mass

[TheGeek]

I have heard that to be traveling at speed of light you need to be messless and to go back in time you need to be in negetive mess. How could something be messless?Negetive mess?

[RyanJ]

I have heard that to be traveling at speed of light you need to be messless and to go back in time you need to be in negetive mess. How could something be messless?Negetive mess?

 

 

A particle known as a photon and another called a graviton are example sof massless particles. I'm not shure how they cna be defined as having no mass (Though I havea asked this question ymself I was confised by the answers I got back).

 

I don't somehting cna have negative mass, since you can't travel at light speed anyway its not really a matter of urgency to find otu either

 

Cheers,

 

Ryan Jones

[5614]

Massless particles (ie. a photon) is massless because when you say "mass" you are referring to the rest mass of the photon.

 

As we know from Einstein a photon always travels at c no matter what.

 

The rest mass is the mass of a particle when it is stationary.

 

Put the 2 together. A photon can never be stationary so it cannot have a rest mass.

 

===

 

As for negative mass it don't exist!

 

Even antimatter has a normal (positive) mass, nothing has a negative mass... how could it!??

[TheGeek]

is that the only way to go back in time, be negative mess?

[VendingMenace]

As we know from Einstein a photon always travels at c no matter what.

 

 

Not true.

 

Light only travels at c in a vacum. When a photon is passing though any other medium, then it travels slower. Ie. light travels slower through glass, and different wavelengths are slowed differently. This is why a prisim splits light into a spectrum.

 

 

What we know from Eistein is that nothing can travel faster than c.

 

In fact, we don't even know this. What we know is that nothing initially traveling slower than c can be accelerated to a speed faster than c.

[Severian]

I have heard that to be traveling at speed of light you need to be messless and to go back in time you need to be in negetive mess. How could something be messless?Negetive mess?

 

To travel backwards in time the particle needs a negative mass-squared, which corresponds to an imaginary mass - not a negative one.

[swansont]

Not true.

 

Light only travels at c in a vacum. When a photon is passing though any other medium' date=' then it travels slower. Ie. light travels slower through glass, and different wavelengths are slowed differently. This is why a prisim splits light into a spectrum. [/quote']

 

"Light travels at c" and "photons travel at c" aren't the same. Light can be slowed, but photons travel at c. In a medium, the photons are absorbed and re-emitted, which accounts for the slowing of the light.

 

What we know from Eistein is that nothing can travel faster than c.

 

In fact' date=' we don't even know this. What we know is that nothing initially traveling slower than c can be accelerated to a speed faster than c.[/quote']

 

"Nothing travels faster than c" is not a completely accurate distillation of relativity. Things "travel" faster than c, but they don't transmit information in doing so.

[TimbaLanD]

A particle known as a photon and another called a graviton are example sof massless particles

 

If photons have no mass, how can it be pulled in to a black hole?

 

Another thing that baffles me is, why can’t we harvest light? If the star light we see in the sky originated form millions and millions of years ago and it’s only reaching us now. It means that light is constantly travelling through space (?). So the light does not decay. It just travels at C through space.

 

If mediums can slow light, surely we can find a medium to “ultra slow” light so that we can actually observe its path of motion (Theoretically).

 

So, why can’t we harvest it? Why can’t we put light in a bottle and keep it on our top shelf?

 

Hope I am not asking silly questions!

[Severian]

If photons have no mass' date=' how can it be pulled in to a black hole?

[/quote']

 

Because gravity acts on energy, not mass. This is one of the perculiarities of Einstein's General Relativity. the only reason why it looks to us like gravity works on mass is because at the speeds we are used to the vast majority of an object's energy is stored in its mass.

 

 

If the star light we see in the sky originated form millions and millions of years ago and it’s only reaching us now. It means that light is constantly travelling through space (?). So the light does not decay. It just travels at C through space.

 

That's right. It can (and does) scatter off interstellar particles, but the space between stars is a pretty good vacuum, so this isn't a huge effect. In fact, the light from the CMBR has been travelling for 13.7 billion years!

 

If mediums can slow light, surely we can find a medium to “ultra slow” light so that we can actually observe its path of motion (Theoretically).

 

Light travels very fast, so it is very difficult to slow it down to our sort of speeds, but in principle it can be done. Bear in mind though, that this isn't really "slowing light" down. Between molecules it will still travel at c, but it is being absorbed and re-emitted by the moelcules/atoms in the medium; it is this absorption/re-emission which takes time and makes the light appear to be travelling slowly.

 

So, why can’t we harvest it? Why can’t we put light in a bottle and keep it on our top shelf?

 

Imagine you had a perfect mirror which reflected 100% of light. Just make the bottle out of that and you have your light store. No need even to slow it down. A lightbulb seems a bit more practical though.

[TimbaLanD]

Imagine you had a perfect mirror which reflected 100% of light. Just make the bottle out of that and you have your light store. No need even to slow it down. A lightbulb seems a bit more practical though.

 

Sev!!

 

Thanks for that... it all made more sense except the last one… I didn’t understand why you were referring to a light bulb because light bulb is constantly emitting light.

 

So, what you are saying is: If I make a perfect mirror which reflected 100% of light (Say in a cubicle) and I stand inside. If I put the light on so it emits light, right? So if I turn it off, the cubicle will be still bright as though if the light bulb is on???

 

Please clarify..

[Severian]

Sev!!

 

Thanks for that... it all made more sense except the last one… I didn’t understand why you were referring to a light bulb because light bulb is constantly emitting light.

 

Yes' date=' that is right. I meant it is easier to make the light when you need it than it is to store it!

 

So, what you are saying is: If I make a perfect mirror which reflected 100% of light (Say in a cubicle) and I stand inside. If I put the light on so it emits light, right? So if I turn it off, the cubicle will be still bright as though if the light bulb is on???

 

Yes. In fact, if you had a perfect mirrored room the light would get brighter and brighter while you left the lightbulb on because the lightbulb would be making more and more photons which bounce around without escaping. When you switch the lightbulb off you would not be making any more photons and the level of light would stay contant. (Of course, you would have to be wearing a perfect mirror-suit too otherwise you would absorb the photons yourself.)

[TimbaLanD]

Yes, that is right. I meant it is easier to make the light when you need it than it is to store it!.

 

I see... it’s cheaper to make photons than to store them!! It would be cool to put the photons in a transparent bottle so that they glow forever!

[swansont]

I see... it’s cheaper to make photons than to store them!! It would be cool to put the photons in a transparent bottle so that they glow forever!

 

You wouldn't be able to see them. Anything you see represents photons being absorbed by your eye.

[TimbaLanD]

You wouldn't be able to see them. Anything you see represents photons being absorbed by your eye.

 

I was actually thinking about that!!! I understand that for us to be able to see it, the light have to travel to the eye, so if photons become motionless, we won’t be able to see anything!! WoW, simple but what an amazing fact!! So what we actually see is never in real time.. There is always a delay!!!

 

While I am looking at my PC Screen, the light reaching my eye is already old and what I am actually seeing is not matched up with what’s actually happening!!! If the distance between the PC and my eye is C, then you will only see my response after 2C!! That is quite amazing to think about!!

 

The eye – What an amazing instrument!!!!!!!

 

Can light have density? I guess if it has no mass, how can it have density but you said that if we keep the light on in the room, it will get brighter and brighter, eventually will it not accumulate mass?

[CanadaAotS]

Actually, I have a question to that extent... can a very large amount of light concentrated (lets say with a storage room similar to the one described above) bend space-time? Does an object have to have rest-mass to induce gravity effects, or would a very large amount of light stored in one place have enough energy to have gravity?

 

Also btw light can be slowed down to "our speeds" apparently light travels very slowly through BEC's (buckyballs).

[CanadaAotS]

http://www.physicscentral.com/action/action-03-04-print.html

 

"Light has been slowed to one mile per hour in... a Bose Einstein Condensate"

 

of course like the guy was saying, they still travel at c there just being absorbed and re-emitted so much that it greatly reduces the apparent speed

[TheGeek]

can photons pass through glass?

if we are in a glass container can photons get to us?

[Jacques]

Depend on the photon frequency

I am not sure but I think UV photon won't go throught glass

[timo]

Actually, I have a question to that extent... can a very large amount of light concentrated (lets say with a storage room similar to the one described above) bend space-time? Does an object have to have rest-mass to induce gravity effects, or would a very large amount of light stored in one place have enough energy to have gravity?

 

As you can see on http://en.wikipedia.org/wiki/Stress-energy_tensor , the relevant quantity that enters the Einstein equations (which relate spacetime curvature to energy) is the energy-momentum tensor (called T, there). Photons add to that tensor so they contribute to spacetime bending (see the energy momentum tensor for an ideal fluid on the same page).

 

If I recall that correctly, the photonic contributions and the fact that they do not come with a rest-mass share is important for cosmology in the early universe. But I´d have to read that up at home to be sure about that.

[Severian]

But I´d have to read that up at home to be sure about that.

 

Frank says you should get back to work and not spend your time researching gravity....

[CanadaAotS]

so, lets say you had a very powerful light bulb that lights into a room of 100% reflective mirrors, and you left it running for say a few thousand years lol.

 

The room would eventually gain a great gravitational pull on its surroundings (assuming a huge amount of energy is eventually trapped in there)?

 

If thats true and lets just say the "lightbulb" pumps out a TW/h (yes one mighty lightbulb lol) it'd take about 17,249 years for the light in the room to have the gravitational pull of myself (70kg) lol.

 

The room would of course have to be a vacuum, and the walls would have to reflect light of all wavelengths and the lightsource would somehow have to be shielded as well (a 1 way mirror I guess), since crazy amounts of energy would be flying around...

 

I guess using light energy to create gravity isn't a very plausible idea lmao

[timo]

Frank says you should get back to work and not spend your time researching gravity....

, he really said this?

[Severian]

, he really said this?

 

No - I was joking. (It is the sort of thing he would say though....)

[TimbaLanD]

so' date=' lets say you had a very powerful light bulb that lights into a room of 100% reflective mirrors, and you left it running for say a few thousand years lol.

 

The room would eventually gain a great gravitational pull on its surroundings (assuming a huge amount of energy is eventually trapped in there)?[/quote']

 

Are we saying that energy has nothing to do with mass? And mass has nothing to do with gravity?

[danny8522003]

Energy has everything to do with mass and mass (well energy) has everything to do with gravity.