# Explaining to an 8-year-old why nothing can travel faster than light

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 explain this in my answer to What is so special about speed of light in vacuum?, though I suspect that answer might be a bit technical for the non-physicist and it will certainly be beyond an eight year old. – John Rennie Oct 19 '15 at 19:21
• "It's not just a good idea; it's the law." – Daniel Griscom Oct 19 '15 at 21:44
• Nobody knows "why". We know experimentally that it is true, and we have mathematical models that provide the mechanism ("how"), but "why" is a philosophical concept, not a scientific one. – m4r35n357 Oct 20 '15 at 21:04
• The question could maybe be formulated as: What could an intuitive explanation of the model sound like so that an 8 year old can understand it? – someName Oct 20 '15 at 21:48

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.

• That's actually the second postulate of special relativity that the speed of light is invariant. To explain it to a child, you can ask yourself, how you can accelerate something as fast as possible and you end up at CERN and the relativistic mass. But nothing explains WHY it is like this. – Timeless Nov 10 '15 at 17:43

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

• To put it as mildly as I can: there is nothing like an 8-year child's question to show adults' limits of knowledge. Less mildly: to me, none of the answers I read makes sense, and not for an 8-year child alone. The only answer I find reasonable is: just because. This is what all our experiments show. The more you try to explain, the worse it gets. I would also add: what makes you believe there is an answer an 8-year child could understand? – Elio Fabri Oct 19 '18 at 14:06
• @ElioFabri A question has an answer and the answer doesn't show up according to age,of course.What matters is how we explain the very complicated in a way as to be understood by the questioner considering the level of understanding the questioner has.Since our questioner is an 8-year old, we have to avoid mathematical explanations and put it in words and not include any technical terms.I gave a thought and what I've given as my answer (last paragraph only)popped into my mind. – ayc Dec 29 '18 at 17:14

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.

As summary: When you accelerated the object to reach to the Speed of Light, the used energy for acceleration is amplifying the amplitudes (at quantum level of substance) of substance vibrations. This amplified vibration causes more resistance. (As the Doppler Effect of the Time as waves) https://www.quora.com/How-is-it-impossible-to-travel-faster-than-the-speed-of-light/answer/Burtay-Mutlu

• -1 This is not at all a mainstream physics answer, but rather pseudoscience. – ahemmetter Oct 19 '18 at 13:11
• Yes, it is not a mainstream answer. Because of that, you decide without thinking and falsifying. Epur si muove. – Burtay Mutlu Oct 19 '18 at 14:01
• Have a look at the physics.SE guidelines regarding non-mainstream physics. – ahemmetter Oct 19 '18 at 14:04
• There is only one major weakness, the Time is accepted as a vector magnitude contrary to mainstream. – Burtay Mutlu Oct 19 '18 at 14:25
• Is there any satisfying answer between uppers? There should be same mechanism behind existence of mass, relavistic mass, momentum, inertia, light speed limiting and time dilation. Because all of that are related to the motion. You can not use different point of views for the same case. Motion is motion. The background mechanism must be same. In this view, I can explain by using some animations. But this site doesn't allow all of them. So, I've linked the answer. – Burtay Mutlu Oct 19 '18 at 14:49