NASA's LLCD Tests Confirm Laser Communication Capabilities In Space 107
An anonymous reader writes "This week, NASA released the results of its Lunar Laser Communication Demonstration's (LLCD) 30-day test carried out by its Lunar Atmosphere and Dust Environment Explorer (LADEE) that is currently in orbit around the Moon. According to the space agency, the LLCD mission proved that laser communications are practical at a distance of a quarter of a million miles and that such a system could perform as well, if not better, than any NASA radio system."
SETI (Score:1)
Does this make what SETI is looking for pointless? Should they instead be looking for lasers if they work better for communication.
Re:SETI (Score:5, Insightful)
The earth orbits where the sun is right now. It takes light about 8 minutes to travel from the sun to the earth.
No. It also takes 8 minutes for changes in the gravitational field to travel from the sun to the earth.
The earth orbits, where the Sun appears to be; as the Sun appears to move, the gravitational field changes.
These changes are delayed by 8 minutes.
The constant 'c' from special relativity is not just the speed of light in a vacuum -- it is also the highest possible speed for any physical interaction within nature, and the conversion factor from changing units of time into units of space.
Gravitational waves, Gluons, Photons, and other massless particles travel at a maximum speed of c.
It is impossible to convey information at a speed faster than c.
There are cases where a wave can propagate faster than c, but no information can be conveyed faster than light.
Re: (Score:2)
Re: (Score:3)
Not that anyone has figured out yet anyway. I wouldn't wish to preclude the possibility.
Re: (Score:2)
Re: (Score:2)
In our current standard model, however, it's pretty much given that information can't be conveyed FTL, not at least becasue that conflicts with causality.
Re: (Score:2)
We live in 4 dimensions. People keep forgetting Time.
And the flow of time is still not completely ruled out to be adjustable, The relativity experiments proved that.
Re: (Score:2)
The earth orbits, where the Sun appears to be; as the Sun appears to move, the gravitational field changes.
The Sun is where it appears to be - we're orbiting it, not the other way around, so if you received a photon from the Sun and sent one back in the same direction, it would hit the Sun - it wouldn't miss it.
Right?
Re: (Score:3)
Wrong. The sun is orbiting the galactic core at about 828,000 km/h, giving it a non-inertial reference frame. In the eight minutes it takes sunlight light to reach us the sun has moved 110,400 km within the galaxy. Then there's the velocity of our galaxy within it's cluster (and our cluster within its super-cluster, etc)
Re: (Score:3)
The distinction here is between linear and rotational movement. The sun's movement for the year (i.e. relative to background stars) is a linear movement (changes in relative position between observer and subject) and thus is affected by relativity and the 8.3 min travel time of light. Since it's only traversing a bit less than one degree per day (360
Re: (Score:2)
That's why I specified rotational movement, which is "real" and thus could be responsible for bizarre anomalies. Presuming the validity of General Relativity, linear movement is irrelevant to the discussion - just say that the sun is standing still while the rest of the universe is moving past and it is so.
So perhaps you can flesh out what I've forgotten and Google refuses to reveal: I could swear that I've heard from a reputable source that there is a measurable discrepancy between where the sun appears t
Re: (Score:2)
Ah, wait a minute - the Earth is moving as well as rotating. So the photons reaching Earth are moving (relative to Earth) with a velocity that is the combination of their initial velocity and Earth's orthogonal velocity. Meaning they should appear to be slightly blue-shifted, and coming from a point slightly more in front of the Earth in it's orbit. Unfortunately I can't remember the details of relativistic velocity addition offhand, so I can't calculate the exact angle of the discrepancy.
Given that, the
Re: (Score:2)
Ah, wait a minute - the Earth is moving as well as rotating. So the photons reaching Earth are moving (relative to Earth) with a velocity that is the combination of their initial velocity and Earth's orthogonal velocity. Meaning they should appear to be slightly blue-shifted, and coming from a point slightly more in front of the Earth in it's orbit. Unfortunately I can't remember the details of relativistic velocity addition offhand, so I can't calculate the exact angle of the discrepancy.
This is referred to as The Aberation of Light [wikipedia.org] , which is an unfortunate name because light aberration has a different meaning [wolfram.com] with regards to optical systems.
Given that, then if gravity travels through space at a finite speed then a similar discrepancy should exist, with gravitons hitting the Earth from slighty "in front", and accelerating the planet in it's orbit.
This makes no sense to me.
Re: (Score:2)
Thank you for that name, now I know I'm not crazy and have a starting point for further research.
>This makes no sense to me.
Okay - so we agree that the sun appears slightly "in front of" it's actual position, yes? And, from the perspective of an Earthbound observer that is a real phenomena - the photons are actually hitting the Earth from a direction other than directly sunward. Agreed?
Now imagine that, in addition to light, those photons carry a tiny bit of pull which will, on impact, pull the object i
Re: (Score:2)
>as if it were the photons itself exerting the pull.
should be
as if the photons themselves were transmitting the pull.
Re: (Score:2)
The thing you need to be careful of is mixing Newtonian ideas (instantaneous force vectors) with quantum or relativistic phenomena. However, there are many similarities between gravity and electromagnetism. If you go through all the math and consider the propagation delay between a moving charged particle and one at rest, it works out that the force vector points towards the instantaneous position of the moving charged particle because the EM field of the moving particle has a velocity-dependent component
Re: (Score:2)
Which is also why civilizations at the core of the galaxy could easily be far more advanced than us because the are traveling at a slower speed than us out here on the rim. Time for them is flowing at a different rate than us.
Re: (Score:2)
And over several milennia that is a HUGE margin.
Re: (Score:2)
There are cases where a wave can propagate faster than c, but no information can be conveyed faster than light.
Interesting, I'd not heard of waves that can propagate faster than c, can you link to more info? If a wave can propagate faster than c then why can't it carry information?
Re: (Score:2)
Usually the issue is that by the time the wave arrives at you, you can't figure out which parts of it are actually information, and which parts of it are noise.
Re: (Score:3)
Re: (Score:2, Informative)
A great example of this that I've seen is: Shine a spotlight at the moon (from Earth) and sweep it across the surface. You can move the spot faster than the speed of light, thus the wave moves faster than c, but no individual photon moves faster than c, and no information is conveyed faster than c.
Re: (Score:2)
Very reasonable, thank you.
Re: (Score:2)
I would suggest looking up the Nimtz double-prism experiment
Apparent faster than c transmission has been observed to be exceeded based on comparison of arrival times b/w groups of waves. transmitted and reflected waves arrived at detectors simultaneously, despite the transmitted light having also traversed an additional distance across a gap
Re: (Score:2)
Interesting, thank you. The wikipedia article says "Nimtz stated that the frequency modulated (FM) carrier wave transported the 40th symphony of Wolfgang Amadeus Mozart 4.7 times faster than light due to the effect of quantum tunneling." So it appears that he did, in fact, transmit information faster than c.
Now I need to read up on quantum tunneling.
Re: (Score:2)
Nimtz stated that the frequency modulated (FM) carrier wave transported the 40th symphony of Wolfgang Amadeus Mozart 4.7 times faster than light due to the effect of quantum tunneling." So it appears that he did, in fact, state that he transmit[ted] information faster than c.
FTFY. Still plenty of contention surrounding the experiment and the many possible interpretations of it, by the looks of things.
Re: (Score:2)
True, it's only what he claims.
Re:SETI (Score:4, Funny)
When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong. - Arthur C Clarke
Re: (Score:2)
Re: (Score:2)
"It is impossible to convey information at a speed faster than c. " It could be possible and we just have not discovered it yet. Unless you know something and just haven't got around to sharing yet. We have a lot of theories and mathematical constructs but if just one of these theories is wrong we would have to re-evaluate all of the assumptions. Especially since some of the baseline theories are based upon mathematical equations that have needed to numerical constants added to make the equations actually w
Re: (Score:2)
The earth orbits where the sun is right now.
It does, but not for the reasons you claim: https://en.wikipedia.org/wiki/Speed_of_gravity [wikipedia.org]
Re: (Score:2)
In other words, because we orbit the sun and not vice versa, the sun is pretty much still in the same place as it was eight minutes ago, and will be in the same place eight minutes from now?
When we look up and see the sun, we're only seeing it how it looked eight minutes ago, but it's in the same apparent position in the sky that it would be whatever the speed of light was.
Is that right?
Re: (Score:2)
From the link:
The Earth does in fact orbit the sun.
Re: (Score:3)
I have heard this. I've also heard that it's an urban myth. And that it's a real phenomena, but that the discrepancy only exists when analyzing the system in terms of Newtonian gravitation, and disappears when analyzed in terms of Relativity. Take your pick.
Regardless, we *are* looking for gravity waves - we've found tightly-orbiting binary star systems that are losing energy at precisely the rate predicted by the emission of gravity waves propagating at light speed (a different speed of gravity wave pro
Re: (Score:2)
Considering that we only recently have figured out that we can't even detect around 9/10 of the matter and energy in the universe I think that there is quite a lot we don't understand.
Re: (Score:2)
No argument. But in fairness, that 90% estimate is based on the assumption that we *do* understand gravity. Even a relatively "simple" and non-controversial reanalysis using Relativity instead of Newtonian Gravity mostly eliminates the galactic rotation problem, which weakens the case for dark matter, and dark energy has an even more tenuous evidence base. And there are several theories that suppose that either or both are actually artifacts of space-time itself, with no need for some sort of mysteriousl
Re: (Score:2)
The force of gravity is faster than light. Why this isn't widely known is a mystery to me (probably breaks someone's pet model - you think science doesn't have heretics?) but it's true. The earth doesn't orbit where the sun was 8 minutes ago. If it did it would have long ago left the solar system. The earth orbits where the sun is right now. It takes light about 8 minutes to travel from the sun to the earth.
Do the math sometime and you'll see that you are wrong (you are right that it orbits where the sun is now, but not for the reasons you think); or at least, do the math from our current models. I don't know what the math would say from whatever model your theory is.
As for heretics and pet models, a summary of where the current pet models stand with regard to being tested can be found here [lbl.gov]. From the conclusion:
All present experimental tests are compatible with the predictions of the current “standard” theory of gravitation: Einstein’s General Relativity. The universality of the coupling between matter and gravity (Equivalence Principle) has been verified around the 10^(-13) level. Solar system experiments have tested the weak-field predictions of Einstein’s theory at the 10^(-4) level (and down to the 2 × 10^(-5) level for the post-Einstein parameter gamma-bar). The propagation properties of relativistic gravity, as well as several of its strong-field aspects, have been verified at the 10^(-3) level (or better) in several binary pulsar experiments. Recent laboratory experiments have set strong constraints on sub-millimeter modifications of Newtonian gravity. Quantitative confirmations of General Relativity have also been obtained on astrophysical and cosmological scales (assuming dark matter and a cosmological constant).
This pet theory is almost 100 year old now, and though there is still lots of interesting things
No and no. (Score:1)
SETI is about detecting ambient electromagentic waves that may be of an "intelligent" origin. Lasers, however, are a specific form of electromagnetic wave. So in some sense, SETI is already looking for them.
Now, lasers used for communications like this are of the utmost quality. Their dispersion (that is, how much the beam of light separates over time and distance) is very minimal. While a flashlight may have a large dispersion measured in meters a few centimeters from the light source, lasers used for comm
Re: (Score:2)
What about reflections. I noticed ambient glows around the areas where a laser pointed is directed at so I assume that some of the light scatters when it is reflected from any surface not a mirror.
So lets assume two space crafts (man made) are using lasers to communicate in an orbit around mars, would we be able to detect and decode the communications from the reflections and scatter without directly watching the crafts or would it disperse so much that it wouldn't be noticeable from the earth.
Re: (Score:1)
Re: (Score:1)
No but then you simply can't intercept Whisker Lasers easily. You need to get a stealthed Recon-drone in the beam to do so.
Re: (Score:2)
Does this make what SETI is looking for pointless? Should they instead be looking for lasers if they work better for communication.
Why would any of those lasers be aimed at us?
Re: (Score:2)
The idea is that any sufficiently advanced interstellar species using lasers for long-distance communications will have enough of them that at any given moment they will be pointed every which way. Much like a normal sophisticated urban human is carrying multiple radio devices.
Although tenuous, it does get away from the background glare problem. Nobody out there is going to see an Earth based laser unless we're using one powerful enough to push a solar sail on an interstellar probe because glare of the s
Re: (Score:2)
Re: (Score:2)
this would be like those pulsar things that would be an extraordinary amount of energy if they radiated in all directions, but reasonable if it were a pulse of directed energy. which begs the question - why are they pointed at earth? one solution is clear. angels. it's a wonderful life!
Re: (Score:2)
Does this make what SETI is looking for pointless? Should they instead be looking for lasers if they work better for communication.
No. SETI should be looking for sharks - duh.
Re: (Score:2)
Optical SETI is an active field of research. It doesn't get talked about as much as radio SETI, in part because it is only recently that optical interferometry arrays became possible, in part because optical telescope time is expensive and in part because the atmosphere limits the quality of data for optics. There are (very recent) developments in autocorrection that reduce atmospherics, but the reality is that until someone parks an optical SKA telescope in space, the quality of telescope data won't be suf
Re: (Score:2)
Yes, but they won't have fins.
and some will take over the laser asking 1 million (Score:2)
after I destroy Washington D.C... I will destroy another major city every hour on the hour. That is, unless, of course, you pay me... one million dollars
It Should Also Be Good For (Score:2)
...dealing with telepathic cats.
The Playzer (Score:1)
I give you: http://theplayzer.com/ [theplayzer.com]
Re: (Score:1)
Good to know that... (Score:1)
gbic (Score:1)
Re:That's Great (Score:4, Insightful)
Re: (Score:2)
cat, just because we dont have a large manned space program does not mean we dont have a space program. I would say that we have found out more about space and moons and planets and stars in the previous 10 years than at any time in the past. Sure it is not as flashy as sending an astronaut to the moon or mars, which I am in agreement with you that we need to be working harder on our manned missions, but to say our space program is dead is just simply wrong
I hope this is not one of those five year technologies.
Slashdot is full of stories about this new technology and that new technology that will be ready in five years. These then just vanish after the story first appears.
After the story is published and we comment, it is forgotten and nobody is held accountable for publishing bullshit.
Maybe someone should do a review of stories published 5 years ago of new technologies that have never materialized. I remember five years ago there used to be so many sto
Re: (Score:2)
Sure it is not as flashy as creating an Immanuel Goldstein out of a nation that's not really such a threat to us, that we often armed and trained first, then spending many times the resources in order to go overseas to shoot up and blow up some brown people with unfamiliar names to keep the military industrial complex satiated, but to say our miniscule, neglected, space program (that no longer inspires the nation) is dead is just simply wrong
Fixed that for you. It must have really been something to have heard Kennedy declare that we choose to go to the moon, not because it is easy but because it is hard. A sense of purpose and a sort of pride that came from technical achievement and engineering marvels rather than the comparatively simple matter of sending the world's strongest military against some of the world's weakest militaries. This generation has nothing quite like that.
That was pre-Boomer America. (Score:1)
I remember 1961 fondly. I was 31 then. You need to understand that those were very different times. That was pre-Boomer America, which I can assure you was much better than the post-Boomer America we have to deal with now. I'm not looking back through rose-colored glasses, either. The people and accomplishments speak for themselves.
Back then, America was a meritocracy. Those who were were good, regardless of origin or background, succeeded. Those who were great excelled. Those who couldn't were rightfully s
Re: (Score:2)
I doubt the veracity of your story. It very much sounds to me like you likely were not in the top 10% of your class, regardless of what you may believe. The talks you got were likely because of this, not because of your gender.
My mother went through the same stuff. She attended University College of Bangor in the late 60s acquiring a BS in Chemical Engineering. From what I gather, while there were some overtly sexist people among both students and teachers, there was also occasionally the attitude that she couldn't possibly be a good student because she was female. Fortunately, she didn't listen to the people who thought they were being helpful.
The Baby Boomer attitude is that everybody should be in the top 10%, whether they deserve it or not, and whether they actually are or not.
No, that isn't the "Baby Boomer attitude". I find it rather interesting how people fin
Re: (Score:2)
Economically, children started to make less than their parents at the same age after the Boomers made theirs.
As I noted, it isn't the fault of the Boomers that there are seven billion people on the planet. The declining premium commanded by developed world labor is not their fault, but just a reality of expanding the global economy to include everyone on Earth.
Politically speaking, Boomers overall are more liberal, and are the ones who voted for Big Government. They're the ones who sided with unions, pushed environmentalism, affirmative action, etc. They tell themselves they were "helping" the world, but in reality they made it worse.
All of those things came before Boomers were even born.
Yet they almost always point their fingers at other usually younger people. It's the lazy kids for not working hard enough or figuring out how to compete with third world labor. It's the damn workers pushing up the cost of business. It's the other people who voted wrong ("I hate Congress, but my guy's doing great"). etc.
Again how does this differ from anyone else?
About the only thing that I think is accurate here is your assertion that Baby Boomers had a big role in building the current society. There's much wrong wi
Re: (Score:2)
But then the Baby Boomers came onto the scene in the mid-1960s. It's safe to say that, as a generation, they have managed to destroy America.
I'd start looking for someone to blame by looking in the mirror. Boomers didn't create the majority of the entitlements, for example. A bunch of that stuff started broken. Nor did they raise the Boomer generation.
Nor did they create seven billion people currently on Earth or the economics and technologies that drive globalism. Those people compete with US workers for jobs and for a lot of industries are simply a better choice - no matter the generation of the US worker. I think how they tried to escape t
Re: (Score:2)
I'd start looking for someone to blame by looking in the mirror.
A Boomer supporter refusing to accept any blame? Color me unsurprised.
As opposed to a Boomer blamer?
Where did GP say Boomers created 7 billion people or globalism? Where did GP even say 7 billion people and globalism were problems? Strawman bro. Strawman.
Not at all. A lot of the alleged Boomer faults are actually due to this significant dynamic.
Not fully, but mostly. It wasn't the young adults who passed legislation for government to back those loans, or heavily regulate the markets and enforce minimum wage so companies can't just hire unskilled students without degrees for cheap, or gamed the political system to put charlatans on the ballot and giving them and only them media coverage, for example.
Well, it wasn't the Boomers either. Passing legislation is the purview of a legislature. And as for Boomers supporting politicians with bad policies - I see nothing to distinguish their bad judgment from the people who came before or after.
Re: (Score:2)
I remember 1961 fondly. I was 31 then. You need to understand that those were very different times. That was pre-Boomer America, which I can assure you was much better than the post-Boomer America we have to deal with now. I'm not looking back through rose-colored glasses, either. The people and accomplishments speak for themselves.
As one of the 'last' of the boomers (b. '64) I'd love to be able to swallow your wholly-generational explanation for the general Suck of things but I cannot. I see too many re-connections of the same bad ideas across generations, often re-sold to the younger under thin new guises. Would that I took a position such as yours, I'd have a few younger generations to blame. But I will not sling it their way either.
A generational explanation does not explain the phenomenon that is Donald Rumsfeld [wikipedia.org] for example, w
Re: (Score:2)
Re: (Score:2)
Ah, just to clarify - the space race was most definitely a military endeavor. We show off our state-of-the-art ICBM technology (carrying a human payload into space) to the Russians, they do the same to us. Rinse and repeat until the Soviet Union imploded, at which point both space programs lost their military importance, and with it most of their funding.
It made for great PR films, but the people that cared about science, exploration, and pushing the boundaries of human accomplishment were always riding o
Re: (Score:3)
That's always been a cute fairy tale put forward by the Pentagon fantasy factory, but it's never been true. The military assisted in the space race, especially in the beginning when they had the only functional launchers, but the necessities of space exploration quickly surpassed the really rather primitive needs of the military. The Apollo 1 booster was already larger and more powerful than any ICBM would ever need to be, and took so long to assemble, prep and fuel that it could never be useful as a weap