why does light have a finite speed

It has "that" value rather than any other value simply because of the design of human scaling, in other words or to specify, it is because of how meter and second is defined by us.

I think the question is why is the constant c have the value it hold independent of the units employed instead of some other value and that is as pointless as asking why any of the other fundamental constants of the universe happen to be their values.

Value of c is not independent of the units employed.
in vacuum c = 299,792,458 m/s

I think you're getting it confused with the idea that the speed of light is independent of a frame of reference.

Well, c is the net result after the interaction between massless particles/waves and the physical properties of a vacuum. Through certain materials, light travels at less than c.

There are a few other physical constants aside from c such as G, h, ε0, etc.

No light never travels at less than c, when travelling through denser mediums it just gets absorbed and re-emitted so when "bouncing" around between particles it looks like its traveling at less than c.

Ok then how can light be " pure" energy and have mass. Btw i could be wrong but i think that in quantum mechanics photons are the smallest possible ammount of light energy, also i heard that they have a relativistic mass, which is the mass they would have if their energy were to be converted to mass. So from what i heard they dont have mass.

So what defines its speed, by what mechanic does it move.

I think i can understand that, but could you possibly elaborate?

Yes indeed C is independent of the units employed .........C is not faster or slower whatever the units that are used to express it.

You need to read a few books on the subject as energy and mass is just two aspects of the same thing as in e=mc^2.

Second comment, light had mass and can be bend in a G field it is call gravity lensing.

Ok if i am wrong could you give me something definitive or quote something because i had also heard that light actually travels straight relative to space time and isn't actually "attracted" via gravity but rather space-time its self bends and as a result light appears to bend but actually travels straight along space-time.

Also what books do you suggest i read? bare in mind none of my comments are meant to be insulting or confrontational or sarcastic, I mean them as genuine questions.

I can understand that, but to me working at the limits of causal explanation is where it is most interesting so even if there isnt a definitive answer it wont stop me askingt he question and it wonts stop me being interested in what ever answer i get.

I gave you the links to the Einstein books that can be freely downloaded off the web and any library will have college level books on the subject that are more up to date.

Could answer the question regarding space-time aswell?

The propagation of its electro-magnetic field over time and space. At time t1 and position p1, you turn on a light source, which emits a rapidly-oscillating electro-magnetic wave. (Remember, that's what light is: an electro-magnetic wave.) Sooner or later, the front of this wave reaches a photodetector. The detector, sensitive to the energy of that field, will detect a rise in this energy at time t2. The speed of light, then, is defined as (p2-p1) / (t2-t1).


Broadly speaking, light moves by a circular process in which a rapidly-oscillating magnetic field induces an electrical field (by virtue of Ampère's law). In turn, this electrical field, oscillating just as rapidly, feeds back into the energy of the magnetic field (by virtue of Faraday's law). The induced fields will arise in a slightly different location as the fields that induced them, and that's how the wavefront moves.

The details of this mechanic are too complicated, technical, and mathematical for a post in an online community. To learn more about them, I suggest you search the web for Maxwell's Equations. Or you could go the whole distance by studying a textbook on electricity and magnetism.

Interesting, the problem is there is just so much and its so daunting, what to read and what not. I am only in sixth form, ( between school and university) do you think its a good idea to start seriously reading now or to wait until im doing this stuff uni.

Read now ! I recommend Brian Cox and Jeff Forshaw:
The Quantum Universe: Everything that can happen does happen.

You have the questions now so why not begin to look at what is known of the field and begin picking up what you can.

It never hurt me to know something of a subject before running into it during a college course and without more of an understanding and background even if the other posters here was completely qualify to explain the subject and I do not think any of us are so quality you at the moment do not have the background.

Oh boy, how can I be any clearer? Sure c is a constant in our given standard unit of measurement, however, the velocity measured, which is a numerical value, is directly dependent to the standard system of units used to measure it. If the standard unit of measurement was defined any differently, the value we have assigned it, to describe the speed of light, will be different. I think that was what dale was asking: why speed of light in vacuum is 299,792,458 m/s and not 100,000,000 x/y. Thus as to why in a vacuum, we say the speed of light is 299,792,458 m/s, and not some other value. Change the standard unit values, you change the c "value" used to describe the velocity of light. The digits in the numbers that make up c is then directly dependent of the standard units that are employed. For c to be truly independent of the units employed it has to be a dimensionless physical constant. If speed of light was truly independent of units employed, then c will have the same value whether I use m/s or ft/in or m/hr, etc...
It is also nonsense to say that the speed of light is now constant just because the SI definitions of units define its numerical value to be constant, because speed of light itself would have different values according to the definition of whatever units you used.