Speed of Light and Size of the universe

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Discussion Overview

The discussion revolves around the apparent contradiction between the speed of light as a universal limit and the observed expansion of the universe, which suggests distances can increase at rates exceeding the speed of light. Participants explore the implications of this expansion, the concept of tachyons, and the nature of movement in the universe.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions how the universe can be 13.73 billion years old with a diameter of 93 billion light years if nothing can move faster than the speed of light.
  • Another participant suggests that while nothing can travel locally faster than light, the expansion of space itself can occur at rates greater than c.
  • There is mention of tachyons, hypothetical particles that are theorized to move faster than light, with a participant expressing curiosity about their implications and existence.
  • A later reply clarifies that relativity does not prohibit faster-than-light travel for certain entities but does impose constraints on how they can behave, such as not being able to slow down to the speed of light.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the speed of light limit and the nature of the universe's expansion. There is no consensus on the existence of tachyons or their characteristics, and the discussion remains unresolved regarding the broader implications of these concepts.

Contextual Notes

Participants acknowledge the complexity of the concepts involved, including the definitions of speed in the context of expanding space and the hypothetical nature of tachyons. There are unresolved questions about the implications of these ideas on our understanding of physics.

Praelly
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Hello, I am quite new to this forum, but have been lurking around reading your posts for some time now.

I have one question that has bothered me for some time. Scientists have been observing the universe with advanced technology, and seem quite certain that their observations indicate the age of the universe is approximately 13.73 billion years old, and that the "diameter" of the observable universe is 93 billion light years. My question is how is this possible if supposedly nothing can move faster than the speed of light? I've read somewhere that the universe itself expands with a speed greater than c, and I've also stumbled upon tachyons. Which also supposedly moves faster than light. I was wondering which "things" are limited to the speed c, and why is it this way?

This does not make any sense to me. I've watched numerous documentaries where physicists say that nothing can move faster than light. Have they not studied the universe's expansion, or is this just a matter of course?

I hope some of you can shed some light on this topic for me, I haven't found any good explanation to this yet..
 
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Hi,

This is quite a common question. My suggestion is to read this article: http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf
and come back with any further questions you may have.
 
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I'll give you a nutshell answer:

Nothing can travel locally faster than the speed of light, but there is no such constraint on points billions of light years apart. It is not the points that are moving faster than c, it is the space between them that is expanding. Locally, those points are relatively stationary.
 
Thanks! That article was very helpful.

But I'm still wondering about the tachyons, if they even exist why can they travel faster than c without receding speed
 
Praelly said:
Thanks! That article was very helpful.

But I'm still wondering about the tachyons, if they even exist why can they travel faster than c without receding speed
Tachyons are hypothetical particles; there is no reason to believe they exist. The point is that relativity does not forbid particles traveling faster than c, but it does forbid them slowing down to c. Also, they would travel backward in time.
 

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