# Proving to someone the speed of light is real

Is there a simple way to prove to someone that light travels at c in a vacuum? I was having a debate with a friend and he said the speed of light isn't real.

## Answers and Replies

Khashishi
Science Advisor
If you microwave a block of soap, you can derive the speed of light from the positions of the hot spots if you know the frequency of the emitter.

Delta2
Homework Helper
Gold Member
Telecommunication technology also proves it, one can watch live broadcasts of athletic events in places that are thousands of miles away, that's because all the tv signals are travelling with the speed of light.

But maybe your friend meant that we cannot accelerate ordinary material bodies (like a car for example) to the speed of light. That is partially true, at least according to special relativity, no matter how much technology evolves we cannot accelerate something to c because then it will have infinite mass. But we can get as close to c (like 99% of c) as we want at least theoretically. But for todays technology even 10%c is too much to achieve.

The accelerator at CERN accelerates particles of the microscopic world (protons) to 99%c though.

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Basically I told him about an article I read about a new technology that could get us to Mars in a few days, I told him it would probably be near the speed of light (just a guess) and then he said the speed of light isn't real, whatever that means.

Khashishi
Science Advisor
The technology to get us to Mars in a few days doesn't exist. Are you sure you heard your friend correctly?

russ_watters
Mentor
Was he talking about the speed of light as a speed limit? Tough to help if we don't know the conversation...

I think he was saying he doesn't believe light is an electromagnetic wave.

Your friend says, supposedly, that the speed of light isn't real. I have don't know what that means. Perhaps if you were more articulate... Here are some possibilities:
1. He means that light does move through space, but that it's speed (in a vacuum) isn't constant
2. He means that light doesn't move through space.
3. He means that light moves instantaneously through space.

Lol That's a good point. My best guess from knowing him is that he doesn't believe in the speed of light being the ultimate speed barrier, and that time dilatation/compression aren't valid. He doesn't seem to agree much with Einstein.

4. He means that there is no limit on the velocity massive objects can reach.
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I'd guess the best answer is that it is unlikely that you can physically demonstrate that light travels at finite speed, that no object can travel faster, and that that speed is constant for all observers. The problem is that you almost certainly don't have (easy, cheap, quick) access to the necessary equipment. You can present a multitude of arguments which rely on experiments done by others. But if that were adequate, I'd expect him (if he was serious) to have already read about those - the Wikipedia article Speed of Light would be one place to start. The most simple and 'active' measure of the speed of light (but not the most accurate!!!) is the laser range finding they do with the Moon. I believe there was just an article on Phys.org this week about it. Where they shine a laser from Earth to the Moon and it reflects back, allowing the distance to the Moon to be computed (and proving light has a speed)

The other facts are more difficult to prove. For instance proving that the speed of light is the same here as it is 2 million light-years away would require us to travel there or communicate with someone there already. Its not going to happen. The Michelson-Morley experiment can be done with fairly crude technology (but would probably cost tens of thousands of dollars, and buying a telescope to do what Rømer did in the 1670's wouldn't be too expensive (but would take a few dedicated years). I can think of no feasible way for you to physically prove you can't accelerate a massive object to over c, (but the LHC is able to accelerate protons only up to 99.9999% of c (IIRC, I have the right number of 9's there...maybe one more?)) but that's not something you're ever gonna build for yourself, obviously. The basic problem here is we are tilting at windmills. HE needs to present an alternative and show evidence that he is correct. Once a specific hypothesis is made explicit, you can provide references (or experts) that totally contradict it. But "proving"? One good definition of proof is: "Evidence sufficient to convince". Being convinced is an internal mental state, you can't force someone to have that state (just think how easy teaching would be if we had a pill or a machine to suddenly give some new knowledge or new skill. How much evidence is necessary to prove to someone who already believes? And how much evidence (say in the existence of God) is necessary for an atheist (who doesn't believe, or rather believes God does NOT exist)? None for the former, and all the evidence in the world won't convince the latter (of course, if he were to talk to God, things might change, LOL)

Laser distance finding (like carpenters or engineers use every day) wouldn't work if light's speed was variable or instantaneous.
The LHC would be accelerating protons above c, if the amount of energy required wasn't asymptotically infinite as speed increases.
But convincing someone who doesn't want to believe? Fools walk in where Angels fear to tread. I'd say don't waste your time. If your friend were a real student of the subject, you wouldn't need to do more than tell him about those key experiments. There are other ways to approach it, by the way. For instance Maxwell's equations (used to explain all of our modern electronic (and electric!) technology) can be used to derive a constant we call c. Since it is a fact of nature, there are countless kinds of evidence for it.

kolleamm
Thanks for the advice. I realize that there are just certain kinds of people where anything they themselves cannot observe in nature they do not believe in. People who believe our own crude common sense, is universal. Proving such concepts to them is probably not likely since the amount of effort required to grasp the reasoning is far too much for them and their interest in the subject.

phyzguy
Science Advisor
The Apollo astronauts left a retro-reflector (basically a mirror) on the moon. Experimenters bounce a light signal off the this mirror on the moon and time how long it takes to go to the moon and back, which is about two seconds. How can you argue with a measurement like this?

kolleamm
phyzguy
Science Advisor
Basically I told him about an article I read about a new technology that could get us to Mars in a few days, I told him it would probably be near the speed of light (just a guess) and then he said the speed of light isn't real, whatever that means.

Mars is a few light-minutes away. Although we don't have the technology to get there in a few days, traveling to Mars in a few days is far below the speed of light.

Vanadium 50
Staff Emeritus
Science Advisor
Education Advisor
2021 Award
Just so I understand, this thread is about convincing someone who isn't here that his opinion, which we don't exactly know, is wrong. Right?

OmCheeto and Drakkith
It's more about me learning what to say next time such a conversation comes up. Nothing I hate more than accepting scientific theories and not knowing how to justify my opinion when someone says otherwise.

Zarich12 and Delta2
ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
It's more about me learning what to say next time such a conversation comes up. Nothing I hate more than accepting scientific theories and not knowing how to justify my opinion when someone says otherwise.

Unless you are an expert in all the numerous fields, and unless you are willing to put time and effort into learning each one of them, at some point, ALL of us have to accept the opinion of experts. I'm a physicist, but I trust my GI specialist when he tells me during my colonoscopy that there are two small polyps removed and that they are benign. (OK, that may have been too graphic, but you get the idea).

The thing about established physics is that, chances are, you ARE using the basic principle in your everyday lives, whether you realize it or not. The speed of light in vacuum is CRUCIAL in your GPS. If we make a small mistake, even a little, the GPS system will be out of whack so quickly, you'll find Chicago located in London! And if your "friend" doesn't think that light can be an EM wave, then he/she needs to visit a particle accelerator where we use RF fields to accelerate charge particles. Or better yet, get him/her to set up an antenna and measure what he/she gets of radio waves.

The thing here is that it take NO effort and no knowledge to simply just say "Oh, I don't believe in that!". One must also produce (i) physics evidence (i.e. experimental evidence), and then (ii) apply a theory to interpret that evidence to support one's belief. This is how science works in either disproving something, or advancing our knowledge. In other words, we don't just say "everything that goes up, must come down. We must also say "when and where it comes down". That, in essence, is physics.

So, if you are intent on dealing with something like this in the future, pay attention to how the argument is made. Did the person simply indicate that he/she doesn't believe in something without justification? Or did that person offers a rational, reasonable argument? Then if it is counter to the established concept, ask him/her for an explanation on how he/she thinks that he/she is correct, while experts in the field is wrong.

Zz.

PeterDonis
Mentor
Thread closed for moderation.

Edit: The thread will remain closed.

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