Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: geordief on 29/08/2021 10:33:23
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(for slow learners :) )
I see there have been recent/ongoing interesting threads already and I doubt this is breaking any new ground ,even semanticly but here I go.
I have heard that the nature of speed of light in a vacuum may be better understood as reflecting the maximum speed of transfer of information in the universe.
So is there any sense in which light and information are joined at the hip?
Can information be transferred
without the mediation of a light signal? ( in either a practical or a theoretical sense)
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Can information be transferred
without the mediation of a light signal?
Yes.
I can wave a huge mass around, and use the gravity waves to transmit information.
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Hi.
We can thump on the ground and send ordinary compression (and transverse) waves through the soil and rock. Some animals are especially good at sensing vibrations (e.g. spiders). These waves travel slower than c and are nothing much to do with light.
Best Wishes.
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Hi again,
Let's look at the wider context of your question:
I have heard that the nature of speed of light in a vacuum may be better understood as reflecting the maximum speed of transfer of information in the universe.
Yes, seems reasonable. Several authors and You Tube content creators recommend that you consider c to be the speed of causality rather than the speed of light. Indeed, light does not always travel at c, it will only do this in a vacuum (this is complicated and has been mentioned in several other threads).
By "transfer of information" you can read "cause something to happen over here as a result of something done over there".
So is there any sense in which light and information are joined at the hip?
Quite possibly. There is a lot of complicated physics involved - not all of which I am aware of or understand. It's also possible to consider this issue as being more a matter of philosophy rather than physics. There are theories which consider information to be more fundamental than energy.
Can information be transferred
without the mediation of a light signal? ( in either a practical or a theoretical sense)
If you broaden your idea of a light signal then perhaps we can answer this with some standard physics. Light is just an oscillation in electric and magnetic fields. The gravitational wave described by Bored_Chemist earlier is just an oscillation in another field (the metric field). The standard model of physics would suggest that all forces or interactions are mediated by fundamental particles (bosons), in it's own turn Quantum Field Theory would represent all such particle interactions with fields. So every interaction could be considered as an oscillation in some field, i.e. something very similar to the propagation of light.
Hope you are well and the above gives you something to start with. Best Wishes.
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What's the relationship between the speed of information and the speed of light?
The speed of light was so-named because light (photons) was the first massless, energetic particle discovered (our eyes can sense them directly).
- We now understand (via Einstein's relativity) that if any massless particle carries energy, it must travel at c in a vacuum (eg gravitons).
- Particles with even a tiny bit of mass (like neutrinos) must travel slower than c.
Information is carried by modulating some carrier:
- It can be vibrations in the air (eg speech) - or vibrations through a string connecting two tins (a kids' telephone)
- It can be electrical signals through a copper wire (eg VDSL2) or through the air or a vacuum (eg radio)
- It can be light through the air (a ship's telegraph) or through an optical fiber (today's internet)
- It can be a stream of neutrinos from the LHC detected at Gran Sasso, or a pulse of gravitons from a black hole merger detected at LIGO.
Information is transferred by modulating some carrier.
- The information can't travel faster than the carrier it is modulated upon.
- How much slower depends in the rate at which you can modulate the carrier (and how quickly your detector can detect the modulation!)
If we take as an example today's common 10Gbps optical Ethernet, transmitted over 20km of single-mode optical fiber:
- The speed of light in the optical fiber (5μs/km = 100μs for 20km) is about 2/3 the speed of light in a vacuum. Information can't travel faster than this.
- The laser can turn on and off about 10 billion times per second, so if you wanted to transmit just 1 bit of information (OFF or ON), you could just switch the laser ON or OFF. The detector could report that bit of information about 0.1 ns after the light turned off - so it very slightly slower than the 100μs it took for the light signal to reach the other end.
- But we usually want to send more than 1 bit - Ethernet typically encapsulates information in larger chunks: frames that are about 12,000 bits long.
- An Ethernet frame takes 1.2 μs to transmit at 10Gbps. The receiving switch only forwards the frame when it has been fully received, so the frame arrives 1.2 μs later than the 100 μs travel time for the light. An internet page takes many Ethernet frames, so it takes even longer.
- When you get to higher speeds or farther distances (like the Voyager spacecraft), noise and various distortions like dispersion become major problems, so error-correcting codes are added. These take even longer to decode the information transmitted.
Claude Shannon worked out the maximum rate that you can send error-free information over a communications channel, and today, some systems are within 1 dB of this theoretical limit. Normally space-based systems are the most expensive, and try to get closest to the theoretical limits; the Low-Density Parity Check code first used on satellites is now integrated into 5G mobile.
See: https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem
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Not indicating relation to the speed of light here.
Quantum computers store and share information not in 2 states of a bit (0 or 1), but in qubits, which also take on so-called superposition states. The quantum state of a qubit is defined by the number of superconducting electron pairs.
https://en.wikipedia.org/wiki/Qubit
Classical information is measured using Shannon entropy, while the quantum mechanical analogue is Von Neumann entropy.