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Author Topic: Why is the speed of light in a vacuum 300 million metres per second?  (Read 21560 times)

Online chris

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What is the reason that the speed of light in a vacuum has the value that it does?

Chris


 

another_someone

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Most technically, these days, because we have now gone and defined the metre in relation to the speed of light and the second, so we now have the situations that the standards body have actually defined what the speed of light must be, and all we can do is change our measure of the metre rather than our measure of the speed of light.

There are other related issues, such as the permittivity and permeability of free space, but whether they actually give a cause to the speed of light, or merely a codependency, is another matter.
 

Offline Soul Surfer

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To paraphrase what another someone has said. the speed of electromagnetic radiation through any medium is a measure of its permittivity  (capacitance per unit volume)  and permeability to a magnetic field.  The vacuum has values of both permeability and permittivity and this effectivley defines the velocity of light.  Prmittivity is the reaction to electrical field stress which is the potential energy part of the equation permeability is the kinetic energy in the magnetic field.
 

Offline lightarrow

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Said in another way: if you could modify the electric and magnetic properties of the void, you would modify light's speed in it. The void's energy is still a very controversial issue (virtual particles, dark energy, ecc); someone says that between two very near metal plates, Casimir effect is due to a decrease in void's energy (less virtual photons), so light's speed should increase there.
 

Offline syhprum

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There is only one answer it is thus because we say it is!
We could have it one meter per second but then the meter would rather inconveniently large for general use (although a nano meter would be about a foot)
 

Online chris

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Sure, but irrespective of what units we apply to it, light still has a maximum speed of a certain value. Why is it that value, 300 million metres per second?

Chris
 

Offline Soul Surfer

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If the answer the physical properties of the vacuum make it that speed does not satisfy you, the same way the temperature pressure and composition of the air make the speed of sound what it is. I cannot say any more than that is what it is!  You might as well say why is Planck's constant the value it is.  Its just part of the properties of things in our bit of the universe.
 

Offline lightarrow

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Sure, but irrespective of what units we apply to it, light still has a maximum speed of a certain value. Why is it that value, 300 million metres per second?

Chris

For essentially the same reason glass' index of refraction is 1.55 and so light's speed is c/1.55: glass' properties. Substitute "glass" with "void".
 

lyner

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Sure, but irrespective of what units we apply to it, light still has a maximum speed of a certain value. Why is it that value, 300 million metres per second?

Chris
The fatuous reply is that it has to be something and it has to be constant. The actual choices of  units for time and distance  was basically arbitrary so the number is nothing significant. 186000miles per second is another way of saying the same thing.
If you are asking why it is constant then you are into Relativity.
A 'reason' for c being an upper limit for speeds is that mass, length and time go whacky as they approach c. In practice, to go faster and faster requires more and more energy - out of all proportion to what you would think of as Kinetic energy.  This is an effect which effects how you get particles to go really fast in an accelerator. Electrons can be accelerated to very near c, at which speed, their masses are many many times their rest mass.
« Last Edit: 06/04/2008 14:21:36 by sophiecentaur »
 

Offline Madidus_Scientia

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So of the energy being put into the electron to make it go faster, is some of that energy converted to mass instead of speed? or does the extra mass seem to come from nowhere
 

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The extra mass 'comes from' the energy that was put into the experiment. This extra mass can be detected because the electron will behave as if it had extra mass when it collides with something. When it loses some KE in a collision, its mass will reduce and it will behave differently in the next collision. Both mass and velocity will be increased when you accelerate an object but, of course, the total KE increase will only be the same as the added energy.
If you look upon mass as a property that an object has which governs how it will interact with another object - rather than 'the amount of stuff inside it' (leave the classical approach behind) then it can make more sense.
You could  say that quantities like Energy and Momentum are more fundamental than Mass. Momentum tends to be conserved during interactions and so does 'energy plus the mass equivalent of energy'.
 

Offline EdwardATeller

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If you take the Planck length and divide it by the Planck time, you get the speed of light in a vacuum.  I know this follows directly from their definitions, but it led me to the following hypothesis, which would neatly answer the OP's question.  If space and time are quantized, with the smallest units being the Planck length and time, then EM radiation simply cannot travel the next Planck length any faster than one Planck time; therefore, the speed of light is Planck length / Planck time = c.

You obviously don't need to quantize space and time to make this work, but you do need to figure out why light can't travel the next Planck length any faster than 1 Planck time to advance this theory.  This is so simple, it must have been suggested already and probably discredited, but the perfect simplicity of this really grabs my imagination.
 

Offline peppercorn

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This is so simple, it must have been suggested already and probably discredited, but the perfect simplicity of this really grabs my imagination.

Hi Edward! And can I offer a warm welcome to the forum :)
Simple as this idea is it is beyond my piecemeal knowledge of fund. physics, but I'm sure there are some here who can tackle it, er.. properly :-X  .....
 

Offline imatfaal

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Hello Edward - I cannot see around the circularity of your argument.  The planck time is the time taken for light to cross a planck length - I don't believe it has a physical meaning apart from that, even the planck length which does have some physical significance is defined in terms of the speed of light.  The reason
Quote
why light can't travel the next Planck length any faster than 1 Planck time
is because that's how its defined.  

For your argument to succeed you need a unit of time that has a physical meaning throughout universe and history and is not defined through use of the speed of light.  you also need to show an argument which explains why space and time are related in such a way that many units of time can be utilised to travelled one unit of length; but that many units of length cannot be travelled in one unit of time.
 

Offline Pikaia

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There is nothing special about light, all particles are subject to the same cosmic speed limit. But why is there a cosmic speed limit at all? Dunno.
 

Offline Bored chemist

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Because Maxwell's equations say (IIRC) it's the reciprocal of the square root of the product of the permittivity and permeability of free space.
Those two quantities are measurable (actually, one is defined by our choice of the definition of the Ampere); if you do the maths you get about 300000000 m/s.
 

Offline Pikaia

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Neutrinos also have the same speed limit, but they are electrically neutral and immune to electromagnetic forces, so why should Maxwell's equations apply to them?
 

Offline QuantumClue

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What is the reason that the speed of light in a vacuum has the value that it does?

Chris

Because the density of spacetime will dictate their velocities. Technically, they will always have a constant velocity, but if you altered the density the speed of light would change 2ec44d4e80bf638cf96255728aa99a70.gif.
 

Offline peppercorn

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...density of spacetime will dictate [photon] velocity.

Can you explain what you mean by density w.r.t. spacetime?
 

Offline QuantumClue

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...density of spacetime will dictate [photon] velocity.

Can you explain what you mean by density w.r.t. spacetime?

The Vacuum contains an energy density. Some scientists like Prof. John Barrow a mathematical physicist among several other scientists have stated that it is possible light speed was atleast 50 times less than what is percieved today due to the vacuum energy variating.
 

Offline peppercorn

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Still, that doesn't answer the question, why 300 and not 200 or 500?

BC explanation seems a good place to start IMO:
Because Maxwell's equations say (IIRC) it's the reciprocal of the square root of the product of the permittivity and permeability of free space.
Those two quantities are measurable (actually, one is defined by our choice of the definition of the Ampere); if you do the maths you get about 300000000 m/s.

Beyond that try reading:
http://en.wikipedia.org/wiki/Anthropic_principle
 

Offline CPT ArkAngel

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C is the relation of energy in space relative to time. Why it has this value is a matter of the units used... With the Planck constant h, this is the most basic constant in my opinion.

the speed of light = 299 792 458 m / s
« Last Edit: 21/11/2010 02:21:02 by CPT ArkAngel »
 

Offline Bill S

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Quote from: QC
Prof. John Barrow a mathematical physicist among several other scientists have stated that it is possible light speed was atleast 50 times less than what is percieved today

Alternatively, Joćo Magueijo and others argue that the speed of light may have been greater in the distant past. YPYM&YTYC!
 

Offline AnswerIs42

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To me the answer is simple:
In our current universe, which is always changing due to the observed expansion of the universe, the speed of light is what it is because it is the point at which any rest energy a particle has (i.e. it's mass) as been fully converted to energy. Once you no longer have mass to "push" their is nothing left of "matter" to measure a velocity on. 

Now e=mc2 shows that as a particle's speed increases so does its mass and that as this continues you need more and more energy to "push" an increasing mass which is why a very small amount of mass is equivalent to orders of magnitude in energy. But if you can reach the point at which all rest energy is used - like in an matter anti-matter interaction, all that was matter and being "pushed" is now pure energy.  Since "speed" is a measure of an "objects" velocity it means we can't measure energy's speed since we have no "object".

My last point is on the "current" speed of light. Note my opening remark on the "current" universe. I know we have two camps as it relates to universe inflation but my thinking is that the speed of light has not and is not always constant as we want it to be in the "standard model". Since my answer is that the "current" speed of light is set by the point at which all rest energy is converted to energy it then follows - why does this point always result in the speed of light we see today?

In other words, why does 'e' equal mc2. Was the current amount of rest energy a partical has always the same? I think not, as this to me is a function of the universe we have at any moment in time and we know the universe is in constant expansion. All current models acknowledge this expansion but still treat its fact as "non-existent" or of "zero-effect". I think this is very short sighted. If the universe is now larger than a second ago then something "is" different. Is their more/less dark energy and/or more/less dark matter in what is now a larger space?

Either way it can not be the same and this means the amount of rest energy for a given amount of mass to reach 'c' however so slight is not the same.  Basically, in a perfect void of space of a given volume you have to have a limit on the total amount of 'stuff' (energy/matter both light and dark) that it can contain.  If that volume is increased or decreased the 'total-stuff' limit has changed. I like to think of this 'stuff' in total as the 'cosmic pressure' or 'zero energy' of the universe.  So any release of rest energy from the particle into space has to 'fight' against this ever present cosmic pressure which would affect in the end the speed of light as the universe changes in size. 
 

Offline Phractality

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Most technically, these days, because we have now gone and defined the metre in relation to the speed of light and the second, so we now have the situations that the standards body have actually defined what the speed of light must be, and all we can do is change our measure of the metre rather than our measure of the speed of light.

Yes; that is the correct answer. To clarify a bit, we originally defined the meter as 1/10,000,000 of the distance along a line of longitude at sea level from the equator to the North Pole. And we defined the second as 1/86,400 of a sidereal day. Later, we measured how many seconds it takes for light to go a certain number of meters. Then we discovered that the speed of light is a more reliable constant than the physical parameters of planet Earth, and it is also easier to measure; so we redefined the meter and second in terms of the speed of light, which had already been measured. Now that the speed of light defines the meter and second, it is possible for the size of Earth in meters to vary, and the length of the sidereal day may also vary.


There are other related issues, such as the permittivity and permeability of free space, but whether they actually give a cause to the speed of light, or merely a codependency, is another matter.

This is a popular misconception. The permittivity and permeability of free space are defined in terms of the speed of light. They are what they are because the speed of light is what it is. There is no physical explanation of how permeability and permittivity cause light to propagate at that speed.

We have an excellent understanding of how acoustic waves propagate in solids. In some aether theories, the speed of light can be explained by the formula for the speed of shear waves in a solid medium. If the aether is a solid, we might expect its inertial density and shear modulus to determine the speed of light, according to the acoustic formula. That can tell is the ratio of the aether's density to its shear modulus, but it can't tell us how great either of those parameters is. Conceivably, permittivity and permeability of free space are analogous to the density and shear modulus of the aether. (The existence of aether has not been disproven; it has only been shown that it is irrelevant as long as there are no faster-than-light phenomena.)
 

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