# Why can light travel at the speed c?

#### mibaokula

A massless photon simply means that all frequencies of light travel at the same speed. The equation is not strictly valid for massless particles, although it is suggestive that such particles can exist.
yeah, but the second equation explains massless particles, right?

#### atyy

yeah, but the second equation explains massless particles, right?
The equation for massless particles is 1/wavelength=constantXfrequency.

#### mibaokula

That's it. You need to use this more general equation, which works for all particles, including massless photons:

$$E^2 = mc^4 + p^2c^2$$

(p is the momentum of the particle.)
i meant the equation which i have now attached in this post

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#### ghwellsjr

Gold Member
try typing 1/0 into a calculator - its undefined. if you think about it this way, even if you had an infinite number of "zeros" you cannot get 1 - hence its not infinity but undefined.

likewise with 0/0, you might think that the answer is infinity as the other guys said there are many answers - infinite answers not just infinity. 1 x 0 is 0. 2 x 0 is 0. 3 x 0 is 0 and so on using this same logic.

so back to my original question, although its been answered, using that particular equation, photons must have an undefined amount of energy
What does your calculator say if you type in 0/0? What does it say if you take the square root of -1?

0/0 is not infinite, it is indeterminate because it can be anything.

#### mibaokula

i just realised i misunderstood your point. i think we meant the same thing. 0/0 is indeterminate because it has an infinite number of answers - i.e. any real number. what i meant to say is that it is not enough to say the answer is 1, 0, or infinity

#### ghwellsjr

Gold Member
i just realised i misunderstood your point. i think we meant the same thing. 0/0 is indeterminate because it has an infinite number of answers - i.e. any real number. what i meant to say is that it is not enough to say the answer is 1, 0, or infinity
If you just ask the question, what is 0/0, there are an infinite number of answers, but that doesn't mean that your equation for the energy of a massless particle has an infinite number of answers, it just means that you need to determine the answer some other way.

#### mibaokula

in other words, that equation doesn't fully describe light (which has no mass)
but would it describe tachyons with negative/imaginary mass (assuming they exist - which i really don't believe at the moment)?