Spyman

I am not an expert, but from my understanding the limits are zero and lightspeed ©. Thus non of the trains will ever reach the speed of light.

Around a Black Hole' date=' outside the Event Horizon there is a Photon Sphere, where light can orbit. http://www.gothosenterprises.com/black_holes/static_black_holes.html So, what is the circumference of the Photon Sphere ?

Just read this in SPACE.com news and though it might help visualize the bending of space-time: Eye-to-Eye with a Black Hole http://www.space.com/scienceastronomy/050711_blackhole_looks.html

The electric current needs to pass through Your body, or part of it, thus it must have one place to enter and one to exit. If Your shoes are well isolated You won't feel any shock because the current can't pass through You. If not isolated You will feel the shock, without someone grabbing You. If both are isolated no one will feel a shock. If the one touching the fence is isolated and the one grabbing is not then both will feel a shock. The path of the electric current through the body and personal internal body resistance is what will make the difference. It could as well be the one touching and getting grabbed that would get the most of the shock. But most common is that the "thoucher" is being grabbed close to the place where the current is entering, on the arm or upper body, and thus have a shorter path through his body, while the "grabber" is the one leading the current down to the ground and thus have a longer path, through his hole body, from the hand to the foot/feets. EDIT: A small electric shock http://www.scienceforums.net/forums/showthread.php?t=12306

Above J.C.MacSwell = 55.

I take a chance on 7.

My wild guess = 12.

The ships relative speed is zero so their relative increase in mass is zero. The attraction against an external observer may be different but the attraction between the ships will be the same for observers and crews. The ships will still need to continuously correct their distance or they will collide due to the force of gravity from their rest mass. EDIT: (Off Topic Question) Doesn't the relativistic mass of a photon attract particles ?

If You where shot out of a cannon with escape velocity Your final velocity would be zero at infinite distance from Earth. As gravity slows You down, You also gets farther away which weakens the gravity field. If fast enough start, gravity will decrease faster than speed. All bodies have a gravity field, it doesn't change unless the shape or mass/energy of the bodie change. The gravity field is not made of light. There is no path from inside to outside, all directions leads into the center. Outside the Event Horizon, where light can escape, light loses energy while climbing out of the gravity well which causes it to get redshifted.

If You had a ladder that went from the surface of the Earth to outer space, then You could leave with any speed You want. If You where to fire a cannon bullet from the surface to outer space it would need the velocity of 11.2 km/sec or more, for not to fall back again. With Black Holes and light it's more complicated but the photon could be viewed as an bullet, it can't be slowed down by gravity but can be pulled sideways. The intense gravity field inside the Event Horizon of a Black Hole is so strong that space-time is curved so hard so the photon is turned back, sideways, before it can escape. If You had a ladder that was strong enough to withstand the strong tidal forces it would still bend so much inside space-time of the Black Hole so both ways would lead down. (Unless You where able to climb faster than c which is impossible.)

BlackDog: A single object still needs a force for it to accelerate but not to maintain speed. BigMoosie: Even if it's gravity there would be tidal forces which could be measured.

Hi Martin' date=' good estimate ! Whether it's 4.5 or 5 tons of TNT it's still a very large firecracker ! I found this: http://www.space.com/missionlaunches/missions/deep_impact_010502.html(In one of the links, in the first link I provided.)

AFAIK They have not yet even succeded in sending information FTL. Information in a letter may be moving Faster Than Light but You can't read it until the letter arrives. To send information backwards in time You need to send one of the entangled particles backwards. (If it's possible and without destroying the entanglement.) Your setup only ends up with one particle older than the other, both are still in the same time frame. (If it's possible to store one of the entangled particles.) I don't know the creditability for this link, but it's interesting: Quantum Entanglement and Causality http://fergusmurray.members.beeb.net/Causality.html

They will (hopefully) release a lot of good images from Hubble and Spitzer space telescopes.(So there is no need to rush and buy a telescope.) With wordings like "Cosmical Fireworks" and the date 4 July, I guess NASA tries to be more popular ... (It's still a very scientific mission and not just a commercial event.) http://www.space.com/deepimpact/

If it was only one single body then that single body and the space around it would be the Universe, and I don't think the Universe can either move or accelerate. But, if we say that we are in an empty part of the Universe, without being able to see or measure anything else then: The object can be moving but since there is no reference the speed can't be measured. If You close Your eyes in a car moving in a straight line, are You able to sense the speed of the car ? Now if the car takes a hard turn or the driver hit the brakes, will You feel the forces ? Well, there has to be someone with some cind of equipment to measure both speed and acceleration. A single body can't do anything else than ceep it's inertia, it's not conscious and have no desire/tools/senses to aquire knowledge. However, the forces would still be acting on the body, if for an example the rotation is rapid enough, the body should be flung apart. Don't be, it's the only way to find out when confused.(And my explanations may not be the best.)

The charge inside a capacitor is between the poles, like in a battery. More, the charge is not a potential, it is electrical energy which includes a potential. A capacitor charged with X Volt is not charged with a potential, it's charged with so much energy that the potential between the poles has raised to X Volt. Lets say a capacitor is charged to (with) 100 Volt: - If the negative pole is connected to the ground then the positive pole will have a potential of +100 V with respect to the ground. - If the positive pole is connected to the ground then the negative pole will have a potential of -100 V with respect to the ground. - If both poles are connected to the ground, then the capacitor will discharge it's energy storage through the ground. - If non of the poles are connected to the ground, then the capacitor will still hold it's energy with the potential of 100 V between the poles. So when touching/connecting to both poles I view the capacitor as an power source and not a electrostatic charge. Capacitors in power applications, like frequency converters, can easily store enough energy to seriously harm or kill a human, if connected to both poles.

Inertia

If only touching one of the poles of the capacitor, thus not creating a complete cicuit ? I agree that it's the wattage across-inside-through the body that causes the harm. The examples from my safetybook is rough and a worst case scenario, simply to give an estimate how dangerous electricity is. Thus 50 V could give 30 mA across-inside-through Your body, in a worst case scenario, which is enough wattage, (1.5 W), to cause harm to Your body, after 5 seconds the harm is considered to be severe from some standard safety level.

With respect to what ? I don't think the Universe is rotating in the example given' date=' only the bodies within it. What revolution do You think the Universe have in the example ? (The same as the Planet, the Moon, the Moons orbit, an average or something else ?) If a body is revolving around the center of mass with the same revolution as the total mass of Universe it should still be measureable. If space-time also is revolving then I can agree that it won't be measureable. But what would space-time revolve with respect to if everything inside is revolving with it ? Spinning can be considered an acceleration because speed is moved from one dimension to another.Forward & Reverse is one and Left & Right is another, (Up & Down is the third and Time the fourth). To complete an circle You move Left, Forward, Right, Reverse with the same decrease of speed in the ongoing direction as increase in the next.

From my understanding the internal body resistance between random different points of induvidial human bodys can be as low as 100 Ohms. The skin when dry and clean can have a resistance up to 2 M Ohms. From my safetybook of "Working with Electricity": Loss of muscle control between 10-20 mA. Difficulty with breathing between 20-30 mA. Possible heart fibrillation between 50-100 mA. Internal burning damage above 1 A. Time dependency for severe damage of body: <50 V 30 mA 5 s 125 V 80 mA 330 ms 230 V 155 mA 180 ms 400 V 280 mA 70 ms So, an electrical shock can be lethal in three ways: 1) Heart stops pumping. (The current overtakes control signals for the heart.) 2) Breathing stops - suffocation. (The current overtakes control signals for breathing.) 3) Severe internal burning degres. (The power from the current transformes to heat.) Most people die of no:1 because AC, (which is normal power supply in houses for most people), causes spasms in Your muscles, thus You problably lose contact fast, but the heart may not restart again. I have never heard of anyone get hurt or dying from an electrostatic charge.

I follow You, but I am not educated in relativity so I can't say for sure. My guess: Light always propegates at c, so if time would speed up so would light, thus no redshift. But it might be considered as an acceleration which could cause the redshift. Also when the Universe was older it was also less expanded which would place the emitter in a higher gravitational potential, which could cause a part of the redshift we see.

How about this analogy: If You have a lake, about 100 m above sea level, (potential). Will You get pushed by the water in the lake or in the sea ? Now connect the two with a river, the water flows, (current). Will You get pushed by the water if entering the river ? The power of the push from the water or the electrical shock, (voltage * current), is what You feel. So without a complete circuit there is no flow and thus no power. With a complete circuit Your body becomes the river. As the human body mostly consists of water with low resistanse, then if the voltage is high enough to penetrate the natural insulanse of the skin, the shock can be considered as trying to stand in a river with the slope of the Niagara Falls.