Q: Why is the speed of light finite?

Physicist: That is such a hard question.  Holy crap.

If you kept the laws of the universe the way they are, but ramped up the speed of light to infinity you’d end up with a surprising array of effects.  Newton would have been right about a lot more (nicely done, old dude), there would be no magnets of any kind, the amount of energy tied up in matter would also be infinite (E=MC2) so you’d have to be extra careful not to bring it near anti-matter, but not too careful because anti-particles probably wouldn’t exist (probably).  Also, all the weirdness of relativity would be out the window.

But, why is the speed of light finite?  I don’t know.  I think this is one of those culdesacs of science.  It is what it is.

The question, as it was originally asked, was about what keeps light from going any faster.  The answer to that question is that there is no faster.  If you shove a stone of mass X and it goes flying off at speed V, then if you shove a stone of mass X/2 it’ll fly off at speed 2V.  So, you might suspect that if you shove a stone of zero mass that it would go flying off at an infinite speed.

Well, that’s pretty much what photons (which have zero mass) do.  If you think of infinite speed as how fast you’d be going if you accelerated forever, then the speed of light is exactly that.  If you got into a rocket that could accelerate forever (using some kind magic fuel, such as the Schwartz), and you let it run for an eternity of two, then you’d be moving at the speed of light.

So it’s not that there’s anything slowing light down, so much as the laws of the universe are such that it doesn’t really make sense to talk about something moving faster.  More here:

Q: What’s it like when you travel at the speed of light?

Q: Why is the speed of light the fastest speed?  What makes light so special?

Also, if you’d like to find more “culdesacs of science” get yourself a toddler during their “Why?” phase, and try explaining something to them.

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16 Responses to Q: Why is the speed of light finite?

  1. Donal McBrien says:

    Out of the many, many questions I’ve seen on this site, that’s the best one yet, well done mate.

    Such an obvious question that nobody has thought to ask it, plus a great answer.

  2. Joel Emanuel Carlstrom says:

    Basically its like this..
    if your body stands still in space and has no contact with gravity you will be going in the speed of light ( in time ), but as you start traveling in space you have to put that energy into your movement in space, which will lead to that you go slower in time.

    The max amount of energy information has is the speed of light, therefor it is impossible for information to travel faster than the speed of light.

    But there are ways inwhich noninformation can travel faster than the speed of light, and also ( if i remember the name correctly which i probably dont ) the speed of impact which you can find in a sissor is faster than the speed of light…

  3. Nainan says:

    Linear speed of light is finite and constant, because that is the highest linear speed at which the agency that moves light (universal medium) can move it. Attempt to increase linear speed of light will increase its frequency rather than its linear speed. Attempt to lower linear speed of light will reduce its frequency rather than its linear speed. Speed of light is finite and constant for any region of space. It may vary from region to region of space.
    For details; http://vixra.org/abs/1103.0026

  4. The Physicist The Physicist says:

    There’s unfortunately no medium for light waves. That’s one of the major tenants of special relativity. The “Universal Medium” you’re talking about was experimentally disproven by the Michelson-Morley experiment (and a lot of follow up work).

  5. christian smith says:

    could there be something that travels faster than light that we have not discovered yet?

  6. The Physicist The Physicist says:

    Assuming that what we know about physics today is even a little accurate; no.
    There are all kinds of things out there yet to be discovered, but even things we don’t know about have to obey physical laws.

  7. Martin says:

    From it’s own frame, does light not travel infinitely quickly? I mean, if distance decreases in the direction of motion to the point that light doesn’t experience time because it always get to wherever it’s going instantaneously. Would we not consider this infinite speed?

  8. The Physicist The Physicist says:

    That’s pretty reasonable. If you’re going to define an infinite speed, the speed of light would be it!

  9. Paul W. Diaz says:

    Perhaps it it linked to the numbers of quantum states an object with no mass can assume, perhaps those states are finite and are all full so it may not matter how much more energy is applied, there’s no more quantum states to fill so a greater velocity cannot be achieved. Just a thought.

  10. Nairn says:

    I think I can supply an answer to this in two ways:

    (1) As light propagates, we have an oscillating electric field. This induces an oscillating magnetic one, which in turn induces an oscillating electric one, … and so on. The speed with which this can be done is dependent on the permittivity of space (the “resistance” to forming an electric field) and its permeability (the “resistance” to forming a magnetic field), and so we have a finite speed of propagation for light. Of course, this just shifts the question to why space is “gluey” to electric and magnetic fields.

    (2) Version 2 – I may be on shakier ground here, and would appreciate a comment from Mr/Ms Physicist. At a quantum level, space is filled with a foam of particles springing into existence for a short time, and then disappearing again. Since many of these (I believe) would be charged, this could slow down electromagnetic wave propagation. [This is all getting a bit hand-wavy].

    At a guess, then, the speed of light and the permittivity (epsilon nought) would depend somehow on Planck’s constant, h. [Treat this with a good deal of wariness]

  11. Just Some Guy says:

    Not that I want to hate on this answer, and I understand that quite honestly it is in many ways beyond the realm of current understanding of science but we can quantifiable measure just how fast the speed of light is. 299 792 458 m / s, Quite fast by any means but demonstrably not infinite speed, also the fact that we can slow it means that it is forced to interact with solid objects, the speed I just stated is the speed of light through a vacuum… Soo.. Essentially my qualm is, if the speed of light should be infinite by any logical reasoning… For example if I am a photon traveling at the speed of light and I add more energy I wouldn’t speed up due to what seems like an arbitrary barrier… But where would that energy go? Assuming I as the photon am a closed system and I am only having more energy continuously added what would occur? Essentially, since it should be possible to add energy to a object with zero mass it should go faster, theoretically it should require 0 mass and infinite energy to go infinite speed but from what I can tell from the laws of the speed of light it requires 0 mass and the energy doesn’t even come into it… Sorry, any sort of explanation would be welcome.

  12. John David Dunson says:

    i’m no physicist, but i’m certain they know about the quantum foam and take it into consideration when calculating the theoretical speed of light in a vacuum. i assume that when they say vacuum, they mean, a space with no quantum foam like the space between quarks in a hadron.

    @Just Some Guy
    part one: speed is relative. for us, it takes light eight seconds to reach earth from the sun, but for the photon, it didn’t take any time at all. it never even had time to realize it existed before it didn’t. if you were somehow able to accelerate to the speed of light, as far as you could tell, i think you would just instantly be dead. you wouldn’t be able to decide to slow down and see where you are because no time would ever pass for you, ever again. from your perspective, even if the universe exists for a hundred trillion more years, that time will pass for you instantly, assuming you don’t smash into anything. therefore, you wouldn’t perceive the time between when you reached the speed of light and when you ceased to exist.

    part two: adding or subtracting energy from light doesn’t change its speed, it changes its oscillating frequency. if you shoot purple (higher frequency) light at one thing and red (lower frequency) light at another, the first thing will heat up faster (demonstrating the fact that higher frequency equals higher energy).

  13. PHYSICATOR says:

    The speed of light being app. 3 x 10^8 ms-1, is one of the basic fundamentals on which the special theory of relativity stands on… science and its theories build on such fundamental postulates… so if this fundamental is to be in another “way”, then a new theory would exist , that again would explain our “observations”… In other words.. the laws of physics would still remain same, just the perspective changes!

  14. Xerenarcy says:

    i think you’re on the right track with both approaches; neither on its own really explains why the limit is what it is in full.

    my understanding of (2) is that virtual particles corresponding with the field appear, interact, and disappear, and that this repetitive behavior defines the properties of a field at any point, as well as its influence on anything moving through the field.

    note that c = 1 / sqrt( [permittivity] * [permeability] )

    to me this reads: the extent that / rate with which electromagnetic fields can geometrically change with respect to time.

    so basically the reason light has a finite speed is because it is an excitation of a field, which means points in space undergo a change in energy. a change in energy requires time, therefore light requires time to propagate (remember that the frame of reference of light is NOT an inertial frame of reference, so it is improper at the least to consider time from light’s point of view directly).

  15. George says:

    The speed of light is what it is because, just like a car engine or a jet engine or a rocket engine or a Higgs particle engine (yet to be constructed for interplanetary travel) is constrained by its propulsion system, just like all engines of motion. It been a self sustained through electro-magnetic interaction (one creating the other to drive it forward) determines its maximum speed. In other words, this interaction or self-sustaining motion causes the delay to the value of c (the speed of light). To gain a deeper understanding, one has to examine how this delay comes about. Implicit in this question is the notion, can we travel faster than the speed of light. Apparently, yes, and it is based on observable phenomena, such as, the age of the universe, 14 billion years old, since the big bang, to the size of the observable universe, about 46 billion light years in radius. So compare 14 billion to 46 billion and ask yourself, how is it so.

  16. Immelman says:

    The speed of light must be infinite. There is no such thing as absolute speed, or constant speed as used to describe light. There is only relative speed, and even then the closing or opening speed between two bodies cannot be attributed to either one in an absolute sense. This applies also to light.

    So measurements of light speed must be subjective. Relativity is subjective in that it indicates the observer’s state of motion as responsible for time effects that produce the universal speed c. But relativity produces endless paradoxes that cannot be reconciled.

    If you used a bathroom scale at a truck weighing facility, all vehicles would weigh 250 lbs. So it is when we measure the speed of light. Something in our physical mechanism only goes c, so we mistakenly attribute that speed to light.

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