h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

i don't know much about physics and i am confused about light being a measure of time. it seems to me that light is only a factor on how time is perceived by the human brain. is time not allways constant and that the only thing changing is how we perceive it? is time not always a constant?

Although our perception of time may vary this is most definitely not what relativity is all about. The changes in the length of time elapsed is measurable using atomic clocks and aeroplanes. I don't know the details but...
Two atomic clocks were started simultaneously and at the same place. One was kept on the ground while another was taken in a plane and flown around the world. After this the times recorded by the two clocks were compared and there was a difference (a miniscule but that's not the point).
These are seriously accurate clocks; the small difference could not reasonably be put down to experimental error - which means something else (Einstein's time dilation) was affecting the readings!
I only know about this from rumours and the like. I seem to recall it was from a reliable source but...
I'm sure someone else will know lots more about this or maybe I'll find out something more definite...
Point is, time dilation is a real effect and not something we can put down to our perception.

hi all time travel is still vastly missunderstood the basic principle is that nothing in the universe is constant exept the speed of light not even time, it has been proved that the huge forces of gravity generated by large planets and black holes can "bend" time and even space itself. heavey stuff. The experiment about the clocks on the plane going around the world and the time being slower then the one on the ground is true and there are plans by nasa to do the same experiment this year at much greater speeds.
to absolutly prove that speed is directly related to time a russion scientist in 2001 acheived time travel he did this by making a laser beem travel over 200 times faster then the speed of light into a cloud of gas, the laser left the cloud 0.000000025 secs befor it entered wow,
on einsteins thoery that nothing can travell faster that the speed of light nothing solid moved only the laser so the theory still stands

Sorry to butt in on your conversation, but if researcher 196748
would go back to there own page and click on the "EDIT PAGE"button and write a little something about yourself , then an "ACE",(Assistant Community Editors), can come and welcome you there properly. Sorry for the interuption.

manda

Hi,

I think, it's not the whole story to say that time isn't constant. To one and the same observer time *is* constant.

At first you have to differ between special and general relativity. The special relativity is the theory of inertial systems; that means of observers who are not in a gravitational field and who are not accelerated. The general relativity takes these possibilitys into account and is therefore -- more general.

In special relativity the flow of time depends on the observer, or more accurate, on the speed of the observer. If I am at the same speed as a friend or any particle, time is all the same for us.

Only if we are not at the same speed, things become difficult or even seem paradox. If both my friend and I show clocks to each other, we both have the impression that our own clock is ticking faster than the other. So if I say something about time, I always have to say in which inertial system I am -- or at what speed.

What I want to say is: Time depends on speed. So does length.


The special relativity for itself doesn't forbid not-solids to move faster than light, that's right. But if you allow any kind of effect to travel faster than light, you'll get very bad paradoxas. It will depend on the observer -- more accurate: on the speed of the observer -- which is cause and which is effect.

Let's assume we have two events (points in space-time) with spacelike distance (not even light could get from the location of one event to the location of the other in the time between the two events). It then depends on the observer to say, which event is before the other. This can be seen nicely in so called "Minkowsky diagrams".

So if you say that it is possible to send information at speeds faster than light, you say that it is possible to send information back in time. This is indeed paradox. And as far as I know, the given experiment can be interpreted in a different way, so that the effective speed is slower than light, so things get clean again.


Einstein's special theory of relativity is a very basic one, and used in *many* circumstances and has been approved very very often. It's almost impossible and unthinkable that it could be wrong in any kind. I have heard, that for GPS it is neccessary to consider the effect of the speed of the satelites on their measurement of time.

Things look completely different for the general theory of relativity. This is some kind of trial to geometrically describe the effects of gravitation. Some predictions are right, but I'm not sure whether some day this theory will have to be corrected.

It's the general theory theory of relativity which perhaps admits time travel, but that doesn't depend on the theory but on the universe. This means that the equations of the theory allow constructions which allow time travel, but there is no known way to tell if such constructions really exist. I don't think that there is any way to build such constructions for mankind any time. Gravitation is very difficult to influence on because there is no negative mass.


Ubick

Please explain why you state that not-solids can move faster than the speed of light? By this I mean 3*10^8 m/s.

I thought that nothing can travel faster than this speed. Even (perhaps most importantly) information. In fact this assertion is so rigorous that it even applies to the quantum effect of two entangled particles. This effect seems to be a non local effect, meaning that the distance between the particles has no bearing on the time at which the measurement of one determines the measurement of the other (they happen at the same time). At first it would seem that this violates the principle I mentioned. Information seems to have travelled instantaneously between the two particles. However, such is the effect of uncertainty that no useful information can ever be extracted (basically, the laws of physics prohibit us from making use of this effect to send messages faster than the speed of light, even if the non-local quantum effect seems to).

I ask only to confirm that there is direct evidence of non-solid objects moving faster than light. I beleive the laser experiment mentioned earlier misquoted the speed of the laser in the gas, as it should have been "200 times the speed of light in the gas" which would be less than 3*10^8 m/s.

Kip Thorne wrote an excellent book about time machines which described a way to create a time machine by making a wormhole and then moving one end creating a time difference between the two ends. However, no one is sure if wormholes are prohibited as some theories suggest that they are destroyed by their own energy.

Anyway, I look forward to hearing your replies and contributing to this site as I have just discovered it.

Hi Researcher 207442,
I am sorry but I dont know the answer (this is not my strong point )
but if you can do me a favore
if you can go back to your own page,and click on the "EDIT PAGE" button,and then write a little something there,that will activate your page and then I can come and welcome you there and show you around,

manda

I'm sorry that my reply comes that late. I have seen your question a few weeks ago, but I haven't found the time to answer up to now.

What I meant with non-solids could travel faster than light (in vacuum) is not that it is indeed possible. It's only that the theory of relativity does not prohibit this directly. It prohibits the faster-than-light transfer of energy. If you try to describe something accelerating faster than light, quite a lot terms diverge. Nothing like this happens as far as the thing you accelerate has no energy or mass respectivly.

But you would still reach some ugly paradoxa if you would allow some kind information to be that fast. So it is believed that all fundamental laws are "local" meaning that no effect takes place faster than light. But we do not know all the world formula, and thus we only can assume this.

I am convinced that there are no non-local effects, too. This even applies to the quantum effect of two entangled particles. There is quite an ugly explanation concerning in quantum theory, which is able to rescue locality.

This explanation uses the fact that it is not possible to find a paradox, until the information of the two measurements comes to one point in space-time. But at exactly this time it is no more a paradox. If you take the measurement as the step from quantum mechanics to the ordinary (classical) world, than it seems not to be permitted to make this step simultanously at two different points in space.

If you do not understand what I said here, don't bother. I haven't found one yet, who does. Probably I'm wrong.

The other question was for objects traveling faster than light. As far as I am informed, there are experiments that state that some faster-than-light(-in-vacuum) information and energy transfer. But this is still a matter of interpretation. The problem is that there is a great loss of energy, and it is only the peak that's faster than light. So in fact, no energy has traveled too fast so far. And if you don't assume the information to be in the peak but in the ascendency of the signal, than there is nothing faster than light. This is how far I have understanded it. I'm hardly able to describe this in German without making drawings. So this one won't probably help you much, I fear.

I don't know very much about wormholes. But it seems quite strange to me that it should be possible to build up a two-end wormhole with both ends in our universe.
And the energies needed won't be in our hands forever. Even the sun would not be enough.

Be careful in reading popular media reports about the faster than light experiment. Although this experiment was widely reported on it has since never been repeated by an independent experimenter and thus cannot be considered a proof of FTL travel. Even at the present time, FTL is only a theoretical possibility.

possibly not bang on topic, but found this off slashdot.
Unfortunately my works pc doesn't have the requisite media player so not sure how good it is. Let me know if anyone has a go.

http://www.spacetimetravel.org/index.html

Has various things on it, including "A tour through the city centre of Tübingen illustrates what we should see when moving at such a high speed". High speed meaning travelling at nearly the speed of light.