# Travelling more than Speed of light

1. Feb 10, 2008

### Habeeb03

is it possible to travel more than speed of light? And What happen if a substance travelling more than speed of light.

2. Feb 10, 2008

### mgb_phys

1, No
2, Anything you like - it doesn't happen

There are hypothetical particles called tachyons with negative mass that travel faster than light but they are just a mathematical game.

3. Feb 10, 2008

### Himanshu

It is impossible for a body object to travell faster than the speed of light. But speed is also relative. If we consider frame dragging in a region close to a rotating black hole and send a projectile in a circular orbit around the black hole, accelerate it to speed close to the speed of light and watch the projectile from a region where the effect of frame dragging is negligible. Wouldn't the object seem to travell faster than light.

4. Feb 10, 2008

### Chip Orr

An object can move faster than light realative to another object but only as a result of space expantion(Hubble Sphere etc.). The objects are not traveling through space, space is creating the movement. You can't travel faster than light through space but you can compared to another object in expanding space.

5. Feb 10, 2008

### marcus

Chip is right. Distances between stationary objects can increase at rates which are many times the speed of light. Indeed this is typical. There is a good Scientific American article that deals with some of these questions:
Misconceptions about the big bang March 2005. Here is a PDF.

http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf [Broken]

Here is an HTML link to the same article

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

The article had some very useful SIDEBARS giving pictorial diagrams with a question together with right and wrong answers explained. For easier access, here are links to individual sidebars.

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p39.gif
What kind of explosion was the big bang?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p40.gif
Can galaxies recede faster than light?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p42.gif
Can we see galaxies receding faster than light?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p43.gif
Why is there a cosmic redshift?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p44.gif
How large is the observable universe?

http://www.sciam.com/media/inline/0009F0CA-C523-1213-852383414B7F0147_p45.gif
Do objects inside the universe expand, too?

Last edited by a moderator: May 3, 2017
6. Feb 10, 2008

### Himanshu

Oo yes! I guess that's the reason that we can see only upto a limited distance into the space. Because after a certain distance the space itself seem to expand faster than the speed of light. So light beyond that point would never reach us.

Thank you marcus. That was a really cool article.

7. Feb 13, 2008

### Sorry!

" So light beyond that point would never reach us."
well it would reach us in some future time when the hubble distance grows large enough to encompass the light... then space is expanding away slower than the speed of light and it can make its journy to earth for us to see it.

8. Feb 13, 2008

### marcus

Sorry, you have grasped a key idea. There is a catch however, an exception to it.

Before 1998 everybody thought the cosm. constant was zero so in the standard nearly flat cosmology model (Omega > = 1) expansion would keep on gradually decelerating and the Hubble radius would keep on extending out farther and farther. then there would be no limit to what we could eventually see. If we could wait indefinitely, eventually all the light aimed in our direction would get here!
This is what you point out.

However after 1998 there was this business of the acceleration, the small positive Lambda, often represented by postulating a small constant dark energy density.

This limits how far out the Hubble radius can extend. It is beginning to plateau, so to speak. It means that stuff that today is more than 16 or so billion LY can no longer send us light.

We are already receiving light from stuff that is now over 45 billion LY away, so we can see a huge piece of the universe. But this limitation is about the future. that is the catch.

In the past, stuff that was receding way faster than c did send us light that succeeded in getting here, because the Hubble radius was extending out rapidly. But now we face a future where eventually the sky will not be so rich in information, regrettably.
Light that is sent today, from a galaxy that is currently 17 billion LY from us, will never (according to the prevailing LCDM model) be able to reach us.

9. Feb 14, 2008

### Sorry!

oh i see. thanks for that correction :p

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