Thinking about Relativity

[Ash_]

So, I still sometimes have a WTF moment when I think about this. The theory (fact now) that time passes differently for an observer and a person/object travelling at the speed of light (or close to it).

So, me thinking about this, is the following scenario correct?

A Space Probe is traveling away from us at for example 80% of c. While doing so it is continually sending a radio signal, eg one beep every second.

For the probe, every second it sends its beep. But as it travels farther away from us at such a high speed, the time it takes for the beep to reach us takes longer each time, so the time between each beep keeps getting longer from our point of view at the receiving end.

Is that right and is that how Relativity works, or is it right, but nothing to do with Relativity...or is completely wrong?

It was the Voyager thread that got me thinking about this.

[Eric Mc]

We can see what happens to radio broadcasts which are influenced by relativity all the time - both with current space probes (which are travelling at a tiny fraction of the speed of light) and distant galaxies, quasars etc, (which are travelling at appreciable fractions of the speed of light).

What happens is that all electromagnetic emissions from these objects (including those in visible light, radio waves, infra-red etc) get stretched to a greater or lesser extent - depending on the speed of recession. This is known as The Doppler Effect and happens with sound as well, although at much lower speed.

When talking about visible light, we refer to "red shift" as light from a receding object shifts towards the lower (red) end of the visible spectrum. Objects approaching would have their light shifted towards the blue end of the light spectrum (blue shift).

The same thing happens to radio and other waves. If you are trying to tune into (say) the radio transmissions from either of the Voyager probes, you need to tune at a slightly lower frequency to the actual frequency being used by the spacecraft as it will have been shifted to a slightly lower frequency due to the Doppler Effect.

[Ash_]

Eric Mc said:

We can see what happens to radio broadcasts which are influenced by relativity all the time - both with current space probes (which are travelling at a tiny fraction of the speed of light) and distant galaxies, quasars etc, (which are travelling at appreciable fractions of the speed of light).

What happens is that all electromagnetic emissions from these objects (including those in visible light, radio waves, infra-red etc) get stretched to a greater or lesser extent - depending on the speed of recession. This is known as The Doppler Effect and happens with sound as well, although at much lower speed.

When talking about visible light, we refer to "red shift" as light from a receding object shifts towards the lower (red) end of the visible spectrum. Objects approaching would have their light shifted towards the blue end of the light spectrum (blue shift).

The same thing happens to radio and other waves. If you are trying to tune into (say) the radio transmissions from either of the Voyager probes, you need to tune at a slightly lower frequency to the actual frequency being used by the spacecraft as it will have been shifted to a slightly lower frequency due to the Doppler Effect.

Ah yes, I understand the Doppler effect and Red/Blue Shift, but my tiny brain didn't make the connection!

So, what I was thinking about is indeed a Relativity related thing.

Edited by Ash_ on Wednesday 25th November 12:53

[deckster]

Well yes and no. The doppler effect is absolutely part of classical physics and needs no particular reference to relativity to explain it. Simple examples like an ambulance driving past you have very little to do with relativity.

However, there is also a relativistic doppler effect, and, well. Feel free to read up on it as it's way beyond me https://en.wikipedia.org/wiki/Relativistic_Doppler...

[Eric Mc]

Carl Sagan had a bash at this in an episode of "Cosmos" back in 1980 using scooters

[Ash_]

deckster said:

Well yes and no. The doppler effect is absolutely part of classical physics and needs no particular reference to relativity to explain it. Simple examples like an ambulance driving past you have very little to do with relativity.

However, there is also a relativistic doppler effect, and, well. Feel free to read up on it as it's way beyond me https://en.wikipedia.org/wiki/Relativistic_Doppler...

Although the equations and stuff go way over my head, I think I get the basics and I think what I said in my original post kind of shows how relativity is at work.....I think.

Thanks Eric, I'll see if I can find that episode (or even the entire show) somewhere.

[Mr Pointy]

Ash_ said:

Although the equations and stuff go way over my head, I think I get the basics and I think what I said in my original post kind of shows how relativity is at work.....I think.

Thanks Eric, I'll see if I can find that episode (or even the entire show) somewhere.

I found this series of YT videos very informative:

https://www.youtube.com/watch?v=1rLWVZVWfdY

[Ash_]

Mr Pointy said:

Ash_ said:

Although the equations and stuff go way over my head, I think I get the basics and I think what I said in my original post kind of shows how relativity is at work.....I think.

Thanks Eric, I'll see if I can find that episode (or even the entire show) somewhere.

I found this series of YT videos very informative:

https://www.youtube.com/watch?v=1rLWVZVWfdY

Thank you, I'll give that a watch.

[V8LM]

Ash_ said:

For the probe, every second it sends its beep. But as it travels farther away from us at such a high speed, the time it takes for the beep to reach us takes longer each time, so the time between each beep keeps getting longer from our point of view at the receiving end.

The problem with this is what happens when the space craft turns around and is then travelling at 0.8c towards us?

[Krikkit]

Mr Pointy said:

I found this series of YT videos very informative:

https://www.youtube.com/watch?v=1rLWVZVWfdY

Dunno if I was lucky, but Special Relativity was taught to me at uni as almost half of a semester-long course, not just an afterthought.

Still a brain melter though, doubly so once you get to General Relativity.

[Beati Dogu]

Eric Mc said:

Carl Sagan had a bash at this in an episode of "Cosmos" back in 1980 using scooters

Didn't Einstein figure it out while on a tram in Vienna or something?

[Eric Mc]

Beati Dogu said:

Didn't Einstein figure it out while on a tram in Vienna or something?

They had very fast trams in Vienna.

[Ash_]

V8LM said:

Ash_ said:

For the probe, every second it sends its beep. But as it travels farther away from us at such a high speed, the time it takes for the beep to reach us takes longer each time, so the time between each beep keeps getting longer from our point of view at the receiving end.

The problem with this is what happens when the space craft turns around and is then travelling at 0.8c towards us?

Bingo! You're right (for my analogy), then the time between beeps gradually gets shorter, until we're back at a 1 second beep at the Rx end.

[Beati Dogu]

Eric Mc said:

Beati Dogu said:

Didn't Einstein figure it out while on a tram in Vienna or something?

They had very fast trams in Vienna.

Hyperloop perhaps.

[MiseryStreak]

Where that analogy becomes really twisted is if your space probe was able to travel at c, which it isn’t as that would create mass/energy at every point along its journey and therefore become infinitely massive/energetic, but let’s pretend.

As anything travelling at 100% c in the physical dimensions is travelling at 0% in the temporal dimension, and has no experience of time, the ‘beeps’ would stop and would not be produced until the probe slowed to turn around. It would then accelerate to c and no further beeps produced until slowing down for its Earth landing.

So it would arrive back on Earth around the same time as the only beeps it produced, which were at its turnaround point. It would be gone for years and we wouldn’t know the probe was OK until just before it landed again. It would also contain just a few minutes of recorded data.

So if you want a probe to be useful and observe things on its travels and not just at the destination, slow it down a bit. Luckily it has to!

Relativity is crazy but it would be even crazier without it, everything would happen all at once.

Now, I’ve got a very powerful laser pointer which I shine onto a distance gas giant. I move the laser pointer very quickly, so that the point of light moves across the face of the planet from one side to the other quicker than light could travel that distance. Is the laser spot exceeding c? If not, how did it get there so fast?

[Ash_]

MiseryStreak said:

Where that analogy becomes really twisted is if your space probe was able to travel at c, which it isn’t as that would create mass/energy at every point along its journey and therefore become infinitely massive/energetic, but let’s pretend.

As anything travelling at 100% c in the physical dimensions is travelling at 0% in the temporal dimension, and has no experience of time, the ‘beeps’ would stop and would not be produced until the probe slowed to turn around. It would then accelerate to c and no further beeps produced until slowing down for its Earth landing.

So it would arrive back on Earth around the same time as the only beeps it produced, which were at its turnaround point. It would be gone for years and we wouldn’t know the probe was OK until just before it landed again. It would also contain just a few minutes of recorded data.

So if you want a probe to be useful and observe things on its travels and not just at the destination, slow it down a bit. Luckily it has to!

Relativity is crazy but it would be even crazier without it, everything would happen all at once.

Now, I’ve got a very powerful laser pointer which I shine onto a distance gas giant. I move the laser pointer very quickly, so that the point of light moves across the face of the planet from one side to the other quicker than light could travel that distance. Is the laser spot exceeding c? If not, how did it get there so fast?

OK, that has completely melted my brain now, so......thanks for that!

[Dr Jekyll]

MiseryStreak said:

Where that analogy becomes really twisted is if your space probe was able to travel at c, which it isn’t as that would create mass/energy at every point along its journey and therefore become infinitely massive/energetic, but let’s pretend.

As anything travelling at 100% c in the physical dimensions is travelling at 0% in the temporal dimension, and has no experience of time, the ‘beeps’ would stop and would not be produced until the probe slowed to turn around. It would then accelerate to c and no further beeps produced until slowing down for its Earth landing.

So it would arrive back on Earth around the same time as the only beeps it produced, which were at its turnaround point. It would be gone for years and we wouldn’t know the probe was OK until just before it landed again. It would also contain just a few minutes of recorded data.

This something I don't get. From the probes point of view it's stayed in one spot and the earth has moved away and back. So an observer on the probe would expect less time to have elapsed on earth.

MiseryStreak said:

So if you want a probe to be useful and observe things on its travels and not just at the destination, slow it down a bit. Luckily it has to!

Relativity is crazy but it would be even crazier without it, everything would happen all at once.

Now, I’ve got a very powerful laser pointer which I shine onto a distance gas giant. I move the laser pointer very quickly, so that the point of light moves across the face of the planet from one side to the other quicker than light could travel that distance. Is the laser spot exceeding c? If not, how did it get there so fast?

The laser spot isn't a 'thing' any more than the bit of Slough I'm looking at or the bit of the Andromeda I look at a second later constitute something moving from one to the other.
Consider a wave hitting a boat and washing over it. It seems like a 'thing' moving from bow to stern. But once the wave hits the boat at an angle it can appear to move from the bow to the stern at any speed, even infinite speed if the wave is hitting the boat broadside on.

[MiseryStreak]

Well you destroyed that little thought experiment without too much trouble. It comes down to semantics really, and that some of the things we apply labels to aren’t really things at all. There are lots of different versions of it that involve trying to communicate information superluminally, but obviously you can’t. For anything to
exceed c is to travel backwards in time, which immediately crashes into the infinite mass/energy paradox, among others.

An observer on the light speed probe would not experience anything whilst travelling at c. But in reality inertial frames can’t exist at c, so it’s wrong to suggest that light ‘experiences’ no time, as there is no experience, nor space and time, at c. Distances shorten to zero and time slows to a stop. The light is actually at all points on its journey at once, but that point doesn’t exist!

Because the speed of light is just a number, and not a particularly big one, it seems intuitive that we should be able to exceed it. But as distances shorten to nothing and time stops, it’s actually instantaneous.

[Dr Jekyll]

The interesting consequence is that if things that travel at the speed of light (like err, light) don't experience time, it may not make sense to talk of them leaving an emitter and sometime later arriving at a detector, They could be considered to be just a relationship between the two.

[Chester35]

Ash_ said:

So, I still sometimes have a WTF moment when I think about this. The theory (fact now) that time passes differently for an observer and a person/object travelling at the speed of light (or close to it).

So, me thinking about this, is the following scenario correct?

A Space Probe is traveling away from us at for example 80% of c. While doing so it is continually sending a radio signal, eg one beep every second.

For the probe, every second it sends its beep. But as it travels farther away from us at such a high speed, the time it takes for the beep to reach us takes longer each time, so the time between each beep keeps getting longer from our point of view at the receiving end.

Is that right and is that how Relativity works, or is it right, but nothing to do with Relativity...or is completely wrong?

It was the Voyager thread that got me thinking about this.

You are correct.

If we ignore the extra distance the beep takes to travel, special relativity says that the clock ticking for the observer on earth received from a spacecraft going away from us will be slower than the clock ticking for the person in that spacecraft if the relative velocity is different.