#### Science Forum

vivify
Science 27 Jul '14 02:28
1. vivify
rain
27 Jul '14 02:28
http://science.howstuffworks.com/science-vs-myth/what-if/what-if-faster-than-speed-of-light.htm

The article in the link above mentions that the mass of an object approaching C starts to have an infinite mass. The energy needed to move such an object at C is infinite, making it impossible to pass C.

You guys already know that. A few questions:

1) Does this mean that light has an infinite mass? If so, why doesn't light hurt us when a flashlight is shown on us?

2) Does this mean that light has infinite energy?

3) Why is only light capable of this speed?

If anyone could help, I'd very much appreciate it.
2. 27 Jul '14 04:25
Originally posted by vivify
http://science.howstuffworks.com/science-vs-myth/what-if/what-if-faster-than-speed-of-light.htm

The article in the link above mentions that the mass of an object approaching C starts to have an infinite mass. The energy needed to move such an object at C is infinite, making it impossible to pass C.

You guys already know that. A few questions:

1) Do ...[text shortened]... y is only light capable of this speed?

If anyone could help, I'd very much appreciate it.
I will answer in a amateurish way, a way to explain to someone without scientific background.

(1) and (2) - If a particle go with the speed of ligth, then its mass is (inf) times its restmass. If the restmass is zero, then it is still zero. So light doesn't have infinite mass when it goes at the speed of light.

(3) - No, not only ligth is capable of this speed. Every massless particle has this property.

Scientifically meaning, this is not entirely true. But in order to explain more scientifically, the answer will easy be non-understandable to an amateur because we have to introduce more scientific concepts which gives rise to yet other questions, and the general ideas get lost.
3. 27 Jul '14 04:362 edits
In his 1905 theory of relativity, Einstein determined that for a body of mass m, the relation between energy E and linear momentum p and mass is:

E^2 - p^2c^2 = m^2c^4.

If one is in a co-moving frame with the object, then p=0, and:

E^2 - 0 = m^2c^4.

Taking the square root of both sides:

E = mc^2.

But one of Einstein's postulates of special relativity is that no observer can ever be in the rest from of a beam of light. Light always moves at the same universal speed c relative to an inertial observer. So there are no observers who are ever in a position to take p = 0 for light. Thus they cannot ever argue that E = mc^2 for light.

(Here I am using ^ for taking a power.)
4. AThousandYoung
West Coast Rioter
27 Jul '14 04:451 edit
Light has momentum but no mass.
5. 27 Jul '14 08:545 edits
Originally posted by AThousandYoung
Light has momentum but no mass.
Yes, that is correct and goes straight to the point for it answers the OP.
The momentum of photons ( which are the particles that light is made of ) are one of the exceptions to the Newtonian rule that:

momentum = mass X velocity

This is because velocity is c in this case and photons have no mass.

The momentum for a photon is given by:

p = hf/c

where p = momentum, h = Planck's constant, f = frequency, c = speed of light.
Note how there is no mention of mass in the above equation.
6. DeepThought
27 Jul '14 14:231 edit
Originally posted by vivify
http://science.howstuffworks.com/science-vs-myth/what-if/what-if-faster-than-speed-of-light.htm

The article in the link above mentions that the mass of an object approaching C starts to have an infinite mass. The energy needed to move such an object at C is infinite, making it impossible to pass C.

You guys already know that. A few questions:

1) Do ...[text shortened]... y is only light capable of this speed?

If anyone could help, I'd very much appreciate it.
It's important to be clear about what is meant by mass. In the early days of relativity they used to distinguish between rest mass and a quantity called equivalent mass. A body with rest mass m travelling with velocity v would have equivalent mass M given by:

M = m / sqrt(1 - (v/c)^2)

However it was realised that M is a redundant variable. Take the equation E = mc^2 and use units where c = 1. The the energy at rest is E = m. The energy when moving at a speed v (as a fraction of the speed of light) is

E = m / sqrt(1 - v^2) = M

So the equivalent mass is just the energy in these units. So we now do not talk about equivalent mass and drop the "rest" from rest mass and just talk about mass.

A photon has zero mass, which means it does not have a rest frame and is constrained to move at the upper bound for speeds in relativity, which is why that upper bound is called the speed of light.

So the article you were reading is out of date. They should not be talking about the rest mass, just the mass. The energy diverges as the speed of a massive particle relative to an observer approaches the speed of light.

1) Light has zero mass. Using the old concepts it has zero rest mass and finite equivalent mass given by humy's formula M = hf/c^2.

2) No a photon has finite energy, to have infinite energy it would need infinite frequency.

3) Gluons, the force carriers of the strong nuclear force, are also massless. Because of the way the strong interaction works it is not possible to see a bare gluon, there are hypothetical particles made up of them called glueballs which would have a mass as QCD is expected to generate a mass gap. All other particles couple to the Higgs field which is one of the two mass generating mechanisms in the Standard Model.
So the reason the photon is massless is because it doesn't interact with the Higgs boson and because it doesn't interact with the strong force. All other particles interact with the Higgs and composites like hadrons gain additional mass due to the strong force.
7. sonhouse
Fast and Curious
27 Jul '14 15:28
Originally posted by DeepThought
It's important to be clear about what is meant by mass. In the early days of relativity they used to distinguish between rest mass and a quantity called equivalent mass. A body with rest mass m travelling with velocity v would have equivalent mass M given by:

M = m / sqrt(1 - (v/c)^2)

However it was realised that M is a redundant variable. Take ...[text shortened]... nteract with the Higgs and composites like hadrons gain additional mass due to the strong force.
What does that phrase 'mass gap' mean?
8. DeepThought
27 Jul '14 15:44
Originally posted by sonhouse
What does that phrase 'mass gap' mean?
It is the energy of the lowest non-vacuum state. In the case of an electron that would be the energy of a stationary electron, which is just the mass in natural units where the speed of light is 1. The Wikipedia article on mass gap is virtually incomprehensible. If you know what a band gap is in a semiconductor, then the concept is essentially identical - and that article is layman readable.
9. sonhouse
Fast and Curious
27 Jul '14 18:491 edit
Originally posted by DeepThought
It is the energy of the lowest non-vacuum state. In the case of an electron that would be the energy of a stationary electron, which is just the mass in natural units where the speed of light is 1. The Wikipedia article on mass gap is virtually incomprehensible. If you know what a band gap is in a semiconductor, then the concept is essentially identical - and that article is layman readable.
Ah, right now I personally am in a band gap. Haven't played in a band in yearsðŸ™‚

Seriously, I deal with band gap in semiconductors all the time, been at that bit, at least the hardware end, for 30 years. Ion Implanters, electron microscopes, plasma etchers, sputtering tools, rapid annealers and the like.

Its interesting to me, for instance, in the ion implanter, the doping does not take place effectively till annealing which forces the dopant atoms to take up residence where the original atom was in the crystal matrix. Before annealing the dopant is just piled up like a plowed up corn field. Afterwords, the whole band gap issue is totally altered by those extra dopants in the crystal matrix. The heating must cause the dopant atoms to vibrate into a lower energy state that happens to be the location of a previous original atom at that location in the matrix. I find that process fascinating.