speed of light

Discussion of theories involving time as a dimension, time travel, relativity, branes, and so on, usually applying to the "real" universe which we live in.

speed of light

Postby DogmaJerk » Fri Jan 16, 2004 11:50 pm

Listen, I have seen the depictions, I know this is not completely related, but how does one see behind oneself while approaching the speed of light? Does it have something to do with the rate at which light hits our eyes? are we being bombarded with an ever-increasing amount with acceleration? and if so, shouldn't we be leaving behind the light bouncing from the stuff behind us? Or is it like a hook? Does the light behind us naturally shine past from our back? never being caught by our eyes because we just aren't looking that way. So then, if we can move fast enough to barge into the light particles, soaring in the same direction we are, we can see what is behind us/

I am not convinced; for this is my own logic.
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Postby Geosphere » Sat Jan 17, 2004 4:55 am

Greetings well met, fellow, hail.

This is so disturbing an answer either it will make it all click, or sound like gibberish.

You will see light as normal behind you even as you recede from the source at near light speed due to the fact that at that speed, time is slowing down for you to make sure the light reaches you.
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Postby RQ » Sun Jan 18, 2004 7:55 am

well the fact that you are moving at the speed of light towards light, or you just sitting there, but light should still be reaching you relative to your own perspective at the same speed, so I wouldn't think that it will do anything special, unless because you time travel back because of traveling at the speed of light, otherwise all that will happen is that it will come to you.
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Postby RQ » Mon Feb 02, 2004 1:57 am

In Einstein's theory of special relativity, he shows that a person in a train throwing a ball would seem to him like a 1dimensional motion, but to a person who's not in motion it would seem like a parabola, thus making a bigger distance. Now the if the distance changed, something else had to change also, so to make the distance over time=the correct velocity, but which one? Now we know that time applies for all objects in space, so we have to have something with a fixed velocity. In the 1860s Maxwell's equations show that the speed of light in a vacuum has to always be the same, so all observers must agree on its speed. now if we replace the ball with light and two mirrors (thought experiment) it would surely have to make the time bigger for the observer. If it increases the distance, it has to increase the time.

This though has problems, since to the person in the train, a person throwing a ball outside would be a parabola?
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Postby Watters » Tue Mar 16, 2004 4:26 am

I liked your discription RQ, but what is the problem. Think you hit it almost bang on the nose.

[Edit by BobXP: that was appaling spelling/grammar :roll: ]
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Postby big_pappa_smurf » Mon Sep 27, 2004 12:27 pm

RQ wrote:In Einstein's theory of special relativity, he shows that a person in a train throwing a ball would seem to him like a 1dimensional motion, but to a person who's not in motion it would seem like a parabola, thus making a bigger distance. Now the if the distance changed, something else had to change also, so to make the distance over time=the correct velocity, but which one? Now we know that time applies for all objects in space, so we have to have something with a fixed velocity. In the 1860s Maxwell's equations show that the speed of light in a vacuum has to always be the same, so all observers must agree on its speed. now if we replace the ball with light and two mirrors (thought experiment) it would surely have to make the time bigger for the observer. If it increases the distance, it has to increase the time.



BUT, what is time? Time could be only our perception. Whos to say whether time actually exists? Check my post "Light speed (and faster...)"
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Postby jinydu » Tue Sep 28, 2004 6:37 am

RQ wrote:This though has problems, since to the person in the train, a person throwing a ball outside would be a parabola?


Problem? No, no problem with that at all. From the viewpoint of Special Relativity, the train's reference frame is just as valid as the person outside's reference frame. Thus, we do expect such a symmetry. In fact, assuming the train is moving on a frictionless track at a constant velocity, Special Relativity says that there is no experiment that will not have such a symmetry.
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Postby jinydu » Tue Sep 28, 2004 6:39 am

big_pappa_smurf wrote:BUT, what is time? Time could be only our perception. Whos to say whether time actually exists? Check my post "Light speed (and faster...)"


According to Special Relativity:

Time = Distance/Speed

Time may be just perception, but without perception, there could be no science.
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Postby houserichichi » Wed Nov 03, 2004 7:23 pm

It may be a moot point, but saying t = d/v leads to a cyclic argument. Distance is measured in units of length, while velocity is measured in units of length/unit time. So by that definition, you can't define time without knowing what it is already. I don't proclaim to know how to define it, but so long as it fits the equations, who's complaining?
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Postby jinydu » Thu Nov 04, 2004 6:17 am

Time can be defined in terms of the number of times a periodic phenomenon occurs. In fact, 1 second is defined as the duration between 9,192,631,770 cycles of a standard Cesium-133 transition. See http://hyperphysics.phy-astr.gsu.edu/hbase/acloc.html
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Postby 3l3ctr0 » Fri Feb 11, 2005 5:37 am

jinydu, that doesnt prove time exists all it shows is that 1 second is precived to have pased when between 9,192,631,770 cycles of a standard Cesium-133 transition has gone by. so when an atom by my understanding of the page, is traped and rotats so many cycles, so much time has gone by.... and another thing to my knolage is that wasint this "clock" created to tell time on a more percisle??.... but plz corect me if im wrong.
Last edited by 3l3ctr0 on Fri Feb 11, 2005 5:59 am, edited 2 times in total.
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Postby jinydu » Fri Feb 11, 2005 5:54 am

Perhaps I should have been more careful with my first sentence of that post. It would be more accurate to say that the measurement of time can be defined using a certain type of Cesium atom.

If by defining time, you mean assigning the word a meaning, this is probably not really possible. Concepts can only be defined by expressing their meaning in terms of simpler or more fundamental concepts. But what word is more basic than time? Let's face it, we all know what time is, just as we all know what position is. Though we may not know everything about how it works, I don't think you would ask someone to define time when he/she uses the word in a conversation because you don't know the meaning of the word.

I think that in vocabulary, as in mathematics, there are some things you have to take as given. You will never be able to build a dictionary where you can define every word without using any circular definitions, since a word can only be defined in terms of other words, and if you keep following the chain, you would eventually have to either stop or repeat yourself. Some words must be taken as given and understood without a definition.
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Postby 3l3ctr0 » Fri Feb 11, 2005 6:02 am

so time can not be explained. but it exists. and we have no idea what it means. i understand i think.
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Postby jinydu » Fri Feb 11, 2005 8:38 am

3l3ctr0 wrote:so time can not be explained. but it exists. and we have no idea what it means. i understand i think.


Time cannot be defined in terms of simpler things. We can explain properties of time.

We all have an intuitive notion of what time means, but we can't define it in terms of simpler concepts, because there aren't any simpler concepts.
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Postby brasileiro » Sun May 08, 2005 5:04 am

Well, RQ has a point.. there is a problem... see? it's all just confusing when you try to explain it. You come across something and say, "But then again..." and keep going... I like the light and mirrors. I have a question for that. How cool would it be to be small enough to stand inbetween the two mirrors and be able to watch the light... but have the sight to be able to watch the light travelling, and not just appearing out of nowhere...? I think that would be so effing wicked awesome!! :D
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Postby Twix18 » Sun May 08, 2005 6:00 am

heres a thought ot consider as well, since the universe is curved than no matter what direction the light goes it will always eventually end back up on itself. since we can agree that this is true we can also assume that other parts of physics have to be able to also follow this law. take time for instance. if it dosent really exist that all the we precive isnt accutally there, we can feel ourslves growing older and we can see the sun rise and set and we know that it passes, and if it passes than it to must evenatlly curve back on itself. so all that we know from birth till death is repeated over and over again, we could be living the same day of our lives over and ver again and not know it becuase all our memories are plugged into this one loop of "time"
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Postby jinydu » Sun May 08, 2005 6:23 am

I suspect you've falled victim to an oversimplication of General Relativity.

General Relativity predicts that the path of light is curved in the prescence of a gravitational field, which in turn is caused by matter. In the absence of matter, spacetime is predicted to be "flat" (i.e. Euclidean). Although spacetime is locally curved due to the existence of stars, galaxies, etc; the universe on the largest scales appears to be flat, at least to an accuracy within our ability to measure curvature. A few years ago, a probe called WMAP made measurements on the overall curvature of the observable universe and found that the universe is flat to within a few percent.

However, even if the Universe were curved on the largest scales (and there is no experimental evidence for this), this does mean that light would return to its initial position. In order for this to happen, you would have to have what's called positive curvature (the classic example being that of a sphere). It's also possible that the universe could have negative curvature, in which case, a light beam would never return to its initial position, even though its path was curved.

For more information, you can look at:

http://map.gsfc.nasa.gov/
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Postby Twix18 » Sun May 08, 2005 6:39 am

ok humor me on this.. what youve described is a ruber table with many differently weighted balls on it... these balls sink into the table and how mar they sink is how much "gravity" they have. no push a marble across the table and the marbel dips into the dips created by the larger balls is that correct?

if so than there is one more object you are forgetting about. the table itself, the whole universe, althought filled almost entirely with empty space it still holds massive amount of gravity on a huge scale. it is just a matter of how big you are tihnking. and then just to spite you about your negitive light beam idea, i fail to see how you can achive negitive light. this is one case where like is like a shape, it cannot take on a negitive form.
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Postby jinydu » Sun May 08, 2005 6:45 am

I see you've read the "popular press" interpretation of General Relativity. Although it is useful as an aid to visualization, the understanding that you get from reading popular press accounts of scientific theories is almost always a far cry from what you get from actually studying the theory in its full glory (which almost always involves mathematics).

While you may think that the whole universe would have some overall curvature, there is currently no evidence to support this hypothesis. Again, take a look at that link I gave, and check out the results of the experiment.

Also, I was talking about negative curvature, not negative light. Again, if you looked at that website, you would see that a sphere has positive curvature, an Euclidean plane is zero curvature, and a "saddle" has negative curvature.
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Postby brasileiro » Sun May 08, 2005 6:48 am

Although there is no evidence to prove it, there is neither the evidence to prove it is wrong...
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Postby Twix18 » Sun May 08, 2005 6:50 am

acctually i havent read anythign on the subject, thats just what came to mind. as for neg curvature... thatws just as audacios as negitive light, a curve can go in every direction, there is no difference if it is negitive or positive curve. it just i curvey.. and ill check out your link later
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Postby jinydu » Sun May 08, 2005 7:02 am

If you studied mathematics in more detail, you would know that surfaces have curvature, and that it is possible to define curvature precisely. If you want to really understand this, take a course in multivariable calculus (which I am doing now).

To brasilerio: True, there is no proof that it is wrong. But it is known that if we base our calculations on the assumption that the universe is flat, the error introduced from such an assumption (if there even is an error) would be at most about 2%, a fairly good approximation. Having said that, later experiments will likely attempt to measure the overall curvature more accurately. If, once again, no curvature is detected, we would be even more confident that the Universe is flat on the largest scales. On the other hand, if curvature was detected, that would be news...
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Postby brasileiro » Sun May 08, 2005 7:05 am

true... true...
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Postby Twix18 » Sun May 08, 2005 7:14 am

ok quick subject change befor ei let you guys go back to beating your head with text books. J- man... you said a 2% chance of being wrong....im going to assume for the moment that becuase of your continueing scintific gibberish that you are not a god fearing person. but what if someone told you that there was a 2% chance god existed. thats a 98% chance that nothign is going to happen when you die... youll just be dead...but then the 2% says your going to have a better life after you die, and youll be abel to spew out countless more scintific gibberish than ever before... which would you want to beleive.. the 98% of dying and staying dead.. or the 2% of living a happily ever after for eternity?

ok back to the head on text book stuff
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Postby brasileiro » Sun May 08, 2005 7:16 am

Oh snap... I think he's got a point there...
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Postby jinydu » Sun May 08, 2005 7:18 am

I think you've misunderstood the meaning of the word "error".

In science, "error" means the extent to which a quantitative measurement differs from actual value of the whatever is being measured. For instance, if you measure your computer's length to be 10.2 centimeters, but the actual length is exactly 10 centimeters, then your measurement has a 2% error.

Quoting from the WMAP website:

The WMAP spacecraft can measure the basic parameters of the Big Bang theory including the geometry of the universe. If the universe were open, the brightest microwave background fluctuations (or "spots") would be about half a degree across. If the universe were flat, the spots would be about 1 degree across. While if the universe were closed, the brightest spots would be about 1.5 degrees across.

Recent measurements (c. 2001) by a number of ground-based and balloon-based experiments, including MAT/TOCO, Boomerang, Maxima, and DASI, have shown that the brightest spots are about 1 degree across. Thus the universe was known to be flat to within about 15% accuracy prior to the WMAP results. WMAP has confirmed this result with very high accuracy and precision. We now know that the universe is flat with only a 2% margin of error.
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Postby Twix18 » Sun May 08, 2005 7:22 am

if you are building a house than a 2% error means jack... but if you are building a micro chip than a 2% error measn the difference between working and a broken peice of junk. so be it an error of .00000001% or 99.9% and error is still an error
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Postby jinydu » Sun May 08, 2005 7:28 am

That's why its important to send future missions to perform measurements to an even higher degree of accuracy.
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Postby wendy » Mon May 09, 2005 4:19 am

It is wrong to consider euclidean or zero-curvature things as flat.

In non-euclidean geometry, flat means "having the curvature of space", the flat plane in S3 has S2 geometry.

A zero-curvature or euclidean surface in hyperbolic geometry is a properly curved thing, being the limit as the radius goes to infinity.

What was shown was that for all intents and purposes, the large-scale curvature of space has a curvature equal to 0, within measurable error. That is, we live on a horochorix (E3 space), all be it pitted by black holes.
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Postby jinydu » Mon May 09, 2005 4:48 am

I defer to wendy on these issues. Her level of education in mathematics is evidently higher than mine (see the Geometry section).
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