Philosophy Now Forum

A show I just watched on Netflix, featuring Steven Hawking as the host, claimed that time runs slower in orbit than it does here on Earth.

Apparently they know this because of super accurate clocks which are part of the GPS satellites. The time difference is very small, but enough that the software has to correct for it, or the GPS system would gradually drift out of focus and become useless.

Ok, here's the chance to show off your physics degree! Or whatever you got. Please educate us further on this if you can.

I'm asking because I'll start collecting social security next month, and I'm wondering if maybe I should be doing so in orbit.

It's about special and general relativity, and no I don't have a degree yet.

In special relativity the faster a mass object travels the slower time will appear to pass for it relatively, like wise in general relativity the lower the gravity relatviely to Earths ~1g the faster time will appear to pass. You can do real world experiments to prove this, the first ones were flying hyper accurate clocks around the world and comparing time: they lost a few millionths of a second relative with earth bound clocks. And putting clocks at the top of sky scrapers, where not only were they revolving about Earth's axis slower but gravitation was less. The whole system of GPS uses a sort of 3d spherical triangulation to put an object in a precise position at a precise time, it doesn't have to take account of speed as such because it knows where it is and how fast it usually travels and also what it's fairly consistent gravity is so it just adjusts the clocks in synch with a base unit by a certain amount of time according to at least 3 or 4 sattelites motion about the Earth that it can receive although there are usually more than 3 or 4. The proof of the pudding is in the eating though, if they don't let the clocks get adjusted it goes out of synch and over time your position on Earth varies so widely that the whole system is useless.

Sadly on Earth you are travelling around the solar system at a fairly constant speed at about 1g and revolving around the galactic centre likewise, so you will I am afraid get the cheque at the same time as everyone else or more or less allowing for where you live and GMT.

The universe song:

http://www.youtube.com/watch?v=yq4uCWtQE24

See here for precise scientific details, and they are at least fairly accurate Monty Python or not.

Thanks for the update and additional info Blaggard.

Although this has little practical use in day to day life, it is nonetheless interesting to reflect that time is not a reliable constant as we typically assume.

According to the show I watched, you'd need an object of huge mass to make time go twice as slow as normal, such as a black hole.

So if I can figure out how to orbit a black hole, I'll live twice as long, and get twice as many checks!

Felasco wrote:Thanks for the update and additional info Blaggard.

Although this has little practical use in day to day life, it is nonetheless interesting to reflect that time is not a reliable constant as we typically assume.

According to the show I watched, you'd need an object of huge mass to make time go twice as slow as normal, such as a black hole.

So if I can figure out how to orbit a black hole, I'll live twice as long, and get twice as many checks!

As long as you are in a different place from other people yes, but sadly the government will probably allow for that. They are clever like that, the tax system is likewise usually fairly bullet proof, but hell loopholes exist for some time at least.

YW though, there has to be some benefit to knowing all this shite.

Felasco wrote:A show I just watched on Netflix, featuring Steven Hawking as the host, claimed that time runs slower in orbit than it does here on Earth.

Check out Hafele-Keating. It actually depends on the orbit whether time runs slower or faster than on the surface.

Just to clarify; what Hafele-Keating did was to put an atomic clock on two aeroplanes, one circled the world heading east, the other went west. The clocks were compared with a third clock that remained on the surface and it was found that the one that went west ran slower to the one on the ground, whereas the one heading east ran faster.
Sounds weird, but if think of what is happening relative to the centre of the Earth, it is less puzzling. Consider that the Earth is spinning at roughly 1000mph at the equator: an aeroplane doing 500mph across the surface in the same direction the world is spinning, is in fact spinning around the centre of the Earth at 1500mph; whereas the plane flying against the spin of the Earth is actually spinning at 500mph, relative to the centre of the Earth. The clock on the surface is spinning at 1000mph. We are used to being stuck on the surface, so we naturally assume that our time is the right one, but it depends on your speed (gravity too, but I'm ignoring that for now).
So what's going on. Well you can get all mathematical and explain it in terms of timelines and geodesics, which is the useful way of doing it in terms of satnav and whatnot. Or you can take the easy option and imagine what is going on inside the atomic clocks. Basically atomic clocks count the vibration of atoms. If you think of the atom as bouncing up and down at a particular speed, then if you are stationary relative to that atom on one of the planes, or on the ground, you see it bouncing up and down at that speed. However, since all three clocks are moving, none of the atoms are bouncing straight up and down, rather they follow a zig-zag path. The faster the clock is travelling, the longer the path the atoms follow and hence the longer it takes and the slower the clock ticks. You don't notice any difference, because the same is true of every atom in your body.

Ah cool I learnt something there thanks for the info.

Sort of a "doppler/red shift" thing or is it something else?

so time relative to energy is govern by the top speed of that energy.so if a photon aproaches another photon front on is there a max closing speed max closing time.

jackles wrote:so time relative to energy is govern by the top speed of that energy.so if a photon aproaches another photon front on is there a max closing speed max closing time.

Yes c. Even when two photons travelling at c approach each other head on they do so at c, it is the speed limit of the universe.

Well, you have to distinguish between what actually happens, and what you perceive to be happening. c is the highest velocity you can measure. In the Large Hadron Collider for instance, there are two counter rotating particle beams that collide at various points. Each beam is travelling at about 3 m a second below c, with an energy of about 7 Tera electron Volts. The combined energy of collisions is 14 TeV. So as far as we are concerned, they are hitting each other at nearly twice the speed of light. If the hadrons could survive the collisions, though, and assuming they had little minds to do some measuring, one thing they might measure is their displacement due to the collision; how much the other particle moved them. That's the same whatever their speed, because the expected increase in a particles displacement, due to it's speed is negated by the particles increase in mass, due to it's speed. In other words, the heavier something is, the harder it is to move.
Fundamentally, all energy is, is the power to move another object.

so photon energy +photon energy in collision = two photon energys.or one photon energy.which?my guess is one photon of energy.

An easier way to understand this concept sensibly, is to realise the word time is simply a label for observable change, so time dilation is how one process of change relates to another.

Well to put it simply it is not possible to look at another photon from its frame of reference because a it travels at c and we are mass objects and b it is undefined anyway, but if we sum the relative velocities using the lorentaz transforms, it turns out the two photons are colliding at c and comoving relatively at c (assuming a vacuum), and any other particle in say a hadron collider at just less than c no matter what speed they are going any mass objects will always be travelling at something less than c and any comoving objects with mass at less than c relatively, there is no point saying they are moving towards each other at 1.999999999...c because they simply are not, that is impossible. Two trains heading towards each other at 60mph will be relatively moving at 120mph, but as you get within fractions of the speed of light time dilation/length contraction effects take over and relatively they no longer are additive.

Look up the twins paradox for further interesting things about time and space dilation. It's not a paradox but it is interesting.

thanks blags but i aready know the easy stuff the same as most others its the trcky stuff i am interested in.

I think James makes a very good point; we don't have any direct evidence of time that we can measure independently of things happening: the world spinning, clocks ticking or atoms vibrating, for example. So despite it's usefulness as an abstract quantity in maths, there is no reason to believe time exists as such.
Photons don't really collide, they behave much more like waves and generally pass through each other unaffected. Fermions, matter particles, on the other hand do collide and regardless of how fast we think they are moving relative to each other, the energy released by Hadrons in the LHC is equivalent to two times a smidge under c. You can draw your own conclusions.