Discussion Forum

What Limits the Speed of Light in a Vacuum?

What is the physical law or property that limits the speed of light to 186,282.397 miles per second in a vacuum? Why can’t it go faster? I’m aware of things like Group Velocity and Einstein’s velocity addition formula, but those things don’t explain the phyiscal limiting property. Does it have to do with the way energy works on the quantum level?


  • It’s not that light speed is limited, but that distance is measured by reference to light. Ie a metre is the distance travelled by light in some fraction of a second. Of course a second is defined with reference to other natural phenomenon and so on.

  • I know this is late but better late than never.. Lights speed limit going thru space is a result of time . D/T=S a simple fractional equation . relativity is a point of view in this case the speed depends on your point of view . From our point of view we see light traveling at 186,000 miles per second but if you could ride on the beam of light because of your speed time would contract to zero your clock would stop . Now take D/T=S distance over time = speed when you put zero in for time you get infinety for speed and this is how the beam of light sees it . While we see the go by us at 186,000 miles per second because of the speed our clock (time) is running differently we see it differently But the beam of light thinks it is traveling at infinety so the speed limit until we can figure out how to alter time and reverse it…. I hope this helps…

  • Dr J’s response restates the question: why is it not 123 instead of 176,282,297? Or some other number? what causes the speed to be what it is?

    I’d encourage speculation on this question. I don’t think accepting it moves us along in our understanding of the universe.

  • The best way to think about it is to accept it as a fact; the speed of light is a universal physical constant, demonstarted by experiment. And then enjoy thinking about all interesting consequences of it like relativity of time.

  • Good question, but I do not believe that science will be able to answer that. Think about the following; why is it 186,282.397 and not 123?

    A more interesting question could be: is the speed of light constant or does it change with time?

  • I don’t think we can answer this question. it’s like asking why the gravitational constant of the universe is what it is. If it were different, the universe would be radically different. perhaps there is some universe somewhere with a different speed of light, perhaps not. Science has trouble answering “Why”

  • The speed of light in a vacuum is a basic property of the universe. It was derived theoretically by Maxwell. Here is a quote from wikipedia:

    The greatest work of Maxwell’s life was devoted to electricity. Maxwell’s most important contribution was the extension and mathematical formulation of earlier work on electricity and magnetism by Michael Faraday, André-Marie Ampère, and others into a linked set of differential equations (originally, 20 equations in 20 variables, later re-expressed in quaternion and vector-based notations). These equations, which are now collectively known as Maxwell’s equations (or occasionally, “Maxwell’s Wonderful Equations”), were first presented to the Royal Society in 1864, and together describe the behaviour of both the electric and magnetic fields, as well as their interactions with matter.

    Furthermore, Maxwell showed that the equations predict waves of oscillating electric and magnetic fields that travel through empty space at a speed that could be predicted from simple electrical experiments—using the data available at the time, Maxwell obtained a velocity of 310,740,000 m/s. Maxwell (1865) wrote:

    This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.

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