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/sci/ - Science & Math

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2229451 No.2229451 [Reply] [Original]

I have a few questions for you to ponder. First one I am reposting
from /b/.
If light is pulled into a black hole, would this indicate that potentially light has mass?

Second question, if the speed of sound increases through denser matter, is it not then possible for sound to potentially travel faster then the speed of light? For example, if I shoot a starting pistol above a pool, at the opposite End of the pool the people under water always hear if first.

In theory, if the water was dense enough, remembering that the deeper the water the denser it becomes, as I understand it the soundwaves should potentially travel faster then light.

>> No.2229459

Photons do have mass...where have you been?

The difference between the speed of sound and speed of light is many orders of magnitude...so no.

Density of liquids doesn't change too much (i.e. barely at all) with pressure.

>> No.2229464

>>2229451
No, the reason why light gets pulled in when it has no mass is because Newtonian physics is fundamentally wrong (though a very good approximation for most earth applications)

General Relativity describes how masses 'Bend' space-time around it. So the light 'falls' into the hole in spacetime. I'm not great at explaining this.

As for the second point, yes, it can travel faster than the speed of light in the medium, theoretically, but it cannot travel faster than the speed of light in a vaccuum.

>> No.2229469

>>2229459
I like his answer better

>> No.2229470

>>2229469
but his answer is wrong

>> No.2229475

>>2229470

Which part is wrong?

>> No.2229477

mass and energy are the same thing, but there's a useful concept called "rest mass." light has mass-energy, but no rest mass (like "solid" substances) because it cannot be at rest.

sound can't travel faster than light can in a vacuum. just because the speed of sound increases with density of the material doesn't mean it does so linearly (it doesn't). however, it may be possible for light to travel slower than sound in a specific medium; i don't know for sure whether or not it could.

>> No.2229482

>>2229475
The part where you say light has mass, and the part where you say there doesn't exist a medium in which sound can travel faster than light

>> No.2229492

>>2229451
Light can't travel through wood, making light's speed 0, or null.

Sound can travel through wood, making it faster than light.

Repost with trollface and profit.

Other than that, there are a lot of assumptions here - i.e. that a mass can have infinite density. Not to mention that sound cannot travel for as far a distance as light, as it dissipates into heat.

>>2229464
And sound can't travel at all in a vacuum. It has no particles to move through.

>> No.2229493

>>2229482

I'm an aerospace engineer and when we're working out perturbations on the orbit of a craft we have to consider solar flux as providing an acceleration on the craft if it has a large surface area - in fact, it can have a larger affect on the acceleration of the craft above 900km altitude than atmospheric effects. Photons hitting the craft exert a force, therefore have momentum. Or in terms of energy, they must have kinetic energy (ergo mass).

What say you?

>> No.2229507

>>2229493
I say whip out them solar sails, baby, we're going to the distant reaches of the galaxy!

And then I'll say, "That's a very good, valid point."

P.S. How do you know it's photons that hit the craft and not heat or other solar emitions?

>> No.2229508

>>2229493

>Shuttle
>Length: 122.17 ft (37.24 m)
>Wingspan: 78.06 ft (23.79 m)
>Assuming more or less triangular shape
>Area: 443 m²
>Assuming full reflectivity
>0.004 Newtons of thrust
>78 tonnes

5.19e-9 Earth gravities of photon pressure is considerable over how much time?

Also, the equation for solar sails is: 2*(wattage * area)/c

Right?

>> No.2229511

>>2229493
Oh you, silly engineer thinks photons have mass.

Special Relativity, energy, rest mass and momentum are connected by
E^2 = p^2c^2 + m^2c^4
And photons have zero rest mass, then
E=pc

Momentum is only defined by p=mv in the newtonian limit, and for a particle such as light, this certainly does not hold.

>> No.2229513

The only way sound would travel faster than light is if the medium that the sound is traveling through was a single atom that was stretched to just over a light year in length.

Also, a black hole is essentially a hole in space. So not just matter gets pulled in, everything gets pulled in. Even energy itself gets pulled in, which I personally believe is just our physical perception of energy within our visible spectrum.

>> No.2229518

>>2229513
i lol'd

>> No.2229522

>>2229507

That's a fair question...but I cannot answer it categorically; I just can't think how it could be anything else.

>>2229508

The equation we use is ap = 4.65x-5*(1+beta)(A/m)(a/r)^2

Where beta is ballistic coefficient

>> No.2229527

>>2229513
>>The only way sound would travel faster than light is if the medium that the sound is traveling through was a single atom that was stretched to just over a light year in length.

This would assume that a regular sized atom effected by a sound wave would move instantaneously. Though I can see sense in a black hole pulling in energy, regardless of mass (though I still don't know what a black hole is).

>> No.2229530
File: 15 KB, 679x427, 1285612818015.png [View same] [iqdb] [saucenao] [google]
2229530

>>2229522

>4.65x-5*(1+beta)(A/m)(a/r)^2
>my underageb& face when

What's the ballistic coefficient?

>> No.2229533

>>2229511
Except for photons momentum is not defined as p=mv, it's defined as a function of the photon's frequency.

>> No.2229545

>>2229533
What I was saying was that I also don't think light really is made up of photons, that's just how we perceive them.

>> No.2229548

>>2229530

Google's your friend but it's (Cd/m)*A...basically a function of the aerodynamic profile of the craft (which gives you the effective frontal area of the craft for light hitting it).

>> No.2229560

>>2229548

Luminosity divided by mass multiplied by area? Is that it? The area that actually receives the thrust?

What's the value for the Sun at Earth Orbit? I know it's 1,4 kw but I wanted to know the number in cd just in case.

>> No.2229563

>>2229533
exactly... because photons don't have mass.
And thats what I said, E=pc, and E=hf

>> No.2229566

>>2229560

Cd = Coefficient of drag (we use these equations for atmospheric drag too)...they just so happen to overlap.

>> No.2229572

>>2229563

Fair enough. If they can have momentum without mass through relativity then it solves the problem.

>> No.2229576

>>2229566

And if it's in space and you assume no drag you just say it's zero?

>> No.2229587

>>2229576

It depends on the model - for a proper analysis we don't remove it but over 800-900km altitude it has a very small effect on the acceleration vector of the craft.

>> No.2229605
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2229605

I like how photons don't have mass in the Standard Model, but in General Relativity light is affected by the curvature of spacetime which means it must interact with the gravitational force. So the Theories tell us that photons interact with the phenomenon of gravity and yet have no real energy density or mass in the traditional sense.

Sounds like a theoretical crisis to me.

>> No.2229611

>>2229605
You obviously don't understand general relativity then. Sorry, but mass is not a necessary condition on being effected by gravity.

>> No.2229612

>>2229587

So for a solar sail craft in inteplanetary space, it would be:

4.65x-5*(1+0)(A/m)(a/r)^2

(A/m) = That's area over mass, right?
(a/r)^2 = area over radius? is that for light intesity, inverse square law etc.?

>> No.2229622

>>2229612

A = surface area
m = mass
a = semi-major axis of orbit
r = orbital radius

>> No.2229631

>>2229622

Oh, so it's only to calculate how swept away the orbitals become? Is there one for spacecraft that are just free-flying to another world or star?

>> No.2229634

You are not aXXo. Why you post movies say aXXo when YOU ARE NOT AXXO!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

>> No.2229641

So photons are massless, but would it be right to say they can still contribute mass to a system through m=E/c^2? They must right? Otherwise the mass of an atom wouldn't decrease when it emits a photon. Even for photons mass and energy must be related.

>> No.2229647

>>2229451
If light had mass it would obviously distort the spacetime in your picture

Light exists in spacetime and follows its contours but does not have a direct influence on it

>> No.2229664

>>2229641
Photons don't have mass, but they have energy. That is why the mass decreases in photon emmision, because of energy mass equivalence

>> No.2229668

>>2229647
>If light had mass it would obviously distort the spacetime in your picture

It does man.

>> No.2229671

>>2229641
Light always moves at the speed of light in all reference frames

E = mc^2 is called the rest mass energy so it is assuming a particle is stationary

Rest mass energy comes from a more general equation which takes into account momentum

But, as I just noted, light always has the same speed regardless of rest frame. The total energy of it changes with its momentum which changes by p = h/l where p is momentum, h is plank's constant, and l is the wavelength of the light.

Increasing the energy and therefor momentum of light only affects its wavelength

>> No.2229674

It is possible to travel faster than light. for example, one light beam is traveling to another light beam. each of them is traveling 300 000 km/sec but, to each other, they're traveling 600 000 km/sec

>> No.2229677

>>2229674
Ok, but that's wrong :3

>> No.2229680

>>2229674
nope.jpg

Special Relativity would like a word

>> No.2229684

>>2229647

>Light exists in spacetime and follows its contours but does not have a direct influence on it

Not true, even though light has zero mass (it has to be exactly zero otherwise the number of polarization spin states would be 3 instead of 2) light enters into Einsteins equations for the Ricci curvature through the stress-energy tensor. Therefore the metric is effected by light. In cosmology this extra effect of light reduces the critical density of the universe.

>> No.2229690

>>2229684
>otherwise the number of polarization spin states would be 3 instead of 2
The two are vertical and horizontal and a third would be about its axis?

The metric is affected by light or by the constant c?

>> No.2229729

>>2229690

>The two are vertical and horizontal and a third would be about its axis?

no, longitudinal (in direction of motion)

>The metric is affected by light or by the constant c?

c=1. The metric of spacetime is affected by the *momentum* of the light.

>> No.2229777

>>2229729
Looks like it'll take a full motivation for me to find the subtleties. Thanks for the head start