A Physics Quiz of a different type

[Ether]

Notes embedded in red below

Originally Posted by DonRotolo

Well, wouldn't you be able to reach c, but not exceed it? That means you CAN reach c.

No, you wouldn't be able to reach c.

Going past c is what requires infinite energy, reaching c needs just a smidgen less than infinity.

Reaching c requires infinite energy.

And yes, we're assuming the object rest mass is nonzero

I see Aren beat me to it.

[the man]

Its an asymptote.

[Ether]

Originally Posted by the man

Its an asymptote.

Specifically, a vertical asymptote.

[DonRotolo]

Someone else offered an explanation as well:

As you gain energy, the mass of the object increases, because energy has weight. This includes kinetic energy. So as speed increases, so does mass, requiring more and more energy to maintain acceleration. At some point, it gets really massy, such that you can't push it hard enough to accelerate it.

I am told that this "weight gain with energy" has actually been measured, but I can't find that reference.

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OK, does a photon have mass? If yes, then it cannot travel at the speed of light, but since it does, at rest it must be massless. (Yes, it has momentum, but that's a special case. In theory you can approach the speed of light using a flashlight as your propulsion unit. Might take a while though...)

Are there any nonzero-mass particles/objects that travel at c? Can there be?

[JesseK]

Eh, why try to travel so fast when we could instead slow light to a crawl? The applications for that seem more pragmatic in our lifetimes than attempting to do anything at light speed.

For instance, could we trap light (and thus its energy) in order to accumulate enough for release when we want it? It'd bring a whole new meaning to 'solar energy'. There's been some work already in that direction and it seems at least a little less complicated.

[Taylor]

Originally Posted by DonRotolo

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OK, does a photon have mass? If yes, then it cannot travel at the speed of light, but since it does, at rest it must be massless. (Yes, it has momentum, but that's a special case. In theory you can approach the speed of light using a flashlight as your propulsion unit. Might take a while though...)

Related, and asked by a student to me a couple weeks ago:
When light is reflected, at the point of reflection, is the speed zero?

[Ether]

Originally Posted by Taylor

Related, and asked by a student to me a couple weeks ago:
When light is reflected, at the point of reflection, is the speed zero?

No. There is no "point" of reflection, because of the quantum mechanical wave/particle duality of the photon.

[Aren Siekmeier]

Originally Posted by DonRotolo

OK, does a photon have mass? If yes, then it cannot travel at the speed of light, but since it does, at rest it must be massless. (Yes, it has momentum, but that's a special case. In theory you can approach the speed of light using a flashlight as your propulsion unit. Might take a while though...)

Are there any nonzero-mass particles/objects that travel at c? Can there be?

The fact that photons travel at the speed of light is exactly what originally motivated the fact that they are massless.

Using a flashlight for propulsion would definitely work (albeit very slowly) because of the momentum of the outgoing photons, but it is no different than any other Newton III interaction; you will still accelerate only asymptotically to the speed of light.