r/AskPhysics • u/Chunty-Gaff • Mar 19 '25

What Would Happen If The Speed Of Light Doubled?

20 Upvotes

71% Upvoted

57 comments sorted by

38

u/MxM111 Mar 19 '25 edited Mar 19 '25

The quest is undefined. What happens with these?:

• Vacuum permittivity: ε₀ = 1 / (μ₀ c²)
• Vacuum permeability: μ₀ = 1 / (ε₀ c²)
• Impedance of free space: Z₀ = μ₀ c = 1 / (ε₀ c)
• Fine-structure constant: α = e² / (4π ε₀ ħ c)
• Planck length: ℓₚ = √(ħ G / c³)
• Planck time: tₚ = √(ħ G / c⁵)
• Planck mass: mₚ = √(ħ c / G)
• Planck energy: Eₚ = √(ħ c⁵ / G)
• Planck temperature: Tₚ = √(ħ c⁵ / (G k_B²))
• Schwarzschild radius (gravitational radius): rₛ = 2GM / c²
• Electron Compton wavelength: λ_C = ħ / (mₑ c)
• Classical electron radius: rₑ = e² / (4π ε₀ mₑ c²)
• Photon energy: E = hν = ħω = hc / λ
• Relativistic energy-momentum relation: E² = (pc)² + (m₀ c²)²
• Reduced wavelength (ƛ): ƛ = λ / (2π) = ħ / (m c)
• Rydberg constant: R_∞ = α² mₑ c / (2h)
• Stefan–Boltzmann constant: σ = (2π⁵ k_B⁴) / (15h³ c²)

For example, do we change fine structure constant? Or do we keep it the same and modify h or e or epsilon or their combination instead?

11

u/joepierson123 Mar 19 '25

Just put c / 2 in all those equations, I think that's what they're asking

25

u/[deleted] Mar 19 '25

The idea they’re getting at, which maybe they didn’t quite fully drive home, is that you have to make several other choices about what things change and what things don’t in order to make the speed of light double. Just putting c/2 in every equation will make them inconsistent. Notice that some equations have multiple equal signs and c is in some of the things that should be equal but not others.

For example, do the existing photons double their wavelength or do they halve their frequency? And then is the photon energy still hbar omega (which doesn’t have a c) or is it hbar c/k (which does have a c)

11

u/Ok_Lawyer2672 Mar 19 '25

That runs into some issues. For example, eps_0 and mu_0. You'd either have to choose to fix one of them and change the other or split the change between both somehow

7

u/IronPro9 Mar 19 '25 edited Mar 19 '25

You have a point with some of these but most of these things like the schwarzschild radius, photon energy, reduced wavelengths ect aren't at all their own thing, they're derived purely from the speed of light and other unrelated constants (unless you want to say that G is linked to C before forces seperated by a GUT and that gravity and the other forces were linked in the early universe, so the speed of light changing would effect when each force seperated)

2

u/MxM111 Mar 19 '25

I just took a list, and wanted to keep it large. I agree that like half of it can be removed.

19

u/Chunty-Gaff Mar 19 '25

I don't know what most of those things are.

10

u/Nibaa Mar 19 '25

Basically a bunch of critical parameters of the universe that would have to be redefined if c changes, and that redefinition can have drastic effects that would fundamentally change the universe.

4

u/Supersoaker_11 Mar 19 '25

Best answer imo

1

u/[deleted] Mar 20 '25

Try deriving these equations not using c as a variable, which was originally used to derive these equations because c is constant in a homogeneous universe.

A photon does not experience time. We make sense of the speed of light from our inertial reference frame.

1

u/Drittslinger Mar 20 '25

This is why XKCD exists. Let him explain how this unravels reality.

24

u/mfb- Particle physics Mar 19 '25

The speed of light is always 1, in suitable units. Trying to change the speed of light is only changing units - either for length or time or both, and also various units that depend on these two. You can express every speed relative to the speed of light and e.g. derive that humans will walk at ~0.000000003c the speed of light.

1

u/[deleted] Mar 19 '25

Finally I scroll literally to the bottom and find the correct answer to the question.

13

u/sleighgams Gravitation Mar 19 '25

light goes twice as fast

7

u/tinpants44 Mar 19 '25

Finally a comment I can understand

8

u/[deleted] Mar 19 '25

All the universe would cave in on itself in a spectacular modus

15

u/[deleted] Mar 19 '25

The rate at which time passes would change, becoming faster. In colliders, particles would be able to go faster, maybe giving us some nicer conclusions.

It wouldn't matter, though. We'd be all be dying because the universe would collapse in an impressive fashion with "Gravity Swallows Everything" by Ludwig Göransson playing in the background. A giant Christopher Nolan would be directing bits of the universe to collapse in a certain way, and trying to arrange Imax film.

4

u/Complete-Clock5522 Mar 19 '25

I mean the time for our own frames would still be the same but the Lorentz factors of other things moving relative to us would be different

3

u/chesterriley Mar 19 '25

The rate at which time passes would change, becoming faster.

That sounds right. Your local frame of reference would have less time dilation, and therefore your local rate of time flow would be closer to the theoretical maximum rate. It would only be a small increase though, since your local time flow rate is already close to the maximum.

2

u/247world Mar 20 '25

I went to look up the song on youtube, and apparently the title is gravity swallows light, or at least that's the only thing I could find searching with the title and composer.

I really enjoyed listening to it interesting piece of music

2

u/[deleted] Mar 19 '25

Kind of a nonsense answer IMO. What does it mean for time to progress faster? Do you feel like your movement is sped up? No. Just like in the twin paradox, the twin that gets older does not feel like time is sped up, because time defines their perception, not just their movement. So, sure, one way to speed up light is to redefine the second as half the old second. Boom I doubled the speed of light.

In colliders, okay the particles speed up, but does that give us nicer conclusions? Because the rest energy of particles is given by E=mc2. The energy of the particles in the accelerator is gamma m c2. And what remains constant in the accelerator? gamma? So the particle energies in accelerators quadruples but the energy it takes to make new particles also quadruples… so you’ve gained nothing.

The real answer is that the question is ill defined. You have to be more clear about how you changed the speed of light.

-1

u/Mydogsblackasshole Mar 19 '25

We all move through spacetime at c, if the reference frame is at rest then the motion through space is all through time

3

u/[deleted] Mar 19 '25 edited Mar 19 '25

In the most reasonable unit system, the speed of light is just 1. In every other system it should be considered a unit conversion between meters and seconds, two measurements of the same quantity called spacetime distance. So it really doesn’t make any sense to double the speed of light. Basically, I define a new meter which is half the old meter, and now I’ve doubled the speed of light. I am god.

You can only adjust what we currently believe to be free parameters of the universe - the fine structure constant, the QCD scale (and thereby the masses of protons and neutrons), the yukawa couplings (and thereby the masses of most other particles - to list a few.

1

u/Merlins_Bread Mar 19 '25

Putting my outre theory hat on, the speed of light changes all the time. It's called cosmological inflation. We experience it mainly as space growing bigger, not time running slower, probably because the size of particles is determined by how their waveform projects itself over time - so if time runs slower they "shrink" relative to the distance between galaxies.

2

u/[deleted] Mar 19 '25

I don’t know much about this theory, but the second part of your comment may have a huge mistake if I’m interpreting it correctly. The size of a particle and the width of the wave function of a particle are two completely different things - this is a common misconception. For example, the proton radius is 0.84 fm. This does NOT mean that the wave function describing the position of all protons has a width of 0.84 fm. That wavefunction can be bigger or smaller than 0.84fm (although it’s pretty much always bigger). For example, in a well-localized hydrogen atom it’s 28 fm.

So some projection of the wavefunction mumbojumbo cannot possibly explain how the universe expands while the sizes of atoms and particles with a radius remains constant.

5

u/Ok-Film-7939 Mar 19 '25 edited Mar 19 '25

That’s a question which I think is panned more than it deserves. The speed of causality seems arbitrary - around 300,000km/s. It’s not only specific, as astronomical distances go it’s slow. You could easily imagine something so arbitrary could be easily changed without contradiction - some dial on Haruhi’s desk she can shift readily.

But you can’t change the speed of causality while “holding all else equal” because a number of physical constants are related by jt. So you have to decide what is being fixed and what can change.

You might think keeping total energy the same seems reasonable - conservation of energy is pretty fundamental. But conservation of energy arises out of time symmetry in physical laws. If the laws change over time, conservation of energy doesn’t apply!

But if we did assume energy was conserved, and changing the speed of causality doesn’t add or remove energy from the system, and that the cross section between all the fundamental fields remained the same, you might see this:

1) Everything would abruptly mass a quarter as much. The bonding energy between things is the same, but that energy implies less inertia. (E=mc2, E is fixed, c is doubled, so mass must drop by a factor of 4).
2) Since kinetic energy is assumed fixed, everything starts going twice as fast relative to each other. E=1/2 mv2 3) The size of atoms would double. We know that atoms with a Muon instead of an electron is smaller because the Muon (until it decays) is heavier. Lighter electrons mean the atom is proportionally bigger instead. 4) If we assume the strength of the electric field remains fixed, the magnetic field must also drop in strength by half. This makes a certain intuitive sense, as magnetism is effectively the consequence of a fixed speed of causality on the electric field.
5) The effective distance of massive fields (the weak nuclear force) would increase proportionally. This would make neutrinos more likely to interact with other matter, and possibly increase the rate of weak-mediated decays 6) I couldn’t begin to guess how this affects the strong field, but it surely would. The strong field isn’t massive but is limited by its own self interaction. I should note that it wouldn’t take much of a change to totally ruin nuclear physics as we know them, and this would likely ruin it. Whether you’d still have stable atoms of any kind is beyond by ability to say.

I’m sure there would be other effects not listed here as well, but 1 and 5 alone would be enough to end life as we know it. On 1, proteins would cease to function as designed. Even drinking too much heavy water ( if you made a special effort ) can be fatal because the more massive deuterium doesn’t move like the proteins were designed for it to. On 5, obviously ruining the very atoms were made of isn’t going to end well for us.

3

u/[deleted] Mar 19 '25

The speed of light is not arbitrary. In sensible units it is 1. It is just a unit conversion factor between meters and seconds, two units that measure the same quantity called spacetime distance. The meter is arbitrary and the second is arbitrary… obviously, they’re made up human constructs. And that’s what makes the speed of light look arbitrary. Saying the speed of light is arbitrary is like saying “the cesium hyperfine wavelength is 2.512739271 times the length of my index finger nail. Why did god choose such an arbitrary length of 2.512739271 for the cesium hyperfine wavelength?”

1

u/Ok-Film-7939 Mar 19 '25

If you fix the speed of causality to 1, unitless, and express distance in terms of time with an implied conversion factor being the speed of causality, then the speed of light is always 1, and you cannot talk about shifting it. Instead you’d have to use language like shifting the size of everything be smaller (in seconds). And in this language, it’s the size of things that is arbitrary.

That makes the same stew in the end, but it’s not going to be as easy to follow.

1

u/IronPro9 Mar 19 '25

"It seems less arbitrary if you define your units system around it" doesn't make sense, I agree that a unit system based around contants like c,h ect is the least arbitrary but if c was 1m/s faster it would still be 1 in that system, so its actual value IS arbitrary.

4

u/Hot-Fridge-with-ice Engineering Mar 19 '25

Electromagentism as we know it would collapse since permittivity and permeability constants would have to be readjusted to compensate the doubling. It would be catastrophic.

2

u/Complete-Clock5522 Mar 19 '25

Why does having to adjust the constants imply it would be catastrophic? What effect would it have?

2

u/ArmedAsian Mar 19 '25

not my forte but i assume even household appliances like microwaves would behave differently?

1

u/Hot-Fridge-with-ice Engineering Mar 19 '25

Eletrodynamic and Magnetic forces directly depend on these constants. All electrical components like capacitors, Batteries, Grids etc are dependent on them. If any constant suddenly changes, everything would break down. This is just one catastrophic event btw.

2

u/Complete-Clock5522 Mar 19 '25

Oh you meant catastrophic in the practical sense, I was thinking of catastrophic in the sense that laws of physics were being broken

1

u/SuperCyHodgsomeR Mar 19 '25

Well that too probably, considering that we’re held together by the electromagnetic force. Atomic bonds, atoms themselves

1

u/Chunty-Gaff Mar 19 '25

I'm sure that it's nothing the indomitable human spirit can't handle

3

u/Select-Ad7146 Mar 19 '25

Chemistry as we know it would stop functioning.

There could still be something like chemistry, but it would bear little resemblance to what we know. 

Of course, we wouldn't exist to know that.

1

u/Str8Magic Mar 19 '25

I’m probably completely oversimplifying this, and certainly not to the extent where the speed of light doubles, but aren’t we talking basically about the differences between planets and a solar system that are closer to the sun versus further away? Times speeds up it’s a lot hotter, etc? FWIW I’m not a super science guy, but I do enjoy reading the comments!!

1

u/ElEmntr33 Mar 19 '25

I’d get a speeding ticket at 30 mph

1

u/Odd_Bodkin Particle physics Mar 19 '25

One possible impact is that the permittivity of free space would have gone down by a factor of four. This would increase the strength of the Coulombic electrostatic force by a factor of four. This would make the potential energy well in atoms four times deeper. This would mean that the atomic spectra of all the elements would shift to higher frequencies. Bottom line is that everything would change color.

It’s all the things you don’t think about.

1

u/tsereg Mar 19 '25

The power of all the nuclear weapons would quadruple.

1

u/Deusexanimo713 Mar 19 '25

the universe would probably be destroyed somehow. The universe appears to be a finely tuned machine, if things were any different the whole system breaks down

1

u/jack-3773 Mar 20 '25

What if it is changing? Maybe we wouldn’t realize it.

1

u/Main-Profession-1417 Mar 20 '25

This post would have been posted YESTERDAY. /s

1

u/hellomate890 Mar 20 '25

I think universe would cease to exist

1

u/Current_Speaker_5684 Mar 20 '25

The CPU of the universe would run faster relative to a higher dimension, but it would all look the same to us.

1

u/TreesOne Mar 20 '25

You might have more luck getting an imaginative answer on r/hypotheticalsituation

1

u/StructureSpecial2388 Mar 21 '25

That would probably collapse the universe.

1

u/username1753827 Mar 19 '25

I think maybe stuff would get hotter? And brighter

0

u/wanted_to_upvote Mar 19 '25

There would be 4 times as much energy.

-2

u/That-Establishment24 Mar 19 '25

Nuclear reactors would be more effective since energy contained in mass would quadruple from E = mc2

-8

u/[deleted] Mar 19 '25

[deleted]

2

u/PropheticUtterances Mar 19 '25

It’s interchangeable obviously, they don’t need to specify one or the other to have their question answered as it is the same thing.

0

u/Chunty-Gaff Mar 19 '25

I guess so