# Light Momentum - Does it Make Sense or Not?

• I
• bob012345
In summary, the speed of light is not constant, and changing its value would result in contradictions.
bob012345
Gold Member
TL;DR Summary
Momentum of Light is weird.
Light is a funny thing. If it could move slower it would have more momentum, not less! How weird is that? Or is that all wrong? I would appreciate comments on that.

bob012345 said:
If it could move slower it would have more momentum, not less!

What are you basing this on?

Momentum of light is Energy/c. If c were smaller, momentum is larger.

Dale
In sensible units, ##c=1## and the magnitude of light's momentum is equal to its energy. ##c## isn't best thought of as the speed of light. It's just the natural conversion factor between units of time and distance. It's a property of massless objects in spacetimes with a Lorentz signature that they travel at that speed.

That's a longwinded way of saying "you can't change ##c##". Research reported as considering a changing ##c## is generally actually considering a changing fine structure constant.

bob012345 said:
Momentum of light is Energy/c. If c were smaller, momentum is larger.
##c## can’t be smaller because it’s a constant. What you’ve found is that if the energy is smaller then so is the momentum, and vice versa. That’s not such a strange result.

Ibix said:
Research reported as considering a changing ##c## is generally actually considering a changing fine structure constant.

And, just to be clear, doing that would not change the relationship bewteen the momentum and energy of light. It would only change, for example, how much energy would be contained in light emitted from a particular physical process, such as a laser using a particular kind of atom.

Ibix
So, in a hypothetical universe where c has half it's value in our universe, assuming we could construct units of the same values when compared in both universes wouldn't light contain twice as much momentum per unit energy? And relativistic massive particles would also have more momentum per unit energy I think.

bob012345 said:
So, in a hypothetical universe where c has half it's value in our universe, assuming we could construct units of the same values when compared in both universes wouldn't light contain twice as much momentum per unit energy? And relativistic massive particles would also have more momentum per unit energy I think.
How do you plan to compare dimensionful values for sameness when they arise in different universes?

If you change the dimensionless fine structure constant while holding all other dimensionless constants unchanged then the question could be sensible (and the answer likely a tautology arising from the chosen system of natural units). As has been suggested already.

Last edited:
bob012345 said:
Summary: Momentum of Light is weird.

Light is a funny thing. If it could move slower it would have more momentum, not less! How weird is that? Or is that all wrong? I would appreciate comments on that.

In special relativity, the speed of light is assumed to be constant. In most cases (as in Einstein's formulation of the theory), this is by postulate, though occasionally different postulates are used and the result is derived rather than assumed.

If one assumes that the speed of light also can change while still assuming it remains constant, one can indeed get logical contradictions. The solution to this is fairly simple - don't assume that the speed of light is both constant, and also able to change, at the same time.

nasu
bob012345 said:
in a hypothetical universe where c has half it's value in our universe

This universe could be our actual universe with standard meter sticks twice as long as the current ones. And in that case your claim would be false: light would have exactly the same momentum per unit of energy that it has now. The specific numbers associated with that momentum and energy would be different, but that wouldn't change any of the actual physics. You can't change physics by changing units.

jbriggs444
PeterDonis said:
You can't change physics by changing units.

And with that, which is basically the short answer to the OP's question, this thread is closed.

## 1. What is light momentum?

Light momentum refers to the momentum carried by photons, which are particles of light. Momentum is a property of moving objects and is defined as the product of an object's mass and velocity.

## 2. Does light have momentum?

Yes, light does have momentum. This was first proposed by physicist Isaac Newton in the 17th century and has been confirmed through various experiments, including the famous photoelectric effect experiment by Albert Einstein.

## 3. How is light momentum measured?

Light momentum is measured using the formula p = E/c, where p is momentum, E is energy, and c is the speed of light. This formula is derived from Einstein's famous equation, E=mc², which relates energy and mass. By measuring the energy and speed of light, we can calculate its momentum.

## 4. Does light momentum follow the same laws as other types of momentum?

Yes, light momentum follows the same laws as other types of momentum. This means that it obeys the law of conservation of momentum, which states that in a closed system, the total momentum remains constant. This has been confirmed through experiments such as the Compton scattering experiment.

## 5. What are the practical applications of light momentum?

Light momentum has several practical applications, including in solar sails, which use the momentum of photons from the sun to propel spacecraft. It is also important in understanding the behavior of light in various optical devices, such as lenses and mirrors. Additionally, the study of light momentum has led to advancements in quantum mechanics and our understanding of the fundamental nature of light.

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