# Why light speed?

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Just now, Silvestru said:

That "universe" would function very differently from ours. Even this slight change in c would have drastic effects on all particles.

So c is fundamentally  a ratio rather than a simple constant?

6 minutes ago, Strange said:

I don't think there is anything fundamental about the ratio itself; speed is just a thing we have invented because it is useful. (Although, seeing Silvestru's answer, I may have misunderstood the question!)

Is spacetime based on a ratio and is this ratio set in stone or could it be different in another universe?

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3 minutes ago, geordief said:

So c is fundamentally  a ratio rather than a simple constant?

Not sure I understand. It is not a mathematical constant it's a physical one. But also my response is in relation tho this new Universe. If you invent a universe it can have any physical laws you desire . c could be slower and our observable universe would be much smaller. Biochemistry would also be affected of course. Basically everything.

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2 minutes ago, Silvestru said:

Not sure I understand. It is not a mathematical constant it's a physical one. But also my response is in relation tho this new Universe. If you invent a universe it can have any physical laws you desire . c could be slower and our observable universe would be much smaller. Biochemistry would also be affected of course. Basically everything.

Yes maybe I am a bit addled (in a waiting queue now for 3 hours)

Of course c is a ratio .Why did I imagine it could be a simple constant (physical as you say)

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8 minutes ago, geordief said:

So c is fundamentally  a ratio rather than a simple constant?

Is spacetime based on a ratio and is this ratio set in stone or could it be different in another universe?

We have now defined c as fundamental (and the second) and then defined our unit of distance in terms of it. But that is a pretty arbitrary decision.

Our measurements of spacetime involve c as a fundamental constant to convert between space and time (the separation between events in space-time is: $s =\sqrt (\Delta x^2 + \Delta y^2 + \Delta z^2 - (c \Delta t)^2)$ [extending Pythagoras' theorem to four dimensions]).

We can't really know what might be possible with a different universe with different laws. Although there is evidence that a universe can only have 3 spatial and 1 time dimensions; it seems any other combination is unstable/impossible.

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6 minutes ago, Strange said:

Although there is evidence that a universe can only have 3 spatial and 1 time dimensions; it seems any other combination is unstable/impossible.

Can you please provide some source for this?  I am not contradicting, I just want to read further on this topic.
Why couldn't there be another temporal dimension? (Of course this discussion is moving dangerously to the limits of physical evidence so please tread lightly )

I thought that "Most physical laws are already written in a dimension-free form."

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41 minutes ago, Strange said:

Speed (of light or anything else) is a ratio between two units: distance and time

9 minutes ago, geordief said:

Of course c is a ratio .Why did I imagine it could be a simple constant (physical as you say)

No wonder I couldn't measure it using my multimeter.

I have been trying for the last hour but I had it set to frequency and wavelength.

15 minutes ago, Silvestru said:

Not sure I understand. It is not a mathematical constant it's a physical one. But also my response is in relation tho this new Universe. If you invent a universe it can have any physical laws you desire . c could be slower and our observable universe would be much smaller. Biochemistry would also be affected of course. Basically everything.

Yes, the point is that there are many connections between different quantities in this or any other universe.

So even a small change to one would affect lots of things.

If c had a different value in another universe then the wavelength of that light would be different.

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12 minutes ago, Silvestru said:

Can you please provide some source for this?  I am not contradicting, I just want to read further on this topic.

Good summary (with a neat diagram) here: https://en.wikipedia.org/wiki/Spacetime#Privileged_character_of_3+1_spacetime

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58 minutes ago, Strange said:

Thanks StrangeI love the conclusion after the entire chapter:

Quote

Hence anthropic and other arguments rule out all cases except N = 3 and T = 1, which happens to describe the world around us.

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1 hour ago, studiot said:

No wonder I couldn't measure it using my multimeter.

I have been trying for the last hour but I had it set to frequency and wavelength.

Did you try the uncertainty settings?

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1 minute ago, geordief said:

Did you try the uncertainty settings?

I'm not sure...

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I think, if you wanted to be able to imagine why something cannot travel faster than the speed of light, you would only need to use this thought experiment.

Say, you started traveling the speed of light and you were traveling at a constant speed, like a photon.  According to SR, you can then say you are actually at rest, and everything else is traveling the speed of light.  Then, if it looks the same as if everything else was traveling the speed of light, everything's time would stop, and everything's distances would contract to zero.  If you were that photon traveling the speed of light, you wouldn't observe any more time or distance to speed up and travel into.  Increasing your speed would require more distance and time to travel in.

It would be like you were in a singularity, but you would probably become a singularity long before you even reached the speed of light.  I have heard scientist are starting to move away from the idea that the mass increase caused by relativity would prevent that.  The speeds of galaxies don't seem to be limited by them containing black holes, but this is usually explained by the relative speed to the space itself which is expanding.  Then a lot of people don't believe in an aether or physical spacetime, so they don't buy into that explanation.

Edited by Conjurer

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5 hours ago, Conjurer said:

I think, if you wanted to be able to imagine why something cannot travel faster than the speed of light, you would only need to use this thought experiment.

Say, you started traveling the speed of light and you were traveling at a constant speed, like a photon.  According to SR, you can then say you are actually at rest, and everything else is traveling the speed of light.

No, you can't. Photons do not have a rest frame. SR fails to describe what is happening at that speed. It does not represent an inertial frame of reference.

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8 hours ago, Conjurer said:

It would be like you were in a singularity, but you would probably become a singularity long before you even reached the speed of light.  I have heard scientist are starting to move away from the idea that the mass increase caused by relativity would prevent that.  The speeds of galaxies don't seem to be limited by them containing black holes, but this is usually explained by the relative speed to the space itself which is expanding.  Then a lot of people don't believe in an aether or physical spacetime, so they don't buy into that explanation.

The idea that you would collapse into  black hole if you got too close to the speed of light is mistaken.   You have to go by the the idea that any object "in motion" is at rest with respect to itself. Even if you go with the idea of Relativistic mass, this mass increase happens only according to the frame which measures the object as moving.  The object doesn't measure itself as moving, thus the object doesn't measure any increase in Relativistic mass, and has no reason to collapse into a black hole.   An event like the collapse into a black hole would have to be consistent for all reference frames.  Thus, if it doesn't form a black hole in it own frame, it doesn't in any other frame either.

I don't see what having black holes at their centers has to do with limiting the velocity of galaxies (especially when the mass of those black holes is only a small fraction of the total mass of the galaxy). What the expansion of space does is allow for very distant galaxies to have apparent velocities that exceed that of light.

If you go the Relativistic mass route,  it is the rate at which the mass increases as the object nears c that is the issue, not the starting initial mass.  It increases without limit. It doesn't matter if you started with 1 kg or the mass of a galaxy.

While physics is steering away from the use of Relativistic mass, it is more of a convention issue and not a matter of changing how they think Relativity works.  "Relativistic mass" is just considered as part of the energy of the object, which increases towards infinity as the object approaches c relative to you.

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17 hours ago, swansont said:

No, you can't. Photons do not have a rest frame. SR fails to describe what is happening at that speed. It does not represent an inertial frame of reference.

One of the main axioms of SR is that ALL objects traveling at a constant speed can assume they are at rest, and of course it doesn't predict anything; everything would just be zero in that frame.  It would end up in a state of superposition, and it would have to be described as a probability of traveling in one direction to be more likely than traveling in every other possible direction at once, like in quantum mechanics...

14 hours ago, Janus said:

The idea that you would collapse into  black hole if you got too close to the speed of light is mistaken.   You have to go by the the idea that any object "in motion" is at rest with respect to itself. Even if you go with the idea of Relativistic mass, this mass increase happens only according to the frame which measures the object as moving.  The object doesn't measure itself as moving, thus the object doesn't measure any increase in Relativistic mass, and has no reason to collapse into a black hole.   An event like the collapse into a black hole would have to be consistent for all reference frames.  Thus, if it doesn't form a black hole in it own frame, it doesn't in any other frame either.

There is no preferred frame of reference, so both observers would detect a mass increase in the others mass.  I was trying to leave mass out of the example on purpose, because I was trying to highlight the idea that a singularity would be caused just due to the length contraction alone in Minkowski Spacetime.  That is because they would both detect each others space contracting, so then most masses would end up closer together just from the length contraction.  Then approaching the speed of light would be more like a game of "trying to dodge the black hole", since everything else apparent mass would increase while they came closer and closer together.  Then I think it is a lot easier to picture the speed of light barrier using length and time dilation instead.

15 hours ago, Janus said:

I don't see what having black holes at their centers has to do with limiting the velocity of galaxies (especially when the mass of those black holes is only a small fraction of the total mass of the galaxy). What the expansion of space does is allow for very distant galaxies to have apparent velocities that exceed that of light.

Galaxies are not predicted to move faster than the speed of light unless they are outside of the visible universe.  The edge of the visible universe is actually the location where the speed of galaxies approach very closely to the speed of light.  Anything beyond that distance is pure speculation.

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3 hours ago, Conjurer said:

One of the main axioms of SR is that ALL objects traveling at a constant speed can assume they are at rest, and of course it doesn't predict anything; everything would just be zero in that frame.

Everything travelling at less than the speed of light. There are two easy ways to see that light speed is not a valid frame of reference: logical and mathematically.

First, the postulates of relativity say that the speed of light is the same for all observers. That would mean that a photon would have to see other photons moving at the speed of light, which would mean they could not all be travelling at the speed of light. A logical contradiction.

Secondly, if you try and use the mathematics of SR for a frame of reference moving at light speed you end up dividing by zero.

3 hours ago, Conjurer said:

There is no preferred frame of reference, so both observers would detect a mass increase in the others mass.  I was trying to leave mass out of the example on purpose, because I was trying to highlight the idea that a singularity would be caused just due to the length contraction alone in Minkowski Spacetime.

But, again, the length contraction is only apparent to the other observer. You are currently travelling at near the speed of light relative to cosmic rays passing by. But you are not contracted (not is your mass increased) by that, so there is no danger of you turning into a black hole.

3 hours ago, Conjurer said:

Galaxies are not predicted to move faster than the speed of light unless they are outside of the visible universe.  The edge of the visible universe is actually the location where the speed of galaxies approach very closely to the speed of light.  Anything beyond that distance is pure speculation.

"we point out confusions regarding the particle horizon, the event horizon, the observable universe'' and the Hubble sphere (distance at which recession velocity = c). We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests."

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1 hour ago, Strange said:

Everything travelling at less than the speed of light. There are two easy ways to see that light speed is not a valid frame of reference: logical and mathematically.

First, the postulates of relativity say that the speed of light is the same for all observers. That would mean that a photon would have to see other photons moving at the speed of light, which would mean they could not all be travelling at the speed of light. A logical contradiction.

Secondly, if you try and use the mathematics of SR for a frame of reference moving at light speed you end up dividing by zero.

But, again, the length contraction is only apparent to the other observer. You are currently travelling at near the speed of light relative to cosmic rays passing by. But you are not contracted (not is your mass increased) by that, so there is no danger of you turning into a black hole.

"we point out confusions regarding the particle horizon, the event horizon, the observable universe'' and the Hubble sphere (distance at which recession velocity = c). We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light. We explain why this does not violate special relativity and we link these concepts to observational tests."

Length contraction actually works.

Time is inverse so division by zero results.

I do try and preface any statements. You can say mathematically or tends towards.

Edited by Endy0816

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8 hours ago, Conjurer said:

There is no preferred frame of reference, so both observers would detect a mass increase in the others mass.  I was trying to leave mass out of the example on purpose, because I was trying to highlight the idea that a singularity would be caused just due to the length contraction alone in Minkowski Spacetime.  That is because they would both detect each others space contracting, so then most masses would end up closer together just from the length contraction.  Then approaching the speed of light would be more like a game of "trying to dodge the black hole", since everything else apparent mass would increase while they came closer and closer together.  Then I think it is a lot easier to picture the speed of light barrier using length and time dilation instead.

So according to you, the Earth should collapse into a black hole just because some distant object is traveling at a high fraction of c relative to us.   Again, relative motion will not produce a singularity in any frame.   If you think it will, you are seriously misunderstanding something.

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9 hours ago, Conjurer said:

One of the main axioms of SR is that ALL objects traveling at a constant speed can assume they are at rest, and of course it doesn't predict anything; everything would just be zero in that frame.  It would end up in a state of superposition, and it would have to be described as a probability of traveling in one direction to be more likely than traveling in every other possible direction at once, like in quantum mechanics...

Both postulates of relativity specifically mention inertial frames of reference. A photon is not in an inertial frame, and can never be at rest.

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On 10/22/2018 at 4:21 PM, Aphile said:

'Why this speed is the limit'

I think the question that you are searching for is "how" and not "why"

Asking the question "why" becomes an endless search for meaning of what is real vs. what is imaginary.

This attempt to solve this "Real" vs. "Imaginary" paradox begins to cause the mind to fall into eventual insanity. It is similar to trying to find a point a circle while you are on the circle if that makes any sense.

However by asking the question "how" and working within the boundaries of the reality which has been created to assist you in understanding the world around you, you will begin to see the analogous circle I had previously mentioned as being a circle.

So in response to your question, "why this speed is the limit" I will respond with 2 questions of my own.

Question 1: How did you ask yourself the question of "why this is the speed of light"?

Question 2: How can you use the answer to question 1 to help you develop new ideas using the question being asked in question 1?

kind of hurts the brain doesn't it, sure hurts mine.

...unless you were simply asking "why this is the speed of light" in regards to the language used in physics then I probably answered the wrong thing here....

Edited by ALine
changed hurt to hurts because I am still thinking about it

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On ‎10‎/‎23‎/‎2018 at 4:02 AM, Aphile said:
On ‎10‎/‎22‎/‎2018 at 10:04 PM, studiot said:

But the OP question (as I understood it) was

Why does light speed have the value it doe?

Not what happens if something goes faster or slower.

I know that but then it depends what you take as fundamental.

Since there are  3 constants involved, only two have to be fundamental.

Relativity was derived using 'light clocks' so light was used there.

And I think that empty space and its properties are more fundamental than light.

Yeah, you got me.

My question is why light speed or fastest speed that anything can travel is that limited rate.

The answer that will satisfy me is the same like the explanation of 'pi' value that we (human) can explain.

Can anyone tell me why this thread has wandered so far off topic?

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14 minutes ago, studiot said:

Can anyone tell me why this thread has wandered so far off topic?

!

Moderator Note

Discussion of holographic principle split of too:

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On 22/10/2018 at 5:21 PM, Aphile said:

Why light speed?

You have asked a question that belongs to a set of three mutually related questions. Let's write the other two.

- Why the elementary charge?
- Why the elementary action? (That is, the Planck constant).

If you raise each question individually, there is no hope of response. If you investigate what the three questions imply together, there is hope.

I have not answered your question. Just now you know that you have raised one of the three fundamental questions in the context of physical constants. That is, in the context of the making of the universe. This is very commendable for a person unfamiliar with physics.

I confess that I am not familiar with the investigation of those three questions. That's why I can not specifically answer what you asked.

Less complications appear when you investigate the questions for the case of the vacuum. In electromagnetic terms, the speed of light in vacuum is expressed in the following way.

$C=\dfrac{1}{\sqrt{\mu_o \ \varepsilon_o}}$

$\mu_o \ \rightarrow$ (magnetic) permeability of the vacuum

$\varepsilon_o \ \rightarrow$ (electric) permitivity of the vacuum

Permeability and permittivity are terms referring to the behavior of the vacuum with respect to the magnetic field and with respect to the electric field, respectively.

Speed C is not the only property related to that pair of terms. The propagation in the vacuum presents a characteristic impedance, expressed in the following way.

$Z_o = \sqrt{\dfrac{\mu_o}{\varepsilon_o}}$

The impedance is measured in ohm.

Only the propagation in the vacuum presents impedance? No. Electric and elecrtronic circuits too. You know there are 4 ohm speakers, 8 ohm speakers, etc. You know that the microphone manufacturers specify the impedance, for example, less than 200 ohm. What is impedance? It is the relation between two magnitudes used frequently, the electric potential difference and the current.

In colloquial terms, we can say that the potential difference corresponds to the electrical amplitude. A singer connects the microphone to the amplifier. Suppose that the microphone input supports a maximum of 0.8 volt. If something is badly predisposed, it can happen that when the voice is emitted with a lot of energy the microphone generates more than 0.8 volt. The amplifier will trim the excess and the sound will lose quality. When an electric field is used to accelerate electrons, the energy that the electrons gain depends on the available   potential difference.

What is the current? 600 thousand trillion electrons constitute more electric charge than a trillion. Amount of charge and number of electrons are equivalent concepts. There are two signs of charge, so you can say the same for protons, positrons, muons and other types of particles that have electric charge. More particle, more charge. If there are electrons traveling and you have a method to measure how much charge crosses the reference plane at a given time, you can divide the charge by time and the quotient is the current.

As stated earlier, impedance is the relationship between potential difference and current. If you divide the first one by the second you calculate the impedance, in the case of a direct current. In that case the word changes, but the concept remains. In direct current it is said resistance and the value is expressed with real numbers. Alternating current varies as a function of time and, in this case, the use of complex numbers is frequent.

In a vacuum, obviously, there is no material thing that is responsible for the characteristic impedance of propagation. This means that the impedance is a property of the electromagnetic field. Why does the impedance appear in material devices? Because there are electromagnetic fields in the internal structure of matter. In each small region of the interior of matter there are electromagnetic fields. The impedance of an amplifier, a microphone, etc. , depends ultimately on the resultant of those constitutive fields.

Why have I devoted attention to the concept of impedance, when the initial question of the thread is related to the speed of light? Because that set of three intimately related questions, mentioned previously, has the following property.

$2 \ \alpha \ \dfrac{h}{e^2} = \sqrt{\dfrac{\mu_0}{\varepsilon_o}}$
$\alpha \ \rightarrow$ fine structure constant
$h \ \rightarrow$ Planck constant
$e \ \rightarrow$ electron's charge

The fine structure constant is a dimensionless term. That is, it does not have units. It is purely numerical.

Each dimensional term, which have units, is established by definition. Definitions are not properties of nature. They are conventional statements, that is, statements voluntarily established by scientists. The virtue of a good scientist is to propose, as far as possible, definitions consistent with what is observed in nature. That does not diminish the conventional character of the definitions.

A dimensionless term is independent of definitions. Where does it come from then? After squeezing the brain, the conclusion is that it truly corresponds to a condition proper to nature. That means that other physicists could use very different definitions of these that we use and, sooner or later, the fine structure constant would be found in the physics they have made. The same dimensionless constant that we find in our physics.

You have put, at the beginning of the thread, a question belonging to a set of three mutually related questions. That set is equivalent to a single fundamental question. Is the next.

A dimensionless term, without units, that is without references to measurable magnitudes, should be calculable in a purely theoretical way, without the need for experimental data. How could someone do that? The person should analyze a phenomenon whose mathematical approach leads to the need to prove a theorem. That is, the need for a purely theoretical development, which does not need to use experimental data. By constructing rectangle triangles and measuring their sides, you can make data tables and come to suspect that the square of the hypotenuse is equal to the sum of the squares of the sides. But it is possible to demonstrate that by developing a theorem, in a purely theoretical way, without the need to measure triangles in practice. Thus the Pythagorean theorem has been proved. To answer the three questions mentioned previously, we need the value of the fine structure constant to be determined by a theorem.

Is that task possible? I bet my life yes.

Which theory allows to prove this theorem? I bet my life that classical electrodynamics allows to do that. (Surely many weapons are pointing towards my body to kill me, haha).

If one assumes that classical electrodynamics allows doing that, a question immediately arises. Why the proof is not in the textbooks, if since the publication made by Maxwell until now more than 150 years have passed? I bet my life that the fault lies with the corporations, that they have maintained tense situations, fierce competitions and wars all over the world, all that time. And they still continue to disturb all human activities and dedications, including the activities and dedications of scientists.

Edited by quiet

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