Its Engineering Jim but not as we know it

[MikeSharp01]

Watch this and be moved by what we can do, how far you can go in 40 years if your are travelling at 40,000 miles an hour, notice how insignificant we are when seen from 20.8 billion Km away and then ponder our stupidity - I think therefore I am just a little worried.

Edited September 5, 2017 by MikeSharp01
finish sentance

[Temp]

Quote

Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space. (Douglas Adams, The Hitchhiker's Guide to the Galaxy)

 

Space blows my mind..

 

Consider an imaginary glass sphere around a star. All the light emitted by the star would pass through the surface of that sphere. So now enlarge the sphere until its the size of say the solar system. The surface area is now massively bigger and the light passing through each square meter is proportionately that much less. At that distance solar panels are too ineffective to power a space craft.

 

Now expand the sphere even more... The star Cassiopeia is 150,000,000,000,000,000 km away. So all of the light from Cassiopeia is effectively spread out over the surface area of a sphere with that radius! That's one heck of a large area. Yet enough photons of light pass through every square mm that we can see Cassiopeia with the naked eye.  

 

Edited September 5, 2017 by Temp

[SteamyTea]

And we don't know how long a photon lasts yet, at least 1 billion, billion years.

And they have a mass of less than two-billionths of a billionth of a billionth of a billionth of a billionth of a billionth of a kilogram.

 

[Crofter]

I should confess to being an absolute space geek. And I think it was dim childhood memories of the Voyager encounters at Uranus and Neptune that started it all off.

These days I get excited about the dawning of reusabiity and, hopefully, the boom in space industries that will follow. And watching a rocket fly backwards through its own plume is pretty cool anyway (for those who haven't watched one yet, SpaceX are launching a DOD payload in a couple of days time, watch the webcast, the landing footage should be amazing).

[Temp]

The fact that amazed me about the SpaceX landings was discovering that the landing rockets can't be throttled. Basically they have to be fired at just the right height. Too high and the rocket stops some feet above the pad and drops the last bit. Too low and it's travelling too fast when it lands.  

 

[Crofter]

1 hour ago, Temp said:

The fact that amazed me about the SpaceX landings was discovering that the landing rockets can't be throttled. Basically they have to be fired at just the right height. Too high and the rocket stops some feet above the pad and drops the last bit. Too low and it's travelling too fast when it lands.  

 

 

Well that's a slight overstatement. They can throttle, but the lowest setting still isn't low enough to hover- the thrust to weight ratio is always more than one. So there is some margin for adjustment but, yes, the basic point remains. You've got to hit the brakes at pretty much the right time. It actually saves quite a bit of fuel compared to hovering, and makes the vehicle less susceptible to cross winds.

[TerryE]

On 05/09/2017 at 22:09, SteamyTea said:

And we don't know how long a photon lasts yet, at least 1 billion, billion years.

And they have a mass of less than two-billionths of a billionth of a billionth of a billionth of a billionth of a billionth of a kilogram.

 

 

Eh?  I assume that this Nick humour. Both statement are true but misleading as is: photons last at least one second and weigh less than a kilogramme.

 

At least the life of the universe, or there'd be no cosmic background radiation, and no theories that I know of postulate a half-life. Photons travel at the speed of light so they must be massless within Special and General Relativity.

[Jeremy Harris]

2 hours ago, TerryE said:

 

Eh?  I assume that this Nick humour. Both statement are true but misleading as is: photons last at least one second and weigh less than a kilogramme.

 

At least the life of the universe, or there'd be no cosmic background radiation, and no theories that I know of postulate a half-life. Photons travel at the speed of light so they must be massless within Special and General Relativity.

 

Sitting on my desk right now (but not spinning) is a Crookes Radiometer.  It spins around quite merrily when the sun is shining on it, and has done for the past 30 years or so.  For  a year or so I pondered on what drove the thing, as, being in a vacuum (or so I thought) it appeared to defy the theories of relativity.  The answer is that it's in an imperfect vacuum, and so there are a few air molecules inside the sphere, enough to be moved and provide motive force to what is a pretty simple heat engine.

 

It is true that photons have zero rest mass, but not that they may always have zero mass.  Things get messily complicated when you get to the behaviour of wave/particle systems at, or possibly in excess of, the speed of light.  Getting your head around a zero rest massless entity (that is never at rest) and that possesses energy and momentum as intrinsic characteristics, is something that took my small brain some time to grasp, and is well off-topic for this forum.

[SteamyTea]

The photon is a 'special case' particle.

It can be kinetically modelled, which makes it look like it has mass, until it comes to rest, or modelled according to the rules of relativity, which are just odd.

[TerryE]

2 hours ago, SteamyTea said:

It can be kinetically modelled, which makes it look like it has mass

 

Being pedantic, it has zero rest mass, but it also has a defined energy E = h/λ and hence a defined momentum = h/λc in the direction of travel, which means that it can be kinetically modelled.