Relativity and time dilation

[996c2]

If all motion are relative, why do you experience time dilation (relative to earth bound people) when traveling in a rocket away and then back to earth?

Surely, from the people on earth point of view, they are traveling away from the rocket and then back to the rocket. What's the difference?

[Einion Yrth]

996c2 said:

If all motion are relative, why do you experience time dilation (relative to earth bound people) when traveling in a rocket away and then back to earth?

Surely, from the people on earth point of view, they are traveling away from the rocket and then back to the rocket. What's the difference?

In brief; acceleration. Slightly longer - useful explanations of the twins paradox.

[James_B]

996c2 said:

If all motion are relative, why do you experience time dilation (relative to earth bound people) when traveling in a rocket away and then back to earth?

Surely, from the people on earth point of view, they are traveling away from the rocket and then back to the rocket. What's the difference?

Two observers travelling relative to one another will both observe the other’s clock running slower than their own.

[SystemParanoia]

Great book using simple maths to explain such a complicated topic.

[Chris_OB]

I think this stuff becomes easier to understand (for a layman, and only very basically - which I still struggle with even after reading everything my underpowered brain will allow me to take in) when you bear in mind that, at light speed (in a vacuum), a photon does not experience time.

Because it's traveling from point A to point B at the fastest speed the universe will allow, time as a measurement of distance travelled is basically zero.

So if you're climb into your Space Type R and gun it using the V-Tech to full effect, you're essentially experiencing less time than those at rest (on Earth).

Then consider the Sands of Time analogy... because you've been whizzing around skipping time, everyone's been decaying at their normal rate whilst you've been cheating the ageing process (in theory - if you were impervious to the radiation of space!)

That's basically my understanding, shallow though it may be... happy to be corrected.

Two facts that blew mind to pieces - satellites have to adjust their clocks for Relativity. And Scott Kelly has a twin...

""So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said"

https://www.space.com/33411-astronaut-scott-kelly-...

[Toltec]

Chris_OB said:

a photon does not experience time

I've wondered about that myself, in a sense that means looking at a distant galaxy is instantaneously exchanging energy with matter billions of years in your past. Looking through a telescope at the night sky means you are touching the past rather than looking at a photograph of it, capturing that photon with your eye is a unique event, even if there are many very similar ones.

[Chris_OB]

Toltec said:

I've wondered about that myself, in a sense that means looking at a distant galaxy is instantaneously exchanging energy with matter billions of years in your past. Looking through a telescope at the night sky means you are touching the past rather than looking at a photograph of it, capturing that photon with your eye is a unique event, even if there are many very similar ones.

Bonkers, isn't it?

And the more you think about it, the weirder it gets. Because the universe is expanding, that light will be redshifted... i.e. that object is moving away at speed enough to alter its colour because the very fabric of the universe is expanding between you and it And that light could be warped, depending on the gravitational field it's travelled through before colliding with your eye...

Then things like; with the speed of light being ~300,000,000 meters per second, it takes around 1.3 second after bouncing off the Moon to hit your eye. And the Moon is 384,000km away...

And it takes EIGHT MINUTES for a photon to leave the Sun and bash into your cornea!

http://joshworth.com/dev/pixelspace/pixelspace_sol...

And Andromeda is 2.5 million light years from us...

It's a rabbit hole that never ends, this stuff.

Edited by Chris_OB on Thursday 14th December 12:15

[Toltec]

Chris_OB said:

Bonkers, isn't it?

And the more you think about it, the weirder it gets. Because the universe is expanding, that light will be redshifted... i.e. that object is moving away at speed enough to alter its colour because the very fabric of the universe is expanding between you and it And that light could be warped, depending on the gravitational field it's travelled through before colliding with your eye...

Then things like; with the speed of light being ~300,000,000 meters per second, it takes around 1.3 second after bouncing off the Moon to hit your eye. And the Moon is 384,000km away...

And it takes EIGHT MINUTES for a photon to leave the Sun and bash into your cornea!

http://joshworth.com/dev/pixelspace/pixelspace_sol...

And Andromeda is 2.5 million light years from us...

It's a rabbit hole that never ends, this stuff.

Edited by Chris_OB on Thursday 14th December 12:15

You try and explain this stuff and why you find it exciting and they just think you are weird!

[AshVX220]

Chris_OB said:

...snip...when you bear in mind that, at light speed (in a vacuum), a photon does not experience time...snip...

So, this is where I really struggle, time is an inherent attribute of speed, speed is always considered as distance covered in a set time, so the thought that because you're travelling at c (which know is (whatever it is) per second/minute/hour) you don't experience the time bit......(mind blown again).

[Moonhawk]

Chris_OB said:

And it takes EIGHT MINUTES for a photon to leave the Sun and bash into your cornea!

From our perspective.

From the photon's perspective, this happens simultaneously.

[SystemParanoia]

Moonhawk said:

Chris_OB said:

And it takes EIGHT MINUTES for a photon to leave the Sun and bash into your cornea!

From our perspective.

From the photon's perspective, this happens simultaneously.

Doesn't Simultaneity breakdown at relativistic speeds

[Chris_OB]

Moonhawk said:

Chris_OB said:

And it takes EIGHT MINUTES for a photon to leave the Sun and bash into your cornea!

From our perspective.

From the photon's perspective, this happens simultaneously.

Yeah I said that in my original post.

I do get a bit carried away and waffly when this topic arises. I forget where I am. Ha.

[Dr Jekyll]

Toltec said:

Chris_OB said:

a photon does not experience time

I've wondered about that myself, in a sense that means looking at a distant galaxy is instantaneously exchanging energy with matter billions of years in your past. Looking through a telescope at the night sky means you are touching the past rather than looking at a photograph of it, capturing that photon with your eye is a unique event, even if there are many very similar ones.

That's what gets me.

From a photons point of view it isn't something that leaves an emitter and might in due course arrive at a detector. It's a relationship between the emitter and the detector.

[V8LM]

Dr Jekyll said:

Toltec said:

Chris_OB said:

a photon does not experience time

I've wondered about that myself, in a sense that means looking at a distant galaxy is instantaneously exchanging energy with matter billions of years in your past. Looking through a telescope at the night sky means you are touching the past rather than looking at a photograph of it, capturing that photon with your eye is a unique event, even if there are many very similar ones.

That's what gets me.

From a photons point of view it isn't something that leaves an emitter and might in due course arrive at a detector. It's a relationship between the emitter and the detector.

Yes, but what colour is it?

[Chris_OB]

V8LM said:

Dr Jekyll said:

Toltec said:

Chris_OB said:

a photon does not experience time

I've wondered about that myself, in a sense that means looking at a distant galaxy is instantaneously exchanging energy with matter billions of years in your past. Looking through a telescope at the night sky means you are touching the past rather than looking at a photograph of it, capturing that photon with your eye is a unique event, even if there are many very similar ones.

That's what gets me.

From a photons point of view it isn't something that leaves an emitter and might in due course arrive at a detector. It's a relationship between the emitter and the detector.

Yes, but what colour is it?

Red.

[Alias218]

SystemParanoia said:

Moonhawk said:

Chris_OB said:

And it takes EIGHT MINUTES for a photon to leave the Sun and bash into your cornea!

From our perspective.

From the photon's perspective, this happens simultaneously.

Doesn't Simultaneity breakdown at relativistic speeds

I have always wondered this. If velocity = distance/time then surely it stands that if time is effectively equal to zero (as light doesn't experience time) then it's velocity is infinite and not 300,000,000 m/s.

Or as you say, does this kind of reasoning fall apart at these speeds? And if so, why from our frame of reference is v = 300,000,000 and from light's frame of reference v = infinite?

I remember reading 'Quantum Theory Cannot Hurt You' by Marcus Chown in my teens and having my puny brain tied in knots trying to make sense of it all, despite it being in laymans terms.

[motco]

Where is the datum for measuring the speed of light if it's always the same relating to this datum regardless of the speed of the light source. Something must be stationary surely, or the speed is meaningless.

[James_B]

motco said:

Where is the datum for measuring the speed of light if it's always the same relating to this datum regardless of the speed of the light source. Something must be stationary surely, or the speed is meaningless.

What are you trying to ask?

If you measure speed, it will generally be relative to some frame. A common one, for example, would be the lab frame, I,e, a set of coordinates in which you say that you and your equipment, the tables, chairs etc are at rest.

One of the main points of relativity is that there is no absolute “at rest”, it’s only relative move,ent that matters.

[Toltec]

V8LM said:

Yes, but what colour is it?

Mukreto™

[RobM77]

ash73 said:

Brian Cox's The Science of Dr Who was on BBC 4 last night - worth a watch

https://www.bbc.co.uk/iplayer/episode/b03hybnv/the...

I watched that and his explanation of time dilation was beautiful I'd encourage the OP to look up the derivation of the Lorentz Transformations - it's basically the maths of what Brian described and is no more than GCSE level, so easy for most to follow.

Basically, the whole thing revolves around the speed of light being seen as constant for all observers. At low speeds for most calculations that we do in everyday life, if someone is moving at speed relative to you, the physics of experiments that you observe them doing all works nicely by adding their speed to the speed of things they're doing. For example, a cricket bowler who can bowl at 100mph would be observed bowling at 150mph if he was on a 50mph train and you and the batsman were stationary. If you run the maths on all the physics and physiology involved in bowling the ball, you just add 50mph to everything and it all works out nicely. The same goes for a clock pendulum ticking back and forth in a moving car - the pendulum is swinging in a crazy way for an observer at the side of the road, but add the speed of the car onto everything and it all works. Before Einstein, this was how we did things.

Einstein realised that if you have to see the speed of light moving at the speed of light and no faster, then to make an observation, you obviously can't just add the relative speed of the two reference frames to the speed of light, like you do with cricket balls on trains, because the speed of light can't change. Instead you have to change the other parameters involved to compensate: time, distance and mass. The amount you have to change them by is related to the proportion of light speed the two reference frames are travelling relative to one another (it's all done to make light speed appear as light speed and no faster or slower), which is why we don't notice the effects in everyday life, because we usually travel really slowly compared to the speed of light. Even the bowler and batsman in my example would experience slight differences due to this Einsteinien effect, but it would be unbelievably tiny. This phenomenon has been observed over and over again and it turns out that Einstein was bang on. One of the simplest demonstrations is that we see some particles generated in the upper atmosphere down here on earth, even though they should have decayed away by the time they reach us - we see them because time travels slower and their decay hasn't occurred by the time they reach us. General Relativity extended this principle to gravity, and again, we've seen those effects repeatedly and they're bang on. Even the extreme measurements that LIGO did of colliding black holes and neutron stars demonstrated a near perfect correlation with Einstein's relativity theories.

As well as running through the Lorentz transformations for SR, I'd recommend reading into GR. It's my favourite theory (just ahead of evolution by natural selection!) because it's so beautiful and so well proven.

Edited by RobM77 on Friday 15th December 15:32