# The Naked Scientists Forum

### Author Topic: If Light was beamed forward from a craft moving at C would it move at cx2?  (Read 2590 times)

#### kettledrum

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##### If Light was beamed forward from a craft moving at C would it move at cx2?
« on: 25/04/2010 18:31:29 »
I have started a topic on another forum which involves the speed of light and was wondering if anyone here could come over at shed some light(lol) on this subject.
the question is- if a space craft is moving at the speed of light and beamed a lazier forward would the light energy be moving at twice the speed of light?
light speed plus the speed of the craft and  if not ? why not?

Mod edit - We would prefer it if you didn't advertise your own forum here, especially in your first post - it looks a lot like spam.
« Last Edit: 27/04/2010 16:37:34 by BenV »

#### graham.d

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##### Re: If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #1 on: 25/04/2010 18:36:20 »
No, light travels at the same speed (in free space) to all observers. The space craft can't ever get to the speed of light anyway. I suggest reading up Einstein's theory of special relativity.

#### quibitheed

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##### Re: If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #2 on: 27/04/2010 12:25:13 »
Come on Graham, what kind of answer is that? Was telling your kids there is no real Santa Claus one of the highlights of your life?

Lets scale it back a little to pander to your pedantry. Lets say the ship is moving at half light speed, which we can observe, and then switches on a forward pointing laser. Would we see a beam that travels 1.5 times light speed? No hiding behind uncle Albert allowed.

#### Soul Surfer

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• keep banging the rocks together
##### Re: If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #3 on: 27/04/2010 13:28:37 »
No the light would be travelling at light speed by all observers whatever their position but the light would of course be doppler shifted to shorter wavelength for the approaching case and longer wavelengthfor the receding case

#### lightarrow

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##### Re: If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #4 on: 27/04/2010 13:35:40 »
I have started a topic on my forum which involves the speed of light and was wondering if anyone here could come over at shed some light(lol) on this subject.
the question is- if a space craft is moving at the speed of light and beamed a lazier forward would the light energy be moving at twice the speed of light?
light speed plus the speed of the craft and  if not ? why not?
the for is at
http://cosmicties.net/newforum/can-you-speed-light-up-to-a-faster-speed/
V = (v1 + v2)/(1 + v1*v2/c2)
V = velocity of laser beam with respect to Earth;
v1 = velocity of space craft;
v2 = velocity of anything which is moving with respect to the spacecraft.
In your case v2 = c = 299,792,458 m/s

Put c in the place of v2 in that formula (the correct formula for velocity composition) and compute V (you don't have to use the numerical value for c).
« Last Edit: 27/04/2010 13:37:13 by lightarrow »

#### graham.d

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##### Re: If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #5 on: 27/04/2010 13:36:42 »
It is just that this question gets answered a lot and, really, it needs an understanding of special relativity to understand the answer. It is much easier to go on to Wikipedia than for me to try to cobble together an explanation. But, here goes...

The theory of relativity is consistent with measured reality and, in particular, that the speed of light is the same for all observers. This seems contrary to observations of moving objects in everyday life, but is nonetheless true. The effects of this are not really noticeable in our normal life because we don't ever encounter such high speeds (light speed in a vacuum = 186,000 miles/second) however there are many cases nowadays where the theory of relativity has to be used because of the precision required. And, indeed, speeds close to light speed are achieved by particles in particle accelerators and by cosmic rays coming from space. In both these cases relativity theory has to be used.

The laws of motion are changed as a consequence of relativity although, at everyday speeds they approximate closely to those of Isaac Newton. A seemingly bizarre consequence is that there is no absolute measurement of the passage of time - if you observe someone who is travelling relative to you then, from your perspective, his clocks would be running slowly compared with yours. Similarly, to him, yours would be running slow. This may seem paradoxical and it is not easy to understand (see the Twin Paradox) but there is, in fact, no paradox. Another consequence is that distances, in your direction of motion, contract (see Lorentz contraction) or, from the perspective of someone watching you pass in your rocket, he would measure your rocket as shorter than you would.

I'm afraid none of these things seem reasonable based on everyday experience but they are true and have been verified by numerous experiments. I'm not good at writing equations on this website but Wikipedia has them if you wish to see the maths behind it.

To go to the half light speed ship; if you fire your light out the front a stationary observer would see the light travel at exactly light speed (no faster). He would see the light change colour to be more blue because of the Doppler effect but the light speed would not be different. Perhaps more interesting is if you fired a particle out also at half light speed with respect to yourself. A stationary observer would see the speed of this particle as 0.8 x lightspeed (if I've done the maths correctly). No mass can be accelerated to the speed of light however you try it - another consequence is that the energy required to accelerate a mass to the speed of light is infinite. Light (photons) can only exist at light speed and are sometimes referred to as having zero "rest mass".

For the non-physicist who wants to understand these things, I think a key issue to grasp is why, in the fist place, we expect things to behave as we commonly observe them when taken to extremes. We are only really built to have intuitive understanding of the world around us sufficient to help us survive and reproduce. There is really no reason why the world should always fit in with the limits of our ability to intuively grasp how it works. Special Relativity is a good example. It leads to the consequence that space and time should be considered as a 4 dimensional whole and that time is not absolute. Distance and duration are not independent but the universe should really be measured as "intervals" between "events" in Space-Time. General Relativity shows that Space-Time need not be "flat" and is curved by the presence of matter. The mathematics of what happens in this case gets very difficult to calculate. And Quantum Mechanics tries to explain what happens and the fundamental level within the matter that makes up the world. Nobody has yet united the two extremes in one consistent theory.

I hope this is more helpful. It took me an hour to write.

#### quibitheed

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##### Re: If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #6 on: 27/04/2010 15:43:05 »
It is indeed. Thank you.

I am no scientist, just a joe average who has an interest in knowing how things tick. Whilst my much abused collection of neurons does tend to compare in an experientially meaningful way they are not limited to thinking the intuitive models that naturally first spring to mind tell the fact. I think I do actually understand to some degree what Einstein was saying in his special theory, and am heartened that he himself found it so confusing too. Whilst the theory does give us the table with which to define experimental observation, and seems to work remarkably well I still feel like it is not the whole picture. And clearly quantum theory, entanglement specifically, shows that information can exceed the limit of C. To escape this hard reality, or so it seems to my poor frequently pickled brain, science keeps quantum and relativity theories apart with the excuse that they are still searching for ways to unify them. I grant they make such a statement with all sincerity but it seems to me the lack of progress is profound because they keep them separate. The whole mindset is actually in opposition to progress. Yet, and yes I know this seems so simplistic and naive, could it be that we by serendipitous accident of language labels already have a meaningful marriage of those two schools? Is relativity related to Schroedinger's cat in the box?

I am no scientist as I will continue to emphasise, but I am an artist and my art draws its synthesis from marrying often disparate elements into a whole. I like to look for connections that are not apparent at first, or even second, glance. And my 'intuitive' sense here screams to me that all the information is there...we are just looking at it the wrong way. Science has become so specialised it often ignores the bigger picture still has much to say. The speed of a photon through vacuum is a given measure that makes little sense to me as a property of the photon. Rether I think we will find the limit is defined by the medium in which the photon finds itself. It is a statement of apparent fact yet the photon itself baffles me. Where does it get its energy? I have seen film of photons being fired through a Bose-Einstein condensate, slowed down to only a few metres per second, yet on re-emerging return to their original velocity. (see  link below) What is going on here? As I am sure you are aware a Bose-Einstein condensate is about the closest thing we can make to a black hole and where we can best see matter behave as though it were on the quantum level. And there we see an apparent breakdown of gravity too. (2nd link) Which takes us back on topic.... this shows that there is evidence that gravity is a property of atomic scale matter alone, that when it descends into quantum incoherence gravity no longer effects it. Yet black holes are the biggest gravitational sources we know of.

So now firmly back off-topic... Maybe I am merely lacking information and/or understanding but it seems there is an inconsistency here. The gravity of black holes is so intense that molecular movement grinds to a halt, just as in a BSE. Black holes are the coldest objects there are. And that the 'matter' should behave similarly to a BSE. What happens to all the photons the black hole captures? If they are stilled they have zero mass and must have transferred their energy to the black hole...which should heat it. Yet as we see in the first clip the photon resumes its original velocity on exit. So the photon is not subject to quantum incoherence in the same way as the superfuid medium. Arghhh... its hard to conclude and explain what I am getting at.... its something like we cannot be sure of what light speed is except as the maximum velocity of a photon in vacuum. But it is the 'vacuum' that defines that value...not the photon...which is as ever a confusing enigma in itself.

If you have persisted in reading this well done.

#### graham.d

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##### If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #7 on: 27/04/2010 17:32:06 »
"And clearly quantum theory, entanglement specifically, shows that information can exceed the limit of C." - Actually this is not thought to be the case; the reasoning is a little complicated and I don't recall it at the moment. I will find a reference if I get chance but no time right now.

How a photon behaves is related to the also non-intuitive Quantum Mechanical concept of a wave-particle. As a wave one can think of it as the progress of a disturbance to an electromagnetic field when it is reasonably easy to reason that this moves through space at one speed but is different in a medium where the electromagetic field is itself distorted by the nature of the medium itself. When an em wave is absorbed the energy can behave as would a particle and be apparently concentrated in a small area. What is interesting is that the wave for a photon is a tangible variation in the electric and magnetic field where the "wave" of any particle (which also behave as a wave-particle) is really one of a probability amplitude and nothing that would be regarded as physical.

Here's a reference to "Spooky action at a distance", as Einstein referred to it, not actually violating special relativity. I don't remember the paper but the author(s) are Ghirardi et al (1980).
« Last Edit: 28/04/2010 08:37:53 by graham.d »

#### lightarrow

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##### If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #8 on: 28/04/2010 14:42:30 »
"And clearly quantum theory, entanglement specifically, shows that information can exceed the limit of C." - Actually this is not thought to be the case; the reasoning is a little complicated and I don't recall it at the moment. I will find a reference if I get chance but no time right now.
It's quite simple: the fact is that we don't know which is the information that will arrive at detector A, entangled with detector B, so even if we want to send to detector B the same information at detector A, we can't do instantaneously, we have to wait that the original signal first arrives at detector A and then send it, conventionally, at speed c, to detector B.

#### The Naked Scientists Forum

##### If Light was beamed forward from a craft moving at C would it move at cx2?
« Reply #8 on: 28/04/2010 14:42:30 »