Spyman

I interpret "traveling" as movement relative something and the Universe is everything everywhere anytime. The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. -> http://en.wikipedia.org/wiki/Universe The Universe is all, you can't "bring it with you" somewhere or sometime else, we are in it and there is no outside. I move around the Earth one lap every 24 hours, but I don't call myself a Earth traveller because of that. The Earth revolves around the Sun, which moves through space around and with the MilkyWay, but I don't call myself a space traveller either. So even if I, IMO, moves through time, I don't call myself a time traveller. So for me "real" time travel is movement relative "normal" movement, and as doing so you wouldn't bring along anything else than what you take with you, all other objects would continue forward in time as normal. (IMO, "normal" movement is staying inside the light cone we have now, the ability to interact with others.) The time traveller leaves our light cone, passes our event horizon, but from his perspective everything would be normal, his clock would continue to tick normal, his body wouldn't stop functioning. But he wouln't have anything to interact with, since everything from the past has moved forward in time.

According to the theory of General Relativity by Einstein, energy bends spacetime equally as mass. The gamma ray is already bending spacetime and the energy released by the annihilation will also bend spacetime to the same degree as the total mass of the electron and the positron. Gravity waves travels with the speed of light in vacuum. Gravitons is still considered a hypothetical particle. http://en.wikipedia.org/wiki/General_relativity http://en.wikipedia.org/wiki/Gravitational_wave http://en.wikipedia.org/wiki/Graviton

That wouldn't be fair to the dog, they shouldn't be allowed to have any animal at all.

A coordinate is not a thing or an observer. But you could place an observer there to measure it's own "objective" time.

So I was correct about the "Key-point" then... Coordinate Systems -> http://en.wikipedia.org/wiki/Coordinate_system EDIT: The point of the coordinate is that it's related to the observer and not the moving object.

Physics is about how things are related. We measure relations.

I don't know if I am able to understand you correct, first you say this is incorrect, and then you continue to say: Where you agree with what I said. Well, I have time in my view of the Universe so I can have changes too... I know meters aren't seconds, and the difference between where and when, I never said they where the same ! Stop twisting my words, please. I saved this one for last, because it seems to be the "Key-point" of the confusion: Did you reread the post I asked you too ? Where I said: Please, explain for me: Why the coordinate (x,y,z) must change if there is a motion of an object. Yes, sure - the position of the object changes, the coordinate of the object changes, but the coordinates themselves don't move. The coordinate where the object was is still real, exists and has NOT moved. (Well, they are relative, spacetime is not rigid or absolute, but thats beside the point.) I view spacetime in 4D, there is motion of objects between all four types of coordinates, both in space and time. Which part in the above sentence is it that you don't understand ? I fully agree.

I don't know, I only gave you the piece in the puzzle I did know... You will have to follow the others explanation. (swansont is the Expert.)

No, I am not, read my post again, please. I said that (x,y,z,t1) coordinate is not the same as (x,y,z,t2) but the (x,y,z) part is, the position in space. And that the difference between (x1,y,z,t) and (x2,y,z,t) is similar to (x,y,z,t1) and (x,y,z,t2). I don't think General Relativity is a "simple" model of spacetime, and it is the best model we have. Yeah, and why don't space have 4d or even 5d ? - Knock it off, I don't know WHY nature is like it is. Such questions won't aid your opinion or shot down mine... Everything we see is from past positions, you are in fact not able to see the actual "now"... (So with your own argument the only real positions is in the past and "now" doesn't exist.) AFAIK, The lack of ability to see or move to positions don't make them less real.

The past, now and future coordinates are of the same cind, placed in the same spacetime continuum, the Universe. The space coordinate (x,y,z) in the past is the same as space coordinate (x,y,z) in the future. Motion is when objects is changing coordinates, not when the coordinates is changing. An event is a specified change at a specified spacetime coordinate. Any change could qualify as an event, at any position, (both space and time). I think that the spacetime coordinate (x,y,z,now) is as real as (x,y,z,now-3days) or (x,y,z,now+3days). Where they exists ? - That would be relative where you are, wouldn't it ? The coordinate -3days is three days back in the past relative now, tomorrow it will be four days back, relative me because I will have moved 1 day forward into the future. It's not very far from saying that a position three meters away will be four meters away if I move one meter. The position where I stand now doesn't vanish because I move one step, it's still there but at a different position relative me. I view Time like a line and we are moving along it, our past and future positions on the line exists too. The question if it is possible to "jump" backwards or forward is still unsettled by science, but we have already proved that we have the ability to slow down or speed up our relative speed in time, (like Edtharans example). Maybe it turns out that "it's recorded on some big universal recording mechanism" so you can visit your grandfather. But then it's likely that you would create a paradox with a major malfunction as a result. In my opinion a time traveller jumping backwards or forward in time will only find empty space, unless there are other time travelling objects or persons out there. The particles that your grandfather was made of has moved and is here now, and they have not moved to the future jet. Unless you don't bring the particles with you, you are not going to be able to interact with them. (And the particles won't reverse their actions and become your grandfather if you bring them back with you either.) I don't see any problems with a infinite set of spacetime coordinates... Everything seems to be moving forward in time, or at least everything we observe, they only photons we can see is the ones emitted in the past and reaching us now, those photons must be moving forward in time too. The photons point of view is irrelevant, if the photon and the observer is moving through time with the same pace then the photon is not able to catch up with the observer. The observer is no longer in the photons world line. The photon might belive it is a time traveller but from our POV it still takes time for a photon to move. If you new beforhand when and where they would pass, you could flash a picture which they could see, but the pass would be instant so they can't reply. Likewise they could flash an image, but they would be gone before you could reply. Any light from our time would reflect of them in the past, outside our light cone. A think a "real" time traveller would be beyond our event horizon, anything else would be something moving forward in time together with us, inside our light cone. Ability to reply to questions not yet asked, implies Fred56s machinery with an infinite row of Edtharans...

I didn't vote on the poll, I don't count my encounters with different breeds... So far during my life, I have encountered a very large sample of different dogs. And I have NEVER been attacked ! (But there have been a lot of "incidents" between my dogs and others.) My experience with dogs and owners are that any trouble is caused by the owner. Further I am inclined to say that owners with "small" dogs are generally more irresponsible. The problem is that "big" dogs causes more harm and are more attractive for "bad" guys. IMHO I think a resonable alternative to banning should be some cind of license. It would filter out a lot of the "bad" guys and also make sure that the owner knows his responsibility. (Both to the animal and the surrondings.)

I don't think we could see or interact with a time traveller, he would not be inside our light cone. (Both the past and the future part.) The plane in your example never left our light cone, but a "real" time traveller would have to. In general relativity, the future light cone is the boundary of the causal future of a point and the past light cone is the boundary of its causal past. http://en.wikipedia.org/wiki/Light_cone @foodchain: My reply was for Fred56, who was asking me where the past spacetime coordinates exists. (If I interpret your post correctly, I think I agree.)

If I have two flashlights and point them in opposite directions, the photons will travel away from me with c. You are not allowed to put an observer on a photon and measure their relative speed, as Severian said. But in my frame, standing between the photons, they are travelling at c in opposite directions, like trains. The photons will "distance" themselves with 2c, from my point of view. (Maybe I should have said "In a valid observers frame"...)

Where is the spacetime coordinates for "now" ?

The rest mass is not variable, the relativistic mass increases as the speed approaches c. The term mass in special relativity is commonly used by physicists to mean a quantity that does not depend on the observer or the inertial frame used to observe it. However, because the term relativistic mass is also used, this occasionally leads to confusion. The invariant mass of an object (also known as the rest mass, intrinsic mass or proper mass) is an observer-independent quantity that is synonymous with mass. On the other hand, the relativistic mass of an object (also known as apparent mass) increases with its speed and therefore depends on one's frame of reference. http://en.wikipedia.org/wiki/Mass_in_special_relativity

If it hurts, it most likely causes some harm and it's probably best to stop... And, (Hehe ), be aware that YT and i_a seem to love playing around with dangerous stuff:

Got to admire your great stamina and endurance, Edtharan ! I think you are worth some reputation points...

Here is a popular science article, for those interested: Dawn is now headed for a February 2009 swing past Mars before reaching its first space rock target, the bright and rocky asteroid Vesta, in August 2011. The probe's novel Xenon ion propulsion system is expected to guide it into orbit around Vesta for almost a year, then send it off toward the icy dwarf planet Ceres -- the largest space rock in the asteroid belt -- for a February 2015 rendezvous. http://www.space.com/missionlaunches/070927_dawn_healthupdate.html There are some indications that the surface of Ceres is relatively warm and that it may have a tenuous atmosphere and frost. The maximum temperature with the Sun overhead was estimated from measurements to be 235 K (about -38°C) on May 5, 1991.Taking into account also the heliocentric distance at the time, this gives an estimated maximum of ~239 K at perihelion. A study led by Peter Thomas of Cornell University suggests that Ceres has a differentiated interior: observations coupled with computer models suggest the presence of a rocky core overlain with an icy mantle. This mantle of thickness from 120 to 60 km could contain 200 million cubic kilometres of water, which is more than the amount of fresh water on the Earth. http://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29 Based on observations made by the Hubble Space Telescope, astronomers think Ceres may harbor not only a thin atmosphere, but also a thick layer of water ice beneath its surface. If true, Ceres may boast nearly six times as much fresh water than is found on Earth. "If we discover evidence of a subsurface ocean, than certainly it would be a high priority to go back and explore in more detail," said Mark Sykes, a Dawn co-investigator at the Planetary Science Institute in Tucson, Ariz. http://www.space.com/scienceastronomy/070925_st_dawn_preview.html Now you got me intrigued, Martin. - There seems to be a small possibility of life down there, below the ice...

You can't tell the difference since all you can measure from utside is mass, charge, and angular momentum according to the "No hair" theorem. Currently we don't know if there is matter inside Black Holes or if what cind of... If there is matter and antimatter inside two BHs then they would probably annihilate each other during an collision and create energy according to E=mc2. Energy bends space equal like mass, so while during the process, mass is transformed to energy, but from the outside all you could see is the merger of two BHs to one larger with the same total gravitational attraction.

But it is still possible to measure how fast two photons are heading towards or away from each other. In the observers frame they have a speed "relative" each other.

I think the spacetime coordinates for the past exists just like the coordinates for the now and the future. So there is a "there" for it to be, but the "there" is likely empty now since objects is moving forward in time. (The previous time is back "there", but the objects are here now.)

List of unsolved problems in Physics -> http://en.wikipedia.org/wiki/Unsolved_problems_in_physics

I hope you realize that Earth is also rapidly moving around the Sun "in 3 hundred some days"... Apophis is not going anywhere, it orbits the Sun, there is NO time limit. And we would still have to go up/down from Earth to reach it, if "catched" in Earth-orbit. For a much lower price, a robotic mission, similar to Spirit & Opportunity on Mars, could be launched. (Spirit have been on Mars since January 4, 2004 and Opportunity landed three weeks after.) They still performs extensive geological analysis of Martian rocks and planetary surface features. http://marsrovers.jpl.nasa.gov/home/ Thats a little more than a few recordings... Here are 3 examples of other such robotic spacecraft missions, on their way right now: Rosetta is a European Space Agency-led robotic spacecraft mission launched in 2004 intended to study the comet 67P/Churyumov-Gerasimenko. Rosetta consists of two main elements: the Rosetta space probe and the Philae lander. The probe is named after the Rosetta Stone, as it is hoped the mission will help unlock the secrets of how our solar system looked before planets formed. The lander is named after the Nile island Philae where an obelisk was found that helped decipher the Rosetta Stone. http://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29 New Horizons is a robotic spacecraft mission conducted by NASA. It is expected to be the first spacecraft to fly by and study the dwarf planet Pluto and its moons, Charon, Nix, and Hydra. NASA may also approve flybys of one or more other Kuiper Belt Objects. New Horizons was successfully launched on January 19, 2006. After a flyby of Jupiter on 28 February, 2007, it is expected to arrive at Pluto in July 2015 before becoming the fifth spacecraft to leave the Solar System. http://en.wikipedia.org/wiki/New_Horizons Cassini–Huygens is a joint NASA/ESA/ASI robotic spacecraft mission currently studying the planet Saturn and its moons. The spacecraft consists of two main elements: the NASA Cassini orbiter, named after the Italian-French astronomer Giovanni Domenico Cassini, and the ESA Huygens probe, named after the Dutch astronomer, mathematician and physicist Christiaan Huygens. It was launched on October 15, 1997 and entered into orbit around Saturn on July 1, 2004. On December 25, 2004 the Huygens probe separated from the orbiter at approximately 02:00 UTC, as confirmed by the Jet Propulsion Laboratory. It reached Saturn's moon Titan on January 14, 2005, where it made an atmospheric descent to the surface and relayed scientific information. It is the first spacecraft to orbit Saturn and the fourth to visit Saturn. http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens EDIT: Links to Don Quijote: Don Quijote is a proposed space probe under consideration by the European Space Agency, which would study the effects of impacting a spacecraft into an asteroid. The mission is intended to test whether a spacecraft could successfully deflect an asteroid on a collision course with Earth. The orbiter is being designed to last for seven years. The mission is still in the planning stages with a launch expected in 2011. http://en.wikipedia.org/wiki/Don_Quijote_%28space_probe%29 http://www.esa.int/SPECIALS/NEO/SEMZRZNVGJE_0.html http://www.esa.int/esaCP/SEML9B8X9DE_index_0.html http://www.esa.int/esaCP/SEM8SUB1S6F_index_0.html

Hehe, I recall it was with me, which don't mean that I think real BH are Schwarzchild ones.

What do you mean with catch, letting it hit Earth or put in orbit ? It's gravity is neglectable compared to Earths. It won't go very far after the pass, Apophis has an orbital period about the Sun of 323 days, and its path brings it across Earth's orbit twice on each passage around the Sun. Any scientific study can still be planned and done on the next pass, if wanted.