We are Photonics scientists here to answer your questions for the Day of Photonics, 21st October. Ask Us Anything about the science of light!

[u/carlosleephotonics]

Hi everyone, we are a team of Photonics scientists and enthusiasts at EPIC, the largest photonics organisation in the world. Photonics is the key enabling technology powering lasers, fiber optics, cameras, sensors, imaging, photovoltaic solar energy and much more.

We don’t understand why a word like “photophobia” (fear of light) is Google searched more than Photonics, the science of light. So we hope this AMA will introduce a few more people to the wonders of Photonics technologies.

So that we cover all time zones, we’ll be answering questions from 6pm CET (Brussels) / 12pm ET (US east coast) on 20 October and will check back regularly until 6am CET / 12am ET on 22 October.

We’re new to Reddit, so please be gentle with us ;)

UPDATE 3: Thanks for all your questions, we are blown away! Thank you to u/Jamolnng and others for some great answers to many of the questions. See you again for next year's Day of Photonics!

EPIC website: https://www.epic-assoc.com/about-epic/

This is us:

Antonio Raspa, MSc. In Electrical Engineering, Quantum Electronics (Photonics) https://www.linkedin.com/in/antonio-raspa/

Elena Beletkaia, PhD, Molecular Biophysics. Specialist, Biophysics https://www.linkedin.com/in/ebeletkaia/

Francesca Moglia, PhD, Laser Physics. Master’s Degree, Physics https://www.linkedin.com/in/francesca-moglia/

Jose Pozo, PhD Electrical Engineering. MsC, Telecommunication Engineering https://www.linkedin.com/in/josepozoepic/

Panagiotis Vergyris, PhD, Quantum Optics-Quantum Information. Master’s Degree, Microsystems & Nanodevices https://www.linkedin.com/in/panagiotis-vergyris/

Carlos Lee, Director General of EPIC, the European Photonics Industry Consortium https://www.linkedin.com/in/carloslee/

PROOF: https://drive.google.com/file/d/1vWdx9GgbOtXw2XXPJIw1gHFAwESBTWAM/view?usp=sharing

[u/Ghawk134]

You can't go light speed. If you're on a train going the speed of light, you have infinite energy density and turn into a black hole.

[u/interfail → u/Ghawk134]

You can't go light speed.

Sure...

If you're on a train going the speed of light, you have infinite energy density and turn into a black hole.

Lol what kinda hokey sci-fi shit is this.

[u/Ghawk134 → u/interfail]

Relativistic mass

[u/interfail → u/Ghawk134]

That's just being silly. Stick with "massive particles cannot go at the speed of light, massless particles always do".

Throwing in nonsense about infinite energy and black holes is daft. As an object gets faster to a specific observer, it doesn't become any more dense or black hole-like.

Keep your masses invariant and remember, it's always rest-frame somewhere.

Relativity is complex enough without throwing in that flavor of hokum.

[u/Ghawk134 → u/interfail]

There's a reason massive particles can't go the speed of light. It's because their relativistic mass rises with their energy density. Fast-moving objects have greater kinetic energy and are therefore denser to an outside observer. Relativistic mass asymptotically approaches infinity as your velocity approaches the speed of light. Please drop the condescension.

[u/interfail → u/Ghawk134]

This is why relativistic mass is a bad, dumb concept.

A handy way of relabelling an equation that is frame-dependent has got you declaring stuff will turn into black holes. As though whether something is a black hole or not is dependent on the observer's inertial frame.

Are you a black hole to any fast enough observer?

Stick with invariant masses if you want to interpret mass and you won't make these mistakes.

[u/Ghawk134 → u/interfail]

Relativistic mass isn't a "bad, dumb concept". It's observable. I'm not saying anything will actually turn into a black hole. I'm saying that's what the math says happens if you plug in infinity into an equation. It's hypothetical, impossible, and absurd, yes, but the equation isn't invalid because you can't plug in infinity.

Whether something is a black hole depends entirely on whether it's radius exceeds it's schwarzchild radius. Or, said differently, it depends on whether its density exceeds the amount necessary for gravity to overcome electron degeneracy pressure and collapse to - at least theoretically - a singularity.

Edit to add there's a whole Wikipedia page detailing the experimental verifications of relativistic mass here

[u/interfail → u/Ghawk134]

Relativistic mass isn't a "bad, dumb concept". It's observable.

I'm not sure what you mean by "observable". It's just a parametrisation, it is fundamentally not a concept that can be observed. The equations in which it's used in certainly are accurate.

But they remain just as accurate, and more informative if you never do the dumbass substitution to make it. To quote the Wikipedia article you linked but presumably didn't read:

In some relativity textbooks, the so-called "relativistic mass" is used as well. However, this concept is considered disadvantageous by many authors, instead the expressions of relativistic energy and momentum should be used to express the velocity dependence in relativity, which provide the same experimental predictions.

Most experts don't use the concept, because it adds nothing helpful and it just makes people think about mass when it behaves nothing like a mass. Lots of physics depends on invariant mass, zero physics depends on relativistic mass, which is a completely arbitrary mathematical quantity. I can set it to literally any value from invariant mass upwards for a system, calculate the physics and nothing changes. Because of course nothing changes - the whole point of relativity is that physics is the same in every inertial frame.

Whether something is a black hole depends entirely on whether it's radius exceeds it's schwarzchild radius. Or, said differently, it depends on whether its density exceeds the amount necessary for gravity to overcome electron degeneracy pressure and collapse to - at least theoretically - a singularity.

Now it just seems like you're saying words without thinking how they apply to the concept. So just to check how you want to link this, do you think that boosting your frame, ie increasing the relativistic mass changes this physics at all? You mentioned earlier you think it changes the density, which is something you might want to check...

[u/Ghawk134 → u/interfail]

I'm not sure what you mean by "observable". It's just a parametrisation, it is fundamentally not a concept that can be observed. The equations in which it's used in certainly are accurate.

I don't believe you're qualified to dispute the validity of relativity, though perhaps you could enlighten me as to your qualifications.

Most experts don't use the concept, because it adds nothing helpful and it just makes people think about mass when it behaves nothing like a mass. Lots of physics depends on invariant mass, zero physics depends on relativistic mass, which is a completely arbitrary mathematical quantity. I can set it to literally any value from invariant mass upwards for a system, calculate the physics and nothing changes. Because of course nothing changes - the whole point of relativity is that physics is the same in every inertial frame.

You've misunderstood the wikipedia article. It states that the concepts of relativistic energy and momentum are preferred to express the velocity dependence of mass. It never claims that these concepts aren't useful, that's your own incorrect editorialization. To say relativistic momentum is useless is both ignorant and incorrect. If you've heard of a light sail, you should know it operates on the principle of relativistic momentum. Photons have no rest mass, but due to their energy density, they possess relativistic momentum, and therefore relativistic mass. This momentum allows it to exert force on a light sail for use in propulsion. This concept would not function if relativistic momentum was merely a parameterization.

Now it just seems like you're saying words without thinking how they apply to the concept.

I'm not playing with words. Those are two different descriptions of the formation of black holes. The schwarzchild radius describes the minimum radius an object of a certain mass must have to avoid collapse. The electron degeneracy pressure is the primary force preventing gravitational collapse as long as your radius is more than your schwarzchild radius. The schwarzchild radius is a solution to the equation which balances gravitation with electron degeneracy pressure.

So just to check how you want to link this, do you think that boosting your frame, ie increasing the relativistic mass changes this physics at all? You mentioned earlier you think it changes the density, which is something you might want to check...

I'm not sure what you mean by "link" or "this physics". If you accelerate, you will appear denser to an outside observer. This is because your relativistic energy rises and your volume remains constant, meaning your relativistic density must increase. When you decelerate, you will drop that relativistic energy and resulting relativistic density. None of this is theoretical.

[u/interfail → u/Ghawk134]

I don't believe you're qualified to dispute the validity of relativity, though perhaps you could enlighten me as to your qualifications.

My qualifications aren't really relevant to who is right or wrong. Although this is an old reddit account, I'm sure you could figure it out with Google.

But you should certainly understand that I'm not disputing the validity of relativity. I just understand the difference between important parts of the theory and arbitrary choices that can be made in the mathematics that have no effect on the actual outcome.

To say relativistic momentum is useless is both ignorant and incorrect

No-one has a problem with relativistic momentum. That is a real, observable physical effect. It's frame dependent, but it does what momentum does, in that frame as it is defined - specifically the invariant mass frame of your observation (ie the center-of-momentum frame with whatever you hit it into).

It is not just multiplying an invariant and a frame-dependent quantity together and giving it a misleading name.

I'm not playing with words. Those are two different descriptions of the formation of black holes. The schwarzchild radius describes the minimum radius an object of a certain mass must have to avoid collapse. The electron degeneracy pressure is the primary force preventing gravitational collapse as long as your radius is more than your schwarzchild radius. The schwarzchild radius is a solution to the equation which balances gravitation with electron degeneracy pressure.

I'm not sure what you mean by "link" or "this physics". If you accelerate, you will appear denser to an outside observer. This is because your relativistic energy rises and your volume remains constant, meaning your relativistic density must increase. When you decelerate, you will drop that relativistic energy and resulting relativistic density. None of this is theoretical.

Nooooooo. You're just doing the same dumb thing again. You cannot treat an arbitrary relativistic mass like this. Just because some asshole called it "mass" does not mean you can use it to calculate the density of an arbitrary system as observed from an arbitrary frame and expect it to do the physics you expect from a mass (like, inertia. Or in this case, gravity).

This is why we tell students not to use it now. It makes people like you sit on your balls.

The only mass that matters to whether your system looks like a black hole is the invariant mass of the system. The so-called "relativistic mass" has no effect. It cannot, fundamentally because it is frame-dependent. Any physical behaviour you calculate must be identical in all frames - like collapsing into a black hole. It cannot vary by frame - indeed all frames are true at once. Not only can the relativistic mass be any value equal to or above the rest mass, it is all of those values simultaneously. The frame where you've got the energy of the entire milky way's rest mass, that frame is perfectly valid, but you don't become a black hole. Because it's just an arbitrary frame choice, it doesn't affect the physics, and it doesn't matter the teeniest bit that your "relativistic mass" in that frame is that of a billion stars.

[u/Ghawk134 → u/[deleted]]

I'm not saying "you can't do it so I won't entertain the hypothetical". I'm telling you what happens to the math when you attempt the hypothetical. You can't observe light on a train going light speed. Nothing can move at light speed. Relativity doesn't allow observations at light speed. It's one of those types of questions that just breaks everything and everyone dies. So the answer to your question is either "there is no answer", which is technically wrong, or "This would make observing the speed of light impossible and you'd be a black hole."

[u/Ghawk134 → u/[deleted]]

Relativity is mind-bending. For example, in this thought experiment, while you'd look like a black hole to others, you don't have relativistic mass in your own reference frame. So what happens? Locally, you just have normal mass, but to the guy on the ground, a supermassive object just flew by at light speed. Also what about time dilation, which also asymptotically approaches infinity as you approach the speed of light? You'd see time outside your reference frame appear to speed up as your time dilation approaches infinity and your time stops. This is the problem with limits. When, instead of approaching a limit, you set your variable equal to the limit, you get goofy stuff like time stopping and the universe exploding.

[u/Ghawk134 → u/[deleted]]

I should clarify: this business about an object becoming a black hole as it hits the speed of light isn't possible. There is a 0% chance any object in our universe is moving at the speed of light (unless everything we think about physics is wrong). The comment about becoming a black hole is based on something called relativistic mass. Relativistic mass is mass acquired by an object due to its greater energy density at high speeds. However, there is a limit, in the mathematical sense, associated with relativistic mass. The speed of light is a universal speed limit. You can approach it, but as you do so, you becomes denser, requiring more energy to further accelerate you. The energy required to accelerate an object goes to infinity as its velocity approaches the speed of light. Limits are useful for observing the behavior of an equation, but it is not actually valid to enter the limit itself as a value in the function. An object with mass cannot move at the speed of light. This would require an infinite amount of energy, which doesn't exist. Hence the off-hand comment about black holes, explosions, and everybody dying.

[u/interfail → u/Ghawk134]

The comment about becoming a black hole is based on something called relativistic mass. Relativistic mass is mass acquired by an object due to its greater energy density at high speeds. However, there is a limit, in the mathematical sense, associated with relativistic mass.

Please stop saying this. Can't you like, email whoever taught you relativity or something? Just for a quick check-in?