Light

[_Simon_]

What is it that makes light move faster than anything else in the universe? Why is it for example, faster than the speed of sound?

 

[Beanze]

I wish I knew, I could only speculate...

Do you mean within our atmosphere or all-round?

I'd say sounds go through vibrations while light does not, I'm not in position to tell you what is and what not due to the fact I know as little as you sorry. Would love to know though!

 

[_Simon_]

I was under the impression that light was the fastest moving particle in space as well as here on earth. So I guess I mean anywhere.

I just want to know what makes it so fast. And I also would like to understand why I´ve been told that time slows down the closer to the speed of light you come. I am so interested in these things but I just cant seem to get my head around it. =)

Then again, I didnt marry my cousin as some did.

 

[MeteorWayne]

Well, light isn't a particle, so that's what makes it the fastest. It is an electromagentic wave, the same as a ridio wave, X-ray, etc.

As to why, whi really knows...the fact is that in a vacuum, any electromagnetoc wave moves at the same speed, which has been shown to be the fastest possible speed in the Universe.

 

[_Simon_]

So are you saying that the an electromagentic wave will move at a constant velocity throughout its journey? It wont matter if the origin is a flashlight or a star? There are no faster and/or slower waves?

 

[MeteorWayne]

That is correct. It makes no difference what the source is. A flashlight, a star, a lightning bug, (or a lightnig stroke), the reflected light of the sun from a planet, a radio wave from a transmitter, an X-ray from any source, the heat from your body (infrared light), or any other EM wave. In a vacuum, they all travel at "the speed of light".

 

[_Simon_]

Ok, that explains alot. Maybe you can answer why time is affected by speed. I´ve read that if I travel at nearly the speed of light for an "earthyear" and then return, you and everyone else for that matter would be older than me.

 

[MeteorWayne]

That's really a seperate subject, and is addressed in a number of other discussions in the Physics, and Space Science and Astronomy fora.

 

[Beanze]

Ok, that explains alot. Maybe you can answer why time is affected by speed. I´ve read that if I travel at nearly the speed of light for an "earthyear" and then return, you and everyone else for that matter would be older than me.

I've heard when you look at something 1 ly away, that's how long it takes for the light to reach you. So if you look 200 light years, it'll take 200 years for the light to reach you.


Edit: Gosh how stupid wasn't that...

If you look at something 200 light years away it'll be like looking 200 years into the past because of the light reflecting back or somet, I'm too stupid for solving this. It's been answered though you'll find it

 

[a_lost_packet_]

Ok, that explains alot. Maybe you can answer why time is affected by speed. I´ve read that if I travel at nearly the speed of light for an "earthyear" and then return, you and everyone else for that matter would be older than me.

Time Dilation

You may want to pose this question in the "Physics" forum.

 

[kelvinzero]

Light can be described in terms of photons, which are particles. As I understand it in quantum mechanics every paticle has wave properties and (I think) vice versa. I have even heard sound being described in terms of 'phonons'.

What makes photons special is that they have no rest mass. Anything with rest mass increases in mass (relative to you) when it moves faster (relative to you), and its mass would approach infinity as it approached the speed of light. Fortunately this would take infinite energy so it doesnt happen.

One other thing I can think of that is meant to move at the speed of light in a meaningful way is gravity.

 

[bushwhacker]

there is the rub gravity doesnt move at the speed of light. seems to me gravity is instantanious.

 

[trumptor]

I didn't know that gravity travelled at the speed of light. I always thought that gravity was instantaneous. So if an object to the left of us and an object to the right merged about a light year in front of us, its gravitational pull from directly in front of us wouldn't reach us for one year? There goes my idea about creating a hyper-sensitive gravity detection device that would be able to transfer and receive instant messages across long distances. :roll:

 

[MeteorWayne]

My understanding (which could be wrong) is that it is assumed that gravity travels at the SOL, but there have been no conclusive experiments that have been able to prove that.

 

[trumptor]

Thanks MeteorWayne. Then I'm assuming that all the forces, weak, strong and electromagnetic...are assumed to travel at c? I never knew that. I always assumed that they just existed and didn't really travel.

 

[MeteorWayne]

I don't know anything about the travel speed of the strong or weak forces. To be honest, the distances where they are active is far too small to measure the speed (less than a nucleus distance). EM radiation travels at the SOL, that we have measured. As I said, gravity has not been measured with any precision AFAIK.

 

[trumptor]

Ok, I've found a bunch of articles online about the controversy and study about the speed of gravity. I'm off to read up on some of it.

Here's just a little from an article from Space.com...

Proving that gravity works at the speed of light would add support to Einstein's General Theory of Relativity and place limits on fringe theories in cosmology. Most physicists are confident that this is the case, but no one has ever confirmed it by direct measurement.

Isaac Newton long ago argued that gravity instead propagates instantaneously. The suggestion has not died. If it were true, a big door would open to wild theories of how the universe might work on the grandest scales, including its possible interaction with other universes or other dimensions. Even a slight difference in the speeds of light and gravity would give theorists nifty wiggle room to craft bizarre ideas about the mechanics of the unseen universe.

http://www.space.com/scienceastronomy/g ... 30116.html

 

[trumptor]

I guess that it was easiest to measure electromagnetic force since it can be turned on and off. Then I suspect that gravitational force will probably also be found to travel at c, even though I think It'd be more interesting if that turns out to not be the case.

I wonder though, if gravity travels at c, then how do black holes have gravity but don't emit light as a result of their escape velocity being greater than the SOL?

Sorry all if I've defected too much from the main point, just got on an inquisitive role. I will let the thread get back to its original intent

 

[kelvinzero]

I wonder though, if gravity travels at c, then how do black holes have gravity but don't emit light as a result of their escape velocity being greater than the SOL?

Haha.. good one! I have not the foggiest clue.

maybe.. and this is a total stab in the dark, the thing with a black hole is that the gravity was already there outside when the star originally collapsed.

What I am saying is, perhaps if there were some way to reach into a black hole and change its mass, we would not know. We can never see what is currently inside, only what it was at the instant it passed the event horison.. infact we never see it passing that point. From out point of view it is frozen without ever quite reaching it. So the gravity we experience from a black hole does not come from inside the event horizon.

 

[yevaud]

Well, the velocity of propagation of Light has nothing to do with it's strength. Otherwise, it would mean that the lower the gravity, the lower the velocity of propagation, wouldn't it? Clearly that isn't so.

 

[MeteorWayne]

If the mass of a black hole were changed, the orbits of the objects around it would change.

Remember, from a LY away, a black hole and a cluster of objects in a small space have the same effect on objects in orbit. They both are calculated (accurately) as a point source with the given mass.

 

[kelvinzero]

If the mass of a black hole were changed, the orbits of the objects around it would change.

Absolutely, eg if the black hole increased in mass by swallowing a star or evaporated by emitting hawking radiation.

Trumptor just asked the interesting question: If gravity is limited to the speed of light, then how can gravity excape a black hole?

I argue that gravity cannot escape the event horizon of a black hole, and the gravity we would nevertheless feel comes from an effect at or above the event horizon. Nothing, not even 'gravitons' if such exist, can be transmitted from mass inside the event horizon to effect mass outside. If there were a way to change what was inside the event horizon after it were formed, this could not change the parameters of the event horizon to an external observer.

 

[ramparts]

Ahhh okay let's get some things straightened out here.

The speed of light isn't something intrinsic to light, it's intrinsic to the fabric of spacetime. You've always heard about time as the "fourth dimension"; well, that's true, but you can't measure time in the same units as space (say, seconds as opposed to feet), so you need to multiply your time measurements by some factor to make it into a length measurement, to make it like a spatial dimension. That factor is the speed of light - multiply a time by the speed of light, and you get a length.

What this means in the context of special relativity is that the speed of light is the universe's speed limit, and the only stuff which can go at that speed limit have to have no mass (anything with mass can get arbitrarily close to the speed of light - say up to 99.999999% of it - but never hit it). As far as we can tell, the only particles which are massless are photons (light particles), gravitons, and gluons, so those are the only guys which will travel at the speed of light. Photons carry the electromagnetic force, gravitons carry gravity, and gluons carry the strong nuclear force, so (at least theoretically) those forces all propagate at the speed of light. Those are three of the four fundamental forces; the other one, the weak nuclear force, is mediated by two particles, called the W and Z bosons, which are both massive, so the weak force is the one that travels slower than the speed of light.

 

[ramparts]

As for a couple of other things that popped up....

It's intuitive to think that gravity propagates instantaneously (hell, Newton did, and the guy invented calculus just to solve a physics problem). But once again Einstein's theory of special relativity puts the kibosh on that. As I said above, nothing can travel above c, and there's a very good reason for that: if a particle travels faster than the speed of light, it can travel backwards in time. Now, that sounds awesome, and physics actually can conceivably allow for some form of time travel (topic for another thread, however!), but there's always one very strong caveat: there must be causality. That is, if event A causes event B, then in no reference frame should I be able to see event B causing event A. If I can send a signal to you faster than the speed of light, you can react to it before I send it, and that's just not allowed. That signal can include, say, me getting rid of some mass and you feeling its gravitational effects. So it would be hugely problematic if gravity travelled faster than the speed of light.

As for the über-interesting question about how gravity escapes from black holes if it's limited to the speed of light... the answer is essentially that gravity never does any "escaping." Normally it's more or less valid to think of forces as working by exchanging particles between two objects: say you have two charged spheres, they send photons back and forth. But while that's a useful picture, it's not exactly what happens. Whether it's electromagnetism or gravity, the fundamental object is called a field. Fields exist throughout space telling you how strong the force is at each point, and in what direction it's pointing. Let's get a little technical for a second. The particle picture comes from "quantizing" the fields: you can think of the particles as little virtual particles popping into existence whenever the field gets a little burst of energy in a particular spot. So they're not real particles that travel all the way from point A to point B. They're ephemeral things, always popping into and out of existence, that trace the fields they represent. And once a field is there, it stays there. So before a black hole forms, the matter that makes it (say a large star) has a gravitational field, and when the star collapses into a black hole, the field remains. If the black hole eats up matter and grows, the field sees that matter before it enters the black hole, and so it takes account of that and becomes a bit stronger. It doesn't actually "see" inside the black hole, it just knows what went in and assumes it's still there. (This means that if some of the mass inside a black hole were to suddenly stop existing, the gravitational field wouldn't change. Thankfully we're pretty sure matter doesn't just cease existing.)

Sorry that's a bit long, but I hope it helps! Feel free to post back with any questions, I'm sure I was unclear about something or other.

 

[trumptor]

Well, I guess there goes my nobel prize :lol:

But if gravity is just caused by matter putting a "dent" in the fabric of space-time, then would it be possible that there is no graviton responsible for the force of gravity, that it is different from the other forces? This is all way above me, so it may be a stupid question :? .