# Stephen Hawking - cultural icon

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1279620.  Thu Mar 29, 2018 11:16 am

This discussion is starting to go round in circles, I think.

 dr.bob wrote: I think you're mistaken here, partly because the speed of light is not just the measure of how far light goes in a set amount of time (weirdly). It pops up all over physics.

Yes. And the metre pops up all over our system of measurement. Not necessarily explicitly, but because other units are defined in terms of the seven SI base units (metre, kilogram, second, ampere, kelvin, candela, mole). And the dimensions have to be the same on both sides of the equation. And the definition of the metre assumes a constant speed of light.

So however you derive the value of the speed of light with respect to the current system of SI units, you will end up concluding that c = 299,792,458 m/s. And that's not physics - it's maths. And it can be proved, quite easily.

 Quote: One example (there are loads like this) are the Einstein field equations. They define the curvature of spacetime according to this equation.

I haven't gone through your particular example yet, because I'm not familiar with the dimensions of the various constants that are used in the equation. But trust me, whatever the dimensions are, it will be possible to deduce logically that the speed of light has that defined value. It has nothing whatsoever to do with experimentation.

I'll look up the definitions as soon as I have time, and put together a demonstration that they lead to the conclusion of a constant speed of light. I'm quite certain I can do this, not because I'm some sort of mathematical genius, but because I know that any other conclusion would be logically inconsistent. And no amount of experimentation can lead anyone to a logical inconsistency.

 Quote: Ive also mentioned (several times) the work of Prof. Joăo Magueijo. His theory of a variable speed of light would manifest itself in the variations detectable in the CMB. That simply involves measuring the temperature of the CMB, again no requirement to redefine the metre.

Well in that case Prof. Magueijo is either using a different system of units to measure the postulated speed of light in the early stages of the universe, or using a value of c other than the defined one.

Quote:
 GuyBarry wrote: Did they have to tear up the entire theory of relativity and throw it in the bin?

No more than Einstein had to tear up the entire theory of Newtonian Mechanics and throw it in the bin.

OK, so what mathematical model are they using then? If you take c to be a variable, then the constant value for the speed of light that's used in relativity will simply be one particular value of that variable - let's say c_0. So the whole theory of relativity will have to be reformulated as a special case of a more general theory where c happens to have the special value c_0. No longer will we have E = mc^2 or any of the other equations where c is assumed to be constant.

Now I don't know - maybe they've managed to find a way of doing this. They could be cleverer than Einstein. I'm certainly not.

But what I do know is that, whatever their theory about the origins of the universe, it's going to have to incorporate the predictions of the whole of Einstein's theory of relativity. Because, as you say, it makes astonishingly accurate predictions about the nature of the observable universe, and to abandon it would be flying in the face of all the data we've collected in the last hundred years to confirm it.

 Quote: How you can read that over and over again, and yet still come to the conclusion that all our problems can be solved if only we'd "abandon the assumptions that we currently regard as fundamental, such as a constant speed of light" simply astounds me. Lots of people have already abandoned that assumption. Did they successfully create a Theory of Everything? Go on, I'll give you one guess.

Then you've clearly misunderstood what I'm suggesting. I am saying that there might be no Theory of Everything, for the following reasons:

(1) In order to explain gravitational effects in the observable universe, one needs to assume a constant speed of light.
(2) In order to explain electromagnetic (and other sub-atomic) effects in the observable universe, one needs to assume a constant speed of light.
(3) In order to understand the origins of the universe (which are unobservable), one needs to assume a variable speed of light.

It is quite possible for (1), (2) and (3) to be true simultaneously. If that's the case, then the quest for a Theory of Everything is doomed. You will never be able to unify gravity and electromagnetism within the observable universe while simultaneously allowing the possibility that the speed of light is variable.

 Quote: Lots of people are working on theories of the universe beyond the observable universe.

Then they're not doing science by my understanding of the concept, because they have no data, and cannot have any data.

If there's evidence of anything at all beyond the observable universe - even by proxy - then that becomes, ipso facto, part of the observable universe. So you're caught in a logical contradiction.

Physicists may be able to do all sorts of clever things, but the one thing they can't do is defy logic.

1279625.  Thu Mar 29, 2018 1:21 pm

OK, I've looked at the definitions of the various constants you mentioned and the units they're defined in.

 dr.bob wrote: We can measure the curvature of spacetime by various experiments such as gravitational lensing.

This is defined by something called the "Einstein tensor" - a multidimensional array of values which have dimension of length^-2. Hence the components would be measured in units of m^-2, the inverse square of a metre - a unit whose value, as already discussed, is dependent on the speed of light.

 Quote: We could check that the Gravitational Constant "G" is not changing by measuring local gravitation effects.

This is measured in units of m^3 kg^-1 s^-2 - involving the definition of the metre, which is dependent on the speed of light.

You simply cannot get away from it. There is no possible way that you could write an equation that determined the value of the speed of light in units that were not dependent on the metre in some way. If it were possible, then you'd be able to rearrange the equation as

c = ?

where "?" had dimensions that didn't involve length. Which is impossible, because c has dimensions of length x (time)^-1. And you can't have a physical equation with different dimensions on both sides.

Or to put it another way, you'd have to be able to write an equation that expressed the metre purely in terms of the other SI base units, or as a dimensionless quantity. And you clearly can't. If you could, the metre wouldn't need to be an SI base unit!

1279638.  Thu Mar 29, 2018 3:26 pm

 dr.bob wrote: Mathematics doesn't care if the speed of light is constant, variable, or made of cheese.

I really want to read a paper that defines the speed of light as 473,682 tons of Emmental!

1279640.  Thu Mar 29, 2018 4:02 pm

GuyBarry wrote:
OK, I've looked at the definitions of the various constants you mentioned and the units they're defined in.

dr.bob wrote:

 Quote: We could check that the Gravitational Constant "G" is not changing by measuring local gravitation effects.

This is measured in units of m^3 kg^-1 s^-2 - involving the definition of the metre, which is dependent on the speed of light.

You could just as easily measure G in fur^3.fir^-1.ftn^-2 and convert to SI later (should you feel it necessary).

The constant c is built in to the structure of spacetime. AFAIK there is no theoretical reason to assign it a particular value, nor one to assume that its observed value is global (Bill Ockham reckons it is though).

1279641.  Thu Mar 29, 2018 4:03 pm

crissdee wrote:
 dr.bob wrote: Mathematics doesn't care if the speed of light is constant, variable, or made of cheese.

I really want to read a paper that defines the speed of light as 473,682 tons of Emmental!

If ever I saw a theory that was full of holes .......

 1279643.  Thu Mar 29, 2018 4:15 pm :-)

1279660.  Fri Mar 30, 2018 4:13 am

 ali wrote: You could just as easily measure G in fur^3.fir^-1.ftn^-2 and convert to SI later (should you feel it necessary).

And when you convert to SI, you've got a conversion factor that involves the speed of light, because of the definition of the metre. You can't escape it.

As long as your final value for the speed of light is expressed in SI units as they're currently defined, it will be 299,792,458 m/s. It might be variable when expressed in other units of course.

1279665.  Fri Mar 30, 2018 5:19 am

 GuyBarry wrote: This discussion is starting to go round in circles, I think.

I don't think "starting" is appropriate here.

 GuyBarry wrote: So however you derive the value of the speed of light with respect to the current system of SI units, you will end up concluding that c = 299,792,458 m/s. And that's not physics - it's maths. And it can be proved, quite easily.

You are focussing your attention far too narrowly on this one definition. As I've said before, the speed of light pops up all over physics. Changing the definition will have huge ramifications, which would produce measurable changes in how the universe behaves. You seem to be flatly ignoring this fact, which is a tad frustrating.

 GuyBarry wrote: But trust me, whatever the dimensions are, it will be possible to deduce logically that the speed of light has that defined value. It has nothing whatsoever to do with experimentation.

The speed of light has everything to do with experimentation. There's nothing in our current theories that logically require it to be a particular value.

 GuyBarry wrote: Well in that case Prof. Magueijo is either using a different system of units to measure the postulated speed of light in the early stages of the universe, or using a value of c other than the defined one.

The latter, obviously. I've been saying that for 5 pages or more. What's your point?

 GuyBarry wrote: No longer will we have E = mc^2 or any of the other equations where c is assumed to be constant.

Why not? Of course we'd still have E = mc^2. That's just an equation. If you vary the value of c, then the values of E and/or m would vary too. You'd still have the same equation, though.

However, if the values of E and/or m varied, then that would be an observable phenomenon.

 GuyBarry wrote: But what I do know is that, whatever their theory about the origins of the universe, it's going to have to incorporate the predictions of the whole of Einstein's theory of relativity. Because, as you say, it makes astonishingly accurate predictions about the nature of the observable universe, and to abandon it would be flying in the face of all the data we've collected in the last hundred years to confirm it.

Now you're starting to understand the scale of the problem here, and why it's unlikely that everything can be fixed if only we'd stop with out dogged "belief" that the speed of light is constant, as you keep suggesting.

 GuyBarry wrote: I am saying that there might be no Theory of Everything, for the following reasons: (1) In order to explain gravitational effects in the observable universe, one needs to assume a constant speed of light. (2) In order to explain electromagnetic (and other sub-atomic) effects in the observable universe, one needs to assume a constant speed of light. (3) In order to understand the origins of the universe (which are unobservable), one needs to assume a variable speed of light.

Do you have any reason for suggesting these are the reasons that we don't have a Theory of Everything?

 GuyBarry wrote: It is quite possible for (1), (2) and (3) to be true simultaneously.

It's also quite possible for none of them to be true.

 GuyBarry wrote: If that's the case, then the quest for a Theory of Everything is doomed. You will never be able to unify gravity and electromagnetism within the observable universe while simultaneously allowing the possibility that the speed of light is variable.

So we're left to conclude that either the quest for a Theory of Everything is doomed, or you've got your reasons wrong. I know which one I find more convincing at the present moment.

GuyBarry wrote:
 Quote: Lots of people are working on theories of the universe beyond the observable universe.

Then they're not doing science by my understanding of the concept, because they have no data, and cannot have any data.

Then you clearly misunderstand science.

The existence of the Higgs Boson was proposed in 1964. Its existence was only proved, and a value for its mass measured, in 2013. In those intervening 50 years, scientists had virtually no data on the Higgs Boson. However, this did not prevent them from creating theories about how it might behave.

To do this, they had to take a best guess about the physical properties of the particle. As I mentioned way back on page 4 of this thread, experiments at the LHC initially proved a lot of theories that had been worked on in the intervening 50 years to be wrong. However, it also proved which ones were right.

Just because scientists don't have data for something, doesn't stop them theorising based on what they do know and eventually coming up with testable hypotheses on whole new areas of knowledge.

1279667.  Fri Mar 30, 2018 6:12 am

 GuyBarry wrote: OK, I've looked at the definitions of the various constants you mentioned and the units they're defined in.

That's nice, but I think you need to look a bit more closely.

 GuyBarry wrote: This is defined by something called the "Einstein tensor" - a multidimensional array of values which have dimension of length^-2. Hence the components would be measured in units of m^-2, the inverse square of a metre - a unit whose value, as already discussed, is dependent on the speed of light.

OK, so are you saying that changing the speed of light will change the length of a metre by exactly the right amount to cancel each other out and produce no observable effect? If so, please show your working, because waving your arms in the air and saying "Oh, there are some metres, so I'm just going to ignore it" is not helping the discussion in the slightest.

GuyBarry wrote:
 Quote: We could check that the Gravitational Constant "G" is not changing by measuring local gravitation effects.

This is measured in units of m^3 kg^-1 s^-2 - involving the definition of the metre, which is dependent on the speed of light.

OK, so are you saying that changing the speed of light will change the length of a metre by exactly the right amount to cancel each other out and produce no observable effect? Again, please show your workings.

 GuyBarry wrote: You simply cannot get away from it. There is no possible way that you could write an equation that determined the value of the speed of light in units that were not dependent on the metre in some way.

You've clearly misunderstood the point I've been making for the past 6 pages. Maybe I'm explaining it incredibly badly.

My point is that changing the speed of light will produce measurable effects in the rest of the universe. I will now show my workings:

I've mentioned before that the speed of light is related to basic constants such as the vacuum permittivity "ε0" and the vacuum permeability "μ0". The relation comes as a result of Maxwell's Equations and is given as:

c^2 = 1/ε0 μ0

So, what happens if we change the speed of light? Let's try, for the sake of argument (and because it makes the maths easier) doubling the speed of light. What effect will this have?

Clearly either ε0 or μ0 (or possibly both) will need to change. So, lets keep ε0 constant and see what happens. If c is doubled, then c^2 will increase by a factor of 4. In order to make the above equation balance, then μ0 will have to reduce by a factor of 4. Is this OK?

Not really. According to Ampere's law, the force between two wires carrying an electrical charge is given as:

F = μ0 L I^2 / 2πr

Where L is the length of the wires, I is the current being carried, and r is the separation between the wires.

Changing the speed of light will reduce the value of μ0 by a factor of 4 as we've seen. However, since the metre is also defined by the speed of light, then the values of L and r will also change. This is what you've alluded to previously. Unfortunately for your argument, they will change by the same amount as each other and, since they're on either side of the fraction, they'll cancel each other out. It doesn't matter if L doubles, because so will r and they cancel out. Or if L halves, so will r, and they cancel out.

So that leaves the current I. This is defined in the new SI in terms of the number of fundamental electrical charges per second. The second is defined in terms of the frequency of a certain type of light, not its speed, so this should be invariant. Thus the force produced by this current will reduce by a factor of 4.

But what if changing the speed of light alters the second? It's possible that the number of wavefronts going past a detector will alter. If it does, it's liable to increase them since the light is moving faster. Therefore it will take half the time for 9,192,631,770 wavefronts to pass a detector (the definition of a second), so the second will also halve. If the second is halved, then the number of fundamental electrical charges per second will also halve, so the value of I will reduce by half, and the value of I^2 will reduce by a factor of 4. Combining this with the reduction in μ0, and the force will reduce by a factor of 16.

That's a measurable difference in the amount of force. But what is a force? It's measured in Newtons which are defined as kg m / s^2. But the new definition of a kg is given by Planck's constant in units of kg m^2 /s. Dividing one by the other gives units of force of m^-1 s^-1. Clearly if we double the speed of light, the metre will halve in length, which will double the force. We're not sure whether the second will change, but it'll either stay the same or reduce by half (see above).

So, while the force will either reduce by a factor of 4 or 8, the change to the units of measurement will actually suggest an increase of a factor of 2 or 4 (depending on if the second changes).

Clearly these units do not cancel each other out, so a clearly observable change in the force between the wires will be measurable despite the units of measurement also changing.

So it's not simply enough to just say "oh look, units of measurement" and brush the whole thing under the carpet. That's a huge oversimplification which ignores the massive effects that changing the speed of light would have on the whole of physics.

1279669.  Fri Mar 30, 2018 6:20 am

 dr.bob wrote: You are focussing your attention far too narrowly on this one definition. As I've said before, the speed of light pops up all over physics. Changing the definition will have huge ramifications, which would produce measurable changes in how the universe behaves. You seem to be flatly ignoring this fact, which is a tad frustrating.

Why woud anyone want to change the definition of the speed of light? It's been measured to a high degree of accuracy up until 1983, to the extent that a proposal was adopted to fix its value at 299,792,458 m/s, and that value has given correct predictions since then. It'd be pretty crazy if the CGPM suddenly decided to change the value to something else.

If genuine evidence is obtained that the speed of light is variable (and I've no idea whether it is or not), the standard will need to be abandoned completely, and the value of c will need to be determined experimentally.

 Quote: The speed of light has everything to do with experimentation.

Not at the moment. It has been experimentally determined in the past, and it may well be so in the future; but at the moment it's a defined value. And given that the CGPM has decided to retain that defined value in its proposed redefinition of SI units, I imagine it's going to retain that value for quite a long time in the future.

 Quote: There's nothing in our current theories that logically require it to be a particular value.

No, there isn't. But because it's been adopted as the standard of measurement of distance, all the measurements of distance we make are implicitly assuming one particular value.

Incidentally, under the current proposals for the redefinition of SI units, the Planck constant will also be given a fixed value, and the definition of the kilogram will depend on that value and on the definitions of the second and metre. So for the first time, the definition of the kilogram will also be indirectly dependent on the speed of light (because it will depend on the metre). There's quite a nice little diagram here.

Quote:
 GuyBarry wrote: Well in that case Prof. Magueijo is either using a different system of units to measure the postulated speed of light in the early stages of the universe, or using a value of c other than the defined one.

The latter, obviously. I've been saying that for 5 pages or more. What's your point?

That the value of c is currently fixed. You can't have a value of c other than the defined one, unless you use a different system of units. If Prof. Magueijo postulated that the value of c was something other than 299,792,458 m/s in current SI units, it would lead to a logical inconsistency. You'd end up being able to deduce that 0 = 1.

I assume that Prof. Magueijo isn't quite so foolish as to have attempted that.

 Quote: Now you're starting to understand the scale of the problem here, and why it's unlikely that everything can be fixed if only we'd stop with out dogged "belief" that the speed of light is constant, as you keep suggesting.

No, I've never suggested that.

Quote:
 GuyBarry wrote: I am saying that there might be no Theory of Everything, for the following reasons: (1) In order to explain gravitational effects in the observable universe, one needs to assume a constant speed of light. (2) In order to explain electromagnetic (and other sub-atomic) effects in the observable universe, one needs to assume a constant speed of light. (3) In order to understand the origins of the universe (which are unobservable), one needs to assume a variable speed of light.

Do you have any reason for suggesting these are the reasons that we don't have a Theory of Everything?

No. That's why I said "might be" rather than "is". It might be impossible to construct a Theory of Everything. It also might not be.

Someone earlier in the thread (PDR?) claimed that there had to be a Theory of Everything. There doesn't have to be one at all. Before Gödel proved his Incompleteness Theorem, some people believed that there had to be an axiomatization of arithmetic that was both consistent and complete. We now know that there can't be one.

That doesn't mean that there shouldn't be research into the possibility of a Theory of Everything; it just means that people should be open to the possibility that there isn't one.

 Quote: So we're left to conclude that either the quest for a Theory of Everything is doomed, or you've got your reasons wrong.

No, we don't have to conclude either of those. Until such time as it's proved that there can't be a Theory of Everything, it seems entirely reasonable to carry on looking for one.

Quote:
GuyBarry wrote:
 Quote: Lots of people are working on theories of the universe beyond the observable universe.

Then they're not doing science by my understanding of the concept, because they have no data, and cannot have any data.

Then you clearly misunderstand science.

Do I? I thought that it was a theoretical consequence of our current understanding of the universe that we cannot receive any signal from any part of the universe more than 45.8 billion light-years away. Not just that we haven't managed to do so yet, but the laws of physics prevent it.

And if the laws of physics prevent us from collecting data from any part of the universe more than 45.8 billion light-years away, then we can't construct any meaningful theories about it, because we know for certain that we will never have any evidence.

If it is possible to gather evidence about parts of the universe more than 45.8 billion light-years away, then doesn't it follow that one of our current theories of the universe is wrong? Either than it's older than we think it is, or that it's expanding at a different rate from the one we think, or that the speed of light is something different from what we think?

 Quote: The existence of the Higgs Boson was proposed in 1964. Its existence was only proved, and a value for its mass measured, in 2013. In those intervening 50 years, scientists had virtually no data on the Higgs Boson. However, this did not prevent them from creating theories about how it might behave.

Yes, but no one had come up with a theory that predicted "It is impossible to ever observe the Higgs Boson". If someone had done so, it would have been pretty silly to continue looking for it.

 Quote: Just because scientists don't have data for something, doesn't stop them theorising based on what they do know and eventually coming up with testable hypotheses on whole new areas of knowledge.

But the issue isn't just that we don't have data. It's that, according to our current model of the universe, we can't have data, ever.

Or to put it another way, that our very model of the universe puts limitations on what it's possible to know about it. If everything we've worked out so far is correct, then we're eventually going to reach the limitations of our knowledge.

Last edited by GuyBarry on Fri Mar 30, 2018 6:50 am; edited 1 time in total

1279670.  Fri Mar 30, 2018 6:37 am

 dr.bob wrote: OK, so are you saying that changing the speed of light will change the length of a metre by exactly the right amount to cancel each other out and produce no observable effect?

No, because you can't change the speed of light within the SI as currently formulated.

 Quote: OK, so are you saying that changing the speed of light...

No, because you can't change the speed of light within the SI as currently formulated.

 Quote: You've clearly misunderstood the point I've been making for the past 6 pages.

You've clearly misunderstood the point I've been making, which is that you can't change the speed of light within the SI as currently formulated.

 Quote: My point is that changing the speed of light will produce measurable effects in the rest of the universe.

I know it will! The whole of physics as currently formulated is dependent on a particular value for the speed of light.

 Quote: I've mentioned before that the speed of light is related to basic constants such as the vacuum permittivity "ε0" and the vacuum permeability "μ0".

Which both, at the time of writing, have defined values. Neither can be experimentally determined, as I said before. Under the CGPM's proposals the value of μ0 will be experimentally determined, but the value of ε0 will be derived from this value of μ0 and the fixed value for c.

 Quote: The relation comes as a result of Maxwell's Equations and is given as: c^2 = 1/ε0 μ0

I know. We've discussed it before.

 Quote: So, what happens if we change the speed of light?

Nothing, because we can't. It's defined!

 Quote: Let's try, for the sake of argument (and because it makes the maths easier) doubling the speed of light. What effect will this have? Clearly either ε0 or μ0 (or possibly both) will need to change. So, lets keep ε0 constant and see what happens. If c is doubled, then c^2 will increase by a factor of 4. In order to make the above equation balance, then μ0 will have to reduce by a factor of 4. Is this OK?

But that's nonsensical. All those constants currently have defined values! You can't change any of them.

If after the CGPM's proposals go through, an increase in the value of μ0 is observed, then the value of ε0 will go down in inverse proportion. That's because their product will still be defined as the fixed value 1/c^2.

 Quote: Changing the speed of light will reduce the value of μ0 by a factor of 4 as we've seen.

No it won't, because μ0 is defined, and anyway you can't change the speed of light.

 Quote: So it's not simply enough to just say "oh look, units of measurement" and brush the whole thing under the carpet. That's a huge oversimplification which ignores the massive effects that changing the speed of light would have on the whole of physics.

I have lost count of the number of times you have talked about "changing the speed of light". I have said again and again and again that our current system of measurement makes that impossible.

My point is simply this: in a system of units that defines the value of the speed of light, it is impossible to make an experimental observation that is consistent with the speed of light being equal to something else. So this whole thing about "changing the speed of light" is a complete irrelevance.

 1279673.  Fri Mar 30, 2018 7:04 am I'm going to take a break from this discussion, I think, since dr. bob and I have obviously been talking at cross-purposes, and I'm sure everyone else is thoroughly bored of it by now. I've learned a lot of interesting stuff and would like to thank dr. bob and all the other participants for their input.

1279706.  Fri Mar 30, 2018 10:05 am

GuyBarry wrote:
 Quote: The speed of light has everything to do with experimentation.

Not at the moment. It has been experimentally determined in the past, and it may well be so in the future; but at the moment it's a defined value.

I think this is needlessly solipsistic. The reason why it has been decided to fix the value of the speed of light is entirely due to a wealth of experimental data. So I think it's misleading to claim that the current value of the speed of light "has nothing whatsoever to do with experimentation."

GuyBarry wrote:
 Quote: There's nothing in our current theories that logically require it to be a particular value.

No, there isn't.

Then why did you just say "it will be possible to deduce logically that the speed of light has that defined value." ?

 GuyBarry wrote: That the value of c is currently fixed. You can't have a value of c other than the defined one, unless you use a different system of units.

You seem to be comparing previous definitions of the SI units to the current ones and calling these "a different system of units." I think this is misleading.

Previously the metre has been defined in terms of the circumference of the Earth, a number of wavelengths of a certain type of light, and now is defined as the distance that light travels in a fixed amount of time. Personally I would not classify these as different systems of units because they're all virtually the same. The length of a metre has not changed markedly in the last couple of hundred years. All that changed is the accuracy to measuring it to the last tiny decimal places.

Prof. Magueijo is free to define the speed of light as something else without leading to a logical inconsistency because he's referring to how quickly light will travel across a set distance that's remained largely unchanged for hundreds of years.

 GuyBarry wrote: I thought that it was a theoretical consequence of our current understanding of the universe that we cannot receive any signal from any part of the universe more than 45.8 billion light-years away. Not just that we haven't managed to do so yet, but the laws of physics prevent it.

That remains to be seen. There may be some way a signal could travel from some other part of the universe, perhaps via wormholes or some other phenomenon that we are as yet unfamiliar with.

 GuyBarry wrote: If it is possible to gather evidence about parts of the universe more than 45.8 billion light-years away, then doesn't it follow that one of our current theories of the universe is wrong?

Our current theories of the universe are definitely wrong. I've already pointed this out. We don't currently know how, but one day we will realise what the error is and create a new, better theory to replace them.

1279711.  Fri Mar 30, 2018 10:18 am

 GuyBarry wrote: You've clearly misunderstood the point I've been making, which is that you can't change the speed of light within the SI as currently formulated.

I think you misunderstand the purpose of SI unit definition.

I know lots of experimental physicists. Many experiments involve accurately measuring distances. Currently I know of nobody who has ever measured a metre by measuring how fast a beam of light travels across the lab. It's simply impractical to do so.

That's not a problem since, as I mentioned above, the length of a metre hasn't markedly changed for centuries. There are lots of ways of measuring a metre, and they'll all give you roughly the right answer. There will always be some error on any measurement, so you just need to choose the method that's accurate enough for the experiment you're performing.

The point of the SI units is to have some kind of independent standard that people can agree on when defining, for example, the metre. However, that doesn't mean it's the only definition. If I use my metre ruler to measure a metre, I'm still roughly correct, even if I'm not 100% in agreement with the SI definition.

GuyBarry wrote:
 Quote: I've mentioned before that the speed of light is related to basic constants such as the vacuum permittivity "ε0" and the vacuum permeability "μ0".

Which both, at the time of writing, have defined values. Neither can be experimentally determined, as I said before.

They can be experimentally determined. I explained how the vacuum permeability can be measured by measuring the force between two wires. The fact that the SI units are defined by committee doesn't stop you determining them experimentally if you wish to do so. That's the beauty of Physics: everything is interrelated, so it's possible to cross-check everything.

 GuyBarry wrote: My point is simply this: in a system of units that defines the value of the speed of light, it is impossible to make an experimental observation that is consistent with the speed of light being equal to something else.

This point is entirely wrong.

As I've pointed out, the definition of a metre is currently given in terms of the speed of light, but the length of a metre has not substantially changed in centuries. Thus, it's entirely possible (if tricky) to measure how much time it takes for a beam of light to travel along a metre ruler.

If the speed of light suddenly doubled one day, you would notice that the time taken for the light to travel along your metre ruler was now half what it was the day before. This is an experimental observation that is consistent with the speed of light being equal to something else.

Nobody is going to try and doggedly stick to the SI definition of a metre and say "since the light is travelling this far in the set amount of time, I'm going to have to move all the markings on my metre ruler." That's not how SI unit definitions work.

You yourself in this thread have given many examples in history where SI units were found to be incorrectly defined. This was tested experimentally and, when the problem was highlighted, the definition was changed. It's done so in the past, and it will probably be so in the future.

 1280077.  Tue Apr 03, 2018 2:45 am XKCD

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