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Two words: inertial dampers.

Two other words: Relativity, and Acceleration.

I've read[1] that if we accelerate consistently at 1G we'll reach 0.77 C in 1 year. However, as we continue to accelerate closer to C, we get more and more relativistic and things get screwy... screwy to the point that I'll estimate it would take about 6 years (that's 6 rocket years, not earth observer years) to get there, with 1G accel and 1G deccel. So, human travel would be extremely feasible.

While a probe could accelerate much harder, I figure it would still take 50 years or so to get results from a probe to confirm it's worth sending people.

1. http://www2.corepower.com:8080/~relfaq/rocket.html

IMO, the only thing we're missing is the "gravity to the rest of it" connection, confounded by the inconvienient fact that gravity appears to be the only force in the universe which is apparently instantainious over galactic distances. Go work any celestial orbital mechanics problem, including the orbit of the earth around the sun, and try and make it work if the gravitational attraction vector is assumed to be toward where the sun appears to be now (as opposed to where it is right now instead where it was 8 minutes ago when that light left the suns position then). By adding any delay, the orbit falls apart, and our earth would have spiraled into the sun many billions of years ago.

Actually, that's a common misconception of people who don't do the math of GR. But you can find many articles about relativity that explain that in fact this claim is false. GR predicts that gravity propogates at the speed of light, and that the orbit of the earth is stable. However, once you put that in, you can't simply use the Newtonian equations and have to use GR properly.

how to add relativistic speeds

Specifically, as to van Flandern, here's the comment I made in response to the earlier guy who brought up the same web page:

van Flandern is a crank who doesn't understand gravitational radiation, despite careful correction. (Though he has done some decent work in other areas.) This issue has been beat to death on Usenet. See, for instance, the FAQ, Carlip's correction of van Flandern, Hillman's archive of a Usenet discussion with van Flandern, etc.

It has been experimentally verified many times over that antimatter behaves exactly like matter in a gravitational field.

The answer is that the Earth is attracted to where the Sun's present position, not its retarded position. (Well actually, its "retarded position linearly extrapolated from its motion to where it is now".) Counterintuitive, but true, and it doesn't violate relativity or constitute FTL propagation of information. See the FAQ.

van Flandern is a crank who doesn't understand gravitational radiation, despite careful correction. (Though he has done some decent work in other areas.) This issue has been beat to death on Usenet. See, for instance, the FAQ, Carlip's correction of van Flandern, Hillman's archive of a Usenet discussion with van Flandern, etc.

If all matter in the universe was in a point, or even a tiny volume, it would collapse into a black hole. Since nothing escapes from a black hole except an extremely tiny trickle of particles via Hawking radiation, the universe could never have undergone a "big bang". Either the "big bang" idea or the "black hole" idea is wrong.

See the FAQ:

"Why did the universe not collapse and form a black hole at the beginning?"

Sometimes people find it hard to understand why the big bang is not a black hole. After all, the density of matter in the first fraction of a second was much higher than that found in any star, and dense matter is supposed to curve space-time strongly. At sufficient density there must be matter contained within a region smaller than the Schwarzschild radius for its mass. Nevertheless, the big bang manages to avoid being trapped inside a black hole of its own making and paradoxically the space near the singularity is actually flat rather than curving tightly. How can this be?

The short answer is that the big bang gets away with it because it is expanding rapidly near the beginning and the rate of expansion is slowing down. Space can be flat while space-time is not. The curvature can come from the temporal parts of the space-time metric which measures the deceleration of the expansion of the universe. So the total curvature of space-time is related to the density of matter but there is a contribution to curvature from the expansion as well as from any curvature of space. The Schwarzschild solution of the gravitational equations is static and demonstrates the limits placed on a static spherical body before it must collapse to a black hole. The Schwarzschild limit does not apply to rapidly expanding matter.

And if you take the estimated mass of the universe and calculate the schwartzchild radius for a black hole of that mass, it's curiously close to the size of the known universe. We are all inside of a black hole right now!

So maybe the steady state theory for galaxy formation is correct after all.

The speed of gravity is the speed of light.

A relevant FAQ.

FAQ.

Light doesn't have mass.

Somebody else posted a cool link, but I don't have moderator points to bump it up. I do, however have karma to burn, so here it is.

Point 2 is odd, I don't understand what they mean by saying that there are no exact strong-field solutions for gravitational waves; certainly you can have very strong-field solutions like black holes with gravitational waves propagating around.

Point 3 appears to be wrong, depending on what one means by a "true" Lagrangian; GR is derivable from the Einstein-Hilbert action and has a very simple Lagrangian, that of the Ricci scalar plus whatever matter fields are around.

Point 4 is wrong. GR does not take the equivalence principle as a separate assumption. It follows from the simple fact that gravity is described by the curvature of a 4D manifold. The equivalence principle really states that "over a local region, spacetime acts Minkowski" -- all that stuff about elevators "in empty space" or "on the surfaces of planets" follows.

Point 5 is extremely questionable. While it is true that GR hasn't been quantized, there are many approaches to doing so -- such as Hawking's Euclidean quantum gravity, the loop quantum gravity approach, etc. And GR has been quantized in dimensions other than four, at least.

I'd advise Slashdot readers to look at the Cosmology and Relativity FAQs, since they probably answer a lot of questions people are tempted to ask.

No, the poster is right. A FTL signal will travel backwards in time according to some observers.

No, photons are massless (according to modern definitions). If you're talking about "equivalent mass" (E/c^2), then that doesn't have to be similar to an electron or even small; it can be anything. (Usually not similar to an electron though... the electron mass-energy is around 0.5*10^6 eV, and we usually see photons that are closer to a few eV.)

Quantum entanglement does not permit FTL communication (if "communication" is defined as "transfer of information").

The ansible in Ender's Game ostensibly makes use of quantum entanglement. Orson Scott Card in turn took the term "ansible" from Ursula K. LeGuin's The Left Hand of Darkness. I don't know how LeGuin's ansible was supposed to work or even if she described the mechanism at all.

It doesn't work.

You're thinking of quantum tunnelling and the Nimtz experiment.

If FTL travel is possible, then effects really can precede causes; this is not merely an illusion due to the finite propagation speed of light. This is disucssed in the following FAQ on time travel".

This is answered in the following FAQ entry, "Does light hve mass?"

I can't access the link, but from the abstract, this doesn't sound like a wormhole (working or not) at all. On the other hand, it might be a description of one of the other usual schemes for FTL communication.

Try the FAQ Is the Big Bang a Black Hole?.

To restate what you just said in my own words: 1) it took 13 billion light years for it to reach us. 2) that means it was 13 billion light-years away from us... 13 billion years ago. 3) it would take at least 13 billion years for it to get that far away from us in the first place 4) so wouldn't that mean that the universe is at least 26 billion years old? That makes sense to me, which probably means that relativity disagrees with it :) I started responding to this post because I thought I knew the answer to your question, but now I am scared and confused. Here's the Relativity FAQ; somebody else figure it out...

It's really called the Large Hadron Collider. It's called that because it's large and it collides hadrons.

Check this... LinuxDVD

I wondered (too) about Linux support for DVDROM (DVDRAM), and I have just found this:
http://linuxdvd.corepower.com/ "LinuxDVD project". Sounds like a start point. Any other relevant URL ?

The closest thing seems to be the linuxdvd mailing list, which has been in existance for a couple of months. They're pretty much only in the early planning/discussion stages, with list traffic seeming to be split about equally between technical jargon and licensing discussion. Any actual hardware or software will probably be at least several months off, and that's contingent on whether they can get licensing worked out.

Theoretically the energetic modus operandi of gravity. Read:
http://www.corepower.com/~relfaq/grav_radiation.ht ml

They don't "overlook" it; it's simply not a problem. See the FAQ. The universe isn't a black hole, either; see the other FAQ.

As to quantum entanglement.. causality still holds in the sense that effects cannot precede causes, nor can you communicate information faster-than-light. There are instantaneous correlations between observables (see the FAQ on the EPR paradox).

The speed of light in a medium is given by its index of refraction, which is the ratio of its speed in vacuum to its speed in the medium (and as its name implies, governs the angle through which a light ray refracts upon entring the medium). For air it's 1.00029 and for water its 1.333.. so in water, light travels 1/3 slower than in vacuum.

Now, as to your specific comments: even in a gravitational field, the local speed of light as measured by any inertial (= free fall) observer is always the same.

The speed of light is not really well-defined in other cases; it depends on what coordinate system you choose, and because there are no global inertial frames in a curved spacetime, there isn't any "best" way of choosing one.

I don't think I'd say that "if you could negate the force of gravity [...] you should see an increase in the speed of light". The speed of light (as that quantity is defined above) is never going to be anything other than 2.997*10^8 m/s^2, no matter what the gravitational field is.

The speed of light also does represent a maximum speed limit in that nothing can travel between two points in a particular spacetime faster than a light ray could travel between those same two points.

Now, as to your specific comments: even in a gravitational field, the local speed of light as measured by any inertial (= free fall) observer is always the same.

The speed of light is not really well-defined in other cases; it depends on what coordinate system you choose, and because there are no global inertial frames in a curved spacetime, there isn't any "best" way of choosing one.

I don't think I'd say that "if you could negate the force of gravity [...] you should see an increase in the speed of light". The speed of light (as that quantity is defined above) is never going to be anything other than 2.997*10^8 m/s^2, no matter what the gravitational field is.

The speed of light also does represent a maximum speed limit in that nothing can travel between two points in a particular spacetime faster than a light ray could travel between those same two points.

Now, as to your specific comments: even in a gravitational field, the local speed of light as measured by any inertial (= free fall) observer is always the same.

The speed of light is not really well-defined in other cases; it depends on what coordinate system you choose, and because there are no global inertial frames in a curved spacetime, there isn't any "best" way of choosing one.

I don't think I'd say that "if you could negate the force of gravity [...] you should see an increase in the speed of light". The speed of light (as that quantity is defined above) is never going to be anything other than 2.997*10^8 m/s^2, no matter what the gravitational field is.

The speed of light also does represent a maximum speed limit in that nothing can travel between two points in a particular spacetime faster than a light ray could travel between those same two points.

Now, as to your specific comments: even in a gravitational field, the local speed of light as measured by any inertial (= free fall) observer is always the same.

The speed of light is not really well-defined in other cases; it depends on what coordinate system you choose, and because there are no global inertial frames in a curved spacetime, there isn't any "best" way of choosing one.

I don't think I'd say that "if you could negate the force of gravity [...] you should see an increase in the speed of light". The speed of light (as that quantity is defined above) is never going to be anything other than 2.997*10^8 m/s^2, no matter what the gravitational field is.

The speed of light also does represent a maximum speed limit in that nothing can travel between two points in a particular spacetime faster than a light ray could travel between those same two points.