Explanation of light speed limit

[Thecla]

TL;DR Summary

How to explain the limit of speed of light to average person

To an average person with high school math knowledge how would you explain in a few words why no object could travel faster than the speed of light ?
Well it's because...

 

[robphy]

….Experiment has suggested that is the case.… and when we built a theory around that, the theory makes predictions that are in agreement with further experiments designed to test that theory.

… and when we built theories around alternatives, they don’t seem to agree at the success rate of relativity.

It could be that the effects are not visible on the scales we test…
…but no alternative is as successful thus far.

 

[Ibix]

To paraphrase the above, there isn't a "why" for that. It's one of the founding assumptions of the theory of relativity. It's only justified because using the assumption produces accurate predictions and (despite active searching) we've never seen evidence to the contrary.

 

[Nugatory]

Well it's because...

Over the years I've tried...
1) "If we graph the kinetic energy of an object as a function of its speed, there's a vertical asymptote at $c$. " This worked with a high schooler who was finishing up Algebra II.
2) "If you could get from point A to point B faster than a beam of light, then there would be someone in the universe who would see your arrival before your departure - and would be able to prevent your departure after they had seen the arrival. " Useful only with an audience that is thinking it's a technological problem, that we need more powerful motors and better spaceships.
3) "The laws of electromagnetism predict, and experiments confirm, that the speed of light is the same for everyone regardless of their relative speed..." It's easy to show that this is incompatible with a relative speed greater than $c$ by considering a flash of light trying to catch up to something traveling faster than light.
4) "No how much force we use to speed something up, the speed increase gets smaller as it gets closer to the speed of light - diminishing returns means that we can't ever quite get there". As with #1, this is the enforcement mechanism for the arguments by contradiction in #2 and #3. The velocity addition formula can be used here if your audience is willing to accept it on faith.

All of these occupy the dubious ground between lies to children, flat out untruths, and grotesque oversimplification. But they all have worked for me at least once - it helps to know your audience. And of course none of these are really answers to the question, they're more different ways of saying "because them's the rules and we have the experiments to prove it"

But seeing as how we're talking about "an average person with high school math knowledge"... I wouldn't give up on just doing it right and deriving the Lorentz transformations.

 

[Mister T]

Summary:: How to explain the limit of speed of light to average person

No matter how fast you chase after a beam of light it will recede from you at speed $c$. You can never catch it. Thus you can never even achieve a speed of $c$ let alone exceed it.

The reason we require an explanation is because our experiences seem to indicate that we can always surpass any given speed if we have a strong enough engine. But the tens or hundreds of thousands of people involved in high energy research see evidence every day that there is a limit to our speed. It's just the way it is, and that is the real reason.

 

[MikeeMiracle]

It's not strictly about light which can be slowed down when passing through another medium like water. The universe appears to have a speed limit known as the speed of causality, how fast after a cause will you see an event. Electromagnetic waves, of which light is one, just happen to travel at this speed through a vacuum, as do gravitational waves.

 

[Ibix]

Electromagnetic waves, of which light is one, just happen to travel at this speed through a vacuum, as do gravitational waves.

It's not exactly "just happen to". It's a consequence of them being massless.

 

[anuttarasammyak]

If there is no limit of speed, I can go to the moon and come back to the Earth instantaneously to see myself. Two of myself can exist at the same time and same place.

 

[haushofer]

If there is no limit of speed, I can go to the moon and come back to the Earth instantaneously to see myself. Two of myself can exist at the same time and same place.

You mean being a teacher.

 

[berkeman]

Thread is closed temporarily for Moderation...

After review, it looks like the OP's question has been answered, and the thread is in danger of going off the rails (even after some cleanup). Thread will remain closed. Thank you everyone for helping the OP.