there is a defined conservation law), it would mean nothing until the final instants before the Big Rip, as energy would still be conserved in all interactions until that moment.
Opportunity beginning its mission at Eagle Crater. Image credit: NASA / JPL-Caltech / MSL team.
From Wow on the cost/effectiveness ratio of missions: “Maybe the difference isn’t the cost, but the effort spent in making the cost effective and ignoring being “cost effective”, which would tend to lead to cutting corners under the *expectation* that these edge cases aren’t important.”
I think it’s also important to keep a little perspective in mind. When we prepare for missions, we try and maximize the chance of success of the main mission, and to make contingency plans for achieving as much of it as possible if there are some malfunctions. Philae was a disaster, but Rosetta was a tremendous success. Opportunity is still a hero; Spirit was less so; the Mars Phoenix lander was a disappointment (failing after only a fraction of its mission was performed); and Mars Climate Orbiter and Mars Polar Lander — both part of “faster, better, cheaper” — both failed spectacularly. It’s easy to point to the biggest successes of that style of missions and go, “SEE!” But the reality is that we’re not as successful as we’d like to be in any of our missions; doing something new always comes with challenges we never could have anticipated.
Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project (http://ift.tt/10eTU31).
From MP on black holes and entanglement: “Is it possible to shoot an entangled particle into a black hole and observe the local particle?”
Yes. And then you’ll learn a tiny bit of information: what one of the spins of one of the particles that went into the black hole was. That, oddly enough, doesn’t teach you anything; angular momentum is one of the types of “hair” that we already know black holes have!
Image credit: Tommaso Dorigo of http://ift.tt/1PoTMB7.
From Denier on infinities and theories: “There are those that say physical infinities are just an artifact of the unfinished character of a theory.”
What most laypeople don’t realize is that pretty much all quantum field theories have infinities in them, and aren’t “complete” theories in any way. You like to think that if you can write down all the possible interactions at all loop-orders and sum them up, you’ll get the full probability amplitudes (and scattering cross sections) of all the things a particle can do. But you won’t; all field theories, if you approach them from this perturbative method, will eventually give you nonsense (infinite) answers. That’s because they give you not a converging series as you go to higher and higher loop orders, but an asymptotic series, which diverges past a certain point.
One of the big, big problems for String Theory, by the way, that people really don’t like to talk about is this: this “feature” of quantum field theories (i.e., this problem of calculation) gets worse with every extra dimension you add. There is still plenty of room for physics to grow.
Image credit: Wikimedia Commons user Stefania.deluca.
From Michael Kelsey on how you measure a galaxy’s rotation curve, and its relative redshift or blueshift: “It only works for relatively nearby galaxies, where the angular size of the galaxy is large compared to the telescope resolution (the “point spread function” or PSF, typically a few arcseconds for ground-based telescopes).
In such a situation, you can use a “long slit” spectrograph, which has an aperture limiter to pick out a narrow strip of the object of interest, and get a spectrum along that whole strip (see the Wikipedia article for a cool picture).
To measure rotation, you don’t actually care about the _absolute_ red or blue shift, but rather the _difference_ between the shift at different points across a galaxy image. If the galaxy is rotating, then the left side will have a shift slightly different, and opposite in sign, compared to the right side, with respect to the average or middle value.”
I can’t believe I haven’t talked about this in this level-of-detail before! So to add onto this, here’s the picture (for nebulae, not galaxies) that Michael refers to:
Long-slit spectroscopy for nebula. Image credit: Wikimedia Commons user Larryisgood, under c.c.a.-by-s.a.-3.0.
For galaxies, we can do this for starlight via standard spectroscopy, we can do it farther out using radio (21-cm) spectroscopy, and we can factor in the tilt of the galaxy as well (because we only measure the edge-on component). We can’t measure the rotational speed directly, or for face-on galaxies, because the timescales are too great. But from a snapshot of a rotating galaxy, this rotation curve is a piece of cake. It’s no wonder, though, that we didn’t really begin to do this in earnest until the 1970s with the work of Vera Rubin: the technology really wasn’t there yet!
This artist’s rendering shows a galaxy being cleared of interstellar gas, the building blocks of new stars. Image credit: ESA/ATG Medialab.
From Omega Centauri on how black holes eat: “[I]f you consider an isolated particle interacting with a BH, either it goes into the event horizon, and vanishes without emitting radiation, or it swings around it, and leaves with the same kinetic energy (relative to the BH) that it arrived with. But matter in bulk, is going to suffer from extreme tidal distortion, which allows it to gain energy. But we still have to conserve overall energy (mass-energy), so some matter must be swallowed or at least left in a more tightly bound orbit, in order for other matter to radiate and/or be thrown out at high velocity.”
Think about this: a black hole is extremely massive but extremely compact. Its event horizon is tiny compared to masses you’re used to. The Earth, for example, is 6371 km in radius and has a mass of 6 x 10^24 kg. If the Earth were a black hole, it would have the same mass but be 0.44 cm in radius, or more than 100000 times smaller in all three dimensions. Most of the space dust passing by our vicinity don’t even collide with our atmosphere; they get gravitationally “slingshotted” out elsewhere. Now scale that up: every factor of 10 in mass gives you another factor of 10 in radius for a black hole.
Image credit: Wikimedia Commons user Lunch, of a 2-D projection of a Calabi-Yau manifold, one popular method of compactifying the extra, unwanted dimensions of String Theory.
From Richard Fraser on growing dimensions: “Suppose for a moment that the 1st dimension segways, a line that on 1 end splits into the second, which explodes into a 3rd dimension, the creation of the 3rd creates motion and distance. If the Big Bang was the birth of the 3rd dimension would universal expansion be the 3rd dimension pursuing time at its maximum dimensional expansion rate? It’s clear working backwards that higher dimensions must be built off lower dimensions, so it indicates that lower dimensional movement gives rise to the dimension above it by reaching its maximum dimensional expansion limit.”
It actually works the other way, in most physical Universes we can construct: it’s very difficult to make a new dimension “spring” out of a lower-dimensional space. How would you turn a piece of paper into a three-dimensional solid, for example? You can crumple it up into a ball, but all you’d have, to an observer on that two-dimensional paper, was a topologically defective sheet of paper with kinks and discontinuities. Instead, it’s easier to have a dimension disappear as it evolves over time, while others grow. A fun example — first worked out by Steve Detweiler (R.I.P.) and Alan Chodos — was to take a five-dimensional Kasner metric and watch it evolve into a four-dimensional expanding Universe.
Image credit: Chodos and Detweiler (1980), retrieved from http://ift.tt/1VV4aaL.
Steve was the guy who taught me GR back in grad school, and I had the opportunity to meet Alan Chodos — an excellent writer and communicator — back in 2004 or so. This is a result that’s periodically forgotten and periodically re-found again, but always interesting. Extra dimensions are messy but fun, and a useful but problematic tool for theorists. Going down is easier than going up!
Public domain timelapse photo by flickr user comedynose (Pete), illustrating fast, relativistic motion. Image retrieved via http://ift.tt/1UkPNLJ.
And finally, from G in an effort to put me out of a job: “The answer to this should be obvious to regular readers.
Spacetime itself is what expands, and at the difference between two sufficiently distant points, the rate of expansion is faster than c. This we observe in the red-shift and the progressive loss of distant galaxies from view.”
“Can the Universe expand faster than the speed of light?” That was the question. And the answer is exactly as G claims. The tough part for most people (non-regular-readers of this blog) to wrap their heads around is that the expansion of the Universe is a rate, but it’s an unusual rate: a speed-per-unit distance. In our Universe’s case, it’s around 70 km/s/Mpc. But the speed of light is 300,000 km/s, so if you’re more than 300,000/70 = 4300 Mpc away (or about 14 billion light years), you’ll appear to recede faster than c. The technical details of the expansion are less than 5% different from this, so this easy rule-of-thumb gets you 95%+ of the way there!
Thanks for a great week, and let’s start the next one imminently!
from ScienceBlogs http://ift.tt/1PoUfn7
Aucun commentaire:
Enregistrer un commentaire