# HST-1 moving faster than light?

1. Sep 8, 2011

I'm no expert in astrophysics so forgive me if this is a trivial question, but I read in (the abstract of) a recent paper in the Journal of Astrophysics and Astronomy that "... It appears that HST-1 moves with an apparent speed of 1.23c±0.91c ..."

Now I want to clarify what this means. I take it c is the speed of light? I have no problem with 1.23c-0.91c but when one says 1.23c+0.91c would it not mean the thing is traveling faster than light?

The abstract is here: http://www.ias.ac.in/jaa/marjun2011/abstracts/04.htm
And the paper is here: http://www.ias.ac.in/jaa/marjun2011/04.pdf

2. Sep 8, 2011

### LJW

The 0.91c is a tolerance, meaning it could be moving as slowly as 0.32c or as quickly as 2.14c. You were right, c is the speed of light (in a vacuum).

Yes, it could mean that the particle could travel faster than the speed of light. While c is widely considered to be a universal speed limit, it's not an impossibility to travel above c. Tachyons come to mind (although being hypothetical) as particles which travel above c. Of course, no particle travelling above c may carry any form of information, which would not be in accordance with special relativity.

3. Sep 8, 2011

Great, thanks! That clears it.
Now I'll go read up on tachyons.

4. Sep 8, 2011

### Ryan_m_b

Staff Emeritus
Hold on a second, this really sounds wrong. What data do you have to support the idea that something can travel faster than c?

If I were you I would wait for the attention of an expert.

5. Sep 8, 2011

### Ryan_m_b

Staff Emeritus
6. Sep 8, 2011

### f95toli

Note also that space itself expands, this means that it is perfectly possible for two object (say stars) to distance themselves from each other by an apparent speed larger than c. Note the apparent, what is happening is that the space between them is increasing so neither object is travelling at a speed larger than c.

An interesting consequense of this is that there are regions in our universe that have forever lost contact, the space between them is expanding faster than c.

7. Sep 8, 2011

### SteamKing

Staff Emeritus
If I measure a value of 1.23 +/- 0.91, I really don't know what the value is that I'm supposed to be measuring.

8. Sep 8, 2011

Thank you, ryan.

It says c, so it's the speed of light, isn't it?

So the apparent speed is really the true velocity of the stars and the rate of expansion of the space between them, combined?

9. Sep 8, 2011

### SteamKing

Staff Emeritus
c is a constant, but that is not what's important. Suppose someone was asked to measure a distance of 1.23 meters, and then came back and said that 1.23 meters is this much +/- 0.91 meter, I don't think he or she would be asked to measure anything again.

10. Sep 8, 2011

### diazona

Although I don't know about this particular object, they're probably talking about the apparent transverse speed - that is, the speed at which the image of the object moves across the sky, taking into account its distance. Under certain circumstances, if the object is moving toward your telescope, the apparent transverse speed can be measured to be faster than light because the light emitted at early times has further to travel than the light emitted at later times. Wikipedia's article on superluminal motion has more details.

As far as the $1.23c\pm 0.91c$, the 0.91c is an uncertainty. They're saying there is a probability of X% that the true apparent transverse speed is between 0.32c and 2.14c, where X is probably either 68 (1 standard deviation) or 95 (2 standard deviation). I don't know which choice is conventional in that field.