speed of light?

[andy_s]

Baron Greenback said:

If gravity alter the path of light, especially black holes, no light escapes the event horizon. The light below some of it is performing a 180 degree path around the back and comes back towards camera.

What happens to the light going straight towards the event horizon does it speed up until hitting the black hole or as the theory goes nothing can go faster and just stays are speed of light?

The life of a photon...

No, it can't speed up as it's going as fast as causation in spacetime allows, what it does do is blueshift [to an outside observer].

Any photons incident to the event horizon will enter and never be seen again, from their perspective. For an outside observer the photon would appear to slow as it approaches the EH until it hangs there.

Think of a BH as a drain for spacetime and the event horizon as the point at which the cascading spacetime goes faster than light*, light always moves at a constant c and spacetime doesn't go faster than c until after the EH. [*Things in the universe can't go faster than light, but 'spacetime' can, except it's now 'timespace'].

'Timespace'? After the EH space and time are exchanged [specifically the radial direction cone interchanges its role with time] so to go back to the EH you'd have to go back in time at this point, all futures lead to the singularity.

You can have a photons orbiting [photons always travel in a straight line relative to the fabric of spacetime, spacetime is curved, so light follows the geodesic], but as the event horizon grows with each increase in mass/energy it'd be very unstable.

Any light emitted some distance from the event horizon and directly away from it will be red-shifted to near infinity [outside observer] depending how far from the EH you are, but the photon is still travelling along spacetime at light speed as spacetime is moving close to light speed but in the opposite direction at this point.

There's weird, then there's black hole weird. [And this is amateur so any boffins with corrections are welcome, hence the convenient gaps, gotta feeling that last point is clunky... ]

Edited by andy_s on Thursday 30th June 19:45

[deckster]

Dr Jekyll said:

It doesn't need energy to keep moving in a vacuum, only to accelerate or decelerate. Orbits only decay because there is a bit of atmosphere up there.

An object in orbit is constantly accelerating due to the resultant gravitational force. GCSE physics

[Fusion777]

deckster said:

An object in orbit is constantly accelerating

This is true. Acceleration is a vector, which means it has a magnitude and direction. The magnitude is constant (assuming a circular orbit), but the direction is constantly changing.

[Simpo Two]

deckster said:

An object in orbit is constantly accelerating due to the resultant gravitational force. GCSE physics

In which case our moon should have reached a bazillion mph years ago and flown off to infinity...

Anyway, if gravity is accelerating the moon then either gravity is being used up... or we have an infinite power supply...

[jules_s]

jules_s said:

What (sort of) buzzed my noodle was the 3d/transition movement of our solar system

I never. ever. considered that.

Weird really. we really are a small point in the larger area - seeing the orbits whilst travelling was - oooh

Probably done before but it still made me think a bit

Like that

[Alias218]

Super Sonic said:

Ayahuasca said:

If I shine a laser at a large object a long way away, say Jupiter, and then quickly move the laser along Jupiter’s equator from one side to the other, the dot of light can travel faster than light.

It's not the same dot.

Ah, but what if the laser beam is replaced by a very long and inflexible stick? Surely then if you’re moving the stick from Earth and the other end moves very quickly from one side of Jupiter to the other instantly, could that exceed the speed of light (if the distance is sufficient)? Or would the inertia of the stick’s mass be so great as to prevent that?

Also, this is a thought I had many years ago as a precocious teen. I put to to more learned this:

1. Speed is defined as d/t
2. Time stops at the speed of light, theoretically
3. Therefore, if t=0 then photons are either stationary, or are travelling infinitely fast (depending on how you want to handle /0.

As mentioned already, if photons are travelling from A>B in an instant from a local perspective does this satisfy that? But then, why if they are travelling infinitely fast, why do they have an objective speed of 300,000km/s from an outside PoV? Why that speed?

I’ve always wondered if they are in fact stationary and what we perceive as the speed of light is the transfer of energy between these stationary photons (consider the wave/particle duality). Photons in the very low energy state are the dark matter we detect, and the dark energy is the “photonic energy” (or whatever) travelling between gaps where there are no photons.

As you can tell, my knowledge of quantum theory is practically zero but I always thought it was an interesting thought experiment if nothing else

Edited by Alias218 on Tuesday 5th July 08:48

[Dr Jekyll]

You couldn't move the end of the stick that quickly.

[annodomini2]

There is no such thing as inflexible, using 30mm carbon tube your stick will weigh iro 139 million tons, so you are not moving it easily.

[Alias218]

Consider me shot down

[Fusion777]

The end of your stick is governed by the same laws of special relativity as everything else- anything with mass can’t reach the speed of light, never mind exceed it.

[deckster]

Fusion777 said:

The end of your stick is governed by the same laws of special relativity as everything else- anything with mass can’t reach the speed of light, never mind exceed it.

To be fair, that's a little circular given the context of the question.

I suspect the answer is something along the lines of: ignoring the engineering challenges, to move the end of a stick by rotating the stick around the other end requires an input of energy, to exert a force. Let's say the speed of the end of stick does approach light speed, in which case the energy required to generate this movement will approach infinity. Therefore to accelerate the end of the stick to light speed will require an infinite amount of energy, which we can't give, and therefore the universe is safe once again. Phew.

[Ayahuasca]

The stick problem can be thought of like spinning around on an office chair. If your arms are stretched out, it takes more energy to spin. I now know that this is because your fingers are in danger of approaching light speed.

[annodomini2]

deckster said:

Fusion777 said:

The end of your stick is governed by the same laws of special relativity as everything else- anything with mass can’t reach the speed of light, never mind exceed it.

To be fair, that's a little circular given the context of the question.

I suspect the answer is something along the lines of: ignoring the engineering challenges, to move the end of a stick by rotating the stick around the other end requires an input of energy, to exert a force. Let's say the speed of the end of stick does approach light speed, in which case the energy required to generate this movement will approach infinity. Therefore to accelerate the end of the stick to light speed will require an infinite amount of energy, which we can't give, and therefore the universe is safe once again. Phew.

It's more that it will snap, but basically if the force applied is below the magnetic interaction limit of the molecules in the material it will move, but above the molecules will compress together.

Should you be able to have a material that could take the stress of the force, the motion would travel along the "stick" in a wave at the speed of light.

But as we have no such material it would break.

[deckster]

annodomini2 said:

deckster said:

Fusion777 said:

The end of your stick is governed by the same laws of special relativity as everything else- anything with mass can’t reach the speed of light, never mind exceed it.

To be fair, that's a little circular given the context of the question.

I suspect the answer is something along the lines of: ignoring the engineering challenges, to move the end of a stick by rotating the stick around the other end requires an input of energy, to exert a force. Let's say the speed of the end of stick does approach light speed, in which case the energy required to generate this movement will approach infinity. Therefore to accelerate the end of the stick to light speed will require an infinite amount of energy, which we can't give, and therefore the universe is safe once again. Phew.

It's more that it will snap, but basically if the force applied is below the magnetic interaction limit of the molecules in the material it will move, but above the molecules will compress together.

Should you be able to have a material that could take the stress of the force, the motion would travel along the "stick" in a wave at the speed of light.

But as we have no such material it would break.

Oh agreed, as a practical experiment it's completely impossible. That's why I said "ignoring engineering challenges". But I figured there must be some more fundamental reason than "we can't make one of those".

[Dr Jekyll]

annodomini2 said:

It's more that it will snap, but basically if the force applied is below the magnetic interaction limit of the molecules in the material it will move, but above the molecules will compress together.

Should you be able to have a material that could take the stress of the force, the motion would travel along the "stick" in a wave at the speed of light.

But as we have no such material it would break.

This is the point, it's a thought experiment about the speed of light being a limit, not an engineering challenge.

The one that I can't get my head round is quantum entanglement in these situations. Checking the status of one particle seems to affect the status of the other entangled one straight away. What happens if your astronaut takes an entangled particle with them and you agree that you will check it's partner at a certain date and time if you want them to do something specific. Immediately after that time they check theirs and figure out what you've done and what you want them to do.
I'm sure it wouldn't work but I can't see why not. It also sounds almost practical while a 400 million mile long stick would give even the maddest of mad scientists pause for thought.

[Fusion777]

You can't transmit any meaningful information through entanglement- have a read on Bell's paradox. It's been quite a while since my degree now, so many of the finer points evade me without trawling through articles.

[scottyfocus]

ChocolateFrog said:

Following on from that as everything is expanding and therefore moving away from us. Stuff that was observable to us yesterday is now out of sight forever today and ultimately in the distant future everything bar very local cluster will beyond our observable sphere.

It's going to be a very dark sky in a trillion years.

What I like about/ am fascinated by that is that an intelligent civilisation evolving a trillion years from now will develop science etc. But they will look up at the sky and see only the stars in their own galaxy. No other galaxy will be within their observable universe and no matter what science they dream up they will never be able to say for sure there is something beyond. The information needed to do so will be lost.

Makes you wonder what information has already passed us by.

[BorkBorkBork]

scottyfocus said:

ChocolateFrog said:

Following on from that as everything is expanding and therefore moving away from us. Stuff that was observable to us yesterday is now out of sight forever today and ultimately in the distant future everything bar very local cluster will beyond our observable sphere.

It's going to be a very dark sky in a trillion years.

What I like about/ am fascinated by that is that an intelligent civilisation evolving a trillion years from now will develop science etc. But they will look up at the sky and see only the stars in their own galaxy. No other galaxy will be within their observable universe and no matter what science they dream up they will never be able to say for sure there is something beyond. The information needed to do so will be lost.

Makes you wonder what information has already passed us by.

If the universe’s eventual demise is due to heat death, then that’s over 100 trillion years from now. If that’s the case, we have appeared surprisingly early.

It reminds me a little of the doomsday argument, which posits that it’s unlikely those humans alive today have appeared early in the lifetime of the species. Maybe the same should apply to this universe, if we are part of a multiverse, that life is unlikely to have appeared in the first fraction of a percent of its lifetime.

https://en.wikipedia.org/wiki/Doomsday_argument

[55palfers]

In 1968 the Moody Blues assured us we can travel faster than light if you want to.

https://www.youtube.com/watch?v=kLynv5gmWtg

[HardtopManual]

The thing that really helped me to understand why the speed of light (/causality) is an upper bound and why it is a constant for every observer, everywhere, is a hyperbolic spacetime diagram - see https://en.wikipedia.org/wiki/Hyperbolic_motion_(r... for an example. Light's representation on a hyperbolic spacetime diagram is an asymptote that other objects can approach, but never reach.