It is a well known fact, that nothing can travel faster than light. I have discussed that fun fact numerous times with my 8 year old son. One day he asked "why?", and I realized that I didn't know the answer myself. So... Why is the speed of light the upper speed limit of everything?
Can it be explained in a way that an 8 year old can understand?
I always thought, that it is actually the other way around. There is a maximum speed limit in our universe. This maximum speed limit of causality is necessary in order to have the world as we have it. So in principle it is possible to have a world with infinite speed, it's just not ours and it would look completely different than ours. And in our world with an upper limit of the speed of causality everything which doesn't have a mass can travel as fast as this limit. It is true therefore that the speed of light is the maximum speed in our universe. However, this is because there is an upper speed limit and the photons have no mass which allows them to travel that fast.
Can it be explained in a way that an 8 year old can understand?
First demonstrate that there exist limits to velocities in various situations.
Take a floating balloon and start adding hanging weights. For a fixed weight there is a terminal velocity, dependent of the size and weight of the balloon-weight system. This can be explained by the reaction of the air to the force of the weight .
Then explain the elementary particle concept, it is elementary particles that cannot get a speed greater than the speed of light. Particularly that light is made up of photons, and the photon is also an elementary particle. All matter is made up by these elementary particles and therefore no matter can go faster than c.
Explain that this limit is an observation , a measurement, and it has been modeled/described by special mathematical descriptions. For mathematical descriptions one can show triangles and other geometrical figures, a maize for example constrains motion and the constraint can be described by geometrical figures.
Go back to the balloon. We know that there is a limiting velocity because of the air and the forces . The elementary particle, if pushed/accelerated finds a resistance, not from air but from space itself. Analogous to the terminal velocity of the balloon the faster it is pushed the more resistance it finds from the intrinsic structure of the space itself. The terminal velocity for the space and time we live in is the value of c, the velocity of light in empty space.
I'm basically explaining without any mathematics.
Because of the equivalence of energy and mass,the energy which an object has due to its motion will add to its mass.In other words,it will make it harder to increase its speed.As an object approaches the speed of light ,its mass rises quickly,so it takes more and more energy to speed it up further.It can never in fact reach the speed of light,because by then its mass would have become infinite,and by the equivalence of mass and energy ,it would have taken an infinite amount of energy to get it there.For this reason any normal object is confined to move at speeds lower than the speed of light .only light or other waves that have no intrinsic mass,can move at the speed of light.
How to explain it to a eight year old?
You can follow the way given by anna v
or,
To put it very much simply and in a funny way:just tell him that you get fatter and fatter if you move faster and faster and hence you wont be able to go any faster where as Light doesn't get fatter and that's why it is the fastest and you can't beat it
I would say there are 2 paths for explanations:
for rockets-like problems you have to go into the relativity stuff, with the contraction of time and length which are just a fact. From that, it's explain everything, from paradoxs, differences from inside and outside, and extra cost of the last "pushes". (just saying alone that puffs get costlier and cause a maximum speed is not far from a lie, as is it a consequence and not a cause).
for energy (signal, waves, information), the fact is that lightspeed is not a property of light but a property of vaccuum (i.e. spacetime medium), quite the same way that speed of waves on water surface or speed of sound is a property of the medium, not the emitter, related to the balance of inertia and "springness" (e.g. compressibility). (ok we are no longer in the age of ether, so it's a bit more complicated for vacuum since the medium we speak about here is with no preferred referential).
I think a good way to approach this is as follows: An object moving at some finite speed needs to have a speed relative to something. You can't say you are moving at 3m/s without saying what you're moving relative to. This is not the case with the speed of light. When something moves at the speed of light it does so relative to everything. The resaon for this has to do with the rate at which clocks tick. It makes no sense to say my clock ticks at 3s/s. That statement alone is nonsense, but you can say "My clock ticks at 3s per second of your clock". According to Special Relativity, any relative speed can be assigned a relative rate at which two clocks are observed to tick. The reason the speed of light is the same relative to everyone is because something moving at the speed of light has a clock that does not tick. It does make sense to say "My clock ticks at zero s/s", you don't need an external reference frame for this because a clock that does not tick doesn't tick relative to every other clock (it's like saying 'this thing has a height of zero"; you don't need to specify meters or feet or light-years, because if the height is zero, it's zero in any unit. The reason you can't have a relative velocity greater than the speed of light is because a clock can't tick less than not ticking at all (you can't be smaller than zero).
If we think about the expansion of the Universe, some galaxies are moving away from us at greater than the speed of light because the space between us is expanding. But this expansion is such that the mathematical description of it does not say that the expansion causes either one of our clocks to stop ticking. Therefore, the physical phenomenon of the recession velocities of galaxies is different than the relative velocity descriptions of Special Relativity. In Special Relativity, the statement that something is moving at the speed of light is equivalent to the statement that that object's clock doesn't tick (and a clock that does tick can never be made to stop ticking without an infinite amount of energy). For the expansion of the Universe, the recession velocities arise from a description of an expansion of spacetime, and this expansion does not have a direct effect on the clocks of the galaxies (i.e. the expansion cannot make a ticking clock stop ticking)
The Michelson-Morley experiment, which should have showed interference patterns when turned different ways relative to Earth's orbit (which would change the velocity of light exiting the source). It didn't. This logical conclusion, per Einstein, is that the speed of light is constant. Even if the source is moving forward when the light it released, light will go no faster. This must mean that time slows down in proportion to velocity (the Lorentz transformations). At the same time, mass increases. At the speed of light, everything would have infinite mass and require infinite power to go faster. So far, so good. Everything is consistent. The problem comes in, according to Michio Kaku, when we try to reconcile quantum theory with gravity. Then the equations go to Hell, so we don't know it all yet.
Son, when you throw your baseball something happens to it. We say it gets boosted. How much was it boosted by? Gosh, how do you measure boost? Let's try the ball moved 10 meters in 1 sec, so it was boosted by 10 m/sec. Ok, you ride your bike at 5 m/sec and you throw the ball forward at 10 m/sec, and I see the ball go by me at 15 m/sec. Now you ride a rocket going at $\frac{3}{4}c$ and from its nose launch another rocket at $\frac{3}{4}c$. Surprisingly, I see the second rocket pass me at $\lt c$, not $\frac{3}{2}c$. We conclude, that the way we decided to measure boost (ie: m/sec) necessitated the introduction of a maximum boost c.
Let's try some different ways to measure boost. As you ride away from me on your bicycle, you point a laser back at me. The laser whose frequency is $\nu_1$ appears redder (ie: lower in frequency by $\delta\nu_1$). This redshift ($z_1=-\frac{\delta\nu_1}{\nu_1}$) is like the lowering of pitch of a train's whistle as it moves away from you. The ball you throw also has a laser on it and it's boost is $z_2$. Now I see the ball's boost is $z_3$ where experimentally $(1+z_3)=(1+z_1)(1+z_2)$. For $z\ll 1$ these boosts are approximately additive and there is no maximum boost in this way of measuring!
We can make these same direction boosts exactly additive by defining $\lambda=\ln(1+z)$. Now $\lambda_3=\lambda_1+\lambda_2$ and there are no limits ($-\infty\lt\lambda\lt+\infty$). The $\lambda$ is called the Lorentz Boost parameter which leads to the Lorentz Group, non-commuting boosts, and a story for another night.
Why is the speed of light the upper speed limit of everything?
Any answer to such a "Why?" question (if there is one) can only depend on the exact meanings of the terms used in phrasing the question. Therefore, more incisive questions to consider first would be
What exactly do we even mean by "speed (of something)"?, or more pointedly: How exactly would we go about measuring "speed (of something)", in thought-experimental principle?, and
What exactly do we mean by "light" (in the given context), in distinction to "some thing", or "any thing"?
Now, an important insight to convey is that, when discussing the basics of the theory of relativity, by "light (being transmitted (in vacuum))" we specificly mean "(the propagation of) a signal front".
This means in simple words that may well be digestible even by very young students:
If two things (let's call them $A$ and $B$) had met each other, and then $B$ took off from $A$ and went on to meet yet another thing, say $Z$, and if $Z$ would really like to know as soon as possible whether $B$ had had a meeting with $A$ before meeting $Z$, or not, then $Z$ can find that out at the very latest by asking $B$ directly when they meet, i.e. when $B$ has completed its trip travelling from $A$ to $Z$; and that's what $Z$ would have to do unless $Z$ found out even earlier, by some "quicker way of signalling", that $A$ and $B$ had met.
The signal front of the meeting between $A$ and $B$ is reaching $Z$ necessarily at least as quick as $B$ travelling from $A$ to $Z$; or even quicker -- that's exactly what we mean by the "front" of the signal; and that's in turn exactly what's meant by "light propagating in vacuum" (to be precise) when it is said that "nothing can travel faster than light (in vacuum)".
(To derive the even stronger statement, "nothing can even travel as fast as light (in vacuum)", requires addressing the more advanced questions
How do we meaasure whether two things, such as $A$ and $Z$, had been at rest wrt. each other?,
How do we compare "distances" between pairs of things which had been at rest wrt. each other?, ...
and finally
Speed equals space (for example meters) divided by time (for example seconds). Since time stops (0) when one moves at a speed of light them meters divided by 0 equals infinity which is a limit.
