Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: nudephil on 01/02/2021 14:18:44
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Tony asks:
The current estimate of the age of the universe is about 13.8 billion years, yet the furthest object known is 13.4 billion light years away from us. That only leaves 0.4 billion years difference. How is this explained?
Also if nothing can exceed the speed of light, surely that implies that the universe is expanding at the speed of light? Or is there an idea that its speed slows the further it travels, which would give us different values of distance based on red shift?
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The universe is mostly empty space, so maybe there's nothing to see at 13.8 byl, or maybe we haven't seen it yet. Indeed why should the most distant object, whatever it is, have receded from us at the speed of light, with us at the center of creation?
The simplest interpretation of c being the maximum speed at which objects can move relative to one another, is that all relative motion must therefore be slower than c, so the universe could indeed be expanding more slowly from any standpoint, but certainly not more quickly.
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The current estimate of the age of the universe is about 13.8 billion years, yet the furthest object known is 13.4 billion light years away from us. That only leaves 0.4 billion years difference. How is this explained?
The furthest visible material known is currently about a proper distance of 44 billion LY away. This is the CMB, and the light was emitted about 380,000 years after the big bang. We cannot see further than that because the universe was opaque before then.
Also if nothing can exceed the speed of light
Nothing can exceed the speed of light locally in an inertial coordinate system. The distances quoted by you and myself above are neither local nor relative to an inertial coordinate system, so the limitation doesn't apply.
that implies that the universe is expanding at the speed of light?
The expansion rate is not a speed. Speed is measured in say km/sec, while the expansion rate is typically measured in km/sec/mpc, which are different units and cannot be compared to a speed like light speed.
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Suppose the speed of light is a natural constant. Throughout the Universe.
In the same way that the speed of sound is a natural constant. Throughout the Earth. Everywhere on Earth, sound travels at the same speed.
But that didn't stop us devising "supersonic" aircraft to exceed the speed of sound.
So couldn't we devise "superluminal" spacecraft to exceed the the speed of light?
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In the same way that the speed of sound is a natural constant.
It isn't.
But the speed of light in a vacuum is a constant.
And we know from both theoretical considerations, and from practical experience, that you can't get anything to go faster.
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In the same way that the speed of sound is a natural constant.
It isn't.
But the speed of light in a vacuum is a constant.
And we know from both theoretical considerations, and from practical experience, that you can't get anything to go faster.
In the 18th Century we knew, from theoretical considerations, and practical experience, that we couldn't get anything to go faster than a galloping horse or a sailing ship.
Therefore we believed it was not possible to transmit messages from England to America in less than three weeks.
However, this belief proved to be wrong.
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Everywhere on Earth, sound travels at the same speed.
Speed of sound in air: 343 m/s
Speed of sound in water: 1,433 m/s
Speed of sound in steel: 5,940 m/s
Speed of sound in diamond: 12,000 m/s
The speed of sound depends on the stiffness and density of the medium.
See: https://www.engineeringtoolbox.com/sound-speed-solids-d_713.html
Suppose the speed of light is a natural constant. Throughout the Universe.
If light enters glass, it slows down in that medium.
If light enters water or diamond, it slows down in that medium.
There is a specific case where subatomic particles can travel faster than the speed of light in that medium; this is responsible for the blue glow of a nuclear reactor. Emitting this Cherenkov radiation slows down the particles, so they no longer exceed speed of light in that medium.
We have not found anything that is less dense than a vacuum.
See: https://en.wikipedia.org/wiki/Cherenkov_radiation
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In the 18th Century we knew, from theoretical considerations, and practical experience, that we couldn't get anything to go faster than a galloping horse or a sailing ship.
No, we did not.
We knew practically, for example, that sound, light and a musket ball or even an arrow could overtake a horse.
And we had no useable theory to say how fast something could go.
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Suppose the speed of light is a natural constant. Throughout the Universe.
In the same way that the speed of sound is a natural constant. Throughout the Earth. Everywhere on Earth, sound travels at the same speed.
But that didn't stop us devising "supersonic" aircraft to exceed the speed of sound.
So couldn't we devise "superluminal" spacecraft to exceed the the speed of light?
The issue is fundamentally different. The speed of light isn't so much about light as it is about the upper limit to the speed of information transfer. It's just that light in a vacuum happens to travel at that same speed. Thanks to special relativity, we know that there are serious problems with the idea of traveling faster than light. One of those is the increase in relativistic mass of an object as it nears the speed of light. It tends towards infinity as you go faster and faster. Even supplying your ship with infinite energy (which is already impossible) will only get you up to the speed of light. You can't give your ship more than infinite energy, so you are prevented from going faster than light.
The other is time travel. Assuming you found a way around the first problem, you will end up traveling into the past and potentially running into time paradoxes.
The Alcubierre warp drive might be able to overcome the first problem because the expansion and contraction of space is not limited in the way the physical movement is. However, one objection I've seen to this is that your ship would have to send a signal to the space in front of you in order to tell it how to warp, and that signal would be limited by the speed of light. So even an Alcubierre drive might be limited by the speed of light.
The time paradox issue might be resolved by the Novikov self-consistency principle: https://en.wikipedia.org/wiki/Novikov_self-consistency_principle Of course, we don't know if it's true or not.