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[Rapstah]

If v = c

Unhandled exception at 0x00000.

 

[ThoseDeafMutes]

Quick question:

Earlier in this thread, it was noted that light, or more specifically, photons, are the only things we've directly observed in the universe to travel at the speed of light, because they are massless particles.

But if photons are massless, why are they affected by gravity in the first place? I know they *are* affected (gravitational lensing), but if the equation is GM1M2 / r^2, why is there any force at all?

Does it have to do with the fact that GM1M2 / r^2 is a low-energy approximation? Or do photons actually have mass? I also remember reading that a bunch of photons hitting a giant solar sail would actually exert a mild force, which implies mass as well.

Under GR/SR, you can think of gravity as the bending of spacetime caused by mass. In order to escape the gravity of an object, you have to have sufficient velocity to "clear" the curve, but in the case of a black hole it's become too great for even something going at the highest possible speed to clear, ergo trapping light.

Gravitons are actually a quantum field theory thing, and I may be wrong but I don't think Relativity actually mentions the existence of a force carrier particle for gravity.


Momentum as p = mv applies to large, slow things (i.e. Newtonian, ordinary sized objects), but the more accurate and general formula for the relationship is:

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


Where E = energy, p = momentum, m = mass and c = speed of light in a vacuum.

Photon momentum is:

p = E/c

Ergo photons have momentum, ergo radiation pressure and solar sails.

 

[szaromir]

Does it have to do with the fact that GM1M2 / r^2 is a low-energy approximation? Or do photons actually have mass? I also remember reading that a bunch of photons hitting a giant solar sail would actually exert a mild force, which implies mass as well.

You are correct, Newton's equation is a weak field approximation. The actual gravity effect isn't like pulling strings between two massed objects - the actual mechanism is completely different. Every object that has energy curves the space around it. So the reason photons are affected by grafity is because they are objects moving on straight lines* through a curved space. So gravitational lensing is measuring the curvature of space.

*it's a bit simplified, but sufficient for our purposes here

 

[szaromir]

Under GR/SR, you can think of gravity as the bending of spacetime caused by mass. In order to escape the gravity of an object, you have to have sufficient velocity to "clear" the curve, but in the case of a black hole it's become too great for even something going at the highest possible speed to clear, ergo trapping light.

Gravitons are actually a quantum field theory thing, and I may be wrong but I don't think Relativity actually mentions the existence of a force carrier particle for gravity.

Gravity is carried through waves in general theory of relativity, however no one knows how to quantize these waves and still obtain expected results. That's why general relativity and QFT are still separate branches.

And you don't need mass to create gravitational field, just energy (so photons generated grafity as well).

 

[Sharp]

Not time travel in the movie sense. The speed at which time passes can change though. An hour on earth and an hour on a starship traveling near the speed of light will seem the same to the people at either place. However, earth people will appear to be going in fast forward to the people on the starship and the people on the starship will appear to be moving extremely slow, or even stuck on pause to the people on earth.

This part ain't exactly true. Because speed is all relative, the earth people will also appear to be moving at near-light speed to the guy in the ship--and hence will actually also seem to be stuck or frozen in time! Don't think about it too hard, your brain will explode.

 

[Zeppu]

This is gravity shown in 2 dimensions. What's shown there is true for every dimension (including time).

Gravity is just there, you cannot instantaneously create mass to generate gravity so the effects of gravity move with the mass itself, but they're always there.

 

[szaromir]

This part ain't exactly true. Because speed is all relative, the earth people will also appear to be moving at near-light speed to the guy in the ship--and hence will actually also seem to be stuck or frozen in time! Don't think about it too hard, your brain will explode.

Why would it explode? It's in accordance with the principle of relativity, if the phenomenom was asymmetrical than it'b be mind boggling.

 

[Sharp]

Why would it explode? It's in accordance with the principle of relativity, if the phenomenom was asymmetrical than it'b be mind boggling.

I think most people have a hard time accepting that if I went on a hugely fast rocket, we would both look slower, shorter and more massive to each other. But then, maybe I'm just projecting my own beliefs onto other people.

 

[szaromir]

I think most people have a hard time accepting that if I went on a hugely fast rocket, we would both look slower, shorter and more massive to each other. But then, maybe I'm just projecting my own beliefs onto other people.

I think if people tried to imagine a frame of reference not attached to any of the objects, but one that's between them and has the same (absolute) speed relative to both of them, they could easily see that both objects should observe the same phenomena relative to each other.

 

[Andeeh]

Wait, wasn't it "proven" that the universe is constantly expanding? if so, does this mean the outer edges of the universe expanding is moving faster then the speed of light since light cannot move past this barrier?

 

[abstract alien]

Wait, wasn't it "proven" that the universe is constantly expanding? if so, does this mean the outer edges of the universe expanding is moving faster then the speed of light since light cannot move past this barrier?

Could it be moving at the same speed?

 

[Rapstah]

Wait, wasn't it "proven" that the universe is constantly expanding? if so, does this mean the outer edges of the universe expanding is moving faster then the speed of light since light cannot move past this barrier?

I thought that the "edge of the universe" was in fact light moving at its typical speed.

 

[Scrow]

Is backwards time travel still considered impossible?

That's the only time travel that matters.

i think there's some stuff about virtual particles doing it

 

[Messypandas]

So if i'm in a car, travelling the speed of light and turn the high beams on - what happens?

 

[Sharp]

So if i'm in a car, travelling the speed of light and turn the high beams on - what happens?

Cars can't travel at the speed of light... and you couldn't flick the high beams on because you wouldn't be moving forward in time. However, if you were traveling at near light speed and flicked the high beams on, the light that came out would seem to be moving at the same speed as if you were on earth and flicked them on. All these equations basically work out so that light always looks like it's the same speed, and everything else warps to accommodate that fact.

 

[TheChewyWaffles]

So if I were to fall into a black hole and I could view the earth with a large telescope, would it be aging very rapidly?

 

[StuBurns]

So if I were to fall into a black hole and I could view the earth with a large telescope, would it be aging very rapidly?

You can't see out of a black hole.

 

[TheChewyWaffles]

Cars can't travel at the speed of light... and you couldn't flick the high beams on because you wouldn't be moving forward in time. However, if you were traveling at near light speed and flicked the high beams on, the light that came out would seem to be moving at the same speed as if you were on earth and flicked them on. All these equations basically work out so that light always looks like it's the same speed, and everything else warps to accommodate that fact.

Huh? I thought redshift is the phenomenon where light changes the way an object appears because of changes in speed relative between two bodies.

You can't see out of a black hole.

Actually, there's no reason I could not see out of a black hole.

 

[StuBurns]

Don't be a smart ass. I know that.

Actually, there's no reason I could not see out of a black hole.

Make up your mind dude.

And you couldn't.

 

[Sharp]

Huh? I thought redshift is the phenomenon where light changes the way an object appears because of changes in speed relative between two bodies.

The light itself appears to be moving at the same speed, but not necessarily with the same wavelength (IIRC). I can't be much more specific than that because I've totally forgotten most of the little quantum physics I ever knew.

And you probably could see out of a black hole, but I have no idea what you'd see. Physics near black holes is hard.

 

[StuBurns]

And you probably could see out of a black hole, but I have no idea what you'd see. Physics near black holes is hard.

If light can't escape, how are you meant to see out?

 

[Sharp]

If light can't escape, how are you meant to see out?

We see things because light reflects off them, and light can certainly enter a black hole. It can't leave, but it's not like we see things by shooting beams of light out of our eyes. That said, the manner in which the light would be distorted would likely make what we saw totally bizarre. Not that it would matter because we'd likely be ripped apart atom by atom by that point.

 

[Zeppu]

Cause light needs to go in for you to see out. But light 'stops' in a black hole, so, um. I dunno.

 

[ThoseDeafMutes]

If light can't escape, how are you meant to see out?

Eyesight doesn't involve firing radar beams and waiting for them to return. If light is coming into the hole and you are inside it (magically not crushed and/or spaghettified), then it can enter your eye and stimulate the rods and cones, creating vision.

 

[Scrow]

So if i'm in a car, travelling the speed of light and turn the high beams on - what happens?

You would never get to turn the headlights on because you would not be experiencing time.

An outside observer would see you flying past at the speed of light, and if they could see you, you would be stuck, with your hand halfway to the light switch, in time.

 

[JoshuaJSlone]

But what about con-time?

Is backwards time travel still considered impossible?

That's the only time travel that matters.

The trouble, to go back to the OP's train example, is there ain't no gettin' off this train we're on.

See, I always get confused at the 'as it gets faster it increases in mass'. Why?

I'll tell you how I imagine it, though I'm not enough into physics to say if this is at all looking at it the right way.

Say you've got two 1 kilogram things, each experiencing 10 years of existence. So, 10 kilogram-years. However, thanks to relativistic differences, Thing A has already experienced 10 years by the time Thing B is only at 5 years. So from Thing B's perspective, Thing A has gone through 10 kilogram-years in a mere 5 years, and so dividing by time Thing A has averaged at 2 kilograms.

 

[Angry Fork]

We're not gonna travel through the universe by FTL, it'll be through some other thing somehow, like ...bending light to our whim, or something. We'll be able to instantly teleport to places without the need of actual travel.

 

[Scrow]

Quick question:

Earlier in this thread, it was noted that light, or more specifically, photons, are the only things we've directly observed in the universe to travel at the speed of light, because they are massless particles.

But if photons are massless, why are they affected by gravity in the first place? I know they *are* affected (gravitational lensing), but if the equation is GM1M2 / r^2, why is there any force at all?

Does it have to do with the fact that GM1M2 / r^2 is a low-energy approximation? Or do photons actually have mass? I also remember reading that a bunch of photons hitting a giant solar sail would actually exert a mild force, which implies mass as well.

gravity doesn't affect photons. it bends SPACE, and photons travel THROUGH that space. it's like bending a hose to redirect water instead of bending the water itself.

 

[Messypandas]

You would never get to turn the headlights on because you would not be experiencing time.

An outside observer would see you flying past at the speed of light, and if they could see you, you would be stuck, with your hand halfway to the light switch, in time.

Woah, heavy. Thanks. So if my primitive mind is understanding this correctly, It would be impossible for me to move inside the car, even on a long enough timeline? My hand would never reach the knob?

 

[StuBurns]

Eyesight doesn't involve firing radar beams and waiting for them to return. If light is coming into the hole and you are inside it (magically not crushed and/or spaghettified), then it can enter your eye and stimulate the rods and cones, creating vision.

But light past the event horizon would be shifted out of the visual spectrum wouldn't it?

 

[Sharp]

But light past the event horizon would be shifted out of the visual spectrum wouldn't it?

Considering how much energy there is coming into a black hole (and emitted by all sorts of sources) I doubt all the light would be shifted out of the visual spectrum. Like I said, I imagine we could see something, I just have no idea what that something would be and doubt that it would have much to do with the universe as we know it.

 

[TheChewyWaffles]

You would never get to turn the headlights on because you would not be experiencing time.

An outside observer would see you flying past at the speed of light, and if they could see you, you would be stuck, with your hand halfway to the light switch, in time.

This isn't true. He would be experiencing time normally. The outside world would be aging a lot faster than him.

 

[Sharp]

This isn't true. He would be experiencing time normally. The outside world would be aging a lot faster than him.

If he were somehow at light speed (which isn't possible) he wouldn't experience time at all. That was what the rather brilliant OP conveyed, in a much easier-to-understand way than the other explanations I'd heard.

 

[Always-honest]

Like, zero bro. At light speed it would only take you minutes to get there, not taking into account time dilation. You wouldn't even have time to blink from your perspective, going that speed.

that's not very exact is it.. ;P

 

[ThoseDeafMutes]

Woah, heavy. Thanks. So if my primitive mind is understanding this correctly, It would be impossible for me to move inside the car, even on a long enough timeline? My hand would never reach the knob?

It doesn't matter what number you multiply zero by, you will never get a non-zero number, regardless of what Big Boss would have you believe. So if you have zero velocity through time (at C) then no amount of time passing will help you move.

What you're asking is essentially if you get exceptionally close to C, then what happens when a beam of light shines from it. The answer is it moves at the speed of light away from you. But as mentioned before, this comes with heavy length contraction / time dilation to make sure you always measure it as that. When you slow down, the length contraction / time dilation eases up, but you still measure it as escaping from you at the exact same speed as when you were going fast.

IIRC, the point C is when your velocity through time become 0, and when you plug in values higher than C your time component becomes imaginary (because working it out involves taking the square root of the velocity, which, if negative, is an imaginary number).

But light past the event horizon would be shifted out of the visual spectrum wouldn't it?

Maybe, but there are way bigger practical problems involved, complaining about that seems silly. The thrust of his question can be answered, just don't take "seeing" literally. Instead discuss what is happening in the outside world from a reference frame such as that.

 

[SappYoda]

So, if we are moving at the speed of light, time pauses? Wouldn't that make us have like infinite speed?

 

[MaddenNFL64]

I like:



Simple and straight to the point.

Yay. Fuck wall of text.

 

[SteveWinwood]

God does it. Pretty easy huh?

 

[ThoseDeafMutes]

God does it. Pretty easy huh?

Why we still got scientists

 

[Sharp]

So, if we are moving at the speed of light, time pauses? Wouldn't that make us have like infinite speed?

Particles with mass can't move at the speed of light, so asking questions like "what would happen if we moved at light speed" or "would we have infinite mass" are sort of missing the point IMO. IIRC the fastest reasonable speed we'd likely to be able to go before any material would be shredded almost instantly by space debris is like 0.1c.

 

[SappYoda]

Particles with mass can't move at the speed of light, so asking questions like "what would happen if we moved at light speed" or "would we have infinite mass" are sort of missing the point IMO. IIRC the fastest reasonable speed we'd likely to be able to go before any material would be shredded almost instantly by space debris is like 0.1c.

Yeah but if time pauses when light is moving why isn't its speed infinite ?

 

[Sharp]

Yeah but if time pauses when light is moving why isn't its speed infinite ?

Photons themselves don't experience time, but that doesn't mean they're moving at infinite speed. It's just that relativistically, light must always be observed by every particle to travel at the same speed, so if you are also traveling at light speed the only way to make the equations work out so that you still experience light as going at light speed is for you and others to (1) have no mass, and (2) not experience time. That way your mass doesn't have to become infinitely huge to everyone else and, since you don't experience anything, you never observe light traveling at any speed other than that of light. It's convoluted but it works.

 

[Escape Goat]

Thank God (or spaghetti monster) for you phyiscs nerds. No way my brain can remember, let alone comprehend, all these laws and theories.

Now if someone can give an easy to understand explanation of string theory and dark matter/energy you win this thread.

 

[Messypandas]

Thank God (or spaghetti monster) for you phyiscs nerds. No way my brain can remember, let alone comprehend, all these laws and theories.

Now if someone can give an easy to understand explanation of string theory and dark matter/energy you win this thread.

If you're interested in this stuff as i am, you might wanna watch the "How long is a piece of string?" with Alan Davies. Its fascinating, explores a lot of topics and quite funny.

Its all on youtube too:

http://www.youtube.com/watch?v=01AT-yt9utw

 

[ThoseDeafMutes]

Thank God (or spaghetti monster) for you phyiscs nerds. No way my brain can remember, let alone comprehend, all these laws and theories.

Now if someone can give an easy to understand explanation of string theory and dark matter/energy you win this thread.

Dark Matter/Energy is the name given to "whatever that thing is that is responsible for the universe's continued expansion despite there being insufficient visible matter to account for it under current physical models." There are many possible explanations, and as we discover more about it we will model and describe it more accurately. Assuming it's like most people think it is currently, then it's sort of like regular matter/energy, except it only interacts using gravity, rather than the other fundamental forces.

Regarding String Theory, pay no attention. It has no empirical basis at the moment, and is something for the physics and maths nerds to mull over. It proposes that the fundamental "unit" of "stuff" are tiny vibrating strings. Other interesting things include the prediction of 11 spatial dimensions that are "compacted" in such a way that we cannot normally perceive or interact with them.

TBH I pay nearly no heed to string theory.

 

[Scrow]

Regarding String Theory, pay no attention. It has no empirical basis at the moment, and is something for the physics and maths nerds to mull over. It proposes that the fundamental "unit" of "stuff" are tiny vibrating strings. Other interesting things include the prediction of 11 spatial dimensions that are "compacted" in such a way that we cannot normally perceive or interact with them.

TBH I pay nearly no heed to string theory.

isn't the idea that string theory will fill the mathematical gaps (problems) that the current model of quantum field theory has?

 

[StuBurns]

M theory is eleven dimensions, String theory is ten.

isn't the idea that string theory will fill the mathematical gaps (problems) that the current model of quantum field theory has?

I believe it's essentially trying to create a model in which current quantum mechanical theory and general relativity can coexist.

 

[The Technomancer]

isn't the idea that string theory will fill the mathematical gaps (problems) that the current model of quantum field theory has?

Yes, but its entirely a numbers game. Its not science if its completely unverifiable, its just speculation really.

 

[Pikelet]

Someone already mentioned this book in the thread, but Brian Greene's 'Fabric of the cosmos' provides a fantastic explanation for all this neat stuff. Highly reccomended if you would like to know a bit about the universe without spending four years at a university.

 

[Jarmel]

So I read the OP and I have a question. Why is light the fastest thing in existence?