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### Author Topic: can the speed of light go twice as fast?  (Read 15573 times)

#### i_have_no_idea

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##### can the speed of light go twice as fast?
« on: 12/10/2005 20:18:23 »
Hi my friend asked me this question "what if you were on an object (hypothetically) going at the speed of light and you sat on top and turned on a flash light what would happen to the light coming out”, this is like if you are in a car going 20mph and you throw a baseball out the window at 20mph then the baseball is moving at 40mph, but light ,according to Einstein, is the universal speed limit and nothing can go faster but can light go twice as fast?

p.s. If possible the light coming out of the flash light would have to be going 1,341,233,258 miles per hour.

#### Ultima

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##### Re: can the speed of light go twice as fast?
« Reply #1 on: 12/10/2005 20:37:15 »
It would appear to travel away from you at the speed of light, but it's still only going at the speed of light. I think thats right anyway we've had this thread over and over, so I doubt the really cool guys who know all the crazy physics will reply to this. I suggest doing a search for "speed of light" there is some good explanations floating around of varying degrees of crazy mathsynous. I think the basic idea is that light always appears to travel at the same velocity regardless of the observer's velocity.... I have no idea if im right this is a guess from my patchy memory glancing at previous threads.

#### Solvay_1927

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##### Re: can the speed of light go twice as fast?
« Reply #2 on: 13/10/2005 00:42:43 »
Phil, welcome to the forum.

There's loads on this forum (as well as across the internet) about this and similar relativity-related questions - where have you been hiding??

One example is:
http://www.thenakedscientists.com/forum/topic.asp?TOPIC_ID=2641

Matt(ultima) - when you say "really cool guys", i assume that's an ironic way of saying "sad anoraks"?

"The early bird may get the worm, but it's the second mouse who gets the cheese."

#### i_have_no_idea

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##### Re: can the speed of light go twice as fast?
« Reply #3 on: 14/10/2005 00:23:44 »
I figured it would be faster to just ask you than to search.

#### Sandwalker

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##### Re: can the speed of light go twice as fast?
« Reply #4 on: 20/10/2005 17:50:42 »
This question is very similar to one of Albert's, his included a mirror!

Its due to spacetime being 'flexible' the faster you go the slower your time passes, to an external observer, and space (in the direction of travel) gets denser (and mass gets heavier!).

The overall effect to the traveller is that he experiences the light travelling at c but 'in the durection of travel' in contracted space, this is in direct proportion to the time dilation seen below.

The external observer sees the light also traveling at c but the traveller existing in a slower spacetime frame.

The light emiting from the torch will be red shifted to the external observer due to its frequency being slowed by the time dilation.

« Last Edit: 20/10/2005 17:59:06 by Sandwalker »

#### tweener

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##### Re: can the speed of light go twice as fast?
« Reply #5 on: 21/10/2005 03:53:17 »
I'll jump in here.  The light coming out from the torch would appear to recede from you at the speed of light.  Of course you would feel like you were stationary.  Now, if you were traveling close to the speed of light relative to another observer, they would see you traveling away at the speed of light, and the light bean you generated traveling at the speed of light.

Light always travels at the speed of light.  The position of the observer is always the "relative" position to which it is measured.  That's why its called the theory of relativity.

----
John - The Eternal Pessimist.

#### Kerala

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##### Re: can the speed of light go twice as fast?
« Reply #6 on: 21/10/2005 09:26:13 »
In fact, the speed of light can vary : if it is inside a material.
What is constant is the speed of light in a vacuum.

In your experiment, indeed, YOU would see light going away from you at the speed of light, AND an outside observer would see both you and light going away at the speed of light...

In fact, the word "see" is even wrong, because that's not what you would see, but what the true speed of light would be in all those frames : always the same.
But what you SEE is different, because you have, on top of it, to take into account the time it takes for light to travel from, for example, a moving ship, to the observer. By the way, to "see" light, either it goes right away into your eye, or, to "see" a ray of light which doesn't arrive straight into your eye, the light has to be diffracted at some point and part of it diverted in the direction of your eye (like when you make a lot of dust in front of a laser ray to make it visible).

I hope what I write is understandable...

Kerala

I love astrophysics movements! ;)

#### McQueen

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##### Re: can the speed of light go twice as fast?
« Reply #7 on: 30/10/2005 09:13:02 »
This is one of the truly amazing things in Physics and it perplexed scientists everywhere until Einstein , decided that , that was that, and proposed that the speed of light was constant. What does this mean. Suppose you have a stationary light bulb the speed at which that light is moving towards you is 300000Km/sec . Suppose that the light was still stationary and you were travelling towards it at 100,000 Km / sec. What should the speed of light be? 300,000 + 100,000km/sec = 400,000km/sec. Wrong it will still be 300,000km/sec. The same would apply if you were moving away from the light source at 100,000km/sec. The speed of light would not be 200,000Km/sec , it would still be 300,000Km/sec. So the speed of light is always constant regardless of your own speed and whether you are moving to , or away from it. This is not how ordinary objects behave. If you were in a car moving at 60Km/hr and another car is approaching at 70km/hr. The cumulative speed would be 130km/hr. Similarly if you were moving away from each other,the difference in speed would be 10km/hr and so on. What do you think ?

#### DoctorBeaver

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##### Re: can the speed of light go twice as fast?
« Reply #8 on: 31/10/2005 03:03:19 »
McQueen - If a humble psychologist may be permitted to poke his nose in, in your instance of the cars, there would be a tiny time distortion. The experiment where 1 of a pair of synchronised atomic clocks was taken in a plane and, on return, was slightly different to the 1 that had remained stationary, proved that, even at fairly modest speeds, the dilation can be measured. The faster one travels, the more pronounced the distortion becomes.

I know this subject has been covered many times, but there's something I have not seen asked before. Imagine there were only 2 objects in the entire universe, 1 being stationary & the other travelling at C. If the travelling object passed the stationary 1 then from the perspective of either it would be impossibe to tell which of them was stationary & which moving. However, the stationary 1 would age more quickly than the moving 1. If Einstein had been on the moving object (yeah, I know that nothing with mass can reach C but bear with me) & at some point, due to the curved geometry of spacetime, again went past the stationary object, he may have concluded that travelling at C would have exactly the opposite effect to that which he actually conceived; i.e. that for an object travelling at C, time passes more quickly. Would his relativistic equations therefore have been completely arse-about-face?

I'm sure there's a profound point lurking in there somewhere but I'm buggered if I can coax it into coming forth in my poor, addled brain

#### Sandwalker

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##### Re: can the speed of light go twice as fast?
« Reply #9 on: 31/10/2005 10:55:24 »
Dr B - I think it depends on initial conditions and forces applied.

If both started in the same spacetime frame travelling at similar speeds relative to each other, and then one having a force applied changing its spacetime frame (Perhaps this is the root of inertia) then you would be able to measure differences, from the initial conditions.

We have no relationship between your two objects until there light cones connected, it would be from this point that we might examine changes (forces applied) to there relative spacetime frames.

but I could be wrong!

I could not join any group that would have me as a member!
« Last Edit: 31/10/2005 11:00:35 by Sandwalker »

#### i_have_no_idea

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##### Re: can the speed of light go twice as fast?
« Reply #10 on: 01/11/2005 00:41:22 »
I thank all of you for your input and Dr. B that was very interesting what you said about the two objects in existance, I've never thought of it that way.

#### McQueen

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##### Re: can the speed of light go twice as fast?
« Reply #11 on: 03/11/2005 19:11:15 »
Doc
I was referring to how puzzling the speed of light must have been to Scientists before Einstein introduced the postulate of the constancy of the speed of light. Naturally once this was accepted , it meant that the theory of relativity , wherein ,the contraction and lengthening  of time and objects became significant also won widespread acceptance.
« Last Edit: 03/11/2005 19:15:44 by McQueen »

#### DoctorBeaver

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##### Re: can the speed of light go twice as fast?
« Reply #12 on: 03/11/2005 21:09:21 »
My uni lecturers hated me because I was always looking at things from a different perspective & asking awkward questions. I think they respected me for that at the same time, though

#### DoctorBeaver

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##### Re: can the speed of light go twice as fast?
« Reply #13 on: 03/11/2005 21:11:27 »
Sandwalker - In the instance I cited, I was assuming that the observer (Einstein in my example) popped into existence when the object he appeared on was already travelling at C. Therefore there would be no need for any external force

We learn from history that we do not learn from history.
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#### DoctorBeaver

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##### Re: can the speed of light go twice as fast?
« Reply #14 on: 03/11/2005 21:13:30 »
& here's another 1. If spacetime really is curved, photons will eventually come back whence they originated. Does that mean that if we wait long enough we will eventually see ourselves as we were n number of years ago?

We learn from history that we do not learn from history.
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#### DoctorBeaver

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##### Re: can the speed of light go twice as fast?
« Reply #15 on: 03/11/2005 21:15:12 »
McQueen - sorry, I misunderstood what you meant

We learn from history that we do not learn from history.
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#### Ylide

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##### Re: can the speed of light go twice as fast?
« Reply #16 on: 11/11/2005 12:16:11 »
Does anyone have a link to the experiments dont a year or two ago where a doped medium was produced in which the light traversed the pathway through the medium at a value of c whose velocity was greater than that of c in a vaccum.

Random FTL factoid:  Speeds greater than c can be measured in media where the refractive index slows c to a value less than a fast-moving particle.  Google the Cherenkov Effect.

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#### another_someone

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##### Re: can the speed of light go twice as fast?
« Reply #17 on: 11/11/2005 20:57:35 »
quote:
Originally posted by DoctorBeaver

& here's another 1. If spacetime really is curved, photons will eventually come back whence they originated. Does that mean that if we wait long enough we will eventually see ourselves as we were n number of years ago?

We learn from history that we do not learn from history.
(Georg Hegel)

The simple answer is, as far as we can tell, no!

The universe is expanding (or so they say), so light leaving our planet would have ever further to travel, and so while it will keep going, it will have ever further to come back.

http://www.news.harvard.edu/gazette/2002/01.10/01-universe.html
quote:

Eventually, the rate of expansion will approach the speed of light, 186,321 miles a second. As far as scientists know, nothing can move faster than that in a local area. But over the gigantic scale of the universe, galaxies moving at close to the speed of light can separate at a relative speed that exceeds the universal limit. At this point, light coming from distant galaxies never catches up to telescopes on Earth because the planet is moving away too fast.

If this is happening to galaxies that are today visible, you can imagine the problems of something trying to circumnavigate the universe.

#### DoctorBeaver

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##### Re: can the speed of light go twice as fast?
« Reply #18 on: 11/11/2005 21:38:50 »
But just hang on a minute. As an object approaches c, the slower it appears to move to an outside observer (isn't that why an object entering a black hole would appear to freeze at the event horizon?), Therefore the faster those galaxies travel, the slower we would see them moving & therefore gain the impression that the expansion was slowing down! And if all that is true, how do we see anything accelerating - as a car pulled away from traffic lights, it should appear to slow down! AAAARRRRRGH I think I'll stick to psychology, it's not quite so baffling [xx(]

#### another_someone

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##### Re: can the speed of light go twice as fast?
« Reply #19 on: 11/11/2005 23:07:36 »
quote:
Originally posted by DoctorBeaver

But just hang on a minute. As an object approaches c, the slower it appears to move to an outside observer (isn't that why an object entering a black hole would appear to freeze at the event horizon?), Therefore the faster those galaxies travel, the slower we would see them moving & therefore gain the impression that the expansion was slowing down! And if all that is true, how do we see anything accelerating - as a car pulled away from traffic lights, it should appear to slow down! AAAARRRRRGH I think I'll stick to psychology, it's not quite so baffling [xx(]

That is not my understanding of things.  From what I can understand (which is not much at all), you are confusing speed with acceleration.  It is not that the faster you go, the slower you appear to go; but that the faster you go, the slower your rate of acceleration, until your acceleration approaches zero as you approach the speed of light.  Thus, as you get to the speed of light, it is not that your speed appear slower, but that your apparent acceleration being zero, you cannot appear to travel any faster.

I think the problem with black holes is that the light itself has to climb out of the black hole, thus the image you see is very very out of date (and, ofcourse, once the object gets below the event horizon, the light never manages to climb out of the black hole, and so you never get to see that bit – the the last image you see is where the object is just at the event horizon, but even that image takes literally forever to reach you).

Maybe I'm talking absolute rubbish (I often am), but that is as I understand it.

#### Solvay_1927

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##### Re: can the speed of light go twice as fast?
« Reply #20 on: 12/11/2005 01:41:09 »
quote:
Does anyone have a link to the experiments dont a year or two ago where a doped medium was produced in which the light traversed the pathway through the medium at a value of c whose velocity was greater than that of c in a vaccum.

Here's a couple of links - but they're both from 2000 - does anyone have anything more recent, or with more details?
http://news.bbc.co.uk/1/hi/sci/tech/841690.stm
http://www.abc.net.au/science/news/stories/s154610.htm

#### DoctorBeaver

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##### Re: can the speed of light go twice as fast?
« Reply #21 on: 12/11/2005 13:58:14 »
Another_someone - I'm not confusing speed with acceleration. Speed is a function of acceleration whereby at any given point in time the speed of an accelerating object has increased from that at any earlier point in time.
The experiment with the 2 atomic clocks (1 in a plane, the other stationary on the ground) showed that from the point of view of the stationary 1, time for the moving 1 had passed slower. At c, this effect reaches its maximum conclusion & to an outside observer, time would cease to pass for the moving object. If no time appears to pass then to the outside observer that object must appear to be stationary.
Oooh, how I love playing with logic in a non-logical world!

#### another_someone

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##### Re: can the speed of light go twice as fast?
« Reply #22 on: 12/11/2005 15:21:37 »
quote:
Originally posted by DoctorBeaver

Another_someone - I'm not confusing speed with acceleration. Speed is a function of acceleration whereby at any given point in time the speed of an accelerating object has increased from that at any earlier point in time.

Speed is a function of acceleration and time, but it is still a different thing (a little like saying that prices and inflation are related, but a drop in inflation, until it becomes deflation, does not mean that things are getting cheaper, it just means they are not getting more expensive as quickly as they were before).

quote:

The experiment with the 2 atomic clocks (1 in a plane, the other stationary on the ground) showed that from the point of view of the stationary 1, time for the moving 1 had passed slower. At c, this effect reaches its maximum conclusion & to an outside observer, time would cease to pass for the moving object. If no time appears to pass then to the outside observer that object must appear to be stationary.
Oooh, how I love playing with logic in a non-logical world!

As I understand it, and it is not exactly clear to me what you meant in your statement (maybe I'm just not reading it properly), the clock that flew on the plane was running slower.  It did not make the plane appear slower to the outside world, but it would make those inside the plane think that everything outside the plane was happening much quicker (you might say that this is the same as people on the outside thinking that what was happening inside the plane was happening too slowly, but it does not alter the speed of the plane itself).

Thus, if a person were accelerated up to the speed of light, they would stop ageing, and to them, the entire lifespan of the universe would pass by them in an infinitesimally short time, but they would still only (to the outside observer) be travelling at the speed of light, no more and no less.  You might say that if time stops for the person who is travelling at the speed of light, that is tantamount to their perceiving that they are travelling at infinite speed, but that is their perception, not the perception of someone of the outside.  Ofcourse, one might say that if they believe they are travelling at infinite speed, then they would also believe that the outside world is travelling at infinite speed relative to them, and thus not constrained by the speed of light – that is where I get lost.

That, at least, is my naïve understanding of relativistic effects.

#### another_someone

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##### Re: can the speed of light go twice as fast?
« Reply #23 on: 12/11/2005 16:00:58 »
quote:
Originally posted by another_someone

Thus, if a person were accelerated up to the speed of light, they would stop ageing, and to them, the entire lifespan of the universe would pass by them in an infinitesimally short time, but they would still only (to the outside observer) be travelling at the speed of light, no more and no less.  You might say that if time stops for the person who is travelling at the speed of light, that is tantamount to their perceiving that they are travelling at infinite speed, but that is their perception, not the perception of someone of the outside.  Ofcourse, one might say that if they believe they are travelling at infinite speed, then they would also believe that the outside world is travelling at infinite speed relative to them, and thus not constrained by the speed of light – that is where I get lost.

Sorry, I must be suffering from Alzheimer's, I am being an utter idiot.  OK, my excuse is that it was many years/decades ago when I last looked at the equations for special relativity (I never did try and understand general relativity).

Although the person travelling at the speed of light sees time on the outside speed up, to the point where his own time appears to stop; he also sees the distances on the outside shrink – so everything on the outside is happening infinitely fast, but is also infinitely small, so the speed over distance still remains at the speed of light and no more.

#### Solvay_1927

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##### Re: can the speed of light go twice as fast?
« Reply #24 on: 12/11/2005 23:56:42 »
quote:
The experiment with the 2 atomic clocks (1 in a plane, the other stationary on the ground) showed that from the point of view of the stationary 1, time for the moving 1 had passed slower. At c, this effect reaches its maximum conclusion & to an outside observer, time would cease to pass for the moving object. If no time appears to pass then to the outside observer that object must appear to be stationary.

DrB, if you're driving past me at the speed of c, I see you as travelling at the speed of c.  But if I could focus on your watch as you drove past me, it would appear to me that your watch has stopped - it wouldn't be ticking anymore.  So no, you don't appear stationary, you appear to be going very fast but with a broken watch!
That's my understanding of relativity, anyway.

As for my links in my last posting above ... has anyone looked at these?  I'm interested to know if they're valid. (Have the experiments been verified and repeated?)  Because I find them a bit confusing - I don't understand why the results DON'T violate relativity theory. They seem to suggest that FTL (faster than light) signalling) IS possible through certain mediums.

Can anyone shed more light (excuse the pun) on these experiments?

Solvay.

"Eagles may soar, but weasels don't get sucked into jet engines."

#### The Naked Scientists Forum

##### Re: can the speed of light go twice as fast?
« Reply #24 on: 12/11/2005 23:56:42 »