What Has Time Travel Done For Us

Absolutely nothing! Yet.

Not true - it has stopped everything from happening at once.

All of us are time travelers. We just all happen to be going in the same direction.

There is one documented case of a time traveller going in the opposite direction. Merlin the Magician was often depicted as living backward in time (which is why he knew all about the future).

Time travel might be possible, but into parallel universes ala MichaelCrichton?''s Timeline.

Time travel might be possible, but only to a maximum distance of one second in reverse.

All of us are time travelers. We just all happen to be going in the same direction.

One way of looking at special relativity is to say that WereAllTravellingAtTheSpeedOfLight. A stationary object is moving into the future at one light-second per second (ie at full speed). An object travelling through space at (3/5)c is still travelling at a total of c, so must be going into the future at only (4/5)light second per second (simple application of Pythagoras); so the clocks travelling with that object will be seen to be running only (4/5) the speed they would at rest. -- RiVer

That's an elegant explanation. Does this mean that light rays are stuck in the present? I turn on my porch light. I continue to move at the speed of light in the time dimension, in the future direction. The photons from my porch light move at the speed of light through space, but they never see the future? -- SteveHowell

For a more complete explanation of this idea, see the book "Relativity Visualized", by Lewis Carroll Epstein (ISBN 093521805X ). It's a great book.

From my surface understanding of it, I think it's less that photons never see the future, and more that the concept of time is an artificial construct that only makes sense if you're observing objects that don't move anywhere near the speed of light. Of course, that includes most objects we care about day-to-day, so time is a pretty useful concept.

It's not as bad as that. As your speed through space approaches the speed of light, the amount of time you experience between perceiving events in the rest of the universe approaches zero. Hence, you can go back home and find your twin brother has grown old, but you have experienced virtually no time, and all that stuff (assuming you have a twin brother). At the speed of light, everything happens at once, you experience no time (assuming you have no rest mass).

High-end physics seems to full of this kind of stuff; I remember being alarmed by the notion that gravity is actually caused when space is warped by the presence of mass.

But everything is moving at some largish fraction of c. As I sit here in my office chair, I am orbiting the earth at about 700 miles an hour. The earth is orbiting its sun at about 18.5 miles per second. The sun orbits the galaxy at some even larger rate. Meanwhile, the galaxy is rocketing away from the point of the Big Bang at some goodly fraction of c. So aren't we all "stuck in time", according to the above reasoning? -- MikeSmith

Be careful of that "point of the Big Bang" stuff; that is a Newtonian notion. There is no point in our 3 spatial dimensions that was the origin of the Big Bang. The origin was a point in 4-space (if it was in fact a point singularity, as it is traditionally considered to be), and all points in the universe coincided there at that point -- so all points in the universe were the spatial center of the Big Bang, and are all moving away from each other, not all moving away from some one particular point in 3-space.
But to the extent that the universe is uniform, there is always a local "not moving" frame, defined by the background radiation. Incidentally, galaxies don't move particularly fast relative to it.
- Locally. There is gravitational clustering, after all. Over a 10 billion light year distance, however...
As to the original question, time dilation is how people see each other's clocks - everyone sees everyone else's clock as slowing down, depending on their relative motions, or if you prefer, the angle between their paths in spacetime.
- Yep, I do prefer; it clarifies things. :-)

Yes, we're all slowed down a bit, but not significantly. Even all these speeds happened be aligned ( 1 000 miles per hour at the equator relative to the center of the earth; 18.5 miles per second relative to the center of the sun; 22 miles per second relative to the galactic center), the total ( 40.5 miles per second ) is only about 0.0002 c, or about 1 / 5000 of light speed. So an observer stationary with respect to the galactic center would observe our clocks as slower (redshifted) by a factor of (1 - v^2/c^2) = 0.99999960 . You'd have to have fairly high precision instruments to detect that. Of course, the counter-intuitive thing is that we'd observe his clocks as slower by the same amount. (EditHint: find the speed of the Milky Way (relative to what) and work that in to this calculation) -- DavidCary

18.5 miles per second relative to the center of the sun; 22 miles per second relative to the galactic center

Is it just a coincidence that these numbers are so similar ?

[Motions relative to galactic centers involve some major open research issues on e.g. dark matter -- central mechanisms are still speculative, so there is no truly solid answer on anything related to any such thing. I don't know if there are any guesses on the question]

Yanking this page back to QuipMode?:

Shouldn't the question be, "What Will Time Travel Have Been Done For Us"?

One of the Hitchhiker's Guide books has a great passage about the new tenses they willen have invented to deal with just these problems.

Remember the comet that destroyed the earth in 1983?

Remember when the moon was blasted out of Earth orbit by an accident at a lunar nuclear-waste dump on September 13th, 1999?

Whenever I step on a British train, I travel further forwards in time on the train than I wanted.

Time travel keeps everything from happening at once.

Time travel makes wrist-watches useful. They tell us how far we have gone and how much time has passed through us.

Are not we Time Witnesses instead of Time Travelers?

If I understand correctly - I am not a physicist (IANAP?)- some interpretations of quantum mechanics hold that anti-matter is simply matter travelling in an advanced path (i.e., backwards in time relative to rest space). This same formulation, if taken literally, implies that light doesn't move at all; rather, what we perceive as a photon's trajectory is only a single aspect of the photon, which has a structure in time as well as space. Or so I gather; CommentsAndCorrectionsWelcome. -- JayOsako

All of that has indeed been said - and Feynman and his advisor did a lot of work in that area, and interestingly found that it only worked if every photon emitter was assumed to always have a matching receiver, no matter how remote in space and time - however, it is difficult to phrase precisely in English without supporting math, so one should be cautious about conclusions drawn from loosely phrased starting points such as that.

Penrose's most recent proposal is to embed strings into a complexified 4-space of twistors, where the points in that space are not the usual semi-intuitive points of space-time, but instead each point consists of the kind of photon trajectory you're alluding to -- those are taken as basic, and what we perceive as space-time is a derived space on top of that.

He says this avoids the need for embedding strings into a high (e.g. 10) dimensional space. The complexification effectively gives the semi-equivalent of 8 pure real dimensions; I don't know how he got rid of the other two, since they were derived as a requirement via group representation. -- dm

Time travel has given us a hundred years of chintzy science-fiction pabulum and will likely give us a hundred more of the same.

We are all travelling backwards in time. We can see where we've been, not where we've going, so we must therefore be facing backwards.

Anyone who has watched a sporting event on Live TV and when talking to someone watching the same via Space Relayed TV, has experienced a sort of time travel, in which you see something happening which another sees later. Also viewing a movie made in 1952 in 2004 is also a sort of time travel. Reading a book can also be time travel of a sort. This illustrates that certain types of Time Travel can be quite beneficial. --DonaldNoyes

Time travel? First invent a @#%&* flying car (consumer grade).

See WhatHasSpaceTravelDoneForUs